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

Galling insects tend to be highly sensitive to changes in their host plants or their environment. Here we analyze the effects of Chaco Serrano forest fragmentation on gall inducing species associated with four native plants species, simultaneously examining area and edge effects as well as the role of host plant availability on such effects. At edge and interior locations in each of nine forest sites in an area gradient in Central Argentina, we estimated herbivory as (1) the proportion of galled plants and (2) the number of galls per plant. Herbivory variations in relation to forest area and edge/interior locations were analyzed with generalized linear models, whereas the influence of plant availability in mediating area and edge effects was assessed by Structural Equation Models. Different responses to fragmentation were observed, depending on the insect species and also on the chosen herbivory indicator. Significant edge and area effects were detected in four and two out of ten performed models, respectively. When significant, edge effects were mostly positive and consistently direct rather than mediated by plant availability; instead, area effects varied from positive and led by plant abundance, to negative and independent of plant availability. Our study provides new evidence of a tendency for galling insects to benefit from edge conditions, while showing less consistent effects regarding forest size. Our results also suggest a very limited role for plant availability as a mechanism mediating fragmentation effects on herbivory by galling insects.