Forest fragmentation reduces parasitism via species loss at multiple trophic levels

FENOGLIO, M. S.; SRIVASTAVA, D.; VALLADARES, G.; CAGNOLO, L.; SALVO, A

ECOLOGY

ECOLOGICAL SOC AMER

Año: 2012 vol. 93 p. 2407 - 2420

0012-9658

Although there is accumulating evidence from artificially-assembly communities that reductions of species diversity result in diminished ecosystem functioning,it is not yet clear how real-world changes in diversity affect the flow of energy between trophic levels in multi-trophic contexts.In central Argentina, forest fragmentation has led to species loss of plants, herbivore and parasitoid insects, decline in trophic processes (herbivory and parasitism) and food web contraction. Here we examine if and how loss of parasitoid species following fragmentation cause decreased parasitism rates, by analyzing food webs of leaf miners and parasitoids from 19 forest remnants of decreasing size. We asked three questions: Do reductions in parasitoid richness following fragmentation directly or indirectly affect parasitism rate? Are changes in parasitism rate driven by changes in the efficiency of the parasitoid community or susceptibility of the leaf miner community, or both? Which traits of species determine the effects of food web change on parasitism rates? We found that habitat loss initiated a bottom-up cascade of extinctions from plants to leaf miners to parasitoids, with reductions in parasitoid richness ultimately driving decreases in parasitism rates. This relationship between parasitoid richness and parasitism depended on changes in the relative abundance (but not incidence) of leaf miners such that parasitoid-rich fragments were dominated by susceptible miner species. Surprisingly, we found that only a small subset of species in the food web was responsible for most of the relationship: namely six Lepidopteran miners that attained exceptionally high densities in some fragments, and their largely specialist parasitoids. Our results show that a full understanding of the functional consequences of perturbations and species loss requires both a multitrophic perspective and a trait-based approach, which together capture some ofthe biological complexity of natural systems.