The measurement of energy and element flux between consumers and their resources is central to our understanding of species interactions. Though it has long been recognized that populations consist of individuals that vary in size or age, uncertainties remain in how stage-structured populations alter rates of energy flux. Two major ecological theories seek to explain variation in trophic interactions - metabolic theory of ecology (MTE) and theory of ecological stoichiometry (ES). In this study, we asked how body size and temperature (MTE), and resource quality (ES) affect per-capita rates of consumption by signal crayfish (Pacifastacus leniusculus). Using a response-surface experimental design, we estimated the functional form of detritus consumption by altering crayfish size, water temperature (10°C, 15°C, 20°C), detritus quality (ash and alder, high and low C:N respectively) and detritus density in a laboratory setting.
Results/Conclusions
In line with predictions from MTE, both crayfish size and water temperature were positively associated with detritus consumption rates. Crayfish had a 2x higher attack rate on low quality ash leaves (C:N = 52) than high quality alder leaves (C:N = 34) at the warmest temperature suggesting an interaction between resource quality and temperature. According to ES, crayfish may increase their feeding rates to compensate for the low nutritive quality of its resource. However, there was no effect of temperature on feeding rates of alder leaves. Feeding rates on ash decelerate with increasing ash leaf density, fitting a type II functional response. Larger, older crayfish have a greater role in detrital processing compared to smaller, younger crayfish and the strength of this interaction is influenced by resource quality. Both metabolic theory and ecological stoichiometry provide a mechanistic understanding of how these factors constrain feeding rates, and ultimately the strengths of stage-structured species interactions.