Samuel Case and Corey Tarwater
Seed dispersal, or the movement of seeds away from parent plants, is an important ecological process in which plants often rely on fruit-eating animals. Specifically, animals consume seeds while eating fruits and then deposit them away from parent plants after digestion. Seed dispersal is affected by the morphology of plant and animal species. For instance, an animal can only consume a seed if its mouth (hereafter gape) is wider than the seed’s width. Further, an animal’s body mass affects how much fruit it may consume, in addition to how far seeds can be dispersed away from parent plants; larger animals often have longer digestive tracts and therefore may disperse seeds farther due to longer digestion times. In flying birds, wing shape may also affect seed dispersal distances. Narrower wings have been linked to farther movements and may also affect the height at which birds feed on fruits.
In the Hawaiian Islands, most native plants are adapted for seed dispersal by birds, but after humans arrived in the islands (~800 years ago), most native seed-dispersers went extinct. Humans introduced many non-native bird species, but the effects of changing bird communities on ecosystem processes have been uncertain. To examine potential changes to seed dispersal, we compared morphological traits (gape width, body mass, and wing shape) between historic and modern communities of fruit-eating birds of the Hawaiian Islands. We found that gape width and body mass have shifted significantly downward across the archipelago, likely reducing the size of seeds consumed, fruit removal rates per animal, and seed dispersal distances. Wing shape did not change between assemblages. We also reviewed recent studies on seed dispersal across the islands and found that larger-seeded plants are presently more likely to be dispersal-limited and of conservation concern compared to smaller-seeded plants, likely in relation to reduced gape widths. Overall, our results suggest that changes to bird communities, from the extinction and introduction of species by humans, may alter ecosystem processes, with non-native species unable to substitute for the roles of extinct species because of differences in morphology.