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1.5: Pollination and Seed Dispersal - Biology

1.5: Pollination and Seed Dispersal - Biology



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An estimated 90 percent of flowering plants depend on pollinators such as wasps, birds, bats, and bees, to reproduce. Plants and their pollinators are increasingly threatened around the world (Buchmann and Nabhan 1995; Kremen and Ricketts 2000). Pollination is critical to most major crops and virtually impossible to replace. For instance, imagine how costly fruit would be (and how little would be available) if its natural pollinators no longer existed and each developing flower had to be fertilized by hand.

Many animal species are important dispersers of plant seeds. It has been hypothesized that the loss of a seed disperser could cause a plant to become extinct. At present, there is no example where this has occurred. A famous example that has often been cited previously is the case of the dodo (Raphus cucullatus) and the tambalacoque (Sideroxylon grandiflorum). The dodo, a large flightless bird that inhabited the island of Mauritius in the Indian Ocean, became extinct due to overhunting in the late seventeenth century. It was once thought that the tambalacoque, a now endangered tree, depended upon the dodo to germinate its hard-cased seeds (Temple 1977). In the 1970s, only 13 trees remained and it was thought the tree had not reproduced for 300 years. The seeds of the tree have a very hard coat, as an experiment they were fed to a turkey; after passing through its gizzard the seeds were viable and germinated. This experiment led scientists to believe that the extinction of the dodo was coupled to the tambalacoque's inability to reproduce. However, this hypothesis has not stood up to further scrutiny, as there were several other species (including three now extinct species, a large-billed parrot, a giant tortoise, and a giant lizard) that were also capable of cracking the seed (Witmar and Cheke 1991; Catling 2001). Thus many factors, including the loss of the dodo, could have contributed to the decline of the tambalacoque. (For further details of causes of extinction see Historical Perspectives on Extinction and the Current Biodiversity Crisis). Unfortunately, declines and/or extinctions of species are often unobserved and thus it is difficult to tease out the cause of the end result, as multiple factors are often operating simultaneously. Similar problems exist today in understanding current population declines. For example, in a given species, population declines may be caused by loss of habitat, loss in prey species or loss of predators, a combination of these factors, or possibly some other yet unidentified cause, such as disease.

In the pine forests of western North America, corvids (including jays, magpies, and crows), squirrels, and bears play a role in seed dispersal. The Clark's nutcracker (Nucifraga columbiana) is particularly well adapted to dispersal of whitebark pine (Pinus albicaulis) seeds (Lanner 1996). The nutcracker removes the wingless seeds from the cones, which otherwise would not open on their own. Nutcrackers hide the seeds in clumps. When the uneaten seeds eventually grow, they are clustered, accounting for the typical distribution pattern of whitebark pine in the forest.

In tropical areas, large mammals and frugivorous birds play a key role in dispersing the seeds of trees and maintaining tree diversity over large areas. For example, three-wattled bellbirds (Procnias tricarunculata) are important dispersers of tree seeds of members of the Lauraceae family in Costa Rica. Because bellbirds return again and again to one or more favorite perches, they take the fruit and its seeds away from the parent tree, spreading Lauraceae trees throughout the forest (Wenny and Levy 1998).


1.5: Pollination and Seed Dispersal - Biology

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seed, the characteristic reproductive body of both angiosperms (flowering plants) and gymnosperms (e.g., conifers, cycads, and ginkgos). Essentially, a seed consists of a miniature undeveloped plant (the embryo), which, alone or in the company of stored food for its early development after germination, is surrounded by a protective coat (the testa). Frequently small in size and making negligible demands upon their environment, seeds are eminently suited to perform a wide variety of functions the relationships of which are not always obvious: multiplication, perennation (surviving seasons of stress such as winter), dormancy (a state of arrested development), and dispersal. Pollination and the “seed habit” are considered the most important factors responsible for the overwhelming evolutionary success of the flowering plants, which number more than 300,000 species.

The superiority of dispersal by means of seeds over the more primitive method involving single-celled spores, lies mainly in two factors: the stored reserve of nutrient material that gives the new generation an excellent growing start and the seed’s multicellular structure. The latter factor provides ample opportunity for the development of adaptations for dispersal, such as plumes for wind dispersal, barbs, and others.

Economically, seeds are important primarily because they are sources of a variety of foods—for example, the cereal grains, such as wheat, rice, and corn (maize) the seeds of beans, peas, peanuts, soybeans, almonds, sunflowers, hazelnuts, walnuts, pecans, and Brazil nuts. Other useful products provided by seeds are abundant. Oils for cooking, margarine production, painting, and lubrication are available from the seeds of flax, rape, cotton, soybean, poppy, castor bean, coconut, sesame, safflower, sunflower, and various cereal grains. Essential oils are obtained from such sources as juniper “berries,” used in gin manufacture. Stimulants are obtained from such sources as the seeds of coffee, kola, guarana, and cocoa. Spices—from mustard and nutmeg seeds from the aril (“mace”) covering the nutmeg seed from the seeds and fruits of anise, cumin, caraway, dill, vanilla, black pepper, allspice, and others—form a large group of economic products.


Fruit & Seed Dispersal

Feature image. A selection of fruits showing structural modifications for different modes of dispersal. Left: Uncarina ankaranensis fruit showing barbs for adherence to animal fur (epizoochory). Center: Box elder (Acer negundo) with winged fruits for wind dispersal (anemochory). Right: Bladdernut (Staphylea colchica) with inflated capsules that may facilitate water dispersal (hydrochory) two individual seeds also shown. Credits: Acer negundo (MHNT.BOT.2007.40.12) and Staphylea cochica (MHNT.BOT.2006.70.4) by Roger Culos and Uncarina ankaranensis (MHNT.BOT.2011.18.22) by Didier Descouens (all from Muséum de Toulous, via Wikimedia Commons, CC BY-SA 3.0). Images modified from originals.

Topics covered on this page:

Introduction

Seed dispersal—the movement of a seed away from its parent plant, often facilitated by a vector (e.g., animals, wind)—has several potential advantages. On the level of the individual, dispersal provides an opportunity for seedlings to establish themselves away from their parent plants, potentially occupying new and/or more favorable habitats. Dispersal also facilitates more genetic mixing in a populatio n because related individuals are less likely to be clustered close to one another.

The unit of dispersal in angiosperms may be the seed itself, or a seed (or seeds) enclosed within a fruit. Fruits or seeds of angiosperms are often modified to enhance dispersal. Dispersal may occur by a number of different means, including gravity (basically, a simple means of dispersal involving the seed falling and potentially rolling downslope a short distance), wind, water, animals, and ballistic dispersal (adaptations that launch seeds from the fruit). Dispersal syndromes are suites of fruit or seed traits that correlate with certain modes of di spersal. For example, wings are associated with wind-dispersal, whereas fleshy structures are associated with animal dispersal. As with pollination syndromes, dispersal syndromes can be used to infer the likely dispersal mode of a particular fruit or seed type. I t should be noted, however, that mode of dispersal may differ from—or may be more variable than suggested by—the structural attributes of a particular type of fruit or seed. Units of dispersal (in this case, fruits or seeds that serve as the units of dispersal) are called diaspores or disseminules.

The earliest angiosperms typically had small disseminules that did not exhibit many specialized modifications to facilitate dispersal. During the Paleogene, fruit and seed size became more diverse, and fossilized disseminules commonly exhibit specialized adaptations to enhance dispersal most notably, fleshy fruits (animal-dispersed), nuts (animal-dispersed), and winged fruits and seeds (wind-dispersed) became diverse and abundant. Fruits and seeds with hairs (wind-dispersed) and spines (often animal-dispersed by adherence) are present but less common in the Paleogene fossil record (discussed here). In fossil angiosperms, dispersal mode is typically inferred from the structural attributes of disseminules (fruits or seeds) and/or the dispersal modes of modern taxa related fossil plants.

Fossil fruits with different adaptations for dispersal. Left: Endocarp (fruit stone or pit) of Suciacarpa xiangae (Late Cretaceous, Spray Formation, Vancouver Island, Canada), possibly adapted for dispersal by ingestion the fleshy structure that likely surrounded the endocarp is not preserved. Scale bar = 2 mm. Center: Nuts (Corylus johnsonii, Eocene, Klondike Mountain Formation, Washington, U.S.A.), probable dispersal by storage/caching. Right: Winged fruits (Dillhoffia cachensis, Eocene, Klondike Mountain Formation, Washington, U.S.A.), probable dispersal by wind. Credits: Suciacarpa xiangae, Holotype, SH 790, Fig. 1A in Atkinson et al. (2017) Int. J. Plant Sci. (CC BY 4.0) Corylus johnsonii, SR 99-07-10 (Kevmin, via Wikimedia Commons, CC BY-SA 3.0) Dillhoffia cachensis, SR 92-17-20 (Kevmin, via Wikimedia Commons, CC BY-SA 3.0). Images modified from originals.

Animal dispersal (zoochory)

Many animal-dispersed fruits are dispersed by vertebrates—especially certain mammals and birds, although fish and reptiles can also act as dispersal agents—or ants. Vertebrate-dispersed fruits and seeds may be fleshy, or may have fleshy coverings ant-dispersed seeds often have nutrient-rich appendages.

Dispersal by ingestion (endozoochory)

Many of the fleshy fruits that humans enjoy—such as raspberries (Rubus) and cherries (Prunus)—are adapted for dispersal by vertebrates. Some fleshy fruits are consumed with seeds intact. The seeds pass through the digestive tract of an animal and are deposited elsewhere germination may be enhanced by weakening of the seed coat as it passes through the digestive tract. Alternatively, seeds may be covered by a hard inner fruit wall (the endocarp, also known as the pit or stone) that is not digested. Sometimes, seeds may be regurgitated rather than passing entirely through the digestive tract. Dispersal through transport in the gut of an animal is endozoochory (Greek endon + zooin = within animal).

In the fossil record, seeds that were probably dispersed via ingestion are often found without the surrounding fleshy fruiting structure, so dispersal by ingestion must often be inferred by comparison to modern plants. Occasionally, however, more direct evidence may be discovered. For example, well-preserved fruits may retain fleshy structures. Seeds may also be found in coprolites (fossilized poop), demonstrating that they were ingested and passed through an animal's digestive tract (see here for one study). Seeds may even be found within a fossilized animal's gut (see here, for example).

