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.
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
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.
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