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==Benefits of seed dispersal==
==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 predation|seed]] and seedling predators and [[pathogens]], which often target the high concentrations of seeds beneath adults.<ref name="Harms">Harms K | Wright, SJ | Calderon, O | Hernandez, A | Herre, EA (2000) Pervasive density-dependent recruitment enhances seedling diversity in a tropical forest . ''Nature.'' '''404''': 493-495.</ref> [[Competition (biology)|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 [[bellbird]]s 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.<ref>Wenny D.G. | Levey, D.J. (1997) Directed seed dispersal by bellbirds in a tropical forest. ''PNAS.'' '''95:''' 6204-6207.</ref> Finally, seed dispersal may allow plants to colonize new habitats and geographic regions.<ref>Malo Juan E. (2008) Extreme long-distance seed dispersal via sheep. ''Frontiers in Ecology and the Environment''. '''4:''' 244-248. </ref>
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 predation|seed]] and seedling predators and [[pathogens]], which often target the high concentrations of seeds beneath adults.<ref name="Harms">Harms K | Wright, SJ | Calderon, O | Hernandez, A | Herre, EA (2000) Pervasive density-dependent recruitment enhances seedling diversity in a tropical forest . ''Nature.'' '''404''': 493-495.</ref> [[Competition (biology)|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 [[bellbird]]s 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.<ref>Wenny D.G. | Levey, D.J. (1997) Directed seed dispersal by bellbirds in a tropical forest. ''PNAS.'' '''95:''' 6204-6207.</ref> Finally, seed dispersal may allow plants to colonize new habitats and geographic regions.<ref>Malo Juan E. (2008) Extreme long-distance seed dispersal via sheep. ''Frontiers in Ecology and the Environment''. '''4:''' 244-248. </ref>

== Types of dispersal ==
===Gravity===
<!--NOTE: I don't expect this section to stay, I've added it so that all 5 types of dispersal have a subheading. Please improve it! -->
Gravity 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 [[apple]]s, [[coconut]]s and [[passionfruit]] and those with harder shells often roll away from the plant to gain further distance. Gravity dispersal also allows for later transmission by water or animal.<ref name="seedsite-gravity">{{cite web|url=http://theseedsite.co.uk/sdgravity.html|title=Dispersal of seeds by gravity|accessdate=2009-05-08}}</ref>

===Wind===
[[Image:Photos-photos 1088103921 Floating.jpg|thumb|right|Wind dispersal of [[dandelion]] seeds]]
Wind dispersal 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.<ref>Gurevitch, J., Scheiner, S.M., & G.A. Fox (2006). Plant Ecology, 2nd ed. Sinauer Associates, Inc., Massachusetts. </ref> The classic examples of these dispersal mechanisms include dandelions (''[[Taraxacum]]'' spp., [[Asteraceae]]), which have a feathery [[Pappus (flower structure)|pappus]] attached to their seeds and can be dispersed long distances, and maples (''[[Acer]]'' spp., [[Sapindaceae]]), which have winged seeds ([[Samara (fruit)|samara]]) and flutter to the ground. An important constraint on wind dispersal is the need for abundant seed production to maximise 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.<ref>Cody, M.L., & J.M. Overton. (1996). Short-term evolution of reduced dispersal in island plant populations. ''Journal of Ecology.'' '''84:''' 53-61. </ref> Reliance on wind dispersal is common among many [[weed|weedy]] or [[ruderal]] species. Unusual mechanisms of wind dispersal include [[tumbleweed]]s.

===Ballistic===
{{main|Dehiscence (botany)}}
Self-dispersal, 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]].<ref>Garrison W. J., Miller, G.L., & R. Raspet. (2000). Ballistic seed projection in two herbaceous species. ''American Journal of Botany.'' '''87:''' 1257-1264.</ref> 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.<ref>Narbona, E., Arista, M., & P.L. Ortiz. (2005). Explosive seed dispersal in two perennial Mediterranean Euphorbia species (Euphorbiaceae). ''American Journal of Botany.'' '''92:''' 510-516. </ref> 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 ([[water]]) and some terrestrial ([[ground]]) [[plant]] species use ''hydrochory'', or seed dispersal through water. Seeds can travel for [[wikt:extreme|extremely]] long distances, depending on the specific mode of [[water]] dispersal.

