Patterns of distribution dispersion

The movements of organisms affect the spatial pattern of their distribution (their dispersion) and we can recognize three main patterns of dispersion, although they too form part of a continuum (Figure 6.3).

Random dispersion occurs when random, regular there is an equal probability of an and aggregated organism occupying any point in space distributions (irrespective of the position of any others). The result is that individuals are unevenly distributed because of chance events.

Regular dispersion (also called a uniform or even distribution or overdispersion) occurs either when an individual has a tendency to avoid other individuals, or when individuals that are especially close to others die. The result is that individuals are more evenly spaced than expected by chance.

Aggregated dispersion (also called a contagious or clumped distribution or underdispersion) occurs either when individuals tend to be attracted to (or are more likely to survive in) particular parts of the environment, or when the presence of one individual

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Figure 6.3 Three generalized spatial patterns that may be exhibited by organisms across their habitats.

attracts, or gives rise to, another close to it. The result is that individuals are closer together than expected by chance.

How these patterns appear to an observer, however, and their relevance to the life of other organisms, depends on the spatial scale at which they are viewed. Consider the distribution of an aphid living on a particular species of tree in a woodland. At a large scale, the aphids will appear to be aggregated in particular parts of the world, i.e. in woodlands as opposed to other types of habitat. If samples are smaller and taken only in woodlands, the aphids will still appear to be aggregated, but now on their host tree species rather than on trees in general. However, if samples are smaller still (25 cm2, about the size of a leaf) and are taken within the canopy of a single tree, the aphids might appear to be randomly distributed over the tree as a whole. At an even smaller scale (c. 1 cm2) we might detect a regular distribution because individual aphids on a leaf avoid one another.

In practice, the populations of all species are patchily distributed at some scale or another, but it is crucial to describe dispersion at scales that are relevant to the lifestyle of the organisms concerned. MacArthur and Levins (1964) introduced the concept of environmental grain to make this point. For example, the canopy of an oak-hickory forest, from the point of view of a bird like the scarlet tanager (Piranga olivacea) that forages indiscriminately in both oaks and hickories, is fine grained: i.e. it is patchy, but the birds experience the habitat as an oak-hickory mixture. The habitat is coarse grained, however, for defoliating insects that attack either oaks or hickories preferentially: they experience the habitat one patch at a time, moving from one preferred patch to another (Figure 6.4).

Patchiness may be a feature of the physical environment: islands surrounded by water, rocky outcrops in a moorland, and so on. Equally important, patchiness may be created by the activities of organisms themselves; by their grazing, the deposition of dung, trampling or by the local depletion of water and mineral resources. Patches in the environment that are created by the activity of organisms have life histories. A gap created in a forest by a falling tree is colonized and grows up to contain mature trees, whilst other trees fall and create new gaps. The dead leaf in a grassland area is a patch for colonization by a succession of fungi and bacteria that eventually exhaust it as a resource, but new dead leaves arise and are colonized elsewhere.

Patchiness, dispersal and scale are tied intimately together. A framework that is useful in thinking about this distinguishes between local and landscape scales (though what is 'local' to a worm is very different from what is local to the bird that eats it) and between turnover and invasion dispersal (Bullock et al., 2002). Turnover dispersal at the local scale describes the movement into

Time 3

Time 4

Time 5

Figure 6.4 The 'grain' of the environment must be seen from the perspective of the organism concerned. (a) An organism that is small or moves little is likely to see the environment as coarse-grained: it experiences only one habitat type within the environment for long periods or perhaps all of its life. (b) An organism that is larger or moves more may see the same environment as fine-grained: it moves frequently between habitat types and hence samples them in the proportion in which they occur in the environment as a whole.

a gap from occupied habitat immediately surrounding the gap; whereas that gap may also be invaded or colonized by individuals moving in from elsewhere in the surrounding community. At the landscape scale, similarly, dispersal may be part of an on-going turnover of extinction and recolonization of occupiable patches within an otherwise unsuitable habitat matrix (e.g. islands in a stream: 'metapopulation dynamics' - see Section 6.9, below), or dispersal may result in the invasion of habitat by a 'new' species expanding its range.

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