Seed dispersal as an ecosystem service by a keystone avian frugivore in New Zealand

Seed dispersal service is the proportion of plant species dispersed by a given disperser. It is important since seed dispersal enables colonisation of new habitats, and because movement of seeds even short distances away from trees can reduce vulnerability of seeds and seedlings to disease, predation and density-dependent effects that occur in the immediate vicinity of parent trees. In New Zealand, indigenous large-fruited canopy tree species are susceptible to recruitment failure because Hemiphaga novaeseelandiae (kerer?; Columbidae) is the only widespread avian frugivore capable of swallowing large fruit and dispersing their seeds, and its survival relies on ongoing predator control. In view of the critical nature of this mutualism, seed dispersal service was assessed by measuring the difference in composition of seeds of indigenous large-fruited canopy tree species that were dispersed by H. novaeseelandiae to under the canopies of co-fruiting large-fruited canopy tree species across the Auckland region in New Zealand. If seed dispersal were effective, seeds of all species should be found under the canopies of all tree species, i.e. low beta diversity of seed rain. Beta diversity was higher than expected at two locations, suggesting incomplete seed dispersal service; this pattern was attributed to interactions between frugivore behaviour and the local distribution of fruit. The third location had lower than expected beta diversity, reflecting dispersal of a complete array of tree species? seeds to sampling sites, plus long-distance dispersal of nonlocal indigenous species of large seeds. Hemiphaga novaeseelandiae appears to be dispersing an array of species of large seeds between co-fruiting tree species at some Auckland locations. However, incomplete seed dispersal service at two of the locations, together with evidence of low recruitment of some species of seeds, could eventually reduce tree species diversity.