Local and Regional Richness of Cynipine Gall Wasps on California Oaks

Regional herbivore richness is defined as the total number of herbivore species associated with a host plant in any part of its range. Local richness is the number of species associated with a host population or clump at a single site. These are roughly equivalent to @a and @c diversity. Understanding the mechanisms operating on local richness required a knowledge of the relationship between these two variables, and, specifically, an answer to the question of whether the same processed determine both regional and local richness. When local ecological interactions are weak and herbivore species are widespread, the correlation between richness at these two spatial scales should be strong. If local interactions are strong and herbivore species have restricted ranges due to habitat specialization, then the correlation should be weak. The relationship between local and regional richness was studied in California in the cynipid gall—forming wasps, which are highly specialized on the oak genus, Quercus. Regressions of presence—absence data suggested that geographic range and host taxonomic isolation explain the majority of regional richness variation in this group. Moreover, the very strong correlation between local and regional richness indicated that there is no upper limit to local richness independent of the regional pool. In terms of @a, @b, and @c diversity,@b (between—habitat) diversity does not increase with @b diversity. Instead, there is an unabated increase in @a diversity as @c diversity increases, suggesting that excess niche space is available on host oaks for additional cynipine species, and that limits to similarity of coexisting cynipines have not been reached. This indicates that sufficient information on the causes responsible for local herbivore richness cannot be gained by studying the local host population in isolation. The spatial dynamics of the system are such that measures of the regional pool from which the local assemblage is drawn must also be taken. Finally, cynipine habitat specialization and local interactions are not predictable from regional richness, but may influence local richness on certain oak species independently of regional richness.