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Seed dispersal
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Seed dispersal is the movement or transport of seeds away from the parent plant. Plants have limited mobility and consequently rely upon a variety of dispersal vectors to transport their propagules, including both abiotic and biotic vectors. Seeds can be dispersed away from the parent plant individually or collectively, as well as dispersed in both space and time. The patterns of seed dispersal are determined in large part by the dispersal mechanism and this has important implications for the demographic and genetic structure of plant populations, as well as migration patterns and species interactions.
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Encyclopedia
Seed dispersal is the movement or transport of seeds away from the parent plant. Plants have limited mobility and consequently rely upon a variety of dispersal vectors to transport their propagules, including both abiotic and biotic vectors. Seeds can be dispersed away from the parent plant individually or collectively, as well as dispersed in both space and time. The patterns of seed dispersal are determined in large part by the dispersal mechanism and this has important implications for the demographic and genetic structure of plant populations, as well as migration patterns and species interactions. There are five main modes of seed dispersal: gravity, wind, ballistic, water and by animals.
Benefits of seed dispersal
Seed dispersal is likely to have several benefits for plant species. First, seed survival is often (but not always) higher away from the parent plant. This higher survival may result from the actions of density-dependent seed predators and pathogens which often target the high concentrations of seeds beneath adults . Competition (biology) 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 . Finally, seed dispersal may allow plants to colonize new habitats and geographic regions .
Types of Dispersal
Wind
Wind dispersal, or anemochory, is one of the more primitive means of dispersal. Wind dispersal can take on one of two primary forms: seeds can float on the breeze or alternatively, they can flutter to the ground. The classic examples of these dispersal mechanisms include dandelions (Taraxacum spp., Asteraceae), which have a feathery pappus attached to their seeds and can be dispersed long distances, and maples (Acer spp., Sapindaceae), which have winged seeds and 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. 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 . Reliance upon wind dispersal is common among many weedy or ruderal species. Unusual mechanisms of wind dispersal include tumbleweeds.
, or autochory, is the physical and often explosive discharge of seeds from the fruit. The seeds are typically ejected from the fruit by elastic contraction of the fruit tissues and often the fruits are shaped such that seeds are flung away from the parent plant . While ballistic dispersal does not often achieve the same distance as animal-dispersed seeds, many ballistic dispersed seeds also have a form of secondary dispersal . Some common examples of species employing ballistic dispersal include the aptly named Touch-me-nots (Impatiens spp., Balsaminaceae) whose fruits explosively dehisce and squirting cucumbers (Ecballium elaterium, Cucurbitaceae) that discharge their seeds in a mucilaginous stream of liquid.
Water
Many aquatic and some terrestrial plant species utilize hydrochory, or seed dispersal through water. Seeds can travel for extremely long distances, depending on the specific mode of water dispersal. For instance, coconuts (Cocos nucifera, Aceraceae)
Dispersal by animals
Animals can disperse plant seeds in several ways. First, seeds can be transported on the outside of animals, 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 . 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 . Epizoochorous plants tend to be herbaceous plants, with many representative species in the families Apiaceae and Asteraceae . 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% . 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 .
Seed dispersal via ingestion by animals, or endozoochory, is the dispersal mechanism for most tree species . Endozoochory is generally a coevolved mutualistic relationship in which a plant surrounds seeds with an edible, nutritious fruit as a reward to frugivorous animals that consume it. Birds and mammals are the most important seed dispersers, but a wide variety of other animals, including turtles and fish, can transport viable seeds. The exact percentage of tree species dispersed by endozoochory varies between habitats, but can range to over 90% in some tropical rainforests . 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 . In the tropics, large animal seed dispersers (such as tapirs, chimpanzees 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 .
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 . The seeds in caches are usually well-protected from other seed predators and if left uneaten will grow into new plants. Finally, seeds may be secondarily dispersed from seeds deposited by primary animal dispersers. For example, dung beetles are known to disperse seeds from clumps of feces in the process of collecting dung to feed their larvae .
See the 5th International Symposium-Workshop on Frugivores and Seed Dispersal (Forthcoming meeting in 2010) Website
Consequences of seed dispersal
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 . Dispersal is also predicted to play a major role in the maintenance of species diversity. 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 .
See also
External links
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