Birds as biodiversity indicators for Europe

Transcription

1 Birds as biodiversity indicators for Europe It is a truism that biodiversity is vital. It is, after all, a measure of life on earth. It is of more pressing concern that biodiversity is being lost at a rate unprecedented in the recent era, and will continue to be lost unless action is taken. There is species loss and habitat loss across ecosystems from the Arctic icecap to tropical rainforest: the so-called Fifth Great Extinction of species really does seem to be happening. Richard Gregory looks at the role birds can play in measuring biodiversity. In 2002, the World Summit on Sustainable Development made a commitment to reduce significantly the current rate of biodiversity loss by At the Gothenburg Summit a year earlier, European heads of state made a commitment to halt biodiversity loss in Europe by that same target year, which would seem to imply two things: that we know what biodiversity is, and that we know how to measure it. The first is more or less true. The second is not. These were targets set with no way of knowing whether or not they were being achieved. It is relatively simple to define biodiversity. It is the sum total of all biotic variation from the level of genes to that of ecosystems 1. The definition spans scales from the tiny to the vast. It includes not only species and species abundance, but genetic variation within species. It therefore ranges from genomic to elephant and continent sized, taking in such things as soil microbes, the confused flour beetle and the genome-base of the domesticated banana en route. (The last, incidentally, if you want something extra to worry about, is seriously weak: there is very little diversity in cultivated bananas, and the wild stock has all but gone.) Since a DNA molecule is about a millionth of a millimetre long and a continent can stretch for thousands of kilometres, the constituents of biodiversity stretch across 13 orders of magnitude and all parts of that scale are important. Measuring biodiversity is therefore not easy. We know it is being lost; but at what rate? And which parts of it are being lost fastest and most significantly? Faced with such variety of scales, of habitats, and of living things within those habitats, no single measure is likely adequately to describe biodiversity as a whole. The gauntlet thrown down to ecologists by the summit targets is to develop summary statistics that accurately and robustly describe trends in components of biodiversity in a way that decision-makers can understand and use. But we do not merely lack a measure of biodiversity; remarkably we lack a basic system to measure progress towards these targets. In particular we lack standard measures of biodiversity and procedures to construct and assess the summary statistics involved. We do have population studies, of various creatures, from various parts and various biomes of the globe. They are highly biased, for all kinds of reasons. Surveys tend Farmland: no place for birds? 106 september2006

2 Osprey: its successful comeback says little about biodiversity. Photo: RSPB to be done on interesting animals predominantly large fluffy ones that are rare. There are good statistics on pandas, orang-utangs and Himalayan snow leopards. On insects, we are vague even about how many species there are, let alone how many are being lost. Estimates for existing species vary between 5 and 10 million a rather remarkable degree of uncertainty. Only 1 million of these species have been scientifically described so far. It is also remarkable how little synthesis of wildlife information there has been. Surveys tend to be national, not international, though few wild creatures pay attention to national boundaries. There is similar lack of synthesis across specialisms. There are enthusiasts for amphibians, and for insects, and for flowering plants, and the diversity of all of these is obviously linked, though not necessarily in simple ways. A study in the Netherlands showed that woodland population trends differed between birds, butterflies and fungi. If birds are increasing but fungi are declining, what can we say about biodiversity? A Finnish study has shown widespread declines in plants, birds, butterflies, bees and dung beetles on farmland. In this case, biodiversity loss is more obvious. The world of biodiversity is complex and generalities are hard to come by. We cannot survey everything. Clearly, we need indicators sampled from the whole. As Dow Jones or FTSE indicators summarise financial trends without all the intricate details, so a single species, or a few selected species, may be able to act as representative of whole sections of the natural environment. They give us biodiversity by proxy. They save us from getting lost in the detail. We need statistical procedures to construct and judge such indicators. Economists are familiar with the techniques required; ecologists, perhaps, are not. What has been missing is a recipe-book to help