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Editor Commentary

Does predator control alter bird populations?

by Mark Whittingham

The answer from three recent papers in the Journal of Applied Ecology suggests the answer is sometimes! It is tempting when one witnesses a magpie Pica pica taking chicks from a nest in your garden or talks to a land manager who has witnessed a carrion crow Corvus corone or a stoat Mustela ermina decimating a breeding colony of birds, to assume a great deal of importance to the effects of predators in controlling populations of their prey. Prey species are often protected either for conservation or for economic purposes (e.g. grouse shooting) and predator control is an important potential option for a land manager.

Past theoretical and empirical studies suggest that the general effects of predation on birds are likely to vary due to a number of factors. For example: some bird species are more susceptible than others to predation; the range of alternative prey species available is important in determining predation rates on any given species; and the response by predators to increased prey density can vary (e.g. be either functional, increased prey density results in individual predators eating more prey, or numerical, more predators are attracted to an area with higher prey density) (for review see Newton 1998). Furthermore, in certain circumstances bird population levels may be regulated by predation and in others limited by predation; these relationships are dependent on other factors, such as habitat, i.e. in ‘better’ habitats predation rates are lowered because prey can evade predators more easily. In short, past work suggests that the effects of predator control on prey populations may be context specific to a particular species, habitat or location.

The three papers just published in Issue 2 of the Journal of Applied Ecology do indeed suggest context-dependent results. Fletcher et al. (2010) present clear-cut evidence from a large-scale, eight-year, replicated experiment that reductions in carrion crows and foxes Vulpes vulpes led to three-fold increases in breeding success of waders (namely, lapwing Vanellus vanellus, golden plover Pluvialis apricaria and curlew Numenius arquata) and red grouse Lagopus lagopus, and subsequent population increases in all four species. This suggests that on UK moorland the control of foxes and crows can have clear positive effects on prey populations and can be used in conjunction with habitat management (e.g. rotational burning of moorland – Tharme et al. 2001) to enhance densities of a range of important bird species.

The two other papers both report a lack of significant (correlative) relationships between predator populations and their prey. Amar et al. (2010) did not find significant relationships between changes in numbers of raven Corvus corax over a twenty year period and a range of breeding upland waders densities (namely golden plover, lapwing, dunlin Calidris alpina, curlew and snipe Gallinago gallinago) although two relationships were close to being significantly negatively linked. Newson et al. (2010) report a general lack of association between 22 avian prey species and their main predators (grey squirrel Sciurus carolinensis, Eurasian sparrowhawk Accipiter nisus , common kestrel Falco tinnunculus and common buzzard Buteo buteo, carrion crow, magpie, Eurasian jay Garrulus glandarius and great spotted woodpecker Dendrocopos major).

These studies highlight two general points. First, similar predators which prey on wader nests in the uplands (e.g. carrion crow and raven) may have differing effects on their prey populations. Secondly, the same species (e.g. carrion crow) can have a significant effect on some species in upland areas (generally >300asl) but not on a wide range of other species mainly in lowland areas. Policy-makers and land managers should be wary of ‘predator control’ as a general tool without the context-specific evidence to back it up.

Mark J Whittingham, Editor, Journal of Applied Ecology