In the 1960s, Rachel Carson preoccupied (to put it mildly) the political and economic powers of the United States with the publication of her book, Silent Spring, an overwhelming essay highlighting the environmental impacts caused by the pesticide industry. Half a century later, the metaphor of that book – a spring devoid of bird song – stands in force as farmland birds keep declining worldwide at the mercy of agricultural practices insensitive to the ecosystem services biodiversity procures. The problem has been best studied in Europe where non-government organisations, such as the British Trust for Ornithology or the Sociedad Española de Ornitología, have been monitoring bird populations for decades, and the European Union has sumptuously financed research and management actions.
Sparrows are the commonest among common birds. Worldwide we see them wherever humans live, travel and take leisure, from mountains and beaches to stadiums and underground trains. These feathery dwarfs snick even through security checks and sliding doors at airports and shopping malls, and it is no one’s surprise to be overflown by one of them carrying a chip from the leftovers of a nearby food court. However, the deterioration of their populations has sown alarm among European politicians, society, and the scientific community.
As an example, Hole and colleagues1 studied four genetically isolated populations of house sparrow (Passer domesticus) on Oxfordshire farmlands (United Kingdom). The number of individuals of one of the populations (University Farm) had decreased by 80% between 1960 and 2000, while the other three populations remained stable. During the coldest months over several winters, the four populations were fed ad libitum, with the number of survivors being counted between several consecutive winters before and after food addition. At the end of the experiment, supplementary feeding matched with an increase in survival only in the historically declining population (Figure 1).
The British biologists hypothesized that the population at University Farm was limited by overwinter food supply potentially resulting from agricultural intensification1. The broader picture numbers the British population of house sparrows at ~ 13 million individuals, after declines by 50 and 60% in rural and urban areas, respectively, since the 1970s2 (Figure 2). This is one among a deluge of cases giving evidence that sustained increases in crop yield per surface area (by means of intensive irrigation, fertilisation and chemical protection) is negatively impacting a variety of resources vital for many farmland birds3. Thus in the last three decades, the censuses of common farmland birds in the European Union (~ 120 species) have shown a sustained reduction4,5 (Figure 2). John Krebs refers to this phenomenon as ‘the second silent spring’6, alluding to Carson’s metaphor, which is affecting birds and the entire biodiversity of our farmlands7,8.
Larger yields, poorer environment
Human beings have turned more than half of the planet’s surface into agriculture, and lifestock has altered most of the remaining land9. In developed countries, the area devoted to crops and pastures has stabilized, but it keeps expanding in developing countries9. Whether developed or developing, agricultural intensification is a major agenda for the majority of countries, partly in an attempt to feed a human population currently nearing 7 billion and estimated to escalate to 9 billion by 205010.
The challenge of food security (and a fair distribution of it) for so many people compromises the environment. I recently touched on how the health of bee populations can determine the quantity of produce. Well, the health of birds is an indicator of the environmental quality of our countryside and indirectly, of the food we consume. Thus in Europe, the worst historical censuses of farmland birds have been observed in countries such as Belgium, France, The Netherlands and the UK where agricultural intensification has rocketed in the last five decades, particularly cereal production4.
The European Union has reacted to the surmounting scientific evidence by adopting those censuses as one of its key indicators assessing the Sustainable Development Strategy. In addition, the Common Agricultural Policy (CAP) has created, inter alia, agro-environment schemes which compensate farmers for following practices that respect the environment, like fertilisation caps or crop timing adjusted to the breeding cycle of birds. However, measures that work for one species at one farm in a given country might well be unsuccessful in others, so agro-environment schemes have heated polemics11-13; nonetheless, the PAC amalgamates a growing 50% of the total billionaire budget of the European Union!
As in many other management scenarios, conservation measures are most effective, not only as a function of political will, but when the causes that drive population decline can be identified. Newton14 has recently overviewed the range of factors unequivocally linked to agricultural intensification that reduce bird reproductive performance and/or survival.
(i) herbicides and intensive lifestocking wipe out invertebrate populations which are prey to insectivore and omnivore species (e.g., cirl buntings Emberiza cirlus);
(ii) subsequently the reduction in those birds cascades to their predators (e.