Interspecific competition in phytophagous insects

The relative importance of interspecific competition is a highly controversial and unresolved issue for community ecology, in general, and for phytophagous insects in particular. Traditionally, two mechanistic forms of competition are cited in ecology, (1) exploitative, and (2) interference:



Recent advancements, however, in our understanding of indirect interactions via plants (induced resistance) and natural enemies (apparent competition) challenge the historical paradigm of competition:



Thus, in the context of this rapidly developing field, I re-evaluated the evidence for interspecific competition in phytophagous insects using a meta-analysis of published studies. The analysis was specifically designed to test the assumptions underlying traditional competition theory, namely that competitive interactions are symmetrical, necessitate spatial and temporal co-occurrence, and increase in intensity as the density, phylogenetic similarity, and niche overlap of competing species increase. Despite finding frequent evidence for competition, I found very little evidence that plant-feeding insects conform to the above-cited theoretical predictions. Virtually every fundamental paradigm that I tested was violated, suggesting a poor concordance between theory and empirical patterns.

Indirect herbivore interactions via plants and enemies are likely to underlie much of the discrepancy between theory and pattern. Until recently most ecology texts emphasized interference and exploitative interactions as the two mechanisms driving competition. My dataset provides weak support for the overall prevalence of these two mechanisms occurring in insect communities. Alternatively, indirect interactions provide the vast majority of evidence for interspecific herbivore interactions (>65% of all observations in the dataset), particularly those involving plants.