Intraspecific mass-scaling of field metabolic rates of a freshwater crayfish varies with stream land cover

We investigated the effect of land cover on the metabolic scaling of the freshwater crayfish, Orconectes rusticus , by comparing the field metabolic rate (FMR) of populations from streams flowing through different natural and agricultural land cover. When data from all streams were pooled, the metabolic mass-scaling exponent was approximately 0.71. However, both the strength and nature of FMR-mass relationships varied among streams (slopes from 0.61 to 0.91). This variability in scaling exponents was significantly correlated with two types of land cover, the proportion of monoculture (row cropping) agriculture (positive slope, P < 0.02, R 2 = 0.75) and the proportion of wetlands (negative slope, P = 0.05, R 2 = 0.57), in the riparian zone of each stream. In a complementary laboratory study, we found the metabolic response of crayfish to differ among animals consuming plant and animal based foods. Crayfish consuming animal-based foods had higher respiration rates than conspecifics consuming plant-based foods. As O. rusticus exhibits variable feeding rates and foraging behavior, differences in the availability and quality of food that accompany changes in catchment land cover provides a potential mechanism for the observed site-dependence of FMR-mass scaling. Intraspecific variability of FMR-mass scaling in stream crayfish and its relationship to catchment land use is further evidence that organismal physiological flexibility and acclimation to specific environments complicates efforts to use general mass-scaling laws to explain disparate ecological phenomena.