Ancient and modern feces showing evidence of seed dispersal. Left: Modern American black bear (Ursus americanus) feces with partially digested fruits and seeds or pits. Right: Moa (Moa) coprolite (fossilized feces) from New Zealand with coprosma (Coprosma) seeds. Credits: Black bear scat (Courtney Celley, USFWS Midwest Region, via flickr, CC BY 2.0) moa coprolite (Jo Carpenter, via The Conversation, CC BY-SA 4.0). Images modified from originals.

In addition to fleshy fruits and fruit appendages, seeds themselves can have fleshy structures that attract animals. In such cases, the seed and its fleshy structure or covering may be consumed whole. In other cases (like ant-dispersed seeds with elaiosomes, discussed below), the fleshy structure alone is consumed, with the seed left behind. Examples of fleshy seed structures are:

  • Aril: An aril is a fleshy or leathery structure that partially or completely surrounds a seed. An example is the red aril that surrounds the nutmeg (Myristica fragrans) seed and yields the spice mace.
  • Sarcotesta: A sarcotesta (from the Greek sarx, flesh) is a fleshy layer of the seed coat. Sarcotestae are perhaps best known from the seeds of some gymnosperms (e.g., the stinky sarcotesta on Ginkgo seeds), but can also be found on seeds of some angiosperms like magnolia (Magnolia) and pomegranate (Punica granatum).

As with fleshy fruits, direct evidence for these fleshy seed structures or appendages may be hard to come by in the fossil record, so comparison to extant relatives may suggest the mode of dispersal.

Dispersal by ants (myrmecochory)

Seed dispersal by ants is known as myrmecochory (Gree, myrmēx = ant). Seeds dispersed by ants often have small fat- and protein-rich appendages called elaiosomes. Ants typically transport seeds with elaiosomes to their nests, where they detach the elaiosomes from the seeds thus, ant dispersal tends to occur over short distances. Ant dispersal, as inferred by the presence of elaiosomes on seeds, is widespread in flowering plants a study estimated that ant dispersal has evolved more that 100 times in angiosperms (see here).

Fleshy/food structures on seeds. Left: Arils on seeds of weeping boer-bean (Schotia brachypetala). Center: Elaiosomes on seeds of prickly burr (Datura innoxia). Right: Pomegranate (Punica granatum) seeds, each with a fleshy sarcotesta. Credits: Schotia brachypetala seeds (JMK, via Wikimedia Commons, CC BY-SA 3.0) Datura innoxia seeds (Stefan.Iefnaer, via Wikimedia Commons, CC BY-SA 4.0) Punica granatum (Anton Croos/Art of Photography blog, via Wikimedia Commons, CC BY-SA 4.0). Images modified from originals.

Dispersal by caching or hoarding

The embryos and stored food within seeds themselves are often attractive to vertebrate dispersers. Caching or hoarding animals, like squirrels and some types of birds (such as jays), gather and cache (store) seeds and/or dry fruits in order to eat them later. Although many fruits and seeds may thus be consumed, some will be ignored or forgotten, providing them the opportunity to grow into new plants. Fruits of hickories, walnuts, and oaks, for example, are dispersed by seed-eating animals. In the fossil record, we may find feeding traces on fruits or seeds. Occasionally, food caches may also be identified.

Animal fruit/seed hoarders. Some animals collect and store fruits or seeds as a future food source this behavior may help disperse seeds. Left: An eastern gray squirrel (Sciurus carolinensis) with a walnut (Juglans) squirrels cache nuts and revisit the caches later. Right: An acorn woodpecker (Melanerpes formicivorus) with an acorn (Quercus). Acorn woodpeckers store acorns in granaries, which are made up of a series of holes drilled in a live or dead trees. Credits: Black squirrel (Grendelkhan, via Wikimedia Commons, CC BY-SA 3.0) acorn woodpecker (Mike's Birds, via Wikimedia Commons, CC BY-SA 2.0). Images modified from originals.

Dispersal by adherence (epizoochory)

Epizoochory (Greek epi + zoion = on animal) is dispersal by adherence to the outside of an animal's body. A common method by which fruits are distributed in this way is to adhere to the fur or feet of a mammal. Fruits adapted for adherence are often covered with hook-like structures, sometimes minute and sometimes large and imposing. It should be noted, however, that spines or hooks may occur in water-dispersed fruits or may be present as defensive structures rather than as structures to aid dispersal.

Dispersal by adherence. The fruits above are dispersed by becoming caught in animal fur or on or in the feet of animals. Left: Fruits of greater burdock (Arctium lappa) the hooked appendages on burdock were the inspiration for the development of velcro. Right: Fruit of grapple plant (Harpagophytum procumbens). Credits: Arctium lappa MHNT.BOT.2004.0.16 and Harpagophytum procumbens MNHT.BOT.2005.0.1243 (Roger Culos, Muséum de Toulouse, via Wikimedia Commons, CC BY-SA 3.0). Images modified from originals.

Wind dispersal (anemochory)

Fruits and seeds that are wind-dispersed frequently have modifications that help slow their descent to the ground and increase the chances that they will be blown laterally by air currents, so that they do not land directly beneath or next to their parent plant. Seed modifications for wind dispersal can include small size and/or light weight, wings, hairs, and/or inflation.

One of the most obvious modifications for wind-dispersal is the wing. Winged fruits are common in the fossil record beginning in the Paleogene. Winged fruits or seeds often have a single wing, in which case the wing may be asymmetrical, or offset to one side of the fruit or seed. If they have more than one wing, the wings many be regularly arranged around the fruit or seed. Often, the structure of the wing or wings will cause a seed or fruit to spin or rotate as it falls (known as autorotation, i.e., self-rotation). Maples (Acer) produce familiar wind-dispersed fruits that spin as they fall. If you live in a neighborhood with maple trees, you can observe this yourself watch the mature fruits as they fall from a tree on a windy day, or pick up the fallen mericarps (fruit halves) and drop them to watch them spin as they fall.

Winged fruits. Left: Winged mericarps (indehiscent parts of a fruit, each developing from one carpel and containing one seed) of amur maple (Acer ginnala) these fruits will rotate while falling. Middle: Winged fruit of Cedrelospermum, an extinct member of the elm family (Eocene, Florissant Formation, Colorado, U.S.A.) the position of the wing probably caused this fruit to rotate. Right: Multi-winged fruit of Chaneya (Eocene, Florissant Formation, Colorado, U.S.A.). Credits: Acer ginnala mericarps (Steve Hurst, hosted by USDA-NRCS PLANTS Database, no copyright) Cedrelospermum and Chaneya (Florissant Fossil Beds National Monument, National Park Service, no copyright). Images modified from originals.

The Javan cucumber (Alsomitra macrocarpa) produces lightweight seeds that are able to glide for long distances on large wings that resemble hang-gliders. Other winged fruits and seeds may rock back and forth as they fall, or follow a spiral path while falling. Lightweight disseminules may have hairs that act as parachutes, allowing them to drift on air currents. Examples include the familiar fruits of the dandelion (Taraxacum), plane tree (Platanus), and cattail (Typhus), as well as seeds of milkweed (Asclepias) and cottonwood (Populus).

Gliders and parachutes. Left: Winged, gliding seed of Javan cucumber (Alsomitra macrocarpa). Right: Achenes of American sycamore (Platanus occidentalis) with tufts of hairs that aid in wind dispersal. Credits: Alsomitra macrocarpa seed (Scott Zona, via Wikimedia Commons, CC BY 2.0) Plantanus occidentalis (Steve Hurst, hosted by the USDA-NRCS PLANTS Database, no copyright). Images modified from originals.

Parachuting dandelion fruits. Video showing how the pappus on the fruit (not seed!) of the dandelion (Taraxacum) helps it float on air currents. Credit: The secret physics of dandelion seeds (Nature Video, via YouTube).

Inflated fruits may also be dispersed by wind. The papery inflated fruits of golden raintree (Koelreuteria, shown below) are often considered to be wind-dispersed, whereas similar fruits of bladdernut (Staphylea, shown at the top of this page) are often said to be water-dispersed. In reality, it is possible that these inflated fruits have more than one mode of dispersal, which could also be determined by the habitat that the parent tree is occupying. (See also the groundcherry/Physalis fruits described below.)

Inflated capsules of koelreuteria/rain tree (Koelreuteria). Left. Capsules of golden raintree (Koelreuteria paniculata). Right. Capsule of Koelreuteria allenii (Eocene, Florissant Formation, Colorado, U.S.A.). Credits. Koelreuteria fruits (E.J. Hermsen, DEAL) Golden-rain tree fruit (Florissant Fossil Beds National Monument, National Park Service, public domain). Images modified from originals.

Water dispersal (hydrochory)

Plants that live in wetland environments or near the ocean may have buoyant, or floating, fruits or seeds. Cranberries (some species of Vaccinium) are low-growing plants found in boggy environments. Their bright red berries are not particularly sweet, and thus probably not terribly attractive to animals. Cranberries do, however, float, which aids in their dispersal in wetland habitats. It has been hypothesized that cranberries evolved from ancestors that had more palatable, animal-dispersed fruits. Humans take advantage of the berries' buoyancy during commercial production, as cranberry bogs can be flooded so that the floating berries can be more easily collected.

Some plants with floating fruits or seeds can disperse long distances over the ocean. The most obvious example of this is the coconut palm (Cocos nucifera), which has large, fibrous fruits that can float to and colonize oceanic islands. Similarly, legumes in the genus Entada produce large, buoyant seeds each seed harbors an air pocket, which enhances it ability to float.

Buoyant fruits and seeds. Left: Workers harvesting cranberries (Vaccinium) fruits, which are naturally buoyant. Right: Seeds of box bean (Entada phaseoloides), a legume with buoyant seeds note the air space in the seed that has been cut open. Credits: Cranberry harvest in New Jersey (Keith Weller, USDA-ARS, via Wikimedia Commons, Public Domain) Entada phaseoloides MHNT seeds (Didier Descouens, Muséum de Toulouse, via Wikimedia Commons, CC BY-SA 4.0).

