Ecosystem consequences of bird declines

Abstract

We present a general framework for characterizing the ecological and societal consequences of biodiversity loss and applying it to the global avifauna. To investigate the potential ecological consequences of avian declines, we developed comprehensive databases of the status and functional roles of birds and a stochastic model for forecasting change. Overall, 21% of bird species are currently extinction-prone and 6.5% are functionally extinct, contributing negligibly to ecosystem processes. We show that a quarter or more of frugivorous and omnivorous species and one-third or more of herbivorous, piscivorous, and scavenger species are extinction-prone. Furthermore, our projections indicate that by 2100, 6-14% of all bird species will be extinct, and 7-25% (28-56% on oceanic islands) will be functionally extinct. Important ecosystem processes, particularly decomposition, pollination, and seed dispersal, will likely decline as a result.

Fig. 1.

Current conservation status of bird species. Number of species in each group is in parentheses. Species that are endangered or more threatened are considered functionally extinct and those that are vulnerable or more threatened are considered functionally deficient. (a) Distribution of extinction-prone species based on primary diet. If omnivores are reclassified based on first diet choice, percentages do not change except for scavengers (32%). (b) More specialization increases extinction risk; r² = 0.851. Specialization index is the product of habitats used and food types consumed. Higher numbers indicate less specialization. Species with an index of 32 or more have been pooled because of small sample sizes. (c) The likelihood of extinction based on primary habitat. Human, human-dominated habitats such as farms, plantations, and towns. If forest birds are taken out, global average drops to 16%. (d) Regional distribution of extinction-prone species. Regions displayed in Fig. 5, which is published as supporting information on the PNAS web site, are as follows: A, Austral; C, Cosmopolitan; E, Eastern Hemisphere; F, Afrotropical; I, Indomalayan; M, Malagasy; N, Nearctic; L, Neotropical; O, Oceania; P, Palearctic; S, South Polar; and Z, New Zealand. Each bird is placed in only one region. Two letters indicate combination regions (e.g., NP includes all bird species found in both Nearctic and Palearctic regions).

Fig. 2.

Predicted percentages of extinct and functionally deficient bird species for 2100 based on scenario 2 (intermediate). Threatened and extinct species are considered functionally deficient. Error bars, not used in a conventional sense, indicate averages of 10,000 simulations of scenarios 1 (best case) and 3 (worst case). See Fig. 1 for category details. (a) Distribution based on primary diet. (b) Specialized species are more likely to be extinct (r² = 0.894) and functionally deficient (r² = 0.879). (c) Distribution based on primary habitat. (d) Distribution among biogeographic regions. Some combination regions were excluded for the sake of clarity.