How might Pacific Island forests be affected by the loss of native seed dispersers and their replacement by novel species?

Background/Question/Methods

As a result of human colonization of the Pacific Islands, most native seed-dispersing animals have become rare or extinct, and a diverse array of alien animals has invaded and proliferated. Because seed dispersal plays a key role in forest community dynamics, this shift has the potential to significantly affect forest community composition. We expected that large-seeded tree species would be disproportionately affected by the change in dispersers, because the loss of native animals was especially severe among the larger animals, and this loss was not offset by the introduction of large-bodied aliens. We also expected that the introduction of novel functional groups of animals (especially rodents) would impose heavy mortality on poorly-dispersed seeds. We combined observational, correlational, and experimental approaches to examine the roles of extinct pigeons and bats, and invasive pigeons and rats, as dispersers of large-seeded rain forest trees in Tonga.  We used observational and experimental approaches to examine the roles invasive passerines, pheasants, and rodents in Hawaiian dry, mesic, and rain forests lacking native dispersers. 

Results/Conclusions

In Tonga, three large pigeons (Ducula spp.) and one bat (Pteropus samoensis) have become extinct, leaving Pteropus tonganus as the only native animal capable of dispersing seeds > 13 mm diameter, and it handled 97% of the dispersed seeds of the 13 large-seeded species studied. A prehistorically-introduced pigeon (D. pacifica) dispersed seeds up to 25 mm diameter, but did not disperse the largest-seeded trees. Rats (R. rattus, R. exulans), were mainly seed predators, but dispersed some species (e.g., Pandanus tectorius). In Hawaii, introduced passerines were effective dispersers for some small-seeded trees, but ignored others, and did not disperse large seeds.  Introduced kalij pheasants (Lophura leucomelanos) had highly variable effects on the seeds of the 29 species they consumed (0-38% survival). Rattus rattus dispersed some large-seeded species, and species with seeds < 1.2 mm diameter (internally), but were mainly seed predators, disproportionately affecting seeds that had not been dispersed away from parent trees. Overall, the risk of dispersal limitation is greatest for large-seeded species, as is the risk of seed predation by introduced animals. Although introduced animals are sometimes effective seed dispersers, their patterns of seed deposition may differ significantly from those of natives.