Buoyancy cannot generally be tested directly in fossil fruits, so water-dispersal of fossil fruits may be inferred based on plant relationships, ecology, and structural comparison to fruits of modern plants. An interesting example is provided by fossil groundcherry (Physalis) fruits that were discovered in Eocene lake sediments of the Laguna del Hunco flora of South America. Groundcherries produce fleshy berries surrounded by an inflated calyx that looks like a paper lantern. The scientists who described the fossil groundcherries speculated that they may have been water-dispersed, and observed that modern groundcherries surrounded by intact lanterns can float (see here). A later experimental study confirmed that the lanterns surrounding modern groundcherries could plausibly aid in their dispersal by water, but noted that they could also aid in wind-dispersal (see here). Since the fruits are also fleshy, groundcherry seeds can potentially be dispersed in three ways: ingestion, water, and wind.

Groundcherry (Physalis) fruits. Left: Modern Cape gooseberry (Physalis peruviana), showing a complete fruit with inflated "lantern" intact and with part of lantern removed to show fleshy fruit within. Right: Fossil groundcherry (Physalis infinemundi, Eocene, Laguna del Hunco flora, Chubut Province, Argentina). Credits: Physalis peruviana (Stefan.Iefnaer, iva Wikimedia Commons, CC BY-SA 4.0) Physalis infinemundi, holotype, MPEF-Pb 6434a, image 61 of 410 (Peter Wilf, via figshare, CC BY 4.0). Images modified from originals.

Explosive dispersal (autochory or bolochory)

Amongst the more novel and exciting ways in which seeds are dispersed is through ballistic or explosive dispersal. In this method of dispersal, the fruit forcibly ejects the seed(s), scattering them for a short distance. The common garden plant Impatiens (also known as balsam, touch-me-not, and jewelweed, amongst other names) is one such plant. It produces capsules. When ripe, an animal brushing by the plant can cause the capsule to open instantly, scattering the seeds. Another plant with dramatic explosive seed dispersal is the squirting cucumber (Ecballium elaterium), which ejects its seeds as the fruit detaches from its stalk.

Ballistic seed dispersal. Video shows ballistic seed dispersal in fruits of Himalayan balsam (Impatiens glandulifera) and squirting cucumber (Ecballium elaterium). Credit: Exploding Cucumbers! (Slo Mo #36, BBC Earth Unplugged, via YouTube).

A spectacular example of explosive dehiscence comes from the tropical sandbox tree (Hura). Upon drying, fruits of this tree explode into pieces, sending the seeds rocketing through the air at up to 70 meters per second (about 230 feet/second) and a distance of up to 43 meters (about 140 feet) from the parent tree (see here)! In addition to the dangers posed by the its exploding fruits, sandbox tree trunks are covered with large prickles and the plant is poisonous as in some other members of the spurge family (Euphorbiaceae), getting the latex in the eye can cause damage. Perhaps this plant is best avoided.

Diagram of the stages in the dehiscence of the sandbox tree fruit (Hura crepitans). A. Immature, dry fruit. B. More mature fruit showing multiple carpels. C. As the fruit dehydrates (loses water), the carpels split open, explosively discharging the seeds. D. An open carpel with seeds. Credits: Image duplicated from and caption modified from Fig. 17 of Sakes et al. (2016) PLoS ONE (CC BY 4.0).

Exploding sandbox tree fruit. Demonstration of explosive sandbox tree (Hura) fruit. It should go without saying: Don't try this at home. Note: This video has no narration. Credit: Hura polyandra explosive seed pod (Mayan Archaeology, via YouTube).

While we obviously cannot observe exploding fruit in the fossil record, we can sometimes infer ballistic seed dispersal in fossil plants based on structural attributes of fossil seeds and fruits as well as plant relationships. For example, many members of the witch hazel family (Hamamelidaceae) have ballistically-dispersed seeds. In these plants, the inner wall at the base of the fruit capsule squeezes the seeds when the fruit is mature, forcing them out of the open fruits. Both fruits and seeds of ballistically dispersed genera in the witch hazel family are found in the fossil record.

Explosive dehiscence of witch-hazel (Hamamelis) fruits. These videos show the explosive ejection of seeds from Chinese witch-hazel fruits (Hamamelis mollis). Whole fruits and fruit endocarps with the outer layers of the fruit wall removed are shown. Note: This video has no narration. Credit: Snippet: This plant can fire seeds with bulletlike force (Science Magazine, from videos by Poppinga et al. 2019 Journal of the Royal Society Interface, via YouTube).

Selected references & further reading

Note: Free full text is made available by the publisher for items marked with a green asterisk.


Department of Biological Sciences, University of Illinois, 845 W. Taylor Street, Chicago, Illinois, 60607, USA

Department of Biology, Lake Forest College, Lake Forest, Illinois, 60045, USA

Department of Biological Sciences, University of Illinois, 845 W. Taylor Street, Chicago, Illinois, 60607, USA

Department of Biology, Lake Forest College, Lake Forest, Illinois, 60045, USA

Department of Biology, Imperial College of Science, Technology and Medicine, Silwood Park, Ascot, Berkshire, SL5 7PY, UK

Summary

Challenges of a sedentary existence

Reproductive imperatives of success and failure

Adjusting to physical and biological reality


Pollination and Seed Dispersal Adaptations

For a seed to successfully grow it must have water, sunlight and soil, just like its parent plant &ndash whether it is a daisy or a redwood. However, it is not always ideal for a seed just to drop to the ground under the parent plant. If it sprouts and grows right next to the parent plant it will be competing for water, nutrients and light (unless it is an annual that will not come back the next season). Ideally the plant wants to spread its seeds as far and wide as possible. This will allow more offspring to spread throughout the habitat and not compete with its own resources.

Plants have developed many ingenious ways to successfully disperse their seeds. We will look at some of the structures they have developed to accomplish those goals and how they work.

Objectives:

Students will observe, study and hypothesize about the adaptations of seed structures to aid in their dispersal.

  1. Hand lens for each team (pair of students)
  2. Paper plates for each seed
  3. Seed samples displayed at different stations (acorns, maple seeds, burrs, nuts in shells, coconut, milkweed pod and seed, berries, grass seeds off wild grass &ndash the more the better)
  4. Fuzzy sock or material at each station
  5. A bowl or cup of water at each station
  6. Paper on clipboards and pencils

Optional: provide a puffball mushroom that is ready to puff out spores for students to see.

  1. Inspect each seed with the hand lens.
  2. Draw a picture of what you see.
  3. Describe its shape and structures.
  4. Explain how these might help it move away from the parent plant.
  5. Does it have structures that will help it move by wind, water, attach to animals, be consumed by animals or be planted by animals? Explain.

Experimenting with Dispersal:

  1. Try touching seed to the sock. Does it stick?
  2. Hold the seed at arms length over the plate and drop it. Does it drop straight down onto the plate, or move away.
  3. Hold the seed in the palm of your hand and blow lightly on it. Does it float away?
  4. Drop the seed in the bowl of water. Does it float? Blow on the water. Does the seed move away in the water?
  5. Is the seed edible to an animal? If the animal eats it, how will this affect where it is dispersed?
  6. If the animal &ldquohides&rdquo it (buries it in the ground), how will this affect where it is dispersed?


Discussion:
Be prepared to discuss each seed and how it responds to your &ldquodispersal&rdquo techniques.


Blowing In The Wind

S eeds provide the vital genetic link and dispersal agent between successive generations of plants. Angiosperm seeds are produced and packaged in botanical structures called fruits which develop from the "female" pistils of flowers. Immature seeds (called ovules) each contain a minute, single-celled egg enclosed within a 7-celled embryo sac. The haploid (1n) egg is fertilized by a haploid (1n) sperm resulting in a diploid (2n) zygote that divides by mitosis into a minute, multicellular embryo within the developing seed. A second sperm unites with 2 haploid polar nuclei inside a binucleate cell called the endosperm mother cell which divides into a mass of nutritive tissue inside the seed. In most seeds the embryo is embedded in this endosperm tissue which provides sustenance to the embryo during germination. In exalbuminous seeds (found in many plants such as the legumes), the endosperm tissue is already absorbed by the time you examine a mature seed within the pod, and the 2 white fleshy halves in the seed are really the cotyledons (components of the embryo). The 2 sperm involved in the double fertilization process originated within the pollen tube that penetrated the embryo sac. The pollen grain (and pollen tube) come from the "male" organs (called anthers) on the same plant or different parental plants in a remarkable process known as pollination. Pollination is also accomplished by the wind (or water), and it may also involve insects in some of nature's most fascinating relationships between a plant and an animal. This is especially true of the amazing fig trees and their symbiotic wasps.

O ne of the important functions of seeds and fruits is dispersal a mechanism to establish the embryo-bearing seeds in a suitable place away from their parental plants. There are 3 main mechanisms for seed and fruit dispersal: (1) Hitchhiking on animals, ( 2) Drifting in ocean or fresh water, and (3) Floating in the wind. This article concerns one of the most remarkable of all seed dispersal methods, riding the wind and air currents of the world.

Wind-dispersed seeds & fruits in different plant families:

Helicopters: A. Box Elder ( Acer negundo , Aceraceae) C. Big-Leaf Maple ( Acer macrophyllum , Aceraceae) E: Evergreen Ash ( Fraxinus uhdei , Oleaceae) F. Tipu Tree ( Tipuana tipu , Fabaceae).

Flutterer/Spinners: B. Empress Tree ( Paulownia tomentosa , Scrophulariaceae) D. Tree Of Heaven ( Ailanthus altissima , Simaroubaceae) G. Jacaranda ( Jacaranda mimosifolia , Bignoniaceae).

1. Gliders

The remarkable winged seed of the tropical Asian climbing gourd Alsomitra macrocarpa . The entire seed has a wingspan of 5 inches (13 cm) and is capable of gliding through the air of the rain forest in wide circles. This seed reportedly inspired the design of early aircraft and gliders.

G liders include seeds with 2 lateral wings that resemble the wings of an airplane. They become airborne when released from their fruit and sail through the air like a true glider. One of the best examples of this method is Alsomitra macrocarpa , a tropical vine in the Gourd Family (Cucurbitaceae) native to the Sunda Islands of the Malay Archipelago. Football-sized gourds hang from the vine high in the forest canopy, each packed with hundreds of winged seeds. The seeds have two papery, membranous wings, with combined wingspans of up to 5 inches (13 cm). They reportedly inspired the wing design of some early aircraft, gliders and kites. Although the seeds vary in shape, some of the most symmetrical ones superficially resemble the shape of the "flying wing" aircraft or a modern Stealth Bomber. According to Peter Loewer ( Seeds: The Definitive Guide to Growing, History, and Lore , 1995), the aerodynamic seeds spiral downward in 20 foot (6 meter) circles, although a gust of wind would probably carry them much farther away.

