Spurious and functional correlates of the isotopic composition of a generalist across a tropical rainforest landscape

We found that 75% of the variance in the fraction of 15N in the tissue of A. araneoides was accounted by one environmental parameter, the concentration of soil phosphorus. After taking into account landscape-scale variation in baseline resources, the most parsimonious model indicated that colony growth and leaf litter biomass accounted for nearly all of the variance in the δ15N discrimination factor, whereas the δ13C discrimination factor was most parsimoniously associated with colony size and the rate of leaf litter decomposition. There was no indication that competitor density or diversity accounted for spatial differences in the isotopic composition of gypsy ants.Across a 500 ha. landscape, soil phosphorus accounted for spatial variation in baseline nitrogen isotope ratios. The δ15N discrimination factor of a higher order consumer in this food web was structured by bottom-up influences - the quantity and decomposition rate of leaf litter. Stable isotope studies on the trophic biology of consumers may benefit from explicit spatial design to account for edaphic properties that alter the baseline at fine spatial grains.It is generally thought that stable isotope ratios vary across terrestrial landscapes. The extent of this spatial variation is enigmatic, and not explicitly built into the design of experiments wherein stable isotopes of carbon and nitrogen are employed to infer the trophic biology of consumers [1]. Typically, a landscape may be sampled broadly, with samples pooled together based on the assumption that baseline isotopic ratios will not vary appreciably within one kilometer (e.g., [2,3]). Nevertheless, to our knowledge there has been no study to test whether baseline stable isotope ratios are consistent across short distances. Moreover, no study has heretofore elucidated the contribution of spatial variation in isotope ratios to the isotopic variation in the tissue of consumers, relative to the effects of biotic factors shaping spatial patterns in isot