This graph has popped up in my timeline a few times over the past few days. It presents a comparison between the biomass of native British breeding birds and the biomass of two species of gamebird, Ring-necked Pheasant and Red-legged Partridge, released into the British countryside each year for the shooting industry. It shows that the biomass of all these gamebirds is around 50% higher than that of all native breeding birds combined. It presents a stark illustration of just one impact that humans are having on the British environment.
As the biomass estimates come from a paper I co-wrote with my long-term collaborator Kevin Gaston (freely available here: https://link.springer.com/journal/10530/20/12/page/1), I am naturally interested in how they are being used. The graph as presented is indeed dramatic, but does mask some analytical issues that I thought it might be worth exploring in a short(ish) blog post.
First off, let’s go through how we calculated the biomass of native breeding birds. Britain is almost unique in the world in having a good enough knowledge of our native birds – thanks to organisations like the BTO and RSPB, and hundreds of dedicated birdwatchers – to be able to produce pretty good estimates of the breeding populations of every species in the country. They are regularly published, most recently by Musgrove, Aebischer, Eaton et al (2013; Population estimates of birds in Great Britain and the United Kingdom. British Birds106:64–100). These estimates may take the form of the number of breeding individuals, pairs, breeding females, territories or nests, but we assumed we could double all bar the first type to calculate the number of breeding individuals for each species. For each species, we then multiplied the number of birds by an estimate of the average body mass of an individual of that species. This gives an estimate for the biomass of each species, which we then added up across all breeding species to give our estimate for the total biomass of native breeding birds – the green bar in the histogram.
We followed the same process to calculate the breeding biomass of alien bird species – that is, species that are not natural components of the British bird fauna, but have populations here because they have been introduced by humans. Pheasants are a good example of this – the species naturally occurs in East Asia, but has been introduced into the British countryside for centuries to enhance the local opportunities for hunting tasty birds. However, Britain has quite a few other alien bird species, including the Indian Ring-necked Parakeets so familiar to Londoners, and the North American Ruddy Duck, whose ongoing eradication for conservation reasons enflames opinions in some quarters.
Most alien bird populations in Britain are the result of historical activities in releasing species from captivity (deliberately or accidentally) that (should) no longer happen, but the situation with alien gamebirds is different. Every autumn, tens of millions of birds, mainly Ring-necked Pheasants and Red-legged Partridges, are released into the British countryside from the pens where they have been reared, so that they can be shot for sport. Estimates of their numbers come from other sources (e.g. Bicknell et al. 2010. Impacts of non-native gamebird release in the UK: a review. RSPB Research Report Number 40). We can nevertheless calculate the biomass of these releases as we did for breeding birds, and doing so gives the brown bar in the histogram.
Having described how we derive the numbers, a few points then on the histogram drawn using them.
First off, as I have mentioned in tweets, the comparison in the picture is not strictly a fair one. It compares breeding populations with total numbers of autumn releases. This is not like with like. In fact, we do not know what the total numbers (and hence biomass) of native birds might be in autumn. At that time of year, a lot of young birds will have been added to the population. Some species raise more than one brood over summer. Youngsters may form a substantial part of the total population (though most will not see Christmas). Some breeding adults may have died during their efforts to produce those young – childcare is hard work, and there are many predators looking to feed their young as well (and let’s not even start on domestic cats). A lot of birds also do not necessarily even try to breed in any given year (or don’t find a mate), and how many of these “floating” individuals there are is simply not known for most species. All of this means that the total biomass of native breeding bird species in Britain at the time when gamebirds are released is probably much higher than the breeding biomass – taking a stab in the dark, potentially twice as much, or maybe even more.
The second point to make is one that I hope you are now starting to realise – our estimates are estimates, and as such couched in uncertainty. For all but the rarest species (which matter least in terms of their biomass contribution), we do not actually know how many breeding birds there are in Britain, so we are basing our analyses on our best guesses. These guesses are generally pretty good ones, we believe, but there are always going to be uncertainties. We try to mimimise those by comparing like with like as much as we can, but we cannot eliminate uncertainty. It is part of science, and coping with it one of the achievements of scientific endeavour. But, our conclusions should always be informed by it.
Take body mass. We have estimates of the size of all British bird species, but these are going to be based on quite small samples of all the birds that have ever lived. How accurate they are is going to depend on how representative those samples are. For rare species this probably doesn’t matter much, but for common species it will. In fact, the mass estimates we used were largely based on females – for some species, males are significantly larger, for others males are significantly smaller.
Overall, it will be errors in the estimates of the commoner, larger-bodied species that we would expect to have the biggest effect on our biomass estimates – species like Pheasants. The impact of these effects could go either way, in terms of the perceived contribution of Pheasants to British bird biomass. Pheasants are one species where males are somewhat larger than females, so depending on what proportion of the birds released in autumn are males, the brown bar in the histogram could potentially be a substantial underestimate of the biomass of those gamebirds.
The third point is that we can quibble about details – and as a scientist, that is something that I am duty-bound to do – but this quibbling does not alter larger picture. Alien gamebirds represent a substantial proportion of the biomass of birds in Britain. Comparing the breeding populations – the fairest comparison we can make with our data – around 20% of the mass of British birds is aliens, and most of this biomass consists of Ring-necked Pheasants and Red-legged Partridges. The massive autumn gamebird releases mean that this proportion is probably a lot higher at that time of year, before shooting, roads, natural predators, and winter take their toll and bring the population down to its breeding level the following spring.
All of this mass of birds cannot but have massive impacts on the natural environment, though the plants and insects they consume, their competition for those resources with native species, the diseases they harbour and help spread, their effects on predator populations, and the knock-on effects of those predators on other species. They will also have effects through the ways in which shooting concerns manage the environment to the benefit of these birds, though the provisioning of food and cover, or the control of predators. These effects will benefit some native species, and be detrimental to others. At the moment, however, we have little real idea what those effects are, or what the British countryside would look like if we did not have the gamebird industry. Regardless of your views about hunting as a pastime, surely that is something we should try to understand.