Biological dispersal refers to those processes by which a species maintains ecosystem. Dispersal relieves pressure for resources in an ecosystem, and competition for these resources may be a selection factor for dispersal mechanisms.[1]
In the latter case, dispersal may simply involve replacement of the parent generation by the new generation, with only minor changes in geographic area occupied.
In most cases, organisms (plants and especially sedentary animals) have evolved adaptations for dispersal that take advantage of various forms of kinetic energy occurring naturally in the environment: water flow, wind, falling (response to gravity).
Dispersal of organisms is a critical process for understanding both geographic isolation in evolution and the broad patterns of current geographic distributions.
Of animals
Most (but not all) animals are capable of locomotion and the basic mechanism of dispersal is movement from one place to another. Locomotion allows the organism to "test" new environments for their suitability, provided they are within animal's range. Movements are usually guided by inherited behaviors.
Non-motile animals
There are numerous animal forms that are non-motile, such as sponges, bryozoans, tunicates, sea anemones, corals, and oysters. In common, they are all either marine or aquatic. It may seem curious that plants have been so successful at stationary life on land, while animals have not, but the answer lies in the food supply. Plants produce their own food from sunlight and carbon dioxide—both generally more abundant on land than in water. Animals fixed in place must rely on the surrounding medium to bring food at least close enough to grab, and this occurs in the three-dimensional water environment, but with much less abundance in the atmosphere. However, that such a life form might be possible is at least suggested by the orb-weaver spiders.
All of the marine and aquatic invertebrates whose lives are spent fixed to the bottom (more or less; anemones are capable of getting up and moving to a new location if conditions warrant) produce dispersal units. These may be specialized "buds", or motile sexual reproduction products, or even a sort of alteration of generations as in certain cnidaria.
Corals provide a good example of how sedentary species achieve dispersion. Corals reproduce by releasing sperm and eggs directly into the water. These release events are coordinated by lunar phase in certain warm months, such that all corals of one or many species on a given reef will release on the same single or several consecutive nights. The released eggs are fertilized, and the resulting zygote develops quickly into a multicellular planula. This motile stage then attempts to find a suitable substratum for settlement. Most are unsuccessful and die or are fed upon by zooplankton and bottom dwelling predators such as anemones and other corals. However, untold millions are produced, and a few do succeed in locating spots of bare limestone, where they settle and transform by growth into a polyp. All things being favorable, the single polyp grows into a coral head by budding off new polyps to form a colony.
Motile animals
Although motile animals can, in theory, disperse themselves by their locomotive powers, a great many species utilize the existing kinetic energies in the environment. Dispersal by water currents is especially associated with the physically small inhabitants of marine waters known as zooplankton. The term plankton comes from the Greek, πλαγκτον, meaning "wanderer" or "drifter".
See also
References
- ^ Irwini, AJ; PD Taylor (2001[[Media:[Example.ogg]]]). "Evolution of Dispersal in a Stepping-Stone Population with Overlapping Generations". Theoretical Population Biology 58: 321-328. Academic Press. doi:10.1006/tpbi.2000.1490. Retrieved on 2007-08-07.
- Hogarth, Peter J. 1999. The Biology of Mangroves. Oxford Univ. Press. 228 p. ISBN 0-19-850222-2
Further reading
- Ingold, C. T. (1971) Fungal spores: their liberation and dispersal Oxford, Clarendon Press 302 p. ISBN 0198541155
- Lidicker, W. Z. and R. L. Caldwell (1982) Dispersal and migration Stroudsburg, Pa. : Hutchinson Ross Pub. Co 311 p. ISBN 0879334355 (Dispersal of animals)
- Bullock, J. M.; R. E. Kenward, and R. S. Hails (editors) (2002) Dispersal ecology : the 42nd symposium of the British Ecological Society. Oxford, UK : Blackwell Science 458 p. ISBN 0632058765 (Animals and plants)
External links
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