The sizes of species’ geographic ranges

Abstract

Summary 1.Geographic range size and how it changes through time is one of the fundamental ecological and evolutionary characteristics of a species, and a strong predictor of extinction risk. However, the measurement of range size remains a substantial challenge. Indeed, there is significant confusion in the literature as to how this should be done, particularly in the context of the distinction between the fundamentally different concepts of extent of occurrence (EOO) and area of occupancy (AOO), and the use of these quantities, including in assessments of the threat status of species. 2. Here we review the different approaches to determining the geographic distributions of species, the measurement of their range sizes, the relationships between the two, and other difficulties posed by range size measurement (especially those of range discontinuities when measuring EOO, and spatial scale when measuring AOO). 3. We argue that it is important to (i) distinguish the estimation of the distribution of a species from the measurement of its geographic range size; (ii) treat measures of EOO and AOO as serving different purposes, rather than regarding them as more or less accurate ways of measuring range size; and (iii) measure EOO including discontinuities in habitat or occupancy. Synthesis and applications.With the availability and collation of extensive data sets on species occurrences, a rapidly increasing number of studies are investigating geographic range size, and particularly how various measures of range size predict macroecological patterns and inform assessments of the conservation status of species and areas. The distinction between EOO and AOO is becoming blurred in many contexts, but most particularly in that of threatened species assessments for Red Listing. Continued progress in these fields demands greater clarity in the meaning and derivation of measures of geographic range size. The two principal measures serve different purposes, and should not be regarded as alternatives that simply differ in accuracy.

Drought-driven change in wildlife distribution and numbers: a case study of koalas in south west Queensland

“…The effect of climate change on the trailing edge of species distribution is predicted to lead to increasingly fragmented populations, some of which may survive in small refugia due to local and regional variations in climate but many of which will face local extinction from extreme events (Parmesan 2006;Thomas et al 2006;Gaston and Fuller 2009). Trailing-edge populations have been shown to be critical to the long-term survival of species because they may contain individuals that can adapt to changing climatic conditions (Wilson et al 2004;Hampe and Petit 2005;Thomas et al 2006).…”

“…However, within a geographic range, there is a complex internal structure of distribution, often termed the area of occupancy, which reflects finer scale variations in habitat suitability, local and metapopulation dynamics and legacies of past events (Brown et al 1996;Gaston and Fuller 2009). This fine scale structure leads to considerable variations in the density and numbers of local populations within the area of occupancy (Gaston and Fuller 2009).…”

“…However, within a geographic range, there is a complex internal structure of distribution, often termed the area of occupancy, which reflects finer scale variations in habitat suitability, local and metapopulation dynamics and legacies of past events (Brown et al 1996;Gaston and Fuller 2009). This fine scale structure leads to considerable variations in the density and numbers of local populations within the area of occupancy (Gaston and Fuller 2009). Changes in environmental conditions, such as climate variability and loss of habitat, affect population dynamics, including dispersal ability, breeding success and mortality rates, and alter population density either temporarily or permanently (Fahrig 2003;Hughes 2003;Parmesan 2006).…”

Integrating species traits with extrinsic threats: closing the gap between predicting and preventing species declines

“…For example, for amphibians (and several other groups) small geographical range size has been consistently identified as the most important predisposing factor to decline [5][6][7]. Other factors, including clutch size, body size and ecological niche, have also been identified as potentially important [8][9][10][11].…”

Distribution and Conservation Status of Phlegmariurus (Lycopodiaceae) in the State of Veracruz, Mexico

“…Furthermore, considering the potential distribution modeled by Maxent, we estimated the Extent of Occurrence (EOO), defined as the area contained within the shortest continuous imaginary boundary which can be drawn to encompass all the known, inferred or projected sites of present occurrence of a taxon, excluding cases of vagrancy [41]. These two measurements represent different aspects of geographical range size: AOO provides information on the area of suitable habitat, whereas EOO provides information on overall geographical spread [42,43].…”