Plant‐Pollinator Interactions in Hawaii: Pollination Energetics of Metrosideros Collina (Myrtaceae)

The most abundant tree species in much of the undisturbed Hawaiian forest was the subject of a 2—yr study on plant—pollinator interactions and energetics. The purposes of the study were (1) to determine the roles of insects and of some endemic Hawaiian birds in the pollination of the tree Metrosideros collina, (2) to test the hypothesis that maximal outbreeding and seed set occur at intermediate levels of nectar availability, (3) to understand the adaptive significance of profuse flowering in this species, and (4) to determine the degree of specialization in the pollination ecology of this species. Endemic Hawaiian birds (Drepanididae) are essential for high levels of fruit—set and outbreeding in M. collina. Fruit—set was much higher in red—flowered individuals when birds were allowed to use inflorescences than when only insects used them. This is apparently caused by partial self—incompatibility, such that maximal fruit—set occurs only with outbreeding, the primary agents of which are the birds. The predominant flower color in the population, the dimensions of floral parts, and copious nectar secretion adapt this species to bird pollination. However, insects effect moderate amounts of pollination and fruit—set. The open flower and the color— and scent—variability within the population may be adaptations for insect pollination in the event that bird pollination fails. The population seems to have differentiated along an elevational gradient, with adaptations for bird pollination increasing proportionally with elevation. Individuals have a generalized pollination strategy, and the population as a whole shows polymorphic variation. Generalization enables M. collina to be the good colonizer that successional patterns and its own geographic distribution show it to be. Red—flowered individuals are partially self—incompatible, but yellow—flowered individuals are totally self—compatible. The yellow—flowered morph may be evolving autogamy. At this stage its breeding system seems to be intermediate between autogamy and outbreeding. Yellow—flowered individuals have a floral structure that may facilitate transferral of pollen to stigmas in the same inflorescence by means of small pollinators, or even without the aid of a pollinator. Birds numbers tend to be constant relative to nectar availability, although temporary deficits and surpluses of these pollinators occur: their inability to respond instantly to changes in the intensity of bloom introduces lags into the system, and these have important consequences for pollination, outbreeding, and gene flow. During some times of the year pollinators are limiting to M. collina, and interspecific competition occurs. Maximal fruit—set and outbreeding do occur at intermediate nectar availabilities. Interspecific competition between species of trees for pollinators is a potential selective force that may explain the character displacement and staggering of flowering seasons of two bird—associated tree species where they co—occur in the Hawaiian forest. Profuse flowering in M. collina results in lowered fruit—set inflorescence apparently because of decreased outbreeding, but the total number of fruits set per tree is high because of partial self—compatibility in most individuals. Thus, the M. collina system does not help explain profuse flowering in mainland tropical species that are totally self—incompatible. Partial self—compatibility in M. collina permits dependable high seed production, adaptive in colonizing species.