Forest fragmentation reduces parasitism via species loss at multiple trophic levels

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

ECOLOGY

ECOLOGICAL SOC AMER

Año: 2012 vol. 93 p. 2407 - 2420

0012-9658

 Although there is accumulating evidence from artificially assembled commu-nities 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 causes decreased parasitism rates, by analyzing food webs of leaf miners and parasitoids from 19 forest fragments of decreasing size. We asked three questions: Do  reductions  in  parasitoid  richness  following  fragmentation  directly  or  indirectly  affect parasitism rate? Are changes in community parasitism rate driven by changes in the parasitism rate of individual leaf miner species, or changes in leaf miner composition, 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 occurrence) of leaf miners such that parasitoid-rich fragments were dominated by leaf miner species that supported high rates of parasitism. Surprisingly, we found that only a small subset of species in the food web could account for much of the increase in parasitism with parasitoid richness: lepidopteran miners that attained exceptionally high densities in some fragments and their largely specialist parasitoids. How specialized a parasitoid is, and the relative abundance of leaf miners, had important effects on the diversity?parasitism rate relationship, but not other leaf miner traits including trophic breadth, body size,  and  mine  shape.  Our  results  show  that  a  full  understanding  of  the  functional consequences  of  perturbations  and  species  loss  requires  both  a  multi-trophic  perspective and a trait-based approach, which together capture some of the biological complexity of natural systems.