Influence of altitude and host-plant species on gall distribution in Colliguaja spp. (Euphorbiaceae) in central Chile

The shrub genus Colliguaja (Euphorbiaceae) shows a characteristic pattern o f altitudinal distribution in central Chile. At lower altitudes it is repre sented by C. odorifera, at middle elevation by the putative hybrid C. salic ifolia and at upper altitudes by C. integerrima. Flower buds of Colliguaja spp. are galled by gall-midges and also show the presence of associated par asitoids and hyperparasitoids. In this study we evaluated the effect of alt itude and Colliguaja species on the distribution and abundance of flower ga lls resulting from parasitoid attack. Analysis of leaf morphology was used to represent either putative hybridisation or a host-plant species transiti on al: different altitudes, while gall prevalence and relative abundance we re measured as dependent variables. Gall prevalence and relative abundance decreased significantly as altitude increased, and leaf morphology indicate d a host-plant transition from C. odorifera through C. salicifolia to C. in tegerrima. Path analysis revealed that altitude had a stronger direct effec t on relative abundance of galls than host-plant, while the influence of ho st-plant hybridisation or species transition was mainly mediated by changes in elevation.