Fitness consequences of habitat selection in terrestrial ectotherms

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Within species, the abundance of individuals varies in space. The geographic range is a function of abundance: its limits are set where population density declines to zero. Abundance is largely a function of local individual survival and reproduction, which together dictate population mean fitness. Therefore, the pattern of spatial variation in abundance is intimately tied to the pattern of spatial variation in population mean fitness. In vertebrates there is a plethora of papers describing habitat use, but surprisingly few tackle the central issue of quantifying the fitness consequences of habitat selection. Terrestrial ectothermic vertebrates offer a unique opportunity to bridge the gap between habitat use and fitness because their organismal performance varies with body temperature which is regulated largely via habitat selection. We thus have an explicit mechanistic link between habitat use and fitness in this group. Much of the habitat selection theory has been developed with vertebrate endotherms in mind and assumes that animals are limited by their ability to harvest resources. But what happens if animals are not limited by their ability to harvest resources, but rather by their ability to process resources? Most vertebrates are ectotherms, and in ectotherms the total amount of energy utilized may be more strongly influenced by thermal constraints on resource acquisition and assimilation than by resource availability. My working hypothesis in this proposal is that, contrary to vertebrate endotherms, the primary driver of population mean fitness in terrestrial vertebrate ectotherms is their ability to process resources, not their ability to acquire resources. This hypothesis can be operationalized at the species range scale (variation in population mean fitness across the geographic range of species), at the landscape scale (variation in population mean fitness across local sites that vary in temperature and resource availability), or at the individual scale (variation in individual fitness as a function of the thermal quality and resource availability within the home range). I propose to conduct tests at all 3 spatial scales using correlative and manipulative field studies of lizards.