Plant-Pollinator Interactions in Hawaii: Pollination Energetics of Metrosideros collina (Myrtaceae)

Abstract

The most abundant tree species in much of the undisturbed Hawaiian forests was the subject of a two year study on plant-pollinator interaction 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 compare the pollination ecology of this species and the degree of specialization in the plant-pollinator community with those of similar mainland systems. Endemic Hawaiian birds (Drepanididae) are essential for high levels of fruit set and outbreeding in M. collina. Fruit set was much higher in redflowered 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. The generalization of the pollination strategy is on both an individual and populational basis, and enables M. collina to be the good colonizer that successional patterns and its own geographical distribution show it to be. Red-flowered individuals are partially self-compatible, but yellow-flowered individuals are totally self-compatible. The yellow-flowered morph may be evolving autogamy. At this stage its breeding system is intermediate between autogamy and outbreeding, with geitonogamy being encouraged due to 1) increased attractiveness to insects relative to the high energy birds by means of color and scent cues, and 2) increased nectar flow which satiates pollinators. Furthermore, yellow-flowered individuals have a floral structure that facilitates transferal of pollen to stigmas in the same inflorescence by means of small size pollinators, or even without the aid of a pollinator. A model is derived that predicts the selective results of various degrees of pollinator limitation on nectar productivity. Bird numbers are more constant relative to nectar availability than would be expected by random sorting, 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 intraspecific 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 several tree species in the Hawaiian forests. Comparison with a similar but more diverse forest community in New Zealand and with mainland tropical forests suggests that the length of flowering season per tree species is inversely related to the number of tree species competing for pollinators. Profuse flowering in M. collina results in lowered fruit set per inflorescence because of decreased outbreeding, but the total number of fruits set per tree is probably 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. In comparison with mainland communities, the degree of specialization in the plant-pollinator relationship seems to be less in Hawaiian forests, although more information is needed on the Hawaiian lobelias and Sophora chrysophylla before such a statement should be made with any certainty.