Galling insects in a fragmented forest: incidence of habitat loss, edge effects and plant availability

Abstract

Galling insects tend to be highly sensitive tochanges in their host plants or their environment. Here weanalyze the effects of Chaco Serrano forest fragmentationon gall inducing species associated with four native plantsspecies, simultaneously examining area and edge effects aswell as the role of host plant availability on such effects. Atedge and interior locations in each of nine forest sites in anarea gradient in Central Argentina, we estimated herbivoryas (1) the proportion of galled plants and (2) the number ofgalls per plant. Herbivory variations in relation to forestarea and edge/interior locations were analyzed with gen-eralized linear models, whereas the influence of plantavailability in mediating area and edge effects was assessedby Structural Equation Models. Different responses tofragmentation were observed, depending on the insectspecies and also on the chosen herbivory indicator. Sig-nificant edge and area effects were detected in four and twoout of ten performed models, respectively. When signifi-cant, edge effects were mostly positive and consistentlydirect rather than mediated by plant availability; instead,area effects varied from positive and led by plant abun-dance, to negative and independent of plant availability.Our study provides new evidence of a tendency for gallinginsects to benefit from edge conditions, while showing lessconsistent effects regarding forest size. Our results alsosuggest a very limited role for plant availability as amechanism mediating fragmentation effects on herbivoryby galling insects.