Large Scale Distribution Patterns

An aspect of spatial patterns which should not be overlooked is the distribution of species at very large, global scales. At such scales, the creation and extinction of species interact with long-range dispersal and large-scale differences in climate to generate patterns of species richness and control the composition of regional species pools. Over time, new patterns are created as species expand into new areas while going extinct in others, and as isolated subpopulations give rise to new species due to evolution.

The basic unit of large-scale ecology is the geographic distribution range. Ranges can be represented in many ways, from the colored blotches known from popular field guides to exact mapping of territories and point counts of individual locations, and the definition of ranges is, like most spatial patterns, strongly scale dependent.

The concept of ranges and their distribution has attracted considerable attention in recent years, because of their profound importance for biodiversity. Identifying and describing patterns of overall species richness and the location of areas with large numbers of endemics (i.e., species with small range sizes) play a key role in the conservation and management of nature, since they are instrumental in directing conservation efforts to the most optimal areas. In addition, an understanding of these patterns is at the center of attempts to predict the result ofthe recent changes in diversity following human habitat destruction and global warming (see Climate Change Models). These patterns all result from the location of species ranges and the manner in which they overlap.

What determines large-scale distribution patterns remains the subject of debate: a long-standing controversy in ecology regards whether the size and location of species' ranges are primarily decided by contemporary ecological (mainly climatic) factors, or by the interaction of dispersal and competition with other species present in the area.

The shape and location of the range at large scales are also mirrored by the distribution of individuals at smaller scales. Toward the edges of a range, populations of the species tend to become more patchily distributed, with more widely separated individual subpopulations. Additionally, peripheral populations are smaller and support fewer individuals, so that the abundance and occupancy of the species co-vary across the range. What creates this pattern is not completely known, but it seems likely that it reflects that the density of habitats with optimal conditions for the organism is higher in the core area than in the periphery.

In this way, large-scale patterns interact with local processes, behavior, and biotic interactions to produce the distributions of plants and animals in nature. The complexity of these distributions underline the increasing realization that a consideration of spatial patterns is a vital part of any comprehensive framework for biology.

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