Resource Use, Competition, and Resource Availability in Hawaiian Honeycreepers

Abstract

This paper tests three predictions about resource selection in competing species developed from theoretical studies of Rosenzweig and others. These predictions are distinct from those predicted by simple, single—species optimal—foraging models. Under single—species models, a species is expected to take only its preferred resource when that resource is abundant and to generalize by also taking poorer resources when its preferred resource is scarce. Rosenzweig's models are of two species which can exploit and compete for two habitat patches. (Habitat patches are defined in a very general way.) There are two competition—based cases which can lead to resource partitioning. In the distinct preference case, one species prefers one resource; its competitor, the other. In the shared preference case, both species share the same preference, but one species, the dominant, will exclude, often behaviorally, the other species, the subordinate, from the mutually preferred resource. In contrast to single—species models, Rosenzweig's models predict three occasions where resource scarcity will be accompanied by specialization: (1) In the shared—preference case, the dominant may be unable to exploit the poorer patch. If it can, it is likely to drive the subordinate to extinction, leaving a one—species system. (2) The subordinate may specialize on clearly inferior resources because the dominant forces it from the better resources. (3) In the distinct—preference case, the scarcity of both resources leads to specialization of both species because interspecific competition (causing specialization) is greater than intraspecific competition (causing generalization). These are the three predictions tested in this paper. During February through June 1979 we recorded the feeding activities of the three commonest endemic nectivores in areas in and adjacent to the Hawaii Volcanoes National Park on the island of Hawaii. The birds fed principally on the flowers of two trees: Metrosideros collina and Sophora chrysophylla. During the study, Metrosideros went from few flowers per tree to peak bloom, and Sophora did the reverse. Both distinct— and shared—preference cases were noted. The former involved Himatione sanguinea on Metrosideros and Loxops virens on Sophora. The latter involved Vestiaria coccinea (the dominant) and subordinates Himatione and Loxops, with the preferred resources being trees with the highest numbers of flowers. Vestiaria only exploited trees with high numbers of flowers and may have had no alternative. Its competitors were forced onto poorer resources by its presence. These results support the theory's first two predictions. The data are also at least consistent with the third prediction: when flowers were scarce, Metrosideros flowers were exploited only by Himatione, and Sophora only by Loxops. However, these results are also explicable in terms of single—species foraging models. The restriction of the behaviorally dominant species to high—quality resources and the temporal and spatial scarcity of such resources suggest that dominant species are more likely to be threatened by human activities than are subordinate species that are capable of using poorer resources. The endemic nectivores that have become extinct or are currently endangered in Hawaii, have all been relatively large, showy, and among the more behaviorally aggressive species.