Foxes & agriculture: Foxes and conservation
Foxes can be a serious threat to native fauna in countries where they have been introduced. However, in countries where they are native, as in majority of the northern hemisphere - including Eurasia, North America and Japan - foxes are a natural predator and pose little threat to the survival of most wildlife.
Effect on endangered species/game birds
A recent report of the Royal Society for the Protection of Birds summarised the available evidence about the effect of predation on wild birds in the UK. The mammalian predators included fox, hedgehog, American mink and grey squirrel (these last two are introduced species in the UK); predatory birds included buzzard, goshawk, hen harrier, peregrine falcon, sparrow hawk, carrion and hooded crows, various gulls, jay, magpie and great spotted woodpecker.
The report found growing evidence that populations of some ground-nesting birds (e.g. curlews, golden plovers, lapwings) and game birds (grey partridges, capercaillie, black grouse, red grouse) are likely to be limited by predation, including by fox predation, perhaps because the nest or young of these species are more vulnerable to predation. In contrast, evidence that predation limits songbirds numbers is, at best, weak.
The best evidence that predators reduce prey numbers comes from studies running experiments at different sites, where at some sites predators are removed but not in others. It is however, important to bear in mind that such studies might be more likely to take place on species or populations that are already thought to be limited by predation (and therefore are more likely to find a positive result). The RSPB report summarised the findings of two reviews on these experimental studies.
In this review 30 studies were examined: some studies removed only some predators, and others removed all predators. The review found that nest survival of prey improved in 85% of studies, prey autumn density (post-breeding population size) increased in 71% and breeding numbers in the following year increased in 59%. Most of the species studied were ground-nesting birds, such as game birds and waterfowl, which, as shown above, might be more vulnerable to predation than other species.
Review with a formal statistical analysis
However, a second review came to a different conclusion. This included many of the studies in the first review, but applied a statistical method that examined not only whether the parameters considered (i.e. prey nest survival, autumn density and breeding numbers the following year) had changed, but also by how much and whether the results of individual studies were reliable.
While predator control increased the survival of young birds on the nest and the size of autumn prey populations, it did not effectively increase the size of breeding prey population the following year. In other words, predator removal had a short-term effect on bird populations that could meet the needs of game management by providing more birds for the coming shooting season, but there were no reliable long-term conservation benefits through long-term increases in population size.
Furthermore, predator removal was effective in increasing population size of stable or increasing prey populations but was ineffective for decreasing prey populations which, in fact, declined further following predator removal. This shows that factors other than predation affect prey population size and that predator removal could be effective in the long-term only in specific cases, such as on island populations.
Other causes of bird declines
There are other factors associated with the decline of bird species in the UK not related to predation, such as habitat change, loss and degradation, and increased use of pesticides. For example, changes in farmland management, such as the loss of hedgerows that provide important nesting cover, high livestock densities (that can trample chicks of ground-nesting birds) and changes in sowing practices are known causes of declines of birds such as skylarks, grey partridges and many more.
Habitat and agricultural changes may also affect bird numbers by reducing their ability to compensate for predation. For example, the switch from spring to autumn sowing that occurred in the UK over the last few decades has reduced the length of the skylark's breeding season and thus the time available to re-nest if the first brood fails. Shorter breeding seasons due to habitat change are also known for snipe and song thrush, whilst it has been suggested that removing deer fences, which are responsible for adult capercaillie deaths due to collisions, would increase chick production by increasing adult survival and thus compensate for losses to predation.
Lastly, habitat changes may affect predator numbers. Fragmented habitats host larger numbers of generalist predators and many studies have shown that predation is favoured near forest edges or in small forest patches compared to inner areas in larger forests. For instance, it is possible that foxes and crows live at higher densities in fragmented than in intact pinewood forests and this may have increased predation on capercaillie nests. Reducing forest fragmentation would lower predator densities without the need for predator control.
In conclusion, predator control seems to be effective only in increasing bird populations in the short-term but to be ineffective in the long-term and even detrimental for bird populations that are already declining. The effect of changes in habitat and agricultural management are known causes of bird declines and more research is needed to implement alternative methods to predator control to boost bird populations.
- Baines, D., Moss, & Dugan, D. (2004) Capercaillie breeding success in relation to forest habitat and predator abundance. Journal of Applied Ecology 41, 59-71.
- Chamberlain, D.E., Wilson, A.M., Browne, S.J. & Vickery. J.A. (1999) Effects of habitat type and management on the abundance of skylarks in the breeding season. Journal of Applied Ecology 36, 856-870.
- Côté, I.M. & Sutherland, W.J. (1997) The effectiveness of removing predators to protect bird populations. Conservation Biology 11, 395-405.
- Gibbons, D.W., Amar, A., Anderson, G.Q.A., Bolton, M., Bradbury, R.B., Eaton, M.A., Evans, A.D., Grant, M.C., Gregory, R.D., Hilton, G.M., Hirons, G.J.M., Hughes, J., Johnstone, I., Newbery, P., Peach, W.J., Ratcliffe, N., Smith, K.W., Summers, R.W., Walton, P. & Wilson, J.D. (2007). The predation of wild birds in the UK: a review of its conservation impact and management. RSPB Research Report no 23. RSPB, Sandy.
- Green, R.E. (1988) Effects of environmental factors on the timing and success of breeding of common snipe Gallinago gallinago (Aves: Scolopacidae). Journal of Applied Ecology 25, 79-93.
- Newton, I. (1993) Predation and limitation of bird numbers. Current Ornithology 11, 143-198.
- Peach, W.J., Robinson, R.A. & Murray, K.A. (2004) Demographic and environmental causes of the decline of rural song thrushes Turdus philomelos in lowland Britain. Ibis 146 (Suppl. 2), 50-59.
- Summers, R.W., Green, R.E., Proctor, R., Dugan, D., Lambie, D., Moncrieff, R., Moss, R. & Baines, D. (2004) An experimental study of the effects of predation on the breeding productivity of capercaillie and black grouse. Journal of Applied Ecology 41, 513-525.