Global beta-diversity is more sensitive to loss of species with small versus large ranges
Beta-diversity -- broadly defined as the variability of communities between locations -- can promote multiple ecosystem functions, insure against loss of function in communities, and impart ecosystem stability. Thus, understanding how beta-diversity will be impacted by ongoing extinction events is of high importance. Previous studies have shown that beta-diversity is more strongly correlated with the distributions of wide-ranged and abundant species than narrow-ranged and rare species. These observations suggest that beta-diversity will be impacted relatively little by incipient extinctions of threatened species, which tend to have narrow ranges and are rare. To investigate this hypothesis, we (i) use a database of the global distributions of almost all known amphibian, bird, and mammal species to empirically assess the relative impacts of extinctions on four classic measures of beta-diversity and (ii) develop a mathematical theory based on geometric probability to understand and predict these impacts.
Contrary to expectations, we find empirically that at global scales beta-diversity is more strongly impacted by extinctions of narrow-ranged species than wide-ranged species. As measured by the change in beta-diversity per species lost, the impacts can be over 1000 times greater for loss of narrow-ranged species than wide-ranged species. This sensitivity to extinctions of narrow-ranged species is evident across all of the taxa that we examined, and for all measures of beta-diversity and in almost all geographic regions that we examined. Our mathematical theory is highly predictive of these empirical observations, and it makes additional predictions about the relationships between beta-diversity and geometric properties of ranges, which we confirm, suggesting that the differential impacts of extinctions on beta-diversity stem from fundamental geometric constraints. Our results show that because loss of narrow-ranged species has an inordinate influence on beta-diversity, incipient extinctions of narrow-ranged species will disproportionately affect beta-diversity, with likely ramifications for the ecosystem services and functions provided by beta-diversity.