Challenges and advances in the study of latitudinal gradients in multitrophic interactions, with a focus on consumer specialization

Abstract

Increases in data availability and geographic ranges of studies have allowed for more thorough tests of latitudinal gradients in trophic interactions, with numerous recent studies testing hypotheses that strength of interactions, herbivory, plant chemical defense, and dietary specialization all increase with decreasing latitude. We review the issues surrounding these latitudinal gradients, discuss some methodological challenges, and provide some caveats relevant to inferences from existing approaches. To examine some potential issues with studies on latitudinal gradients in dietary specialization, we simulate a latitudinal gradient of communities that increase in diversity and specialization towards the equator then test the power of different sampling designs for detecting the gradient. Based on this simple simulation, as well as apparent incongruities in the literature, we conclude that subtle differences in sampling design can be responsible for failure to detect existing gradients. Despite calls for rejecting some latitudinal gradient hypotheses, it is clear that a great deal of careful research remains to determine important correlates of the well-established latitudinal gradient in diversity. In particular, future studies should focus on replicated gradients, greater emphasis on continuous sampling, and use of taxonomic controls that allow for meaningful analyses across latitudes.