How Large Is a Species' Geographic Range?

Many patterns of community structure will not be understood without reference to processes operating at regional and geographical scales (Cornell 1985a, b, Ricklefs 1987, Brown and Maurer 1989, Lawton 1990). This realization doubtless has been a prime motivation behind many recent studies of patterns in the sizes of species geographic ranges, and how these interact with such characteristics as local abundances, body sizes and trophic habits (Brown 1984, Brown and Maurer 1987, 1989, Bowers 1988, Gaston 1990, Gaston and Lawton 1990a, b, Pagel et al. 1991). It is thus somewhat curious to find that few of these studies explicitly state what they believe the size of a species geographic range, measured by whatever method, actually represents. This situation is made the more troublesome because different studies seem to be measuring different things. Thus, whilst Reaka (1980) quantifies species geographic ranges in terms of the numbers of degrees of latitude and longitude over which they have been recorded, and regards the numbers of 50 X 5? quadrats in which they were recorded as a measure of another aspect of their distribution, Schoener (1987) and Ford (1990) use numbers of occupied quadrats as a measure of geographic range size. This paper examines some of the ways in which the size of species geographic ranges could be measured, what these measures mean, and how they relate to one another.