Species richness of herbivorous insects on Nothofagus trees in South America and New Zealand: The importance of chemical attributes of the host

Abstract

Abstract Several studies have evaluated the relative contribution of various host-plant attributes to the species richness of the associated insect herbivores, with and without the inclusion of the phylogeny of the host species for Northern hemisphere trees. In general these studies reached the same conclusion: tree availability (range and abundance) was a good predictor of insect species richness, although chemical attributes of the trees were not tested. The present study evaluates the relative contribution of ecological attributes of host-plant species within the Southern hemisphere genus Nothofagus, to the species richness of their associated insect fauna in South America and New Zealand. The variables included were: area of distribution (a), including longitudinal (rln) and latitudinal ranges (rlt), architectural complexity (c) and phytochemical attributes of the species, including chemical diversity (d) and chemical uniqueness (u). The analysis by independent contrasts revealed that the latitudinal range is an important factor, which explained the insect richness associated with Nothofagus for all guilds and taxonomic orders, except for the sap feeders. Compared to the non-phylogenetic analyses, including host-plant phylogeny led to the inclusion of additional variables in the regression equations. Phytochemical uniqueness of the host plants was an important factor to explain insect species richness. For example, Nothofagus alessandrii, the species with the most unique chemistry, had very few and mostly specialised herbivores, whereas Nothofagus dombeyi, with the least unique chemistry, had the highest number of related insect species. We conclude that, in addition to geographic range and phylogenetic relatedness, studies of insect herbivore diversity must also examine plant chemical composition.