Climate and population regulation : The biogeographer's dilemma

Based upon a conceptual dichotomy between those environmental influences on a population which vary in their effect with population density, and those which do not, it is widely held that only density-dependent factors can determine equilibrium abundance or "carrying capacity" for a population under field conditions. Since the direct effects of climate on mortality and birth rates are commonly thought to be independent of density, that interpretation means that correlations between climatic factors and long-term average population density are presumed to reflect only indirect effects of climate, acting, for example, on resource availability. When density-dependent and density-independent processes are carefully defined, however, and their expected consequences derived, it becomes clear that the equilibrium abundance for a species can usually be altered by any environmental factor which has a consistent effect on either birth rate or mortality rate, regardless of whether that action itself depends upon density of the species. This implies that the concept of "carrying capacity" must be extended, to include possible direct, density-independent effects of climate on birth and death rates; these conclusions follow from a very simple model which appears to be extremely robust. Hence, geographic trends in average population density, which are so familiar to the biographer, may well be due in a straight-forward manner to the influences of climate, even if climate were to act only in a densityindependent manner.