Geographic and population variation in pollinating seed-consuming interactions between senita cacti (Lophocereus schottii) and senita moths (Upiga virescens)
Jn. Holland et Th. Fleming, Geographic and population variation in pollinating seed-consuming interactions between senita cacti (Lophocereus schottii) and senita moths (Upiga virescens), OECOLOGIA, 121(3), 1999, pp. 405-410
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.