Trends in Ecology & Evolution
ReviewIncreasing awareness of avian ecological function
Section snippets
Birds as ecological actors
Birds are the best-studied class of organism and various investigations have established their significance as important mobile links (see Glossary) in the dynamics of natural and human-dominated ecosystems (Box 1; Figure 1) 1, 2, 3, 4, 5, 6. Birds also benefit humans by providing important ecosystem services [7], such as: provisioning services via game meat for food, down for garments and guano for fertilizer; regulating services by scavenging carcasses and waste, by controlling populations of
Genetic linkers
A hornbill (Figure 1a) swallowing fruits and defecating viable seeds away from the parent tree and a hummingbird (Figure 2b) pollinating flowers while foraging are important ‘genetic information linkers’ [1] because they transport plant genetic material via seed dispersal and pollination, respectively. These functions also result in ecosystem services for humans. Birds pollinate dozens of crop species [26] and avian seed dispersal is particularly important for big-seeded tropical tree species
Resource linkers
By transporting minerals and nutrients in their guano, birds can be vital resource linkers [1], particularly between marine and terrestrial [6], and between terrestrial and wetland [11] ecosystems. This ecological function provides the ecosystem service of crop fertilization, which can occur thousands of kilometers away from the original source of the nutrients.
Trophic process linkers
Birds that are trophic-process linkers connect habitats by serving as primary (Figure 1, Figure 2) or secondary (Figure 1, Figure 2) consumers across habitats [1]. The ecological function of predation is considered an ecosystem service if avian predators reduce agricultural pests and increase yields [5] or if they limit pest activity through fear [23]. Scavengers provide sanitary services, such as carcass disposal, waste recycling, indirect population control of scavenging mammalian disease
Non-trophic process linkers
Birds that are non-trophic process linkers (Figure 1, Figure 2) supply or facilitate an essential process that influences ‘the physicochemical environment rather than the trophic web’ [1] and are analogous to allogenic ecosystem engineers [83]. Such birds can indirectly generate ecosystem services, for example by providing frugivorous and predaceous birds with nesting cavities and nectarivorous birds with food resources [16].
Conclusions
Birds are important but ecologically little-known actors in many ecosystems. Avian seed dispersal might be the ecological function that affects the greatest number of species, especially considering its importance for late successional tropical trees with large seeds. Consequently, studies of the botanical implications of large avian frugivore extinctions are sorely needed. Compared with seed dispersal, bird pollination is an order of magnitude less common, but still important in regions such
Acknowledgements
Christensen, Koret, Moore Family and Winslow Foundations, National Geographic and Wildlife Conservation Societies, and Walter Loewenstern funded the research that inspired this paper. I am grateful to D. Wheye for creating such an excellent illustration. I thank K. Al-Khafaji, G.C. Daily, P.R. Ehrlich, W.F. Laurance, C. Peterson, R. Pringle, S. Renner and N.S. Sodhi for their valuable comments, and C. Kremen, K.A. Lythgoe, R. Nathan and an anonymous reviewer for their helpful reviews.
Glossary
- Ecosystem engineers
- organisms that directly or indirectly modulate the availability of resources to other species by causing physical state changes in biotic or abiotic materials [83].
- Ecosystem services
- the benefits that humans obtain from ecosystems [7]. If an avian ecological function, such as insect consumption, results in material benefits for humans, such as pest control, it is also considered an ecosystem service.
- Functional group
- the primary diet-based grouping that parallels the main
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