Anyone’s game: do gamebird releases lead to increases in generalist predators?

In their new research, published this week, Henrietta Pringle and colleagues recommend the recording of gamebird releases and predator numbers.

Every year, 40-50 million non-native gamebirds (ring-necked pheasant Phasianus colchius and red-legged partridge Alectoris rufa) are released in the UK, equivalent to around 46000 tonnes of biomass. Fewer than half these birds are shot, with the remaining birds predated, scavenged or surviving to breed or to be predated in subsequent years. This means there is potentially a large food resource available to predators and scavengers; a resource that has increased year-on-year as the numbers released have grown.

By removing the density dependence of ‘natural’ predator-prey systems, gamebird releases could act to sustain predator and scavenger populations above the levels they would normally reach without such injections of biomass, particularly as releases generally occur in late autumn, when other food sources in the wider countryside may be scarce. If gamebird releases do inflate predator numbers, this might alter predator-prey dynamics, increasing predation pressure on some vulnerable species. While other aspects of game management, such as measures to enhance habitat and food availability, have been reported to have benefits for wider biodiversity, the impacts of the releases themselves are relatively understudied, despite being flagged as a critical policy-relevant question and subject to frequent discussion in the scientific literature and media.

002873-Red-legged Partridge-Harding
Red-legged partridge by John Harding (BTO)

Our aim in this study was to examine evidence for the first link in the chain; whether gamebird releases have any effect on numbers of avian predators and scavengers (hereafter predators), namely buzzard, jay, magpie, raven and combined numbers of carrion / hooded crow. In the absence of spatially precise, quantitative data on gamebird releases, we used three different datasets as proxies. A register of gamebirds held in captivity provided by the Animal and Plant Health Agency (APHA) probably informs best about the numbers actually released, although not precisely about release locations, while data from the Bird Atlas 2007-11 and the BTO/JNCC/RSPB Breeding Bird Survey (BBS) give the abundance and distribution of ‘wild’ gamebirds in the countryside. These ‘wild’ datasets are more removed from the numbers released, as the free-roaming gamebird populations will comprise those that survive after release and the shooting season, and those that have naturalised from previous releases. They do, however, more accurately represent the resource available to predators than the APHA data.

After controlling for variation in land-use and habitat quality, we found numerous positive associations between the abundance of gamebirds (both reared and ‘wild’) and predators. In particular, buzzard and jay were positively associated with pheasant numbers, whether these Pheasants were ‘wild’ or captive.  Wild gamebird abundance (pheasant, red-legged partridge and both combined) were also positively associated with numbers of crows. Population growth rates told a similar story; for buzzard, jay, crows and raven, the rates of year-to-year population change were all more positive in areas of high gamebird abundance. Note that this does not mean that populations of these species necessarily increased where more gamebirds were present, but that the rate of change was more positive: a species could still be in decline, but at a slower rate where there are more gamebirds.  Analysis of associations between reared and ‘wild’ gamebirds indicated that patterns of spatial variation in the abundance of free-roaming, non-native gamebirds across Britain are determined by gamebird releases, more so than they are influenced by variation in land-use or habitat. This gave us confidence that the associations between predators and ‘wild’ gamebirds reflected associations between predators and releases.

We also found a negative association between the abundance of magpies and wild gamebirds, suggesting an interaction with game management and control activities; in areas of high gamebird release, large-scale predator control may reduce predator numbers. However, the predominance of positive associations between predators and gamebirds suggests levels of control associated with releases generally do not overcome the positive effects of resource provision. An alternative explanation could be that in areas of high gamebird abundance, magpies experience higher competition and nest predation from crows, leading to the observed negative association.

While there are many other factors shaping predator abundance, such as fine-scale habitat variation, availability of other food sources, and game management activities, our results suggest that large-scale variation in avian predator populations is positively affected by gamebird releases. This could have implications for species vulnerable to predation, like curlew, but these indirect effects of releases would need to be thoroughly tested. Such tests could include regulation of releases on a trial basis, to determine effects on ground-nesting birds, for example. As a first step towards a better understanding of the impacts of releases, and the complex interactions acting on them, we suggest that the compulsory recording of releases and the number of predators controlled would be valuable.

The full article, Associations between gamebird releases and general predators is free to read for a limited time in Journal of Applied Ecology.

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