The insect herbivore assemblage on Solanum campylacanthum and its indirect interaction with large mammalian herbivores

Background/Question/Methods

Insects are the most diverse and one of the most abundant group of organisms. Prediction of insect community dynamics is crucial to management and conservation efforts due to their abundance and ubiquity. This process is often complicated by indirect interactions of insects with other organisms through their shared food plants. Furthermore, such indirect interactions can be influenced by environmental factors such as temperature and precipitation gradients. I am studying the indirect interaction of insect herbivores on Solanum campylacanthum (Sodom apple) with large mammalian herbivores (LMH) at the Mpala Research Centre, Kenya. I am using a set of large mammal exclusion plots that exclude mammalian herbivores based on three size categories (Mega, Meso, Total, and Open), along a precipitation gradient (High, Medium, and Low). I am comparing the densities of S. campylacanthum among the exclusion plots and censusing the insect assemblages on them. I predicted that the abundance, species richness, and species diversity of insects would be the highest in plots where all sizes of LMH herbivores are excluded, and the lowest in control plots. I also expected this discrepancy between treatments to be the greatest in plots at the Low precipitation region.

Results/Conclusions

S. campylacanthum density was significantly higher when all LMH were excluded (Total) than in the control (Open) plot or when only Mega LMH (elephants and giraffes) were excluded. Precipitation level had no effect on S. campylacanthum density. Similarly, insect herbivore abundance and species richness on S. campylacanthum were higher in Total and Meso exclusion plots than in Mega exclusion and control plots. Because impala is the most dominant Meso LMH at Mpala, it is likely the mammalian species contributing to the observed pattern. Impalas commonly consume the fruits of S. campylacanthum at the study site, potentially reducing resource for frugivorous insects on the plant. My results suggest that mammalian herbivores can indirectly compete with insect herbivores, but this relationship does not appear to be affected by precipitation levels.