A model to predict the influence of insect flower constancy on interspecific competition between insect pollinated plants

Abstract

Honeybees and bumblebees, among the most important pollinators in many plant communities, are known to exhibit flower constancy. When foraging, bees do not sample flowers randomly as they encounter them, but rather they exhibit a strong preference for one species of flower, and will ignore other species that they encounter. The species favoured depends on previous foraging success: ifa species of flower has previously provided a high reward, it will be favoured to the exclusion of others. Various models have been developed to describe the optimal foraging pattern of flower feeding insects, which explain, at least in part, observed behaviour. However, they do not consider the importance consequences of flower constancy for competition between plants dependant upon insects for pollination. Bees are most likely to favour a common species which they encounter frequently, and will fly directly between individuals of the favoured species, and thus provide efficient pollination. Conversely, they are unlikely to develop a preference for a scarce flower which necessarily is rarely encountered. A simple two-flower species model is developed which predicts the proportion of insects which will favour each of the two species for particular flower densities, assuming that individual insects favour the species which provides them with the greatest reward per time. The model predicts that scarce plant species will receive no pollinators below a threshold density of reward, and that scarce plants must provide a relatively huge reward per flower to achieve pollination. The threshold is lowered at high insect densities when the reward offered by the more common species becomes depleted. The implications of flower constancy to plant communities are discussed.