Journal of Biogeography. In press.
The distribution-abundance (i.e. density) Relationship: its form AND CAUSES in a tropical mammal order, Primates.
A.H.HARCOURT1, S.A. COPPETO2 & S.A.PARKS3.
1. Aim. Across a wide variety of organisms, taxa with high local densities ('abundance') have large geographic ranges (distributions). We use primatology's detailed knowledge of its taxon to investigate the form and causes of the relationship in, unusually for macroecological analysis, a tropical taxon.
2. Location. Africa, Central and South America, Asia, Madagascar.
3. Methods. To investigate the form of the density-range relationship, we regressed local density on geographic range size, and also on female body mass, because in the Primates, density correlates strongly with mass. To investigate the biological causes of the relationship, we related a) abundance (density x range size) and b) residuals from the density-range regression lines to various measures of (i) resource use, (ii) reproductive rate, and (iii) potential specialization. All data are from the literature. Analyses were done at the level of species (N = 140), genera (N = 60) and families/sub-families (N = 17). We present various levels of results, including for all data, after omission of outlier data, after correction for phylogenetic dependence, and after Bonferroni correction of probabilities for multiple comparisons.
4. Results. Regarding the form of the relationship, Madagascar primates are clear outliers (high densities in small ranges). Among the remaining three realms, the relation of density to range is weak or non-existent at the level of species and genera. However, it is strong, tight and linear at the level of families/sub-families (r2 = 0.6, F1, 10 = 19, P < 0.01). Although among primates, density is very significantly related to mass, at no taxonomic level is range size related to body mass. Consequently, removing the effects of mass makes little to no difference to density-range results. Regarding the biology of the relationship, only traits indicative of specialization are associated with abundance (meaning numbers): rare taxa are more specialized than are abundant taxa. The association is largely via range size, not density. Across families, no traits correlate significantly with the density-range relationship, nor with deviations from it, despite the strength of the relationship at this taxonomic level.
5. Main conclusions. We suggest that in macroecology, analysis at taxonomic levels deeper than that of the relatively ephemeral species can be appropriate. We argue that the several purely methodological explanations for the positive density-range size relationship in primates can be rejected. Of the various biological hypotheses, those having to do with specialization-generalization seem the only applicable ones. The fact that the relationship is entirely via range size, not via density, means that while we might have a biology of range size, we do not yet have one of the density-geographic range relationship. It is probably time to search for multi-variate explanations, rather than univariate ones. However, we can for the first time for at least primates suggest that any association of abundance or range size with specialization is via number of different sub-taxa, not average degree of specialization of each sub-taxon. The implication for conservation is obvious.