RSPB Scotland's Senior Conservation Scientist Staffan Roos describes the findings of a new study that he and his RSPB colleagues Jennifer Smart, David Gibbons and Jeremy Wilson recently published in the journal Biological Reviews.
People often ask me whether the decline of farmland birds is caused by the increasing numbers of buzzards they see in their local area. Often they follow up their question with "surely, there are too many predators now". Clearly, many people equate rising predator numbers with falling prey numbers, which may not always be the case. These questions also show that people care about what is happening to our birds. For decades, scientists have also been interested in the effects of predators on their prey with several thousand studies published across the world.
Different interest groups often quote their favourite studies that – conveniently – support their "pet theory". But is there a more objective way to make sense of all the different studies, which often reports conflicting results? The answer is "yes", and most scientists do this as a review of the literature, which synthesizes the findings of many studies into an objective overview of the topic. It is becoming increasingly common that these reviews contain statistical analyses that summarise the findings of the many studies, based on the strength of the results, whether the effect is positive or negative and the statistical rigour of each study.
The "effects of predators on prey" is a good subject for such a review, because there are a large number of studies and the topic has not been reviewed recently (but see this RSPB report from 2007). It is also very relevant for the RSPB, because we know that sometimes the species we are trying to conserve are affected badly by predation, and predator management is costly and occasionally ethically challenging. We therefore need to have impartial and evidence-based facts to focus our predator management on species and situations where it could actually make a difference. For a good summary of RSPB's vertebrate control policy, see this blog by Martin Harper, RSPB's Director of Global Conservation.
With all of this in mind, we set out with two aims: to examine whether the claims that the number of predators have increased in the UK was true, and to identify in what circumstances bird species were limited by predation. If you are really interested in how we did the study you can of course look at the methods in our paper, but here we just discuss the results.
What did our study show?
We found that many species of predatory mammal and birds have increased in the UK during the last decades. For many species, especially the birds of prey, this is of course a great conservation success story with many recovering after the ban of certain toxic pesticides, better protection of birds of prey in some, but not all areas and successful reintroduction programmes. But how does the UK compare to other European countries? Well, the density of foxes and crows, two generalist predators often highlighted as important predators of birds, were very high in the UK. In fact, our crow density was the highest and our fox density was the second highest in Europe. The density of magpies, a bird predator often implicated in songbird declines, was intermediate compared to other European countries.
Figure 1. The densities of crows (hooded and carrion crows combined) is higher in the UK than in any other European country according to the new study. The picture shows a carrion crow.
Are high and increasing densities of predators affecting birds in the UK?
Our review found 81 relevant scientific papers and reports. Within these studies, there were 908 cases where the effect of a predator on changes in the numbers of a prey species had been measured. Overall, in 15% of these cases predators had a limiting effect on the prey species. However, there were large and significant differences among different prey when they were organised into functional ecological groups. We found strong evidence for predation limiting seabirds, gamebirds and waders, with 81, 43 and 25% of cases, respectively, showing that predators had a negative and population limiting effect on these groups of birds. We also showed that the species limited by predation shared similar ecological and life-history characteristics. For example, they tend to only make one nesting attempt per year, they begin breeding late in life, they are long-lived species and they are ground-nesting. We found no evidence of predation limiting pigeons, raptors & owls, woodpeckers and songbirds.
Figure 2. Waders, such as this lapwing, was one species group reported to be limited by predation. They nest on the ground and their chicks are flightless for around 30 days, which makes their eggs and chicks vulnerable to both mammalian and avian predators.
Which predators were reported to limit prey numbers?
We were also able to look at which predators were responsible for limiting bird populations. When foxes, crows and other predators were managed simultaneously, compared to when only a single species of predator was managed, this was more likely to result in increasing populations of the prey species. This suggests that there is strong evidence for "compensatory predation". This is when the declining density of one predator, perhaps through active management of that predator, does not result in higher prey numbers, because a different predator "steps in" and depredates the prey in the absence of the original predator. We also found strong evidence that non-native predators, notably American mink, brown rat and hedgehog (the latter two being non-native on many Scottish islands) can limit their prey via predation on mainly eggs and nestlings. Interestingly, the number of cases showing that grey squirrels and birds of prey could limit their prey was rare.
Figure 3. The density of red fox is very high in the UK compared to other European countries. The new study showed that there was strong evidence that red foxes could limit populations of ground-nesting birds. However, the evidence also suggested that management of foxes alone was not as effective in protecting ground-nesting birds as when foxes, corvids and other predator species were managed simultaneously.
How will RSPB use the results?
The very high densities of foxes and crows in the UK suggests that our countryside is capable of supporting large numbers of these predators, and we currently spend a lot of time and money managing these predators through fencing and lethal control. An alternative approach would be to try to understand why we have such high densities compared with other parts of Europe and then to manage aspects of the landscape to limit opportunities for these predators. We discuss possible landscape-wide approaches in our paper. For example, patches of woodlands and forestry can provide breeding areas for generalist predators, and can therefore increase the predation pressure on birds nesting in the surrounding landscape and there is some good evidence from studies to show that waders avoid nesting close to woodlands. In addition, the near-absence of top predators in many parts of the UK (e.g. golden eagle, goshawk, lynx and wolf, just to mention some very charismatic species that are widespread in continental Europe) means that the natural "top-down" control of intermediate-sized predators (i.e. intra-guild predation) is just not happening at the same scale as in mainland Europe. Similarly, the release of around 50 million gamebirds into the UK countryside every year for shooting purposes provide lots of food for predators like foxes, thereby increasing the numbers of these predators that can survive and thrive in our landscape. In spring, when gamebird numbers are at their lowest, it is likely that foxes and other generalist predators have to turn to native species to feed themselves and their young, which add pressure on ground nesting birds of conservation concern. In our paper, we argue that better landscape planning and regulation of non-native gamebird releases, as well as encouragement of natural intra-guild predation processes might be a more effective, cheaper and sustainable solution to reduce the populations of generalist predators. However, this is not a "quick fix", and the traditional techniques of lethal control and predator fencing to minimise predation on key bird populations might be the only remedy in order to protect what we have in the short-term.
We expect that these results will be used to make our predator management more focused, to help us to communicate our policies around predator management to interested parties in a better way, and to generate new avenues for research that could help us to provide these longer-term sustainable landscape solutions.
The full reference to the paper:
Roos, S., Smart, J., Gibbons, D. W. and Wilson, J. D. In press. A review of predation as a limiting factor for bird populations in mesopredator‐rich landscapes: a case study of the UK. Biological Reviews. https://onlinelibrary.wiley.com/doi/abs/10.1111/brv.12426.
Staffan Roos is a Senior Conservation Scientist with the RSPB's Centre for Conservation Science. Staffan is working mainly on the ecology of raptors, predator-prey interactions, and conflicts between raptor conservation and shooting interests. An overview of his publications are found on Google Scholar.