Host diversity and environmental variables as determinants of the species richness of the parasitoids of leaf-cutting ants
Because of the obligatory relationship between endoparasitoids and their hosts, we presume that hosts exert strong selection pressure on parasitoids. One prediction is a positive relationship between host diversity and parasitoid richness. This relationship could be the product of resource availability which could lead to more opportunities for speciation, or could represent shared responses to the environment by both groups. Location
Argentina and Paraguay. Methods
We sampled a 1800-km transect to test for a correlation between the richness of leaf-cutting ant hosts and their phorid parasitoids. Regression models were used to assess if host and environmental variables could explain phorid species richness at nest, hectare and locality spatial scales. We used canonical correspondence analysis (CCA) to explore if there were similar responses of phorid species to particular host and environmental variables at different spatial scales, and partial CCA to separate the relative importance of both groups of variables. Results
Phorid richness was positively correlated with host richness. Host richness/abundance accounted for 20–53% of the variation in parasitoid richness at the hectare and locality scales of analysis, with most of the variation accounted for by ant abundance. We were not able to assess the prediction at the nest scale as only one phorid species was found at most nests. Climatic variables did not explain phorid species richness once host variables were in the models. Partial CCA showed that host-related variables accounted for most of the variance associated with phorid species ordination at the nest and hectare scales, but not at the largest grain, the locality, where climatic variables were more important. However, most phorid species did not show particular positions along the climatic gradient. Main conclusions
The association between parasitoid richness and host richness and abundance, and the overall weak associations with environmental variables, suggest that these host variables are key factors influencing parasitoid speciation.