Short-term effects of canopy openness on insect herbivores in a rain forest in Guyana

1. A before/after-control/impact experiment (BACI) was used to examine the shortterm effects of canopy openness on insect herbivores foraging on the s eedlings of five rain forest species in an unlogged forest in Guyana. Durin g the first and second study years, insects were collected monthly from 250 sites, representing 9750 seedlings spread over a plot of 1 km(2). At the o nset of the second collecting year, half of the parent trees at the sites w ere felled, creating an average canopy openness of 26%. 2. Comparisons of the control and impact sites before and after the felling showed that overall insect abundance, particularly of psyllids, and specie s richness increased at the impact sites, whereas evenness decreased marked ly. These responses were consistent with the prediction that intermediate d isturbance promotes high diversity in tropical systems. 3. Twenty-nine per cent of insect species analysed showed a significant res ponse to felling, with some populations doubling or halving at the impact s ites after felling. The abundance of most species increased, although respo nses varied even between congeners. 4. A canonical correspondence analysis at the impact sites after felling in dicated that most species responded to an increase in leaf production by se edlings, not to canopy openness per se. Species declining at the impact sit es were more likely to feed indiscriminately on young and mature foliage an d probably reacted to changes in microclimate. 5. The study illustrates the difficulty in identifying higher taxa, umbrell a or indicator species that adequately integrate the responses of insect he rbivores to forest disturbance. Because the integrity of insect communities in the understorey of this forest depends more on minimizing damage to pla nt resources than on canopy openness, we found some support for the conserv ation value of reduced impact logging in tropical wet forests.