Benefits and costs of floral visitors to Chilopsis linearis:: pollen deposition and stigma closure

In pollination mutualisms, nectar-robbers are usually considered antagonists; visitors that enter flowers (legitimate visitors) are usually considered mutualists. However, nectar-robbers may provide some benefits to plants, whereas legitimate visitors may inflict some costs. The costs and benefits of floral visitors to Chilopsis linearis (desert willow) were compared by number of pollen grains deposited and their effect on stigmas. Because these plants are self-incompatible and pollen-limited, they depend on visitors for services by pollinators to reproduce. On a per-visit basis, only one legitimate visitor, Bombus sonorus (bumblebees) generally benefited plants in terms of pollen deposition. However, no species of visitor was consistently beneficial; every one was at least sometimes ineffective in terms of pollen deposition. Chilopsis had sensitive stigmas that closed immediately upon touch and sometimes reopened later. Whether stigmas remained permanently closed or reopened depended on number of pollen grains deposited and tended also to be affected by source either, outcross or self. Legitimate visitors sometimes cost plants by causing stigma closure without depositing enough pollen to set a fruit. When abundant, a visitor such as Apis mellifera (honeybees) that was only occasionally beneficial on a per-visit basis may have provided a greater benefit to plants as a population than a more effective, but rare visitor. In contrast, nectar-robbers did not benefit plants by depositing pollen grains, but they also did not inflict costs on plants by causing stigmas to close without adequate pollen. There are two general implications of these results. First, individuals of species that appear to be mutualists can vary greatly in the benefits that they give to their partners. Second, apparent mutualists can inflict costs on plants that apparent exploiters (nectar-robbers) do not.