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. Patches in the environment that are created by the activity of organisms have life histories. 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. Patchiness may be a feature of the physical environment: islands surrounded by water, rocky outcrops in a moorland, and so on. 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). it is patchy, but the birds experience the habitat as an oak-hickory mixture. 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. MacArthur and Levins (1964) introduced the concept of environmental grain to make this point. 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. 1 cm2) we might detect a regular distribution because individual aphids on a leaf avoid one another. 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. 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. in woodlands as opposed to other types of habitat. At a large scale, the aphids will appear to be aggregated in particular parts of the world, i.e. Consider the distribution of an aphid living on a particular species of tree in a woodland. 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. The result is that individuals are closer together than expected by chance. The result is that individuals are more evenly spaced than expected by chance.Īggregated 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 individualįigure 6.3 Three generalized spatial patterns that may be exhibited by organisms across their habitats.Īttracts, or gives rise to, another close to it. 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 unevenly distributed because of chance events. 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 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).
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