Seed dispersal by terrestrial frugivorous birds

Background/Question/Methods

Seed dispersal of vascular plants is among the most important ecosystem services provided by birds.  Plants and their avian dispersers form part of a complex mutualistic network fundamental to maintaining biodiversity and community structure. We present a preliminary review of this mutualism in the context of ecosystem services with the goal of evaluating the consequences of population declines of seed-dispersing bird species for plant communities and ecosystem function. This review is a first step towards assessing value to human society of ecosystem services.

Results/Conclusions

Nearly 1/3 of all bird species in at least 48 families are frugivorous to some degree; about 1350 species are primarily frugivorous and another 1800 species are partially frugivorous. About 25% of these bird species are extinction-prone. Birds disperse plants in at least 79 families of Angiosperms including about 5000 genera and perhaps as many as 80,000 species (out of about 250,000 species). Most plants dispersed by birds are trees and woody shrubs along with some lianas, vines, and herbaceous plants (and a few gymnosperms).  In many woodland and forest biomes birds are primary drivers of plant succession and recruitment. The principle benefit of seed dispersal by birds is movement of some seeds away from areas of higher seed/seedling mortality. The consequence of declining disperser populations is typically drastically reduced plant reproductive success rather than plant extinction. If such declines continue species composition and age distribution of plants and communities will be fundamentally altered; bird-dispersed and late-successional plants become less common and older, wind-dispersed and generalist plants increase at all age categories and dominate younger age classes. The loss of avian seed dispersers and resulting consequences for plants they disperse will likely be most severe in the tropics and on oceanic islands. Most bird dispersal activities fall into the category of supporting services. While birds disperse seeds of plants with direct economic use (lumber and landscape species), most plant species dispersed by birds are important for supporting other ecosystem services (carbon sequestration, erosion control). For this reason it is difficult to assign economic value to seed dispersal by birds.