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Biological dispersal
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Biological dispersal refers to those processes by which a species maintains or expands the distribution of a population. Dispersal implies movement—movement away from an existing population (population expansion) or away from the parent organism (population maintenance). This is necessary for members of a population because organisms of the same age require all of the same resources within an ecosystem. Dispersal relieves pressure for resources in an ecosystem, and competition for these resources may be a selection factor for dispersal mechanisms.
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. More significantly, dispersal enables the species population to occupy much of the available habitat, thereby maximizing resources in its favor and providing an edge against local adverse events.
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 plantsUnlike animals, plants are limited in their ability to seek out favorable conditions for life and growth. Consequently, plants have evolved many ways to disperse and spread a population through their seeds or spores (see also vegetative reproduction). Those properties or attributes that promote the movement of the next generation away from the parent plant may involve the fruit more so than the seeds themselves.
GravityThe effect of gravity on the dispersal of seeds and spores is simple: heavy seeds drop downward from the parent plant, though not very far by themselves. Encasing seeds in a rounded fruit promotes gravity driven movement away from the parent. Spores, being much lighter, are more influenced by physical movements in the environment, especially those of wind and water, and therefore less strictly subject to gravity alone. Gravity may be sufficient agent for plants growing on steep slopes, but upslope movement of a population can be a problem. Plants such as gymnosperms that utilize gravity for dispersal often rely on additional dispersal adaptations favoring zoochory or anemochory; few rely on gravity alone.
MechanicalNumerous species have evolved mechanical means to overcome the tendency of a seed to drop close to its parent. Seedpods are often shaped so that the seeds are flung away from the parent plant with considerable force as the seedpod matures.
Examples of fruit with mechanical dispersal mechanisms:
- Oxalis corniculata – capsule, as it dries, becomes sensitive to disturbance, ejecting tiny seeds in an explosive discharge.
WindAnemochory, or wind dispersal, is probably the most primitive form of dispersal besides gravity alone. Wind is reliable, but plants must produce seeds to ensure that a sufficient number will land by chance in a suitable location, making anemochory an inefficient means of dispersal. Wind dispersal is common among ruderal species, including some important agricultural and horticultural weeds such as hawkweed, horseweed, and dandelion. Like many members of the Asteraceae, or sunflower family, they produce seeds with a feathery "parachute" called a pappus that aids wind dispersal,allowing them to be carried over distances
WaterPlants that grow in water (aquatic and obligate wetland species) are likely to utilize water to disperse their seeds. For example, all mangroves disperse their offspring by water. Hydrochory is the dispersal of seeds by water; a plant which uses this method in its life cycle is termed a hydrochore. Rhizophora demonstrates an unorthodox method of propagation called vivipary: the embryo is retained on the plant until after germination; in essence, a dry seed is not produced. The hypocotyl of the germinating seedling (now called a propagule) bursts through the fruit and hangs, poised for continued growth. In R. mangle, the hypocotyl can reach a length of 20 to 25 cm; and in R. mucronata lengths up to 1 m have been recorded. Eventually, the seedling separates from the fruit, leaving its cotyledons behind, and—floating horizontally on the water surface—is carried away by tidal or river flow. After a month or two, the propagule turns vertical in the water. Once the hypocotyl of a propagule "feels" bottom or strands, roots start to develop and leaves appear at the upper end (Hogarth, 1999).
Adaptations commonly seen in littoral plants are those that promote flotation of the fruit, allowing the seed to be carried away on the tide or ocean currents. Examples would be:
- Cocos nucifera – the coconut produces a large, dry, fiber-filled fruit (a fibrous drupe) capable of a long survival adrift at sea.
- Calophyllum inophyllum – Alexandrian laurel or kamani produces a globose fruit that is almost cork-like.
Terrestrial plants may also have their seeds dispersed by raindrops.
By animalsMany plants rely on zoochory, dispersal by animals. Animal dispersal is broadly divided into two categories: endozoochory, transport internally, and epizoochory, external transport. Endozoochory is generally a coevolved mutualistic relationship in which a plant surrounds seeds with an edible, nutritious fruit as a reward to animals that consume it. Seeds within the fruit are often protected by tough outer coating; many seeds have such thick protection that they cannot germinate until they are scarified by digestion. This keeps the seed from sprouting while still in the fruit and reduces competition with the parent plant. Birds and mammals are the most important seed dispersers, but a wide variety of other animals, such as box turtles and fish can transport viable seeds. Some animals that disperse may also eat the seed. Many rodents (such as squirrels) hoard seeds in hidden caches; those left uneaten can grow into a new plant.
Epizoochory, or transport on the outside of an animal, is often a commensalistic relationship in which the plant benefits but the animal does not. Plants such as burdock and cocklebur have fruits with recurving hairs or spikes that cling to fur or feathers so that passing animals will carry them away. Many species in the genus Bidens are called "beggar's ticks" because their achenes stick in clothes using special barbed awns. Many members of the Apiaceae (carrot family) such as Torilis, Caucalis, and Daucus have spikes or spiky hairs on their fruits.
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 animalsThere 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, p?a??t??, meaning "wanderer" or "drifter".
See also
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)
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