The most well-known and perhaps most influential macroecological relationship is the latitudinal diversity gradient (LDG). The increase in species richness from the poles to the tropics has long fascinated naturalists, ecologists, and evolutionary biologists. Species richness also plays a central role in our understanding of the structure and dynamics of ecological networks. Several hypotheses for the LDG have logical extensions for how network structure should vary with latitude. We reviewed published studies of ecological networks across latitudinal gradients and analyzed network structure of aquatic food webs found in the leaves of the widespread pitcher plant Sarracenia purpurea (latitudinal range from 29.92° to 58.49°) to test the following hypotheses: i) food webs at lower latitudes should have longer chain lengths due to increased productivity in tropical regions; ii) species are more specialized in ecological networks at lower latitudes and as a result, should have lower linkage density than high latitude networks; iii) ecological networks at lower latitudes should have greater turnover in network structure and composition when compared to their temperate counterparts.
Results/Conclusions
We found mixed support for all three hypotheses in the literature we reviewed. For pitcher plant food webs, we found: i) weak evidence that chain lengths were longer in low latitude food webs; ii) no evidence of greater specialization at low latitudes; and iii) greater turnover in network structure in low latitude food web networks. Our results suggest that the type of ecosystem and network are important factors in exploring latitudinal patterns of ecological network structure. Furthermore, we conclude that direct observations of interactions (particularly for food web networks) at all locations along a latitudinal gradient, as opposed to basing interaction structure on literature, life history, or interactions in other locations, is essential for elucidating latitudinal patterns of structure in ecological networks.