Pollination Energetics and Foraging Strategies in a Metrosideros-Honeycreeper Association

Abstract

Evidence suggests that birds, specifically certain nectar-eating species of Drepanididae, are necessary for proper pollination, outcrossing, and high levels of seed set in Metrosideros collina on the island of Hawaii. Selection pressures acting upon the honeycreeper-M. collina association in the Keauhou Ranch forests on Mauna Loa fluctuate through the year. At the peak of the M. collina blooming season in late spring, nectar is relatively abundant to the honeycreepers, which means that the most important pollinators are potentially limiting to M. collina. Resultant intraspecific competition has selected for high rates of nectar secretion to attract pollinators; a ceiling in the population has been placed on these rates by the fact that seed set declines at very high flow rates, apparently because of decreased outcrossing due to pollinator sedentariness in such trees. At other times of the year when M. collina flowers are less abundant, a surplus of pollinators occurs, which results in high levels of seed set but in competition for food among the nectar-eaters. Loxops virens, and also Himatione sanguinea to a lesser extent, can turn to insectivory under these circumstances and therefore need not possess a highly specialized and efficient nectar-foraging strategy, but the more nectar-dependent Vestiaria sanguinea establishes interspecifically:defended territories in M. collina and visits flowers on its territory in a timed sequence. An hierarchy of interspecific aggression seems to be aligned among the three honeycreeper species in order of degree of nectar dependency.