Effects of human mediated disturbances on exotic forest insect diversity in a Chilean mediterranean ecosystem

Abstract

At the current rate of exchange of goods and people among geographic areas, the introduction of insect species into new habitats represents an increasing threat to insect diversity. The situation is especially acute in Mediterranean ecosystems where the high human population density incurs multiple sources of disturbance and high propagule pressure. In this study, we characterize the relationship between native and exotic forest insect richness and evaluate how human-mediated disturbances can influence this relationship in the Mediterranean central Chile. Exotic and native species richness were positively correlated across the study area, suggesting similar effect of environmental variables on both assemblages over large scales. When the effect of human-mediated disturbances was evaluated using generalized linear and additive models, we found that native richness, human population density and habitat diversity were the most important variables affecting exotic richness. Moreover, we detected strong nonlinearities in the effect of some variables. For instance, the influence of human population density on the exotic richness followed a threshold function, where below 1,000 hab/km², the proportion of exotics in the community grew rapidly with increasing human density, but above this threshold density, human population did not produce further increases in exotic richness. Two important conclusions arise from these results: first, there is a positive effect of human-mediated disturbances on the exotic richness in central Chile, and second, the key role that human population density has on the invasibility of insect communities in rural and semi-rural Mediterranean areas.