2. Parachutes

An individual parachute of western salsify ( Tragopogon dubius ) showing an umbrella-like, plumose crown of hairs (pappus) above a slender one-seeded fruit (called an achene). These fragile units can become airborne with the slightest gust of wind, and can literally sail across valleys and over mountain slopes.

Western salsify or goatsbeard ( Tragopogon dubius ) showing dense, puff-like cluster of numerous parachute seeds (one-seeded achenes). Each achene has an umbrella-like crown of plumose hairs and may literally be carried into the atmosphere by strong ascending air currents.

A population explosion of western salsify( Tragopogon dubius ) near Mono Lake, on the east side of the Sierra Nevada of Central California. This ubiquitous species is actually native to Europe and Asia.

P arachutes include seeds or achenes (one-seeded fruits) with an elevated, umbrella-like crown of intricately-branched hairs at the top, often produced in globose heads or puff-like clusters. The slightest gust of wind catches the elaborate crown of plumose hairs, raising and propelling the seed into the air like a parachute. This is the classic mechanism of dispersal for the Eurasian dandelion ( Taraxacum officinale ) and includes numerous weedy and native members of the Sunflower Family (Asteraceae). A giant Eurasian version of the dandelion called salsify or goat's beard ( Tragopogon dubius ), is one of the most successful wind-travelers in North America. Its seeds have literally blown across mountain ranges, colonizing vast fields of open land in the western United States. Three weedy species of salsify ( T. dubius , T. pratensis and T. porrifolius ) have been introduced into the western United States, 2 with yellow dandelion-type flowers and one with purple flowers. The latter, purple-flowered species ( T. porrifolius ) has a large, edible tap root with a flavor resembling oysters, hence the name "oyster plant."

Inflorescence and mature, seed-bearing head of the Eurasian dandelion ( Taraxacum officinale ). The slightest gust of wind catches the elaborate crown of plumose hairs, raising and propelling each seed-bearing achene into the air like a parachute. This successful weed thrives in a wide range of climates and has become naturalized throughout North America.

I n some parachutes, the crown of silky hairs arises directly from the top of the seed (not on an umbrella-like stalk). Again, the Sunflower Family (world's largest plant family with about 24,000 described species) contains many weedy representatives with this type of parachute seed. One of the most troublesome weeds of farm land in the western United States is wild or thistle artichoke ( Cynara cardunculus ). The large seed head of this weedy composite releases hundreds of parachute seeds which fly through the air and invade vast areas of grazing land with spiny, perennial bushes that literally take over. The large leaf stalks (resembling giant celery stalks) are edible and are sold under the name of "cardoon." Populations of wild artichoke often contain so much variation between spiny and non-spiny plants, that some experts believe that they belong to one variable species. In fact, some botanists believe that the cultivated artichoke ( C. scolymus ) may be a cultivated variety of the wild C. cardunculus . Incidentally, the delicious artichoke is really a cooked flower head in which the outer bracts (phyllaries) and central basal portion (receptacle) are dipped in butter and eaten.

Brown puffs ( Stebbinoseris heterocarpa ), formerly Microseris heterocarpa , an interesting member of the sunflower family. A closely related species is called silver puffs ( Uropappus lindleyi syn. Microseris lindleyi ). In Stebbinoseris the pappus paleae are bifid at the apex. Unlike the weedy dandelions ( Taraxacum ) and salsify ( Tragopogon ), this is a native species in California.

A nother plant family which has evolved this parachute method of seed dispersal is the Milkweed Family (Asclepiadaceae). Hundreds of parachute seeds (each with a tuft of silky hairs) are produced within large, inflated pods called follicles. So abundant are the silky hairs, that they were actually collected and used as a substitute for kapok during World War II. Kapok comes from masses of silky hairs that line the seed capsules of the kapok tree ( Ceiba pentandra ), an enormous rain forest tree of Central and South America. Kapok is used primarily as a waterproof filler for mattresses, pillows, upholstery, softballs, and especially for life preservers. The floss silk tree ( Chorisia speciosa ), another member of the Bombax Family (Bombaceae) also produces large seed capsules lined with masses of silky hairs. This tree with its distinctive thorny trunk and showy pink flowers is commonly planted in southern California. The seeds of kapok and floss silk trees are embedded in these silky masses which aid in their dispersal by wind however they probably belong in Section 5 below ( Cottony Seeds & Fruits ).

T he Dogbane Family (Apocynaceae) also includes members with seed pods (follicles) and parachute seeds similar to those of milkweeds. One of the best examples is Nerium oleander , a drought-resistant, Mediterranean shrub planted throughout southern California. The foliage contains a powerful cardiac glycoside that can permanently relax the heart muscle.

Parachute seeds escaping from the follicle of Nerium oleander . The crown of silky hairs arises directly from the top of the seed (not on an umbrella-like stalk. Unlike the seed-bearing achenes of the sunflower family (Asteraceae), these are true seeds.

3. Helicopters (Whirlybirds)

The South American tipu tree ( Tipuana tipu ) has one of the most unusual legumes in the world. Unlike the fruits of most members of the legume family (Fabaceae), the 3rd largest plant family, the fruits of this tree have a distinctive wing that causes the legume to spin as it falls from the rain forest canopy.

H elicopters (also called Whirlybirds) include seeds or one-seeded fruits (samaras) with a rigid or membranous wing at one end. The wing typically has a slight pitch (like a propeller or fan blade), causing the seed to spin as it falls. Depending on the wind velocity and distance above the ground, helicopter seeds can be carried considerable distances away from the parent plant. The spinning action is similar to auto-rotation in helicopters, when a helicopter "slowly" descends after a power loss.

N umerous species of flowering trees and shrubs in many diverse and unrelated plant families have evolved this ingenious method of seed dispersal, good examples of convergent evolution. Representative examples of helicopter seeds and one-seeded fruits (called samaras) include the Maple Family (Aceraceae): Maples and box elder ( Acer ) Olive Family (Oleaceae): Ash ( Fraxinus ) Legume Family (Fabaceae): Tipu tree ( Tipuana tipu ) and the Protea Family (Proteaceae): Banksia and Hakea .


An interesting one-seeded winged fruit that spins as it falls through the air. It is called "whirling nut" and belongs to the genus Gyrocarpus in the family Gyrocarpaceae. Note: This genus of tropical trees, shrubs and lianas is often placed in the hernandia family (Hernandiaceae). The unusual fruit shown above was collected and photographed at Ho'omaluhia Botanical Garden on the windward side of Oahu in the Hawaiian Islands. Identification provided by Ricarda Riina, Botany Department, University of Wisconsin.

Spinning fruits (seeds) from Thailand: A. Diptocarpus alatus (Diptocarpaceae). B. Diptocarpus obtusifolius . C. Gluta ( Melanorrhoea ) usitata (Anacardiaceae). The latter species is also called the Burmese lacquer tree and is well-known as a source of lacquer. The resin is chemically similar to the Japanese lacquer tree. Like the Japanese lacquer tree and poison oak, the resin canals also contain urushiol, a mixture of toxic phenolic compounds that cause a cell-mediated immune response in some people. Identifications courtesy of Dr. Tomiki Sando, Thailand.

A lthough they are classified as gymnosperms with naked seeds arising from woody cones rather than flowers, the Pine Family (Pinaceae) contains many genera with winged seeds, including Pinus (Pine), Abies (fir), Picea (spruce), Tsuga (hemlock), and many additional genera. When shed from cones high on upper branches, they fly over slopes and across deep canyons. The natural reforestation of conifers following fire is proof of the flying ability of seeds from nearby forested slopes.

M aples have a double or twin samara composed of 2 winged one-seeded fruits (double samara) joined together at their bases. When they break apart, each winged fruit flies like a typical helicopter seed. Although the Legume Family (Fabaceae) is the third largest plant family with over 18,000 described species, the vast majority of legumes do not have winged seeds or fruits. The South American tipu tree ( Tipuana tipu ) is a notable exception, with beautiful yellow blossoms that give rise to pendant, samara-like legumes, each with a large wing on the lower end. The dried, winged legumes spin so neatly in the air that they could be marketed as a child's toy.

T he remarkable Protea Family (Proteaceae) of Australia contains some truly amazing genera with winged seeds, including Banksia and Hakea . Although they are flowering plants, banksias produce a dense flower cluster (inflorescence) that gives rise to a cone-like structure containing many woody carpels. Each carpel bears 2 winged seeds and the entire cone-like structure superficially resembles a pine cone. In fact, some banksias release their seeds following fire and even resprout from subterranean lignotubers like chaparral shrubs.

4. Flutterer/Spinners

The native range of hopseed bush ( Dodonea viscosa ), a member of the Soapberry Family (Sapindaceae), extends from Arizona to South America. It is also commonly cultivated in southern California. The papery, winged fruits flutter and spin in the air, and may be carried short distances by the wind.

The jacaranda tree ( Jacaranda mimosifolia ) of northwestern Argentina. Like many other members of the Bignonia Family (Bignoniaceae), the papery, winged seeds flutter and spin as they are carried by the wind.

A lthough their mode of dispersal is similar to single-winged helicopter seeds, the flutterer/spinners include seeds with a papery wing around the entire seed or at each end. When released from their seed capsules they flutter or spin through the air. Whether they spin or merely flutter depends on the size, shape and pitch of the wings, and the wind velocity. This method of wind dispersal is found in numerous species of flowering plants in many different plant families. Some examples of flutterer/spinner seeds include the Quassia Family (Simaroubaceae): Tree of heaven ( Ailanthus altissima ) Figwort Family (Scrophulariaceae): Empress tree ( Paulownia tomentosa ) Bignonia Family (Bignoniaceae): Jacaranda ( Jacaranda mimosifolia ), catalpa ( Catalpa speciosa ), desert willow ( Chilopsis linearis ), yellow bells ( Tecoma stans ), bower vine ( Pandorea jasminoides ), violet trumpet vine ( Clytostoma callistegioides ), and the fabulous trumpet trees ( Tabebuia serratifolia and T. ipe ) Elm Family (Ulmaceae): American and Chinese elms ( Ulmus americana and U. parvifolia ) Soapberry Family (Sapindaceae): Hop seed ( Dodonea viscosa ) and the Goosefoot Family (Chenopodiaceae): Four-wing saltbush ( Atriplex canescens ).

