Density of Insect Galls in the Forest Understorey and Canopy: Neotropical, Gondwana or Global Patterns?

Abstract

Gall-forming insects reach highest diversity, abundance and survivorship on sclerophyllous vegetation. This pattern was recently reviewed and shown as a habitat rather than ecosystem effect. We tested the hypothesis that upper forest canopies are probably the best suitable habitat for gall-forming insects in any tropical vegetation, comparing the wet rainforest of Panama (Neotropical), and the subtropical forest of Australia (Australian). We further tested whether foliage/plant community traits could influence this gall distribution pattern, and we investigated the effect of host family size and evolutionary age. Foliage traits, leaf chewing herbivory, and gall abundance and survivorship were measured using vertical cylindric transects from the understorey to the canopy. In both Panama and Australia, leaf sclerophylly increased significantly with sampling height, while free-feeding herbivory decreased inversely. Gall distribution and survivorship responded significantly to sclerophylly, but distribution between understorey and canopy varied between study sites. The probability of gall survivorship increased with increasing leaf sclerophylly as death by fungi, parasitoids or accidental chewing were greater in the non-sclerophyllous vegetation in the understorey of both study sites. However, number of galls, proportion of infected sampled plants, and proportion of host species against total sampled species were all greater in Panamá than in Australia. On the other hand, the Australian forest had a fauna much more concentrated on fewer hosts, with 80 % of galls infesting six host species. The present study supports the existence of a global positive effect of sclerophylly on gall establishment and survivorship in the upper canopy of tropical and subtropical forests.