Orchids, bees and spiders: neutral effect of a predator on a plant-pollinator mutualism

QUINTERO C

Boulder, Colorado

Encuentro; Ecology and Evolutionary Biology Spring Symposium; 2006

University of Colorado at Boulder

Plant reproductive success is the result of many interacting factors. Among those, floral features like the reward - principally nectar- are very important as predictors of reproductive success. However, net reproductive success is also regulated by top-down effects. Tritrophic systems that include mutualistic associations, like plant-pollinator interactions, can be very susceptible to the impact of a third trophic level through direct effects or indirectly through modifications in their interaction. Rewardless orchids usually have low reproductive success due to weak plant-pollinator association. A predator of flower visitors can modify this relationship with important effects on the population dynamics of both plant and pollinators. I analyzed the direct and indirect effects of a crab-spider (Misumenops pallidus), which preys on floral visitors, on the reproductive success of Chloraea alpina, a rewardless orchid. During the 2001 reproductive season I monitored 300 plants and recorded reproductive success of the orchid through female and male function, crab-spider presence and its effect on the floral visitors’ assemblage. These results were compared with female and male reproductive success obtained from sampling during six flowering seasons (1998-2003) in the same natural population. The results indicate that the effect of M. pallidus on the reproductive success of C. alpina is neutral. Apparently, its prey were not part of the pool of insects which behave as effective pollinators, and the presence of the spider on a flower did not affect the likelihood of reproductive success of an inflorescence. The main factor determining low reproductive success (6%) was the scarcity of pollinators. We conclude that pollen limitation due to pollinator scarcity can be the consequence of the rewardless condition. In addition, in stochastic systems in which the likelihood of interaction among the three trophic levels is low, direct interactions play a more important role than indirect interactions on the reproductive success of one species.