Generalization vs. specialization in the pollination system of Hormathophylla spinosa (Cruciferae)

This study describes the interaction between Hormathophylla spinosa, a crucifer shrub, and its pollinators, analyzing the spatiotemporal variability of the pollinator assemblage as well as the foraging behavior and effectiveness of the common pollinators. The study was carried out in the high mountains of the Sierra Nevada, Spain, over four years (1988-1991). We selected three populations of H. spinosa located along a wide altitudinal gradient. This plant species was visited during the four years of the study by at least 70 species of insects belonging to five orders and 19 families. The assemblage of floral visitors of H. spinosa was composed mainly of ants and different species of solitary bees and flies, although most pollinators were comparatively rare. There were striking similarities between most pollinator species in the distributions of flight distance between consecutive flowers, which were leptokurtic and highly skewed. Moreover, almost all visitors acted as pollinators, depositing pollen grains enough for seed production. Insects visiting flowers of H. spinosa can be divided into two guilds: nectarivores (small flies and ants) and pollinivores (bees and hoverflies), which differ in pollination attributes. Flower-visitation rates were quite different between the faster pollinivores and the slower nectarivores. All nectarivorous species moved almost exclusively between flowers of the same plant, whereas pollinivores sometimes moved between plants; and pollinivores deposited more pollen grains per visit than did nectarivores. H. spinosa interacted most frequently with the nectarivorous Proformica longiseta, Fannia scalaris, and Exechia dorsalis in the two lowest elevation populations of Sierra Nevada, and with the pollinivorous Colletes sp. and Eristalis tenax in the highest elevation population. The mutualistic interaction between H. spinosa and its pollinators is a generalized system, where the traits of the flower allow almost every floral visitor to act as a true, effective, pollinator. The lack of differences in per-visit pollination effectiveness and the functional equivalence of very different pollinators may help to maintain this generalized system.