The effect of ecological factors on the mating system of a South American shrub species (Helicteres brevispira)

Mating systems are influenced by several ecological factors, including plan t density, number of flowers per plant, and pollinator movements. In this s tudy, we investigated the simultaneous effects of these three factors on th e mating system of a self-compatible Brazilian shrub species: Helicteres br evispira St. Hil. Outcrossing rate is directly correlated with plant densit y. Changes in the number of flowers per plant affect outcrossing rate throu gh their effect on the density of flowers. Variation in foraging behaviour of hummingbird pollinators is a consequence of the interaction between plan t density and number of flowers per plant. Territorialist pollinators are c ommon in high density areas but visit few flowers on each plant, thereby pr omoting outcrossing. In areas of low plant density, trapliners and rare ter ritorialists visit several flowers per plant, thus increasing selfing. Our results indicate that outcrossing rate is a dynamic parameter, with the ext ent of variation depending on a number of ecological factors. In succession al species such as those in the genus Helicteres, demographic changes may b e accompanied by alterations in mating system parameters, with concomitant effects on the genetic structure of populations.