Seed dispersal

November 11, 2012, 11:37 pm
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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.  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[1]. 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[2].  Finally, seed dispersal may allow plants to colonize new habitats and geographic regions[3].

Types of Dispersal

Gravity

The simplest form of seed dispersal utilizes gravity.  In this scenario, seeds drop from the parent plant and are typically not dispersed very far.  Species with gravity- dispersed seeds typically lack obvious rewards (i.e., fleshy fruit) or extra structures (i.e., winged seeds) [4]. One example of a gravity-dispersed seed is European Beech (Fagus sylvatica, Fagaceae), although the seeds are also dispersed by scatter-hoarding rodents[5].  Some species rely upon gravity for primary dispersal, but following this initial dispersal event, they are further dispersed by another vector (secondary 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[6].  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[7]. Reliance upon wind dispersal is common among many weedy or ruderal species. 

Ballistic

Ballistic seed 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 [8].  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[9].  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) can travel up to thousands of kilometers on the ocean before settling on land and germinating. Mangrove species (Rhizophora spp., Rhizophoraceae) all utilize water to disperse their propagules, although mangroves display vivipary (the seed germinates prior to detachment from the parent plant).  In general, water dispersal has been identified as the principal dispersal agent for freshly deposited seeds (primary dispersal) and remobilizing previously dispersed seeds (secondary dispersal)[10].  An interesting case of secondary dispersal involves the invasive species ''Ailanthus altissima'' (Simaroubaceae), which is typically wind-dispersed.  As ''A. altissima'' expands its range, however, its dispersal and germination is being enhanced by water transport[11]

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[12]. 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[13]. Epizoochorous plants tend to be herbaceous plants, with many representative species in the families Apiaceae and Asteraceae [12]. 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[13].

Seed dispersal via ingestion by animals, or endozoochory, is the dispersal mechanism for most tree species [14]. 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[15]. The exact percentage of tree species dispersed by endozoochory varies between habitats, but can range to over 90% in some tropical rainforests [14]. 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[16].  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[17].

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[18]. 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[19].

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[20]. 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].

 

Note: This article uses material from the Wikipedia article Seed dispersal that was accessed on November 17, 2008. The Author(s) and Topic Editor(s) associated with this article may have significantly modified the content derived from Wikipedia with original content or with content drawn from other sources. All content from Wikipedia has been reviewed and approved by those Author(s) and Topic Editor(s), and is subject to the same peer review process as other content in the EoE. The current version of the Wikipedia article may differ from the version that existed on the date of access. This article is licensed under the GNU Free Documentation License 1.2. See the EoE’s Policy on the Use of Content from Wikipedia for more information. 

 

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. ^ Seidler, T.G. & J.B. Plotkin. (2006). Seed dispersal and spatial patterns in tropical trees. PLOS Biology. 4:e344.
  5. ^ Sagnard, F., Pichot, C., Dreyfus, P. & B. Fady. (2007). Modelling seed dispersal to predict seedling recruitment: Recolonization dynamics in a plantation forest. Ecological Modelling. 203: 464-474.
  6. ^  Gurevitch, J., Scheiner, S.M., & G.A. Fox (2006). Plant Ecology, 2nd ed. Sinauer Associates, Inc., Massachusetts.
  7. ^ Cody, M.L., & J.M. Overton. (1996). Short-term evolution of reduced dispersal in island plant populations. Journal of Ecology. 84: 53-61.
  8. ^ Garrison W. J., Miller, G.L., & R. Raspet. (2000). Ballistic seed projection in two herbaceous species. American Journal of Botany. 87: 1257-1264.
  9. ^ 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.
  10. ^ Gurnell, A., Thompson, K., Goodson, J. & H. Moggridge. (2008). Propagule deposition along river margins: linking hydrology and ecology. Journal of Ecology. 96: 553-565.
  11. ^ Kowarik, I. & I. Saumel. (2008). Water dispersal as an additional pathway to invasions by the primarily wind-dispersed tree Ailanthus altissima. Plant Ecology.
  12. ^ a b c Sorenson, A.E. (1986) Seed dispersal by adhesion. Annual Review of Ecology and Systematics. 17: 443-463.
  13. ^ a b Malo Juan E. (2008) Extreme long-distance seed dispersal via sheep. . Frontiers in Ecology and the Environment.
  14. ^ a b Smallwood J. (1984) Ecology of Seed Dispersal. Annual Review of Ecology and Systematics. 13: 201-228.
  15. ^ Corlett, R.T. (1998) Frugivory and seed dispersal by vertebrates in the Oriental (Indomalayan) Region. Biological Reviews. 73: 413-448.
  16. ^ Terborgh, J. (1986) Community aspects of frugivory in tropical forests. Frugivory and Seed Dispersal. Springer. 416 pp.
  17. ^ Chapman, CA |Onderdonk, D.A. (1998) Forests without primates: primate/plant codependency. Conservation Biology. 45: 127-141.
  18. ^ Forget, P.M. | Milleron, T. (1991) Evidence for secondary seed dispersal by rodents in Panama. Oecologia. 87: 596-599.
  19. ^ 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.
  20. ^ Lensink, R. | Neubert, M.G. (2003) Demography And Dispersal: Life Table Response Experiments For Invasion Speed. Ecology. 84: 1968-1978.

Further Reading

Glossary

Citation

(2012). Seed dispersal. Retrieved from http://www.eoearth.org/view/article/51cbeedc7896bb431f69ac54

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