Galling arthropod diversity in the subtropical neotropics: Espinilho savannah and riparian forests compared

Abstract

Xeric sites with scleromorphic vegetation appear to have higher gall-inducing arthropod richness as revealed in studies of local faunas across the world, and the harsh environment hypothesis (HEH) was proposed to explain this. However, plant richness also seems to influence galling arthropod richness positively, which appears paradoxical on a biogeographical scale. To test the HEH in southern Brazil, a sampling regime distinct from the usual local survey was adopted: eight transects were sampled during 90 min counting all galls; this was repeated three times over one year for the same transects. Xeric espinilho savannah (five transects) and mesic Uruguay River riparian forests (three transects) in subtropical Brazil were compared. Fifty-nine gall morphotypes on 15 host families were recognized out of 12,355 individual galls. Riparian forests were richer (individual-based rarefaction) and had higher gall equitabilities (bootstrapped 95% CIs); this contradicted the HEH. The plant richness hypothesis was generally supported because forests were recorded as richer in both plants and host plants. Vegetation types had different galling and host plant compositions; the latter may drive the former. Species pools appear distinct among environments, even adjacent ones, given the low similarities found. This further negates the ecological process of preference for xeric sites proposed in the HEH.