practitioners to create robust indicators for biodiversity. What then makes for a good indicator of biodiversity? For indicators to be effective at a general level they must be able to meet scientific and practical criteria which often compete. Scientific credibility is one: the indicators must be representative, sensitive to environmental change but not unstable, and capable of timely quantitative measurement. They also need to be easily understood by non-experts, and easy and economical to collect. Indicators should also be useful and useable that is, they should give some indication of why the numbers they provide are fluctuating, so that policy decisions can attempt a remedy. The current decline in house sparrow population in the UK is an example. Numbers here have fallen by 64% over 35 years. The reasons for this are still largely mysterious (though suggestions have been made); it is hard, therefore, to know whether they are a reflection of a general biodiversity decline, or a special case. Hence, as a guide for action to conserve biodiversity, the humble house sparrows are not any help. We need to be able to generalise from an indicator, and to establish a link to the driving causes. Without ecological knowledge of what makes a species thrive or decline it is not much use as an indicator. The output we want is a number, or a map, that provides, reasonably correctly, the big picture. It will not give all the answers. Fundamentally, we are looking for a bridge between policy and science. september2006 3(3)_04 Gregory_Birds.indd /08/ :20:35

3 An umbrella species would be ideal: a single species whose thriving or failure can be followed and whose conservation could be expected to confer protection over a larger suite of co-occurring species and their ecosystem. If we save the spotted owl the focus of a long campaign in North America the whole forest system and its inhabitants are saved along with it. It is an attractive idea, and easy to convey to decision makers and to the public. The only downside is that it does not often work. Finding a single species that genuinely represents the diversity of a whole ecosystem is difficult. One might be tempted to focus on the rarities since they have an intrinsic worth: if they thrive, things must be getting better. But it does not necessarily work that way. When we devised an indicator for Britain based on birds, we excluded the rarities because, perversely, they were doing so well. This is actually no surprise or accident because there has been a significant investment in their conservation. The osprey is a classic example. In 1954, there was just a single nesting pair; today there are about 200. It has been a major conservation success, some of it achieved by round-theclock protection of nest-sites. The populations of the 42 rarest native species in the UK, those with fewer than 500 breeding pairs, increased by over 2% between 1973 and 1988; but it would be absurd to pretend that biodiversity has increased by that amount, or at all. So rarities alone do not work as good indicators. Nor are increases among the commonest species alone necessarily good indicators of increasing biodiversity. Paradoxically, they may show the reverse. When ecosystems degrade, a few generalist species that can thrive in a wide variety of man-modified habitats take over from a large number of specialist species, ones which need absolutely precise conditions, or ecological niches to survive. The process of wholesale change is called biotic homogenisation. The diminutive lesser-spotted woodpecker and the larger white-backed woodpecker are extreme specialists in Europe, living in mature forests with a good deal of dead wood, the primary source of their insect food. The skylark is a farm specialist needing sparse open vegetation for nesting in the spring and stubble seeds through the winter. Similarly, the yellowhammer needs the same seed supply in winter, but dense vegetation and hedges for nesting. Many of these birds have fixed and special requirements. Under biotic homogenisation, regional differences in plants and animals begin to disappear, small populations of native species become extinct, and a few generalist species dominate. The chaffinch is a British generalist which is increasing; amongst four-footed creatures, perhaps the ultimate generalist is the rat. So neither the rarest nor the commonest species are reliable indicators of biodiversity. In the focal species approach, populations of several different species are combined to act as umbrellas to protect a community. At this point the thing does start to become useful. We know birds can be good umbrella indicators of the broad state of wildlife and the countryside, on practical and scientific grounds. They are diverse, widespread, and mobile; they are present in all habitats, worldwide; they are high in the food chain, which makes them sensitive to changes lower down if insects decline, so do the birds that feed on them; they are sensitive to persistent