Abstract
BackgroundWe examined species composition of forest and bird communities in relation to environmental and human disturbance gradients on Lakeba (55.9 km2), Nayau (18.4 km2), and Aiwa Levu (1.2 km2), islands in the Lau Group of Fiji, West Polynesia. The unique avifauna of West Polynesia (Fiji, Tonga, Samoa) has been subjected to prehistoric human-caused extinctions but little was previously known about this topic in the Lau Group. We expected that the degree of human disturbance would be a strong determinant of tree species composition and habitat quality for surviving landbirds, while island area would be unrelated to bird diversity.Methodology/Principal FindingsAll trees >5 cm diameter were measured and identified in 23 forest plots of 500 m2 each. We recognized four forest species assemblages differentiated by composition and structure: coastal forest, dominated by widely distributed species, and three forest types with differences related more to disturbance history (stages of secondary succession following clearing or selective logging) than to environmental gradients (elevation, slope, rockiness). Our point counts (73 locations in 1 or 2 seasons) recorded 18 of the 24 species of landbirds that exist on the three islands. The relative abundance and species richness of birds were greatest in the forested habitats least disturbed by people. These differences were due mostly to increased numbers of columbid frugivores and passerine insectivores in forests on Lakeba and Aiwa Levu. Considering only forested habitats, the relative abundance and species richness of birds were greater on the small but completely forested (and uninhabited) island of Aiwa Levu than on the much larger island of Lakeba.Conclusions/SignificanceForest disturbance history is more important than island area in structuring both tree and landbird communities on remote Pacific islands. Even very small islands may be suitable for conservation reserves if they are protected from human disturbance.
Highlights
The 25,000+ islands in the tropical Pacific Ocean represent discrete terrestrial habitats that vary greatly in size, isolation, origin, and age [1,2]
This paper addresses one of those gaps in West Polynesia (WP), a region that includes Fiji, Tonga, Samoa, and the outlying islands of Rotuma, Wallis & Futuna, and Niue
Because prehistoric and historic deforestation has been a major driver of vertebrate extirpations and extinctions in WP [1], we expected that the human disturbance gradient would be a strong determinant of habitat quality for landbirds [32,33,35] as well as tree species composition [22]
Summary
The 25,000+ islands in the tropical Pacific Ocean represent discrete terrestrial habitats that vary greatly in size, isolation, origin, and age [1,2]. Because prehistoric and historic deforestation has been a major driver of vertebrate extirpations and extinctions in WP [1], we expected that the human disturbance gradient (agroforestry, plantation forestry, and native forest at varying stages of secondary succession) would be a strong determinant of habitat quality for landbirds [32,33,35] as well as tree species composition [22] Those previous studies found that on oceanic islands with long histories of human impact, the composition of ecological communities varies to a greater degree as a function of the intensity of human disturbance than it does along environmental gradients (substrate, topography), especially when those environmental gradients are short. As noted in [36] and references therein, nonsignificant correlations between species number and area may be underreported in the literature, they would be as informative about species-area relationships as positive correlations
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