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Wind dispersal of dandelion seeds.

Biological dispersal refers to a species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population genetics, and species' distribution¹²³. Understanding dispersal and the consequences on an ecosystem level requires understanding on the type of dispersal, the dispersal range of a given species, and the dispersal mechanisms involved.

Dispersal from parent population

Types of dispersal

In general there are two basic types of dispersal: density independent and dependent.

Density Independent dispersal

In many cases, organisms have evolved adaptations for dispersal that take advantage of various forms of kinetic energy occurring naturally in the environment. This is refered to as density independent or passive dispersal and operates on many groups of organisms (some invertebrates, fish, insects and sessile organisms such as plants) that depend on animal vectors, wind, gravity or current for dispersal⁴⁵.

Density Dependent dispersal

On the other hand, it is now well recognized that density dependent or active dispersal for many animals largely depends on factors such as local population size, resource competition, habitat quality, habitat size, etc⁶⁷. Due to population density, dispersal may relieve pressure for resources in an ecosystem, and competition for these resources may be a selection factor for dispersal mechanisms⁸.

Dispersal of organisms is a critical process for understanding both geographic isolation in evolution through gene flow and the broad patterns of current geographic distributions.

Dispersal Range

Dispersal range refers to the distance a species can move from an existing population or the parent organism. An ecosystem depends critically on the ability of individuals and populations to disperse from one habitat patch to another. Therefore, biological dispersal is critical to the stability of ecosystems.

Environmental Contraints

Few species are ever evenly or randomly distributed within or across landscapes. In general, species significantly vary across the landscape in association with environmental features that influence their reproductive success and population persistence⁹¹⁰. Spatial patterns in environmental features (e.g. resources) permit individuals to escape unfavorable conditions and seek out new locations¹¹. This allows the organism to "test" new environments for their suitability, provided they are within animal's geographic range. In addition, the ability of a species to disperse over a gradually changing environment could enable a population to survive extreme conditions (i.e. climate change).

Dispersal Barriers

A dispersal barrier may mean that the dispersal range of a species is much smaller than the species distribution (e.g. habitat fragmentation due to human land use). However, there are also natural barriers to dispersal that limit species distribution (e.g. mountain ranges, rivers, etc.).
On the other hand, human activities may also expand the dispersal range of a species by providing new dispersal routes for invasive species (e.g. roads).

Dispersal Mechanisms

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.

Plant Dispersal Mechanisms

See Seed Dispersal [1]

Animal Dispersal Mechanisms

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.

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

The majority of all animals are motile (see locomotion for details). Although motile animals can, in theory, disperse themselves by their spontaneous and independent locomotive powers, a great many species utilize the existing kinetic energies in the environment, resulting in passive movement. 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

• Dormancy - 'dispersal in time'
• Gene flow
• Rafting event
• Island hopping
• population distribution
• species distribution
• Population ecology
• Landscape ecology
• Metapopulation
• Population modeling
• Disturbance
• Habitat fragmentation
• Competition (biology)

References

1. ^ DUNNING, J. B. J., STEWART, D. J., DANIELSON, B. J., NOON, B. R., ROOT, T. L., LAMBERSON, R.H. & STEVENS, E. E. (1995). Spatially explicit population models: current forms and future uses. Ecological Applications
2. ^ Hanski, I. & Gilpin, M. E. (eds.) (1997). Metapopulation biology : ecology, genetics and evolution, Academic Press, San Diego.
3. ^ HANSKI, I. (1999). Metapopulation Ecology, Oxford University Press, Oxford.
4. ^ Maguire Jr., B., 1963. The passive dispersal of small aquatic organisms and their colonization of isolated bodies of water. Ecol. Monogr.
5. ^ NATHAN, R. (2001). The challenges of studying dispersal. Trends in Ecology & Evolution.
6. ^ JOHST, K. & BRANDL, R. (1997). The effect of dispersal on local population dynamics. Ecological Modelling
7. ^ Bowler, D. E. & Benton, T.G. (2005). Causes and consequences of animal dispersal strategies: relating individual behaviour to spatial dynamics. Biol. Rev.
8. ^ Irwini, A.J. & Taylor, P.D. (2007). Evolution of Dispersal in a Stepping-Stone Population with Overlapping Generations. Theoretical Population Biology
9. ^ Groom MJ, Meffe GK, Carroll CR. 2005. Principles of Conservation Biology. Sunderland (MA): Sinauer Associates, Inc.
10. ^ Martin TE. 1998. Are microhabitat preferences of coexisting species under selection and adaptive? Ecology
11. ^ Turner MG, Gardner RH, O’Neill. 2001. Landscape Ecology: In theory and practice, pattern and process. New York (NY): Springer Science.

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)
• Turner MG, Gardner RH, O’Neill. 2001. Landscape Ecology: In theory and practice, pattern and process. New York (NY): Springer Science.
• Groom MJ, Meffe GK, Carroll CR. 2005. Principles of Conservation Biology. Sunderland (MA): Sinauer Associates, Inc. 793 p.
• Hanski, I. (1999). Metapopulation Ecology, Oxford University Press, Oxford.
• Hanski, I. & Gilpin, M. E. (eds.) (1997). Metapopulation biology : ecology, genetics and evolution, Academic Press, San Diego.
• Levin, S.A. 1974. Dispersion and Population Interactions. The American Naturalist. 108:960, p.207.

External links

• Fruit and seed dispersal images at bioimages.vanderbilt.edu
• Interactive model of movement of plant species induced by climate change

Wikipedia content modification information:

• This page was last modified on 12 November 2008, at 14:40.

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