pollutants, which become more concentrated as they travel along the chain of consumption; their ecology is well studied and on the whole well understood, so the driving forces behind their fluctuations can be identified. They are also relatively easy to see and to count. Many people all over Europe are willing and able to count them, many of them volunteers who nevertheless have huge expertise. Put simply, there are a fair number of birds, and a lot of skilled bird-watchers. Birds as indicators have another advantage: crudely, they have appeal. The public like birds. Few politicians would go out on a limb to save an unremarkable beetle, but in skylarks and nightingales there is mileage. The danger is that we use birds because we can, and do so without careful thought. There are downsides to birds as indicators. They may not necessarily reflect the health of all the other taxa in their domain. Some are migratory, so can be hard to link populations with specific drivers on the ground. If swallows decline, is the problem in Africa, in Europe, or somewhere en route en route being a km migratory path? In general, birds are middling in body mass, so slower than smaller creatures to respond to change. Some species can be positively counter-indicative: eutrophication of wetlands algal blooms caused by excess nutrients is detrimental to biodiversity in the round, but some wildfowl find it beneficial and their numbers increase. A critical question is the degree to which trends in wild bird populations mirror those in the rest of nature. Here, oddly, the information available to answer the question is thin. A British study found that plants, birds and butterflies declined together, but butterflies by more 2, which suggests that birds are a conservative indicator, erring on the side of complacency; but it is difficult to tell without more detailed studies. However, even given such caveats, birds, for Europe, are about the best indicators of the general state of biodiversity that we have. The pan-european common bird monitoring (PECBM) scheme was set up precisely to explore bird population trends as indicators of biodiversity capable of measuring Sparrows do not help us measure biodiversity 108 september2006

4 Corn bunting: a farmland bird with a message. Photo: RSPB the same woodland/farmland split. It shows (Figure 1) that between 1970 and 2004, populations of woodland species declined by 12%. The decline in farmland species, however, was very Population index (1970 ) All common birds (n=111) Woodland birds (n=33) Farmland birds (n=19) Figure 1. The UK wild bird indicator (source: Royal Society for the Protection of Birds British Trust for Ornithology/Department for Food and Rural Affairs) september2006 3(3)_04 Gregory_Birds.indd progress towards the 2010 targets. It collates data from 18 countries to create an index that can be updated annually and thus provide feedback to policymakers on a reasonable time scale Selecting the right combinations of species is at the heart of it. 33 species of woodland birds were chosen to make up one indicator, and 19 farmland birds for another. The farmland species included skylark, yellowhammer and corn bunting; woodlanders had the goldcrest, pied flycatcher and redstart. The full list is available from PECMB3. These two habitats were chosen because agricultural land and grassland make up roughly 50%, and boreal and temperate forest 30%, of the land surface of Europe4. All the chosen species were reasonably abundant, so counts could be reasonably accurate; all are dependent to some degree on their particular habitat for nesting or feeding. In other words, the generalists were excluded from the habitat indicators. The UK wild bird indicator was a forerunner of the PECBM scheme, using a slightly different selection and number of species but with much greater. Over the same period populations fell by 44%, and this against a background of an increase of all common species of 9%. Our indicator therefore has done its job: it shows very clearly where action most needs to be taken. Decline of wildlife on lowland farmland has emerged as one of the issues in British nature conservation. The driving force behind the decline is agricultural intensification and specialisation of methods. The UK government has responded with a clear plan of action, including the countryside stewardship scheme, which gives incentives and rewards for bird friendly farming practices. Extending such indicators to cover the whole of Europe hits the problem of national boundaries. Birds take no notice of them; bird census-takers do. Of the 18 European countries whose annual breeding bird surveys contribute to the PECBM scheme, some use line transects, some use point count transects, some use territory mapping to count birds; some use combinations of all three. The selection of the plots they sample can be by free, random, or systematic choice; the newer schemes have better designs. Statistical sophistication is needed to combine these in a meaningful way. Nevertheless, the data is there, and it is useable. TRIM (Trends and Indices for Monitoring Data) is a program developed for analysis of count data from wildlife populations (program and manual available for free download from Within each country, for each species, for example the skylark, it uses loglinear Poisson regressions to produce a national index of the percentage increase or decrease in population since a given base year. Each country index is then weighted for that country s sky /08/ :21:22

