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, r2 = 0.83, F1,22 =
107.585, P < 0.0001), but there was no significant correlation between galling
richness and altitude in mesic habitats (for all plant categories, F1,18 = 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.
KEY WORDS: altitudinal gradients, environmental stress, insect galls, insect herbivory,
Serra do Cipó.

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