Geographic and population variation in pollinating seed-consuming interactions between senita cacti (Lophocereus schottii) and senita moths (Upiga virescens)

Interspecific interactions can vary within and among populations and geogra phic locations. This variation can subsequently influence the evolution and coevolution of species interactions. We investigated population and geogra phic variation in traits important to pollinating seed-consuming interactio ns between the senita cactus (Lophocereus schottii) and its obligate pollin ating moth (Upiga virescens), both of which are geographically restricted t o the Sonoran Desert. Female moths actively pollinate senita flowers and ov iposit onto flowers. Their larvae consume developing seeds and fruit of flo wers pollinated by females. Traits important to this interaction include fr uit set from moth pollination, fruit survivorship, and costs of fruit consu mption by larvae. We studied these traits for five populations at two widel y separated geographic locations. On average, 37% of flowers set fruit, 22% of flowers produced mature fruit, and larvae consumed 25% of immature frui t pollinated by female senita moths. Senita cactus and senita moth interact ions were strongly mutualistic in all populations that we studied. Although one population had statistically lower fruit set and fruit production than the other four, all five populations were qualitatively similar in fruit p roduction, costs, and patterns of fruit survivorship. Hand-pollination expe riments suggested that fruit set was resource-limited in all but this one p opulation. Apparent pollen limitation in the one population explains the qu antitative differences in fruit set and fruit survivorship among the popula tions. As predicted by theory and exemplified by the senita mutualism, spec ialized and/or obligate interactions vary little among populations and geog raphic locations.