Large avian frugivores in the Philippines show linear responses to improvements in forest quality

Abstract

Large avian frugivores are important in ecosystem function, but are seriously threatened across the tropics. To conserve them we must understand their habitat needs and the effects of improved forest management on individual species and the community as a whole. We recorded the presence/absence of 18 parrot, pigeon and hornbill species along nearly 500km of transects at 24 sites in Luzon, Philippines, and used logistic GLMMs to identify bird-habitat associations based on topographical, forest structure and floristic data taken at 1227 habitat plots. We then searched for more complex relationships and thresholds in species responses along forest quality/restoration gradients using GAMMs. Frugivore species richness was highest in forest with large-girthed trees, although some small-scale agricultural disturbance was tolerated or even favoured. Importantly, richness was highest in forests on flat ground, areas which are usually the first to be converted to agriculture. Individual species were positively associated with large trees but responses to floristic gradients were more variable. Very few species had complex relationships with forest quality; for the great majority, the probability of occurrence increased linearly along the forest quality/restoration gradient. While the precise benefits in terms of seed dispersal, and costs of management, at different points along the quality/restoration gradient are likely to be themselves complex, avian frugivores benefit proportionately from step improvements right along the gradient. Thus, any actions to improve forest quality on Luzon, from reforesting the most degraded lands to preventing degradation of relatively healthy forests, are likely to benefit frugivores.