restricted access I.12 Geographic Range
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I.12 Geographic Range Kevin J. Gaston OUTLINE 1. Range size 2. Range edges 3. Range structure 4. Fundamental units No species occurs everywhere. Indeed, most are absent from the vast majority of sites across the globe. Those areas in which a species does occur constitute its geographic range. As such, the geographic range is one of the fundamental units in ecology. The sizes and distribution of geographic ranges give rise to patterns of species richness and change in species composition from site to site, and combined with their abundance and trait structure give rise to other spatial patterns in assemblages. Likewise, temporal changes in assemblages on both short and long time scales follow from changes in the size, position, and structure of geographic ranges. GLOSSARY area of occupancy. The area within the outermost geographic limits to the occurrence of a species over which it is actually found extent of occurrence. The area within the outermost geographic limits to the occurrence of a species intraspecific species-abundance distribution. The frequency of areas within a species’ geographic range in which it attains different levels of abundance range edge or limit. The outermost geographic occurrences of a species, usually excluding vagrant individuals species–range size distribution. The frequency of species with geographic ranges of different sizes 1. RANGE SIZE The sizes of the geographic ranges of species vary dramatically and can be characterized in two fundamentally different ways. Extent of occurrence is the area within the outermost limits to the occurrence of a species, and area of occupancy is the area over which the species is actually found. The latter will tend to be consistently smaller because no species is distributed continuously across space even within the broad geographic limits to its occurrence. The finer the spatial resolution and the shorter the time period over which area of occupancy is measured, the smaller will be the area over which the species is documented to occur, and the greater this disparity will be. At one extreme lie those, predominantly freshwater or terrestrial, species that are currently found occurring in a single small habitat patch (often with only a very small number of individuals), which are thus narrowly distributed in terms both of extent of occurrence and area of occupancy . At the other extreme lie some marine organisms . Species of microorganisms may be widespread across the oceans both in terms of extent of occurrence and area of occupancy, whereas some large-bodied species of vertebrate may have large oceanic distributions in terms of extent of occurrence but, because of the relatively low numbers of individuals, not area of occupancy. Species–Range Size Distributions Both within and across major taxonomic groups, the geographic ranges of the majority of species are relatively small, and only a very few are widespread. Indeed, within such groups species–range size distributions , the frequency of species with ranges of different sizes, are almost invariably strongly right-skewed. One important consequence is that the vast majority of occurrence records result from a small number of species. For example, by one estimation, at a spatial resolution of approximately 100  100 km, the 10% most globally widespread extant species of birds account for 50% of occurrence records. Given that the ratio of extents of occurrence to areas of occupancy may often be proportionately larger for rare species than for widespread ones, that is, they occupy their ranges less densely, the dominance of occurrence records by widespread species may increase when documented at finer spatial resolutions. This dominance may explain why it is the more widespread rather than, as often assumed, the restricted species that contribute disproportionately to spatial variation in species richness and related macroecological patterns. Phylogenetic Constraint The average sizes of geographic ranges can vary markedly between species in different major taxonomic groups. Thus, among nonmarine vertebrates, species of fish and amphibians tend naturally to have smaller ranges than do mammals, and mammals smaller ranges than do birds. However, within taxonomic groups, the extent to which the geographic range sizes of species exhibitphylogeneticconstraintiscontentious.Certainly range size is not as strongly conserved as are body size and many life history traits. Even where significantly conserved, it typically remains impossible to predict with any accuracy the range size of a species from that of its sister species or other close relatives, suggesting that such heritability has limited practical value (e.g., in estimating the range sizes of species whose distributions have not been well documented). This would tend to follow if the...


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