The [[water lily]] is an example of such a [[plant]]. Water lilies' beautiful [[flower]]s 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]]. <ref>{{cite book | last = Fulbright| first = Jeannie K.| authorlink = | coauthors = | title = ''Exploring Creation with Botany''| publisher = Apologia Educational Ministries, Inc| date = 2004| location = 1106 Meridian Plaza, Suite 220| pages = 74| url = | doi = | id = | isbn = 1-932012-49-4}}</ref>
The [[palm tree]] is another excellent example. [[Palm tree]]s often grow very near [[ocean]]s. [[Ocean]] [[current]]s are powerful and stretch from one [[continent]] to the other. This is the road the palm trees use to move its seeds ([[coconut]]s) to their new home. When a coconut lands in the [[ocean]], it can [[float]] quite a long time, being carried thousands of [[mile]]s away, even to a new [[continent]], to grow.

[[Mangrove]] trees live right in the [[water]]. Their [[seed]]s fall from the [[tree]] and grow [[root]]s 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 [[island]]s as dirt and other things collect in their [[root]]s, making little bodies of land.<ref>{{cite book | last = Fulbright| first = Jeannie K.| authorlink = | coauthors = | title = ''Exploring Creation with Botany''| publisher = Apologia Educational Ministries, Inc| date = 2004| location = 1106 Meridian Plaza, Suite 220| pages = 75| url = | doi = | id = | isbn = 1-932012-49-4}}</ref>

===Dispersal by animals===
Animals can disperse plant seeds in several ways. 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.<ref name="Sorenson"> Sorenson, A.E. (1986) Seed dispersal by adhesion. ''Annual Review of Ecology and Systematics.'' '''17:''' 443-463.</ref> 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 [[hair]]s covering the [[seed]].<ref name="Malo">Malo Juan E. (2008) Extreme long-distance seed dispersal via sheep. . Frontiers in Ecology and the Environment.</ref> Epizoochorous plants tend to be herbaceous plants, with many representative species in the families [[Apiaceae]] and [[Asteraceae]]. <ref name="Sorenson"/> 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%.<ref name="Sorenson"/> 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.<ref name="Malo"/>

Seed dispersal via ingestion by animals, or endozoochory, is the dispersal mechanism for most tree species.<ref name="Smallwood">Smallwood J. (1984) Ecology of Seed Dispersal. ''Annual Review of Ecology and Systematics.'' '''13:''' 201-228.</ref> 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 and fish, can transport viable seeds.<ref>Corlett, R.T. (1998) Frugivory and seed dispersal by vertebrates in the Oriental (Indomalayan) Region. ''Biological Reviews.'' '''73:''' 413-448.</ref> The exact percentage of tree species dispersed by endozoochory varies between [[habitats]], but can range to over 90% in some tropical rainforests.<ref name="Smallwood"/> 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.<ref>Terborgh, J. (1986) Community aspects of frugivory in tropical forests. Frugivory and Seed Dispersal. Springer. 416 pp.</ref> 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 [[frugivore]]s from poaching and habitat loss may have negative effects on the tree populations that depend on them for seed dispersal.<ref>Chapman, CA |Onderdonk, D.A. (1998) Forests without primates: primate/plant codependency. ''Conservation Biology.'' ''45:'' 127-141.</ref>

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.<ref>Forget, P.M. | Milleron, T. (1991) Evidence for secondary seed dispersal by rodents in Panama. ''Oecologia.'' '''87:''' 596-599.</ref> 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.<ref>Andresen E. | Levey, D.J. (2004) Effects of dung and seed size on secondary dispersal, seed predation, and seedling establishment of rainforest trees. ''Oecologia.'' '''139:''' 45-54.</ref>


==Consequences of seed dispersal==
==Consequences of seed dispersal==

Revision as of 17:13, 1 February 2010

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. Some plants are serotinous and only disperse their seeds in response to an environmental stimulus.

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 and seedling predators and pathogens, which often target the high concentrations of seeds beneath adults.[1] 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.[2] Finally, seed dispersal may allow plants to colonize new habitats and geographic regions.[3]

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.[4] 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.[1] Seed dispersal is essential in allowing forest migration.

See also

References

  1. ^ a b Harms K | Wright, SJ | Calderon, O | Hernandez, A | Herre, EA (2000) Pervasive density-dependent recruitment enhances seedling diversity in a tropical forest . Nature. 404: 493-495.
  2. ^ Wenny D.G. | Levey, D.J. (1997) Directed seed dispersal by bellbirds in a tropical forest. PNAS. 95: 6204-6207.
  3. ^ Malo Juan E. (2008) Extreme long-distance seed dispersal via sheep. Frontiers in Ecology and the Environment. 4: 244-248.
  4. ^ Lensink, R. | Neubert, M.G. (2003) Demography And Dispersal: Life Table Response Experiments For Invasion Speed. Ecology. 84: 1968-1978.
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