Diversity of insect-induced galls along a temperature-rainfall gradient inthe tropical savannah region of the Northern Territory, Australia

Evidence regarding the effect of temperature and rainfall on gall-inducing insects is contradictory: some studies indicate that species richness of ga ll-inducing insects increases as environments become hotter and drier, whil e others suggest that these factors have no effect. The role of plant speci es richness in determining species richness of gall-inducing insects is als o controversial. These apparent inconsistencies may prove to be due to the influence of soil fertility and the uneven distribution of gall-inducing in sect species among plant taxa. The current study tested hypotheses about de terminants of gall-inducing insect species richness in a way different to p revious studies. The number of gall-inducing insect species, and the propor tion of species with completely enclosed galls (more likely to give protect ion against heat stress and desiccation), were measured in replicate plots at five locations along a 500-km N-S transect in the seasonal tropics of th e Northern Territory, Australia. There is a strong temperature-rainfall gra dient along this transect during the wet season. Plant species lists had al ready been compiled for each collection plot. All plots were at low elevati on in eucalypt savannah growing on infertile soils. There was no evidence t o suggest that hot, dry environments in Australia have more gall-inducing i nsect species than cooler, wetter environments, or that degree of enclosure of galls is related to protecting insects from heat stress and desiccation . The variable number of gall-inducing insect species on galled plant speci es meant that plant species richness did not influence gall species richnes s. Confirmation is still required that low soil fertility does not mask tem perature-rainfall effects and that galls in the study region are occupied p redominantly in the wet season, when the temperature-rainfall gradient is m ost marked.