Spatial variation in the density of ectotherms

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Within species, abundance (population density) varies markedly in space and in time. Accordingly, explaining spatial and temporal patterns in the abundance of species is one of the major goals of ecology. The abundant center hypothesis posits that species should be most abundant near the center of their geographic ranges, but the empirical data do not support it. Thus, there is a clear need for alternative hypotheses to predict better the spatial patterns in the abundance of animals.Predicting spatial patterns in the abundance of animals has important theoretical and applied implications. On the theoretical side, a better prediction of spatial patterns in abundance may help determine what drives broad spatial patterns in biodiversity. On the applied side, a better ability to predict spatial patterns will help define critical habitats for species at risk.Abundance at broad spatial scales is largely dictated by climate. Within the geographical range of species, however, habitat selection can strongly influence the distribution and abundance of individuals. The spatial heterogeneity of resource distribution provides an opportunity for organisms to maximize their fitness by choosing the highest quality habitat available. The suitability of a habitat is determined by the mean success rate of individuals that occupy the habitat, and depends on factors such as food abundance, shelter availability, and density of competitors.Much of the habitat selection theory assumes that animals are limited by their ability to harvest limited resources. But what happens if animals are limited by their ability to process resources? Most animals are ectotherms, and in ectotherms the total amount of energy available may be more strongly influenced by thermal constraints on resource assimilation than by resource acquisition. My working hypothesis is that the primary driver of abundance in ectotherms is their ability to process resources, not their ability to acquire resources.The long-term objective of my research program is to determine what dictates spatial patterns of abundance in ectotherms. To meet this objective, I document patterns of abundance in space and I test hypotheses about mechanisms that could generate these spatial patterns. My objective over the next grant cycle is to determine whether it is the ability to acquire, or the ability to process, resources that best predicts abundance of ectotherms. I will address this objective at three spatial scales: at the scale of the geographical range (variation in abundance across the range), at the scale of the landscape (variation in abundance across habitat gradients), and at the scale of the individual home ranges (variation in fitness between home ranges varying in quality).