A ny discussion of flutterer/spinners would not be complete without mentioning the quipo tree ( Cavanillesia platanifolia ), a massive rain forest tree in the bombax family (Bombacaeae) native to Panama. The enormous winged fruits of the quipo tree flutter through the air, carpeting the ground beneath the huge canopy of this striking tropical tree.

The quipo tree ( Cavanillesia platanifolia ), a remarkable rain forest tree in the bombax family (Bombacaceae) with huge winged fruits. This massive tree is native to Panama.

S ome of the most beautiful flowering trees of the New World tropics belong to the Bignonia Family (Bignoniaceae). They typically produce long, slender (cigar-shaped) seed capsules containing masses of flat seeds with papery wings at each end. [The beautiful jacaranda of Argentina has flattened, circular seed capsules.] The lovely yellow bells ( Tecoma stans ) is native to Mexico and the Caribbean region, and is the official flower of the U.S. Virgin Islands. Some of the South American trumpet trees, including the pink-flowered Tabebuia avellanedae (listed as T. ipe in some references) and the yellow-flowered Tabebuia serratifolia , are also called ironwoods or axe-breakers (quebrachos) because of their dense, hard wood. The latter species is called "pau d'arco" and its wood actually sinks in water, with a specific gravity of 1.20. In South America, trumpet trees drop their leaves during the dry season and produce a profusion of pink or yellow blossoms. The crowns of these huge timber trees resemble gigantic floral bouquets in the midst of the forest. As with so many tropical species, some of the trumpet trees inhabit rain forest areas that are seriously threatened by slash and burn agriculture, large plantations of exportable products, and the general annihilation of the South American rain forests.

O ther South American species of Tabebuia are also referred to as pau d'arco, including the pink-flowered T. impetiginosa and T. avellanedae . According to The New York Botanical Garden Encyclopedia of Horticulture Volume 10, 1982, T. avellanedae is a synonym for T. impetiginosa , and T. ipe " is so closely similar to T. impetiginosa that it can scarcely be more than a variety of that species." These attractive pink-flowered species are commonly used as landscape trees in temperate regions.

T he powdered inner bark of these pink-flowered species of pau d'arco is sold as a popular herbal remedy that reportedly stimulates the immune system. According to a book by Kenneth Jones ( Pau d'Arco: Immune Power From the Rain Forest , Healing Arts Press, 1995), this valuable herb has been proven successful in the treatment of certain cancers, allergies associated with the Candida yeast syndrome, and in disorders involving a weakened immune system.

Specific Gravity

5. Cottony Seeds & Fruits

Fuzzy brown cattail spikes ( Typha latifolia ) contain dense masses of tiny seeds, each with a tuft of silky hairs. Each spike contains about a million seeds. They are shed by the millions in a cloud of white fluff.

C ottony seeds and fruits include seeds and minute seed capsules with a tuft (coma) of cottony hairs at one end, or seeds embedded in a cottony mass. Some of the examples in this group are very similar in function to parachute seeds, but probably are not carried as far by the wind. Many plant families have this type of wind dispersal, including the Willow Family (Salicaceae): Willows ( Salix ) and Cottonwoods ( Populus ) Cattail Family (Typhaceae): Cattails ( Typha ) Evening Primrose Family (Onagraceae): Willow-Herb ( Epilobium ) and California fuchsia ( Zauschneria ) Bombax Family (Bombaceae): Kapok tree ( Ceiba pentandra ) and floss silk tree ( Chorisia speciosa ) and the Sycamore Family (Platanaceae): Sycamore ( Platanus ).

I n the California sycamore ( Platanus racemosa ), a common riparian (streamside) tree throughout the state, the one-seeded fruits (achenes or nutlets) are produced in dense, globose heads. The spherical heads hang from branches like little balls. Individual achenes have a tuft of hairs at the base which probably helps in their wind dispersal. Seeds of the South American kapok tree ( Ceiba pentandra ) and floss silk tree ( Chorisia speciosa ) are embedded in dense masses of silky hairs inside large woody capsules. This undoubtedly helps to disperse the seeds when seed-bearing masses of hair are carried by the wind. In tropical regions of the New World, the kapok grows into an enormous rain forest tree with a massive buttressed trunk. Kapok hairs are coated with a highly water-resistant, waxy cutin layer. The empty lumen (cavity) inside each hair is larger the cotton hairs hence, the hairs are lighter. Unlike cotton hairs, kapok is difficult to spin and is not made into textiles. It is used primarily as a waterproof filler for mattresses, pillows, upholstery, softballs, and especially for life preservers. A kapok-filled life jacket can support 30 times its own weight in water.

O ne fuzzy brown cattail spike may contain a million tiny seeds. Each seed has a tuft of silky white hairs and is small enough to pass through the "eye" of an ordinary sewing needle. They are shed in clouds of white fluff and float through the air like miniature parachutes. A cattail marsh covering one acre may produce a trillion seeds, more than 200 times the number of people in the world. The fluffy seeds have been used for waterproof insulation and the buoyant filling of life jackets. In addition, each plant produces billions of wind-borne pollen grains in fact, so much pollen that it was used as flour by North American Indians and made into bread. Cottonwoods and willows also produce masses of seeds, each with a tuft of soft, white hairs. Since they are dioecious, with pollen-bearing male and seed-bearing female trees in the population, only female trees produce the actual cotton. During late spring and summer in the western United States, the cottony fluff from cottonwoods resembles newly fallen snow. Because the wind-blown fluff can be quite messy in cultivated parks and gardens, male trees are generally planted. The discriminatory label of "cottonless cottonwood" refers to a male tree.

6. Tumbleweed (Russian Thistle)

T he common tumbleweed or Russian thistle is a rounded, bushy annual introduced into the western United States from the plains of southeastern Russia and western Siberia in the late 1800s. The name "thistle" comes from the stiff, sharp-pointed, awl-shaped leaves. Although it is depicted in songs of the old west, this species is a naturalized weed in North America. It is listed in most older references as Salsola kali or S. pestifer however, the Jepson Flora of California (1993) lists it as S. tragus . Russian thistle belongs to the goosefoot family (Chenopodiaceae), along with many weedy species and some valuable vegetables, including beets ( Beta vulgaris ), goosefoot ( Chenopodium album ) and spinach ( Spinacia oleracea ).

A large tumbleweed ( Salsola tragus ) in San Diego County, California. Tumbleweeds are pushed along by the wind, scattering thousands of seeds as they roll across open fields and valleys. A tumbleweed of this size is difficult to hold on to during a strong wind storm.

T umbleweed is a prolific seeder and rapid seed germination and seedling establishment occurs after only a brief and limited rainy season. A single plant may produce 20,000 to 50,000 seeds within numerous small fruits, each surrounded by a circular, papery border. Mature plants readily break off at the ground level and are pushed along by strong gusts of wind. As they roll along hillsides and valleys, the seeds are scartered across the landscape. Tumbleweeds often pile up in wind rows along fences and buildings. This is a troublesome weed in agricultural areas because it literally covers the farm land with bushy, prickly shrubs. One interesting use for this plant in arid regions of the American southwest is for a "snowman" at Christmas time. Three proportionally sized tumbleweeds are used to make the head, thorax and main body of a "snowman." Another suggested use is to compress tumbleweeds into logs and use them for firewood.

A tumbleweed "snowman" in San Diego County made from three dried plants of Salsola tragus .

7. Miscellaneous

Squirrel-Tail Grass ( Elymus elymoides ), formerly named Sitanion hystrix is an attractive grass native to the mountains and plains of the western United States. Seed-bearing sections (spikelets) of the flower spike (containing one-seeded fruits called grains and very long awns) are carried short distances by the wind. Although not as efficient fliers, the long awns function like the parachute bristles (pappus) of composites.

T his miscellaneous category of wind-blown seeds and fruits includes plants that really don't fit the above 5 categories. The Grass Family (Poaceae) includes a number of species with plumose flower stalks that fragment into seed-bearing spikelets that blow into the wind. Some of these species have become troublesome weeds in southern California, including the South African fountain grass ( Pennisetum setaceum ). Although this tufted perennial makes an attractive, drought-resistant landscaping plant along walkways and roads, it is becoming a widespread weed in disturbed areas of San Diego County. Another species, called squirrel-tail grass ( Elymus elymoides ), resembles a weedy introduced grass, but it is actually a native perennial of dry, rocky mountains and open land in the western United States. To appreciate its airborne seeds, you really must see this grass during a strong gust of wind on the eastern slopes of the Sierra Nevada during late summer.

M ountain mahogany ( Cercocarpus minutiflorus ), a native shrub in the chaparral of southern California, produces a rather unique wind-blown fruit. The one-seeded fruit (achene) has a persistent, feathery style that glistens in the sunlight. Although they usually don't travel very far, the achenes are blown into the air by strong gusts of wind during the dry, fire season of late summer and fall. This species is not related to the West Indian mahogany ( Swietenia mahagoni ) or the Honduran mahogany ( S. macrophylla ), members of the true Mahogany Family (Meliaceae). Mountain mahogany actually belongs to the Rose Family (Rosaceae) and produces very hard wood that sinks in water when dry. In fact, the wood of a montane species ( C. ledifolius ), has a specific gravity of 1.12, as heavy and dense as ebony ( Diospyros ebenum ).

Wayne's Word Drift Seed Links:

    Jones, K. 1995. Pau d'Arco: Immune Power From the Rain Forest . Healing Arts Press, Rochester, Vermont.


