Isotopic evidence for variation in niche breadth across a latitudinal gradient

Background/Question/Methods Classic biogeographic theory posits that niche breadth is positively associated with latitude.  It has been qualitatively suggested that decreased variability in climate and population dynamics have contributed to intensified species interactions and decreased niche breadth in low latitude habitats.  However, difficulties quantifying niches have made this hypothesis difficult to assess empirically.  Recently, the isotopic niche, or the area obtained by plotting δ¹⁵N against δ¹³C, has been proposed as one index which can be used to approximate niche breadth.  C and N isotopic ratios have been important tools for examining several aspects of niche breadth such as variation in physiology, foraging location, and proportions of certain prey species consumed. However, limitations in the ability to assess species interactions unequivocally, and a lag in the development of appropriate mathematical models have detracted from widespread support of the isotopic niche concept.  To address these limitations and examine the utility of isotope niche models we used mixing models standardized by resource base to assess variation in isotopic niche breadth across a latitudinal gradient using data collected in a field survey and a literature review. 
Results/Conclusions Results suggest that body size, ontogeny, and phylogeny have a strong influence on isotopic niche space.  Nonetheless we found a decrease in isotopic niche breadth across a latitudinal gradient for herbivorous arthropods.  This suggests that climactic stability and host specificity result in decreases in isotopic variation for this group.  Difficulties isolating a resource base signature for detritivores, and high variation in trophic shift of δ¹⁵N values for predators made a relationship between latitude and isotopic niche difficult to detect for these trophic levels.  Our findings contribute to the evaluation of the isotopic niche approach as one means of understanding food web dynamics across broad spatial scales.  Isotopic niche methods are a potentially powerful contribution to facilitate testing biogeographic theory because they integrate physiology, nutritional ecology and species interactions on a common continuous axis associated with patterns of species distributions.