Diversity of insect-induced galls along a temperature-rainfall gradient in the 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 gall-inducing insects increases as environments become hotter and drier, while others suggest that these factors have no effect. The role of plant species richness in determining species richness of gall-inducing insects is also controversial. These apparent inconsistencies may prove to be due to the influence of soil fertility and the uneven distribution of gall-inducing insect species among plant taxa. The current study tested hypotheses about determinants of gall-inducing insect species richness in a way different to previous studies. The number of gall-inducing insect species, and the proportion of species with completely enclosed galls (more likely to give protection 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 the Northern Territory, Australia. There is a strong temperature-rainfall gradient along this transect during the wet season. Plant species lists had already been compiled for each collection plot. All plots were at low elevation in eucalypt savannah growing on infertile soils. There was no evidence to suggest that hot, dry environments in Australia have more gall-inducing insect 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 species meant that plant species richness did not influence gall species richness. Confirmation is still required that low soil fertility does not mask temperature-rainfall effects and that galls in the study region are occupied predominantly in the wet season, when the temperature-rainfall gradient is most marked.