Direct and indirect effects of pollinators and seed predators to selection on plant and floral traits

Although flowering traits are often assumed to be under strong selection by pollinators, significant variation in such traits remains the norm for most plant species. Thus, it is likely that the interactions among plants, mutualists, and other selective agents, such as antagonists, ultimately shape the evolution of floral and flowering traits. We examined the importance of pollination vs pre-dispersal seed predation to selection on plant and floral characters via female plant-reproductive success in Castilleja linariaefolia (Scrophulariaceae). C. linariaefolia is pollinated by hummingbirds and experiences high levels of pre-dispersal seed predation by plume moth and fly larvae in the Rocky Mountains of Colorado, USA, where this work was conducted. We first examined whether female reproduction in C. linariaefolia was limited by pollination. Supplemental pollination only marginally increased components of female reproduction, likely because seed predation masked, in part, the beneficial effects of pollen addition. In unmanipulated populations, we measured calyx length, flower production, and plant height and used path analysis combined with structural equation modeling to quantify their importance to relative seed set through pathways involving pollination vs seed predation. We found that the strength of selection on calyx length, flower production, and plant height was greater for seed predation pathways than for pollination pathways, and one character, calyx length, experienced opposing selection via pollination vs seed predation. These results suggest that the remarkable intraspecific variation in plant and floral characters exhibited by some flowering plants is likely the result of selection driven, at least in part, by pollinators in concert with antagonists, such as pre-dispersal seed predators. This work highlights the subtle but complex interactions that shape floral and vegetative design in natural ecosystems.


Results

Interaction between hornet and diaspore of S. tuberosa

Diaspore, elaiosome and seed carrying experiments indicated that the remaining number of diaspores, elaiosomes, and seeds was 1.30 ± 0.42 (n = 10), 1.40 ± 0.37 (n = 10), and 10 (n = 10) at the KBG population. There was a significant difference between the amount of materials remaining following different treatments (F2,27 = 236.305, P < 0.001, one-way ANOVA). However, no significant differences were found between the numbers of remaining diaspores and elaiosomes (P = 0.997, one-way ANOVA, Tamhane's post hoc). The results imply that diaspores and elaiosomes elicited significantly more carrying behavior in V. velutina than do seed controls (Fig. 3). Hornets never carried seeds without elaiosomes. Furthermore, elaiosomes elicited the same amount of carrying behavior as intact diaspores (Fig. 3). The obtained results imply that elaiosomes rather than seeds elicit carrying behavior in hornets. Similar results were found in the FN population (Fig. 3). The record of foraging behavior of 67 tagged hornets indicated that the number of revisits was 29.67 ± 8.40 times at the same site. In an extreme case, one individual of V. velutina was observed to revisit capsules of S. tuberosa > 128 times within 7 d. However, no nest-based recruitment behavior of hornets could be observed in this study, because the tagged hornets did not recruit other individuals from the same colonies. Only tagged hornets visited capsules during the observation period.

Olfactory cues from capsule of S. tuberosa attract hornets

Bioassays 1–3 indicated that olfactory cues alone, or visual + olfactory cues attracted more hornets than the controls however, visual cues alone presented similar attractive ability to the control (Fig. 2b). In bioassay 1, the nonparametric chi-square test indicated that approach, landing and approach + landing behaviors of hornets to sample and control were significant differences (approach: χ 2 = 8.895, df = 1, P = 0.003 ˂ 0.01 landing: χ 2 = 56.889, df = 1, P ˂ 0.001 approach + landing: χ 2 = 65.154, df = 1, P ˂ 0.001). Bioassay 2 revealed that, if only the visual cue was presented to V. velutina in a dual-choice bioassay, the hornets approached the sample or control, but they seldom landed on either (Fig. 2b). The results of bioassays 4–6 indicated that olfactory cues, and visual + olfactory cues attracted significantly more hornets than visual cues alone, however, olfactory cues alone may show a similar attraction for hornets as the combination of olfactory and visual cues (Fig. 2b). The results revealed that olfactory cues from elaisomes attracted hornets to approach and land on the capsules (Movie S3). More details of the statistical results of bioassays were presented in Fig. 2(b).

Hydrocarbons from elaiosome attract seed dispersers of S. tuberosa

The total mass of extracts from one elaiosome is c. 26.92 ± 2.20 μg (n = 29), However, no hydrocarbons could be found on the surface of the seeds. We only measured the relative hydrocarbon composition (%) of leaves because of the irregular leaf area. A total of 21 compounds could be identified by GC-MS from the elaiosome and the leaves of S. tuberosa (Table 1). These compounds from elaiosome and leaves were straight-chain saturated and unsaturated hydrocarbons. GC-EAD revealed 10 biologically active compounds in extracts from elaiosomes (Fig. 2c). The composition of hydrocarbons differed significantly between elaiosomes and leaf samples and no active compounds could be detected in the extracts from the leaves (Table 1). Behavioral experiments showed that V. velutina significantly preferred the odors of the elaiosome extracts and the EAD-active compounds compared to the control (Fig. 2d). However, the hornets showed no preference for the extracts obtained from the elaiosomes vs the EAD-active compounds (approach: χ 2 = 1.383, df = 1, P = 0.24 landing: χ 2 = 1.829, df = 1, P = 0.176 approach + landing: χ 2 = 3.01, df = 1, P = 0.083). More details of the statistical results of bioassays were presented in Fig. 2(d).

Abundance (%)
Compound Elaiosome (n = 29) Leaf (n = 5)
Nonadecane 0.48 ± 0.17
Eicosane 1.27 ± 0.19
(Z)-9-Heneicosene 0.25 ± 0.08
Heneicosane 12.88 ± 0.66
(Z)-9-Docosene 1.38 ± 0.35
Docosane 11.60 ± 0.72
(Z)-9-Tricosene 17.06 ± 1.45
Tricosane 31.55 ± 0.91
(Z)-9-Tetracosene 0.95 ± 0.16
Tetracosane 6.73 ± 0.31
(Z)-9-Pentacosene 2.34 ± 0.44
Pentacosane 6.00 ± 0.42
Hexacosane 0.85 ± 0.08
(Z)-9-Heptacosene 0.06 ± 0.02
Heptacosane 1.04 ± 0.22 2.95 ± 1.48
Octacosane 0.45 ± 0.15 1.15 ± 0.44
Nonacosane 4.23 ± 1.13 19.76 ± 2.15
Triacontane 0.12 ± 0.07 4.22 ± 0.66
Hentriacontane 0.87 ± 0.32 50.50 ± 1.69
Dotriacontane 4.97 ± 0.91
Tritriacontane 16.45 ± 1.92
  • Abundances are given as mean ± SE and electrophysiologically active compounds are shown by shading.

Interaction between hydrocarbons and chemosensory proteins of hornets

The expression pattern of VvelCSP1-2 genes in V. velutina

The transcript expression levels of VvelCSP1-2 genes in various tissues of adult hornets were measured by qRT-PCR with tubulin gene control. As shown in Fig. 4, the real-time qPCR results showed that both VvelCSP1 and VvelCSP2 are highly expressed in the antennae, suggesting a major olfactory role of these proteins in the V. velutina workers. Additionally, the expression levels of VvelCSP1-2 genes in the head (without antennae) were significantly higher than those in other tissues such as thorax, abdomen, legs or wings (Fig. 4). This highlights the involvement of these proteins in gustation in the adult hornet.

Sequence analysis and recombinant expression of VvelCSPs

The VvelCSP1-2 genes contain an open reading frame (ORF) of 384 and 378 bp, respectively. The predicted amino acid sequences of VvelCSP1-2 have the typical four-cysteine signature of insect CSPs with a signal peptide of 22 and 17 amino acid residues at the N terminus, respectively, which are believed to form two disulfide bridges and the hydrophobic domains (Fig. S1). Moreover, both of the CSPs showed a common cysteine sequence motif of C1-X6-C2-X18/19-C3-X2-C4 (Fig. S1). Hence, they display the characteristics of a chemosensory protein family, which is similar to AcerCSPs (Li et al., 2016 ). The calculated molecular masses of mature VvelCSP1-2 proteins were 12.35 and 12.41 kDa, and the calculated isoelectric points of mature VvelCSP1-2 were 8.436 and 8.299, respectively. The amino acid sequences of two CSP genes show obvious similarities with previously identified CSPs in other Hymenoptera (Fig. S1). VvelCSP1 is similar to AcerCSP1 (GenBank: ACI03402.1) and Ssp.CSP1 (GenBank: ALG36154.1) with identity values of 32.28% and 37.01%, respectively, whereas VvelCSP2 is similar to MpulCSP2 (GenBank: AQN78396.1) and CcinCSP4 (GenBank: ARN17835.1) with identity values of 65.60% and 74.40%, respectively. The neighbor-joining tree revealed that these CSP sequences from distinct insect species can be divided into two subgroups (Hymenoptera CSPs and Lepidoptera CSPs) (Fig. S2). Both of the VvelCSP1-2 are clearly separated from each other by phylogenetic analysis and are most closely related to CSPs from Hymenoptera CSP subgroups. The recombinant VvelCSP1-2 proteins were abundantly expressed in E. coli BL21 (DE3) after IPTG induction and the SDS-PAGE results were consistent with the expected sizes of the VvelCSP1-2 proteins (Fig. S3).

Fluorescence binding affinities of VvelCSP1-2 proteins in V. velutina

The binding curves and Scatchard plots indicated that the binding of the fluorescent ligand to the VvelCSP1-2 proteins increased with increasing concentrations of the N-Phenyl-1-naphthylamine (1-NPN) (Fig. 5). VvelCSP1-2 proteins were determined in fluorescence competitive binding experiments with nine synthetic volatile chemicals from the elaiosome of S. tuberosa which elicited obvious electroantennogram responses. As shown in Fig. 5, both of the VvelCSPs also showed excellent binding capacities to nine synthetic ligands Ki values were 3.32, 3.61, 1.58, 1.69, 1.97, 1.93, 1.85, 2.48 and 3.71 μM for VvelCSP1, and 1.36, 1.88, 1.43, 2.05, 1.40, 2.19, 1.45, 1.24 and 2.64 μM for VvelCSP2, respectively (Table 2). In comparison, most of the volatile chemicals showed a high affinity to the VvelCSP1-2 proteins, indicating that CSP proteins seem to be involved in the olfactory recognition function of V. velutina.

VespCSP1 VespCSP2
No. Compounds IC50 (μM) Ki (μM) IC50 (μM) Ki (μM)
1 Nonadecane 4.74 3.32 2.71 1.36
2 Eicosane 5.15 3.61 3.74 1.88
3 Heneicosane 2.27 1.58 2.86 1.43
4 Docosane 2.40 1.69 4.01 2.05
5 (Z)-9-Tricosene 2.81 1.97 2.78 1.40
6 Tricosane 2.76 1.93 4.35 2.19
7 Tetracosane 2.64 1.85 2.87 1.45
8 Pentacosane 3.53 2.48 2.45 1.24
9 Hexacosane 5.27 3.71 5.27 2.64
  • IC50, ligand concentration displacing 50% of the fluorescence intensity of the VespCSPs/N-phenyl-1-naphthylamine complex Ki, dissociation constant.

