SEX-BIASED HERBIVORY IN EPHEDRA-TRIFURCA - THE IMPORTANCE OF SEX-BY-ENVIRONMENT INTERACTIONS

We examined patterns of herbivory in the dioecious desert shrub, Ephed ra trifurca (Ephedraceae), along a small watershed in the northern Chi huahuan Desert. We documented significant (p= 0.002) sexual dimorphism in densities of Lasioptera ephedrae and L. ephedricola (Diptera: Ceci domyiidae), with male E. trifurca supporting higher densities than did females. Densities of gall midges varied significantly (p<0.001) amon g four sample populations of E. trifurca arrayed along the watershed a nd exhibited an inverse gradient with elevation. There was also a sign ificant p=0.006) sex-by-site interaction such that the degree of inter sexual variation in gan densities decreased down the watershed. Variat ion in gall densities and intersexual variation in gall densities exhi bited inverse trends with variation in soil water and inorganic nitrog en concentrations along the watershed.