Arthropod abundance and community structure in contrasting Amazonian forests

Herbivorous insects are hypothesized to promote tropical plant diversity by exerting different pressures on plants across gradients of resource availability. Nonetheless, few studies have characterized insect communities across the broad gradient of soil fertility that occurs in the Amazon basin. Contrasting predictions can be made for insect abundance across gradients of soil fertility in tropical forests. On the one hand, we predict that the overall abundance and biomass of phytophagous insects will increase with increasing soil fertility due to increased productivity. On the other hand, the effects of soil fertility may be tempered by higher trophic levels, eliminating or even reversing these trends. Here we test these alternative predictions and hypothesize that different insect orders and feeding guilds will show contrasting responses to the same soil fertility gradient. We performed a 1-year survey using four entomological traps in twelve 0.5-ha plots across white-sand, clay terra firme and seasonally-flooded forests in Peru and French Guiana that well represent the range of soil fertility, forest structure, and floristic compositions found throughout the Amazon region. Here we present results for the abundance and biomass of seven insect orders (Lepidoptera, Blattoptera, Coleoptera, Diptera, Orthoptera, Hemiptera and Hymenoptera) from this survey in addition to preliminary analyses of feeding guild abundance within particular families and orders. Preliminary results on 54000 individuals of arthropods suggest that the effect of habitat type on overall biomass of phytophagous insects is consistent with our prediction, with terra firme clay forest supporting the greatest biomass in both countries.