Contents

Seed dispersal is likely to have several benefits for different plant species. First, seed survival is often higher away from the parent plant. This higher survival may result from the actions of density-dependent seed and seedling predators and pathogens, which often target the high concentrations of seeds beneath adults. [2] Competition with adult plants may also be lower when seeds are transported away from their parent.

Seed dispersal also allows plants to reach specific habitats that are favorable for survival, a hypothesis known as directed dispersal. For example, Ocotea endresiana (Lauraceae) is a tree species from Latin America which is dispersed by several species of birds, including the three-wattled bellbird. Male bellbirds perch on dead trees in order to attract mates, and often defecate seeds beneath these perches where the seeds have a high chance of survival because of high light conditions and escape from fungal pathogens. [3] In the case of fleshy-fruited plants, seed-dispersal in animal guts (endozoochory) often enhances the amount, the speed, and the asynchrony of germination, which can have important plant benefits. [4]

Seeds dispersed by ants (myrmecochory) are not only dispersed short distances but are also buried underground by the ants. These seeds can thus avoid adverse environmental effects such as fire or drought, reach nutrient-rich microsites and survive longer than other seeds. [5] These features are peculiar to myrmecochory, which may thus provide additional benefits not present in other dispersal modes. [6]

Finally, at another scale, seed dispersal may allow plants to colonize vacant habitats and even new geographic regions. [7] Dispersal distances and deposition sites depend on the movement range of the disperser, and longer dispersal distances are sometimes accomplished through diplochory, the sequential dispersal by two or more different dispersal mechanisms. In fact, recent evidence suggests that the majority of seed dispersal events involves more than one dispersal phase. [8]

Seed dispersal is sometimes split into autochory (when dispersal is attained using the plant's own means) and allochory (when obtained through external means).

Long distance Edit

Long-distance seed dispersal (LDD) is a type of spatial dispersal that is currently defined by two forms, proportional and actual distance. A plant's fitness and survival may heavily depend on this method of seed dispersal depending on certain environmental factors. The first form of LDD, proportional distance, measures the percentage of seeds (1% out of total number of seeds produced) that travel the farthest distance out of a 99% probability distribution. [9] [10] The proportional definition of LDD is in actuality a descriptor for more extreme dispersal events. An example of LDD would be that of a plant developing a specific dispersal vector or morphology in order to allow for the dispersal of its seeds over a great distance. The actual or absolute method identifies LDD as a literal distance. It classifies 1 km as the threshold distance for seed dispersal. Here, threshold means the minimum distance a plant can disperse its seeds and have it still count as LDD. [11] [10] There is a second, unmeasurable, form of LDD besides proportional and actual. This is known as the non-standard form. Non-standard LDD is when seed dispersal occurs in an unusual and difficult-to-predict manner. An example would be a rare or unique incident in which a normally-lemur-dependent deciduous tree of Madagascar was to have seeds transported to the coastline of South Africa via attachment to a mermaid purse (egg case) laid by a shark or skate. [12] [13] [14] [6] A driving factor for the evolutionary significance of LDD is that it increases plant fitness by decreasing neighboring plant competition for offspring. However, it is still unclear today as to how specific traits, conditions and trade-offs (particularly within short seed dispersal) effect LDD evolution.

Autochory Edit

Autochorous plants disperse their seed without any help from an external vector, as a result this limits plants considerably as to the distance they can disperse their seed. [15] Two other types of autochory not described in detail here are blastochory, where the stem of the plant crawls along the ground to deposit its seed far from the base of the plant, and herpochory (the seed crawls by means of trichomes and changes in humidity). [16]

Gravity Edit

Barochory or the plant use of gravity for dispersal is a simple means of achieving seed dispersal. The effect of gravity on heavier fruits causes them to fall from the plant when ripe. Fruits exhibiting this type of dispersal include apples, coconuts and passionfruit and those with harder shells (which often roll away from the plant to gain more distance). Gravity dispersal also allows for later transmission by water or animal. [17]

Ballistic dispersal Edit

Ballochory is a type of dispersal where the seed is forcefully ejected by explosive dehiscence of the fruit. Often the force that generates the explosion results from turgor pressure within the fruit or due to internal tensions within the fruit. [15] Some examples of plants which disperse their seeds autochorously include: Arceuthobium spp., Cardamine hirsuta, Ecballium spp., Euphorbia heterophylla, [18] Geranium spp., Impatiens spp., Sucrea spp, Raddia spp. [19] and others. An exceptional example of ballochory is Hura crepitans—this plant is commonly called the dynamite tree due to the sound of the fruit exploding. The explosions are powerful enough to throw the seed up to 100 meters. [20]

Witch hazel uses ballistic dispersal without explosive mechanisms by simply squeezing the seeds out at 28 mph. [21]

Allochory Edit

Allochory refers to any of many types of seed dispersal where a vector or secondary agent is used to disperse seeds. These vectors may include wind, water, animals or others.

Wind Edit

Wind dispersal (anemochory) is one of the more primitive means of dispersal. Wind dispersal can take on one of two primary forms: seeds or fruits can float on the breeze or, alternatively, they can flutter to the ground. [22] The classic examples of these dispersal mechanisms, in the temperate northern hemisphere, include dandelions, which have a feathery pappus attached to their fruits (achenes) and can be dispersed long distances, and maples, which have winged fruits (samaras) that flutter to the ground.


An important constraint on wind dispersal is the need for abundant seed production to maximize the likelihood of a seed landing in a site suitable for germination. Some wind-dispersed plants, such as the dandelion, can adjust their morphology in order to increase or decrease the rate of germination. [23] There are also strong evolutionary constraints on this dispersal mechanism. For instance, Cody and Overton (1996) found that species in the Asteraceae on islands tended to have reduced dispersal capabilities (i.e., larger seed mass and smaller pappus) relative to the same species on the mainland. [24] Also, Helonias bullata, a species of perennial herb native to the United States, evolved to utilize wind dispersal as the primary seed dispersal mechanism however, limited wind in its habitat prevents the seeds to successfully disperse away from its parents, resulting in clusters of population. [25] Reliance on wind dispersal is common among many weedy or ruderal species. Unusual mechanisms of wind dispersal include tumbleweeds, where the entire plant (except for the roots) is blown by the wind. Physalis fruits, when not fully ripe, may sometimes be dispersed by wind due to the space between the fruit and the covering calyx which acts as an air bladder.

Water Edit

Many aquatic (water dwelling) and some terrestrial (land dwelling) species use hydrochory, or seed dispersal through water. Seeds can travel for extremely long distances, depending on the specific mode of water dispersal this especially applies to fruits which are waterproof and float on water.

The water lily is an example of such a plant. Water lilies' flowers make a fruit that floats in the water for a while and then drops down to the bottom to take root on the floor of the pond. The seeds of palm trees can also be dispersed by water. If they grow near oceans, the seeds can be transported by ocean currents over long distances, allowing the seeds to be dispersed as far as other continents.

Mangrove trees grow directly out of the water when their seeds are ripe they fall from the tree and grow roots as soon as they touch any kind of soil. During low tide, they might fall in soil instead of water and start growing right where they fell. If the water level is high, however, they can be carried far away from where they fell. Mangrove trees often make little islands as dirt and detritus collect in their roots, making little bodies of land.

Animals: epi- and endozoochory Edit

Animals can disperse plant seeds in several ways, all named zoochory. Seeds can be transported on the outside of vertebrate animals (mostly mammals), a process known as epizoochory. Plant species transported externally by animals can have a variety of adaptations for dispersal, including adhesive mucus, and a variety of hooks, spines and barbs. [26] A typical example of an epizoochorous plant is Trifolium angustifolium, a species of Old World clover which adheres to animal fur by means of stiff hairs covering the seed. [7] Epizoochorous plants tend to be herbaceous plants, with many representative species in the families Apiaceae and Asteraceae. [26] However, epizoochory is a relatively rare dispersal syndrome for plants as a whole the percentage of plant species with seeds adapted for transport on the outside of animals is estimated to be below 5%. [26] Nevertheless, epizoochorous transport can be highly effective if seeds attach to wide-ranging animals. This form of seed dispersal has been implicated in rapid plant migration and the spread of invasive species. [7]

Seed dispersal via ingestion by vertebrate animals (mostly birds and mammals), or endozoochory, is the dispersal mechanism for most tree species. [27] Endozoochory is generally a coevolved mutualistic relationship in which a plant surrounds seeds with an edible, nutritious fruit as a good food for animals that consume it. Birds and mammals are the most important seed dispersers, but a wide variety of other animals, including turtles, fish, and insects (e.g. tree wētā and scree wētā), can transport viable seeds. [28] [29] The exact percentage of tree species dispersed by endozoochory varies between habitats, but can range to over 90% in some tropical rainforests. [27] Seed dispersal by animals in tropical rainforests has received much attention, and this interaction is considered an important force shaping the ecology and evolution of vertebrate and tree populations. [30] In the tropics, large animal seed dispersers (such as tapirs, chimpanzees, black-and-white colobus, toucans and hornbills) may disperse large seeds with few other seed dispersal agents. The extinction of these large frugivores from poaching and habitat loss may have negative effects on the tree populations that depend on them for seed dispersal and reduce genetic diversity. [31] [32] A variation of endozoochory is regurgitation rather than all the way through the digestive tract. [33] The seed dispersal by birds and other mammals are able to attach themselves to the feathers and hairs of these vertebrates, which is their main method of dispersal. [34]

Seed dispersal by ants (myrmecochory) is a dispersal mechanism of many shrubs of the southern hemisphere or understorey herbs of the northern hemisphere. [5] Seeds of myrmecochorous plants have a lipid-rich attachment called the elaiosome, which attracts ants. Ants carry such seeds into their colonies, feed the elaiosome to their larvae and discard the otherwise intact seed in an underground chamber. [35] Myrmecochory is thus a coevolved mutualistic relationship between plants and seed-disperser ants. Myrmecochory has independently evolved at least 100 times in flowering plants and is estimated to be present in at least 11 000 species, but likely up to 23 000 or 9% of all species of flowering plants. [5] Myrmecochorous plants are most frequent in the fynbos vegetation of the Cape Floristic Region of South Africa, the kwongan vegetation and other dry habitat types of Australia, dry forests and grasslands of the Mediterranean region and northern temperate forests of western Eurasia and eastern North America, where up to 30–40% of understorey herbs are myrmecochorous. [5] Speed dispersal by ants is a mutualistic relationship and benefits both the ant and the plant. [34]

