Influence of landscape complexity on the strength of predator-prey interactions

Background/Question/Methods

Model simulations of biotic communities emphasize the importance of trophic interaction strengths (IS) between consumers and resources. However, parameterizing these models has been restricted by a lack of empirical food web data. Moreover, neither empirical studies nor mathematical models of food webs have yet to fully incorporate the spatial landscape heterogeneity that exists in nature. In our study, we utilized the natural complexity of a floodplain ecosystem to investigate aquatic food webs in the context of a heterogeneous landscape mosaic. Our aims were to (1) calculate IS between fish predators and their invertebrate prey in six different habitat patches of the floodplain mosaic, (2) evaluate how predator-prey IS varied among these patches, and (3) investigate the influence of landscape heterogeneity (via the aggregation of patch-scale food webs into floodplain meta-food webs) on the distribution of predator-prey IS. The population-level IS used in our analyses (defined as the proportion of prey production consumed by a predator population) was calculated using empirically derived estimates of invertebrate prey production, fish predator production, and fish dietary contents. 

Results/Conclusions

Similar to findings from other studies, our results showed that the distribution of predator prey IS for individual habitat patches was skewed toward weak interactions, and as a result, average interaction strength was consistently low (0.09 - 0.16). However, IS for individual predator-prey combinations were highly variable across habitat patches, and as a result, strong predator-prey interactions at a single location were generally balanced by much weaker interactions at other locations in the floodplain. At the landscape scale, combining local food webs resulted in an average IS that was at the lower end of the range found within individual habitat patches (0.09). In particular, increased landscape complexity appeared to increase the proportion of very weak predator-prey interactions, and decrease the proportion of strong interactions. If hypotheses regarding the distribution of interaction strengths in food webs are correct (i.e., weak interactions promote stability), then these findings have important implications for the influence of landscape heterogeneity on community stability, and highlight the importance of studying food webs in a landscape context.