Niche shifts in New Hebridean birds

Abstract

Niche breadth varies with competitive pressure. When most of the pressure experienced by a species is from a single close competitor, its niche tends to be broader in the absence than in the presence of the competitor. When 'diffuse competition' (the combined effects of numerous more distant competitors) is more significant, the niche tends to be broader on islands with few than with many species. We summarize all known examples of such niche shifts for New Hebridean birds, based on comparing conspecific populations on different New Hebridean islands or New Hebridean populations with those in another archipelago. The commonest niche shifts are spatial ones affecting type of habitat, altitude or vertical foraging range. Other shifts affect size and number of islands inhabited: shifts from small to large islands and shifts in 'incidence'. The remaining types of shifts are in diet and in abundance. We conclude that competition is the proximate cause of distributional limits for most New Hebridean species. We hypothesize that rapid behavioural responses of colonists to altered competition mainly affect spatial parameters of niches. Subsequent slower genetic changes modify foraging technique and diet and may produce shifts into forest and montane forest according to so-called taxon cycles. Further work is needed to test directly this proposed sequence of behavioural and genetic changes in colonists and to compare the importance of interspecific aggression and depletion of resources as proximate mechanisms of competitive exclusion.