Seed predators, which include many rodents (such as squirrels) and some birds (such as jays) may also disperse seeds by hoarding the seeds in hidden caches. [36] The seeds in caches are usually well-protected from other seed predators and if left uneaten will grow into new plants. In addition, rodents may also disperse seeds via seed spitting due to the presence of secondary metabolites in ripe fruits. [37] Finally, seeds may be secondarily dispersed from seeds deposited by primary animal dispersers, a process known as diplochory. For example, dung beetles are known to disperse seeds from clumps of feces in the process of collecting dung to feed their larvae. [38]

Other types of zoochory are chiropterochory (by bats), malacochory (by molluscs, mainly terrestrial snails), ornithochory (by birds) and saurochory (by non-bird sauropsids). Zoochory can occur in more than one phase, for example through diploendozoochory, where a primary disperser (an animal that ate a seed) along with the seeds it is carrying is eaten by a predator that then carries the seed further before depositing it. [39]

Humans Edit

Dispersal by humans (anthropochory) used to be seen as a form of dispersal by animals. Its most widespread and intense cases account for the planting of much of the land area on the planet, through agriculture. In this case, human societies form a long-term relationship with plant species, and create conditions for their growth.

Recent research points out that human dispersers differ from animal dispersers by having a much higher mobility, based on the technical means of human transport. [40] On the one hand, dispersal by humans also acts on smaller, regional scales and drives the dynamics of existing biological populations. On the other hand, dispersal by humans may act on large geographical scales and lead to the spread of invasive species. [41]

Humans may disperse seeds by many various means and some surprisingly high distances have been repeatedly measured. [42] Examples are: dispersal on human clothes (up to 250 m), [43] on shoes (up to 5 km), [40] or by cars (regularly

250 m, singles cases > 100 km). [44] Seed dispersal by cars can be a form of unintentional transport of seeds by humans, which can reach far distances, greater than other conventional methods of dispersal. [45] Cars that carry soil are able to contain viable seeds, a study by Dunmail J. Hodkinson and Ken Thompson found that the most common seeds that were carried by vehicle were broadleaf plantain (Plantago major), Annual meadow grass (Poa annua), rough meadow grass (Poa trivialis), stinging nettle] (Urtica dioica) and wild chamomile (Matricaria discoidea). [45]

Deliberate seed dispersal also occurs as seed bombing. This has risks, as unsuitable provenance may introduce genetically unsuitable plants to new environments.

Seed dispersal has many consequences for the ecology and evolution of plants. Dispersal is necessary for species migrations, and in recent times dispersal ability is an important factor in whether or not a species transported to a new habitat by humans will become an invasive species. [46] Dispersal is also predicted to play a major role in the origin and maintenance of species diversity. For example, myrmecochory increased the rate of diversification more than twofold in plant groups in which it has evolved because myrmecochorous lineages contain more than twice as many species as their non-myrmecochorous sister groups. [47] Dispersal of seeds away from the parent organism has a central role in two major theories for how biodiversity is maintained in natural ecosystems, the Janzen-Connell hypothesis and recruitment limitation. [2] Seed dispersal is essential in allowing forest migration of flowering plants. It can be influenced by the production of different fruit morphs in plants, a phenomenon known as heterocarpy. [48] These fruit morphs are different in size and shape and have different dispersal ranges, which allows seeds to be dispersed for varying distances and adapt to different environments. [48]

In addition, the speed and direction of wind are highly influential in the dispersal process and in turn the deposition patterns of floating seeds in the stagnant water bodies. The transportation of seeds is led by the wind direction. This effects colonization situated on the banks of a river or to wetlands adjacent to streams relative to the distinct wind directions. The wind dispersal process can also affect connections between water bodies. Essentially, wind plays a larger role in the dispersal of waterborne seeds in a short period of time, days and seasons, but the ecological process allows the process to become balanced throughout a time period of several years. The time period of which the dispersal occurs is essential when considering the consequences of wind on the ecological process.


Methods

We conducted a systematic search of peer-reviewed journal articles including all records until June 2016 that focussed on the effects of human forest disturbance on ecological processes involved in plant regeneration (i.e., pollination, seed dispersal, seed predation, recruitment and herbivory). We are aware that other processes, such as fungal or soil microbial interactions, may also play a role, but only few case studies have studied their effects on plant regeneration across a disturbance gradient 50,51 . At each step of this study we followed the guidelines of quality criteria for meta-analyses listed by Koricheva and Gurevitch 52 . We searched the Web of Science using combinations of the following keywords “((fragmentation OR disturbance OR hunting OR defaunation) AND (pollination OR seed_set OR seed_dispersal OR seed_removal OR seed_predation OR pilferage OR germination OR establishment OR recruitment OR herbivory) AND (tree OR shrub))”. The search resulted in 4,242 journal articles. Furthermore, we added publications that were cited in meta-analyses of the effects of forest disturbance on specific processes (pollination 6,53,54 seed dispersal 5 predation 23,55 recruitment 23 herbivory 29,30,56,57 ).

We searched the abstracts of the pre-selected publications for matching the following criteria: 1) study investigates the effect of forest disturbance on pollination, seed dispersal, post dispersal seed predation, recruitment or herbivory of at least one woody plant species 2) study compares near-natural or protected forest to one of the following types of disturbed forest: fragmented forest, forest edge, defaunated forest, forests hunted for bushmeat, logged forest, or secondary forest 3) study reports on observational or experimental field data. We excluded studies in which isolated trees were compared to natural forests, studies in which fire events caused forest disturbance, and studies on cultivated crops or invasive species.

Out of 339 publications that matched these criteria, we retained 145 studies from which we were able to extract mean values, standard deviations and sample sizes of the comparison between near-natural and disturbed forests (Supplementary Dataset). If necessary, we extracted data with graphical software 58 . We chose response variables that have been used in previous meta-analyses and have been proven to reflect the respective processes of plant regeneration. For pollination, we used pollinator visitation and seed set 6,54 for seed dispersal, we used seed-disperser visitation, disperser seed removal and seed-dispersal distance 5 (we only included primary seed dispersal, since quantitative studies on the effects of human disturbance on secondary seed dispersal are scarce in the literature) for post-dispersal seed predation: rodent seed predation, insect infestation and predator seed removal 23,55 for recruitment: germination, seedling establishment, seedling survival and sapling establishment 23,59 for herbivory: herbivore abundance, leaf damage and leaf loss 23,55 .

We used the following protocol for data extraction: (1) In case the original paper studied a gradient of forest disturbances, we used the end-points of the gradient (i.e., continuous forest vs. smallest fragment forest interior vs. forest edge protected forest vs. forest with bushmeat hunting near-natural forest vs. highest degree of logging near-natural forest vs. secondary forest) to define a two level factor 55 . (2) If several plant species were investigated in the same original study, we included both in the data set and accounted for non-independence with a random effect for study (see data analysis section). Likewise, if the same plant species was investigated in more than one study, we included both in the data set and accounted for non-independence with a random effect for species (see data analysis section). We extracted the latitudinal and longitudinal position of the study site from the original publication and annotated each site with biomes based on the WWF Terrestrial Ecoregions 10 . We also extracted measures of seed size and seed mass from the publications. If no seed measurements were specified in the publication, we either extracted data from other publications on the same plant species, or we obtained seed measurements from the TRY data base (www.try-db.org) 60 . For 92 out of 247 plant species, we could only acquire data on seed mass. We conducted a linear regression using the species for which we had information on both seed size and seed mass (seed size and seed mass both log-transformed, n = 83 species, r 2 = 0.77, β = 0.32) and predicted the seed size for those species where only seed mass was available (see Supplementary Dataset for predicted seed sizes). We obtained taxonomic classification from The Plant List Version 1.1 (www.theplantlist.org) for all 247 plant species, which belonged to 70 families and 159 genera. Furthermore, we retained information on pollination and seed-dispersal syndromes of each plant species from the publication or additional literature. More than 96% of the plant species in our data set depended on animals for pollination and/or seed dispersal, reflecting the high proportion of animal-dependant plants in natural ecosystems 16,17 .

Data analysis

To examine the effect of forest disturbance on plant regeneration, we built two models that compared disturbed versus non-disturbed forest across all original studies. The first model tested the overall effect of human forest disturbance on plant regeneration. The second model tested whether disturbance differentially affected plant regeneration processes and additionally included absolute latitude and longitude as a measure of the geographic location of the study sites and seed size as a correlate for life-history strategy. We also tested for a potential publication bias that was not detectable (see Supplementary Methods 1).

All analyses were conducted with the statistical programming language R 61 using the metafor package 62 . We calculated Hedge’s d as an estimate of the mean standardized difference and the corresponding sampling variance for each comparison between near-natural and disturbed forests (see Supplementary Dataset) 62 . This estimator is robust against small sample sizes and unequal sampling variances 13 . In the comparison between near-natural versus disturbed forests, we defined effect sizes as positive when the ecosystem process was beneficial for plant regeneration and negative if it was detrimental for plant regeneration. That is, we characterized more pollination, seed dispersal, recruitment and less seed predation and herbivory with positive effect sizes and vice versa.

We fitted random effects models accounting for variability in sampling methods among studies 62 . Both models included the study site as additional random effect to account for non-independent samples from the same site. We accounted for phylogenetic relatedness by including plant species and genus as random effects. We also added a spatial autocovariate as fixed effect to control for spatial autocorrelation 63 . After including the spatial autocovariate, we could not detect spatial autocorrelation in the model residuals (see Supplementary Methods 2). Effect sizes were weighted by the inverse sampling variances 62 . In both models, we used REML approximation of the model estimates 62 .

To test the robustness of our findings, we additionally built both models using a Bayesian approach 64 . In this case, we accounted for phylogenetic relatedness among plant species with a taxonomic tree and kept the model structure otherwise identical. For both models, the results were qualitatively identical (see Supplementary Methods 3).


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