Trait-based community assembly from a multitrophic perspective: Bottom-up or top-down regulation?

Background/Question/Methods

Classic trait-based community assembly theory suggests that the similarity of traits among co-occurring species is shaped by two processes: abiotic filtering and competition. Abiotic filtering, mostly dominating in stressful environments, promotes similarity, while competition, dominating in productive environments, limits similarity. In line with this theory, several studies have found within-community trait similarity to decline along productivity gradients. However, these studies have always focused on species from single trophic levels. Here, we studied how interactions between different trophic levels affect trait similarity patterns along environmental gradients. We proposed three hypotheses for the main drivers of variation in trait similarity patterns of grasses and herbivorous grasshoppers along environmental gradients: 1) environmental control of both, 2) bottom-up control of grasshopper trait similarity, and 3) top-down control of grass trait similarity.

To test these hypotheses, we collected data on the community composition and trait variation of grasses and grasshoppers in 50 plots in a South African savanna. We used Structural Equation Models to investigate how the spacing and range of within-community trait values of both grasses and their insect herbivores respond to (1) changes over rainfall and fire frequency gradients and (2) the trait similarity patterns of the other trophic level.

Results/Conclusions

Our analyses revealed that within-community similarity of grass traits was highest towards low rainfall and high fire frequency areas (environmental control), while showing little evidence for top-down control of grass trait similarity patterns by grasshoppers. Variation in grasshopper within-community trait range patterns, on the other hand, was mostly directly driven by vegetation structure and within-community grass trait range patterns (bottom-up control), while environmental factors had only indirect effects via plant traits. This study shows the potential to include trophic interactions in trait-based community assembly theory.