What effect might annual releases of non-native gamebirds be having on native biodiversity?
Henrietta Pringle discusses how she and her colleagues used existing datasets to explore this question and the complex interactions that may drive effects on ecosystems.
We don’t know for certain how many birds are released each year, but estimates based on the National Gamebag Census by the Game and Wildlife Conservation Trust put the figures at around 41–50 million. This equates to around 45,000 tonnes of biomass: the combined weight of these Pheasants and Red-legged Partridges. To give this figure some sort of perspective, this is more than twice the combined weight of all of our native breeding birds. With this quantity of non-native birds being released each year, what effects might this be having on our ecosystem?
How many gamebirds are released?
We don’t know for certain how many birds are released each year, but estimates based on the National Gamebag Census by the Game and Wildlife Conservation Trust put the figures at around 41–50 million. This equates to around 45,000 tonnes of biomass: the combined weight of Pheasants and Red-legged Partridges. To give this figure some sort of perspective, this is more than twice the combined weight of all of our native breeding birds. With this quantity of non-native birds being released each year, what effects might this be having on our ecosystem? We know that game management in general can have positive effects; after all, measures to enhance agricultural habitat for gamebirds are likely to be advantageous for other species. The creation of woodland rides can also be of benefit to butterflies, but what about the impacts of the gamebirds themselves? Firstly, their foraging habits could have effects. For example, they might alter hedgerow structure which is important for nesting birds or reduce the numbers of invertebrates available for other birds to eat. They may also increase disease or parasite transmission. However, what we wanted to look at in this study was whether the extra food resource represented by these millions of gamebirds might have an impact on predatory and scavenging birds that feed on them. We know that the majority of birds released do not end up in game bags, but are predated, scavenged, hit by vehicles, or survive to become part of the breeding population. They are therefore a large additional food resource that is potentially available to other species. Gamebird releases typically occur in late autumn, when other food may be scarce, so is it possible that predator populations are being sustained above levels they would otherwise reach without releases? If this is the case, could this have implications for other species that are vulnerable to increased predation pressure?
Is it possible that predator populations are being sustained above levels they would otherwise reach without releases?
Does the science indicate impacts of gamebirds?
To answer the question of whether gamebird releases have any effect on predator numbers, we were hampered by the lack of available data on exactly how many birds are released in different places. To overcome this, we used a combination of datasets to represent releases. A register of captive gamebirds provided by the Animal and Plant Health Agency gave us the best idea of where gamebirds are held pre-release, while numbers and distributions from the Bird Atlas and Breeding Bird Survey told us about ‘wild’ gamebirds in the countryside, and therefore the actual food resource available to predators. While all of these approaches have their weaknesses – not all birds on the APHA register will be released, and the wild datasets will include birds that have survived post-release and those that have been naturalised from previous releases – together they help to build the best possible picture of releases. Then we tested to see whether there were any links between these releases and numbers and population changes of predators and scavengers: Buzzard, crow (Carrion and Hooded combined), Jay, Magpie and Raven.
By examining how well the ‘wild’ and captive gamebird datasets matched up, we showed, perhaps unsurprisingly, that gamebirds in the wild are largely found where they are held in captivity (and therefore presumably released, Fig. 1). This suggests that our approach is an appropriate one to use. Then, whether we used captive or wild data as the source of gamebird numbers, we found that generally there were more predators (specifically Buzzard, Jay and crows) in areas of high gamebird abundance, and that the predator population growth rates were more positive where there were more gamebirds. In other words, whatever the background change in these predator populations (Buzzard, crows, Jay and Raven) over time, there was an additional uplift where gamebirds were present (Fig. 2).
In contrast, we found that Magpie abundance decreased as gamebirds increased, suggesting a possible interaction with game management and control activities. In areas of intense gamebird release, large-scale predator control may reduce Magpie numbers, or habitat management may not favour them, or perhaps there are negative effects from the increases in the other predators. However, in most cases, predator numbers increased with gamebirds, suggesting both underlying positive effects and that levels of predator control associated with game management generally do not overcome the benefits of the extra food resource provided by gamebird releases.
Have we answered the gamebird question?
Our results suggest that gamebird releases boost predator numbers at a national scale. However, the ecology is very complex: with more gamebirds, there may be more habitat provision, supplementary feeding and predator control, which will all influence predators, gamebirds and other prey in different ways. To separate these different factors would really require an experimental set-up where releases are controlled, which would be extremely challenging, and only useful if it is practical for these aspects of land management to be conducted independently. However, what would really help our future understanding of impacts of releases would be better knowledge of exactly how many birds are being released, and how many predators are being controlled. This could be achieved through making recording these compulsory.
..what would really help our future understanding of impacts of releases would be better knowledge of exactly how many birds are being released, and how many predators are being controlled.
Could gamebird releases be responsible for declines in other birds?
Simply put, we don’t know. To be certain, we’d need to know (1) that releases are inflating predator numbers, (2) that this translates into other birds (e.g. ground nesters) experiencing much higher predation than they would without releases and (3) that the predation has caused populations of these other birds to decline. We’ve found some evidence for (1) but, as mentioned, this is not definitive. There are many factors that shape predator numbers, such as availability of other food, access to good quality nesting habitat and the abundance of, and therefore competition with, other species. Similarly, bird declines may be attributable to many factors, such as agricultural intensification, loss of habitat, or decline in food availability. We know that there are some local effects of reduced predation increasing productivity ground-nesting birds , but national analysis by BTO found little evidence for avian predators causing large-scale declines of a suite of birds. More recently, however, Curlew declines were found to be greatest in areas of high crow abundance, suggesting that increases in predators associated with gamebird releases could have played a role. What is needed now is better recording of release activity to make future studies like ours more definitive, but it is timely to consider the regulation of gamebird releases as a conservation tool, at least on a trial or experimental basis. For example, is it possible for conservation and shooting interests to cooperate so that releases are fewer and take place away from sensitive habitats, or that a higher proportion of birds are shot? We hope further research can shed more light on these kinds of questions.
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