5 Population index (19 ) European trend of the skylark lark population size before it is combined with skylark indices from other countries; missing counts are estimated from similar sites or countries. The end result is a pan-european skylark index, of the change in their populations year on year (Figure 2). Skylarks, one of the farmland species, fell Europe-wide by % between 19 and Trends for each species are available from The indicators are produced by combining the indices for the 19 or the 33 bird species that make them up. A geometric scale is used to average them: an index increase from to 200 is equivalent but opposite to a decrease from to 50. If more species decline than increase, each at the same rate, then the index goes down, and vice versa. The end result is a measure of change in species composition Figure 2. Pan-European trend of the skylark (source: European Bird Census Council Royal Society for the Protection of Birds BirdLife International Statistic Netherlands) Population Index (19 ) 1 the first-ever pan-european indicators for biodiversity (Figure 3). The indices show that some farmland specialists, such as goldfinch and whitethroat, are increasing. Many lapwings, turtle-doves, corn-buntings are on a downward slide. But the farmland indicator as a whole in 2003 stands at 28, from a baseline of in 19. In other words, two decades have seen a decline of nearly 30% in farm specialist populations. The common non-specialist birds, on the other hand, are up by 28%. The results parallel those of the UK, and seem to show our indicator working well on a wider scale and show also that the process of biotic homogenisation is at work in Europe. European woodland birds are also down, by 13%, again echoing the UK figure. That the Other common birds (25) Common forest birds (33) Common farmland birds (19) decline in farmland species is less severe in Europe than in Britain could be a consequence of more traditional farming practices in the new-accession, former Eastern block, members of the EU Poland, Czech Republic and others and the data can be analysed to check this. On average, there is indeed greater stability of populations in the Eastern countries, and a pronounced East West difference, with both farmland and forest birds doing less badly in the east, and forest specialists actually increasing there. Again, there are obvious implications for policy-makers as the new accession countries bring their agricultural practices into line with those of the West. The threat to birds and other wildlife is real. The indices and indicators will be updated in Autumn 2006, and thereafter annually. Wild bird indicators have already had an impact on policies and the indicators have been taken up as official statistics for the EU. A note of caution is that no single index will capture the state of nature perfectly. Other and more precise information at finer resolutions will be needed to give a more complete suite of indicators for nature; but PECBM has shown the way by demonstrating that population trends of common birds can provide a simple, realistic way for measuring biodiversity targets, and one that has statistical rigour. It took international co-operation on a continental scale; it has measured only one small part of bird biodiversity, which itself is only a tiny fraction of the astonishing diversity of all living things, but it is the first truly representative indicator for biodiversity, and is a great leap forward for nature conservation. References 1. Purvis, A. and Hector, A. (2000) Getting the measure of biodiversity. Nature, 5, Thomas, J. A., Telfer, M. G., Roy, D. B. et al. (2004) Comparative losses of British butterflies, birds and plants and the global extinction crisis. Science, 303, Pan-European Common Bird Monitoring Scheme (2006) State of Europe s Common Birds Prague: CSO/RSPB. 4. Tucker, G. M. and Evans, M. I. (1997) Habitats for Birds in Europe: a Conservation Strategy for the Wider Environment. Cambridge: BirdLife International Figure 3. Pan-European wild bird indicators (source: European Bird Census Council Royal Society for the Protection of Birds BirdLife International Statistics Netherlands) Dr Richard Gregory is head of monitoring and survey and conservation science at the RSPB, based at Sandy in Bedfordshire. He oversees a range of bird monitoring projects and analyses including the annual status report, the state of the UK s birds. He is also Chairman of the European Bird Census Council. 110 september2006