Sex-biased herbivory in Ephedra trifurca: the importance of sex-by-environment interactions

We examined patterns of herbivory in the dioecious desert shrub, Ephedra trifurca (Ephedraceae), along a small watershed in the northern Chihuahuan Desert. We documented significant (p=0.002) sexual dimorphism in densities of Lasioptera ephedrae and L. ephedricola (Diptera: Cecidomyiidae), with male E. trifurca supporting higher densities than did females. Densities of gall midges varied significantly (p<0.001) among four sample populations of E. trifurca arrayed along the watershed and exhibited an inverse gradient with elevation. There was also a significant (p=0.006) sex-by-site interaction such that the degree of intersexual variation in gall densities decreased down the watershed. Variation in gall densities and intersexual variation in gall densities exhibited inverse trends with variation in soil water and inorganic nitrogen concentrations along the watershed.