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A.H. Harcourt, S. A. Coppeto & S.A. Parks

In Journal of Biogeography


Aim: To determine and explain the biological traits that distinguish rare from common taxa.
Location: Primate distributions throughout the globe.
Methods: We compare the biology of rare with common primate taxa. Rarity is defined by 1) small size of geographic range; 2) small geographic range plus low local population density; and 3) small geographic range plus low local density plus narrow habitat specificity. After the first comparison of size of geographic range with various biological traits, globally and per continent, extremes of rarity and commonness per continent are identified, and then combined for a global analysis. Tests are done both with genera treated as independent data points (N = 62), and with phylogeny controlled for by use of an independent contrasts test. The literature indicates that extinction risk in vertebrates, including primates, often correlates with high resource requirements, slow population recovery rate, and specialization. The three measures of rarity are therefore compared to these three general traits, measured as body mass, local density, annual range size, group size (resources), interbirth interval, maximum intrinsic rate of natural population increase (recovery), and dietary variety, variety of habitats occupied, maximum latitude, and morphological variety measured as number of species per genus (specialization). All data come from the literature. Because several measures are compared, probabilities are Bonferroni corrected.
Results: Rarity in primates consistently correlates with only indices of specialization, and not with indices of high resource use, or slow population recovery rate. The first two indices of rarity associate significantly with all four measures of specialization; the first index associates with all four measures when phylogeny is accounted for, and the second index with two measures (maximum latitude and number of species per genus). The third index of rarity associates significantly with maximum latitude, but not after phylogenetic correction. The four indices of specialization are strongly interrelated: in stepwise regressions on geographic range, maximum latitude shows the strongest effect, followed by dietary variety and number of species per genus and, finally, habitat variety.
Main Conclusions: The most commonly demonstrated traits of susceptibility to extinction are those of high resource use, slow recovery rate, and specialization. And yet, while rarity is an inevitable precursor to extinction, specialization is effectively the only trait found to correlate with rarity in this study. Why the other common indices of risk of extinction do not correlate with rarity, we do not know. If no reader knows either, then we have a long way to go in understanding determinants of biodiversity, the processes of extinction, and hence the processes of evolution.