Tests of hypotheses on patterns of gall distribution along an altitudinal gradient

We investigated the distributional pattern of galling insects along an altitudinal gradient in southeastern Brazil. We sampled insects on 33,000 herbs, 3,520 shrubs, and 1,760 trees, at 44 sites across an elevational gradient of 700 m, in order to test four hypotheses: (a) harsh environment hypothesis that predicts higher galling species richness (GSR) in xeric than in mesic habitats; (b) altitudinal gradient hypothesis that predicts a negative correlation between GSR and altitude; (c) plant richness hypothesis that predicts a positive correlation between GSR and plant species richness (PSR); and (d) plant architecture hypothesis that predicts a positive correlation between GRS and the structural complexity of plants. Only hypothesis (a) was completely confirmed by our study; galling species richness was higher in xeric than in mesic habitats (Wilcoxon test = 5.5816, P < 0.0001). The altitudinal gradient negatively influenced galling richness in xeric habitats (for all plant categories, r(2) = 0.83, F-1.22 = 107.585, P < 0.0001), but there was no significant correlation between galling richness and altitude in mesic habitats (for all plant categories, F-1.18 = 2.251, P > 0.05). The mean number of galling insects was higher on woody than on herbaceous plants, but did not differ between shrubs and trees, refuting the plant architecture hypothesis. The plant richness hypothesis was rejected because the correlation between GSR and PSR could have been influenced by the distribution of plant species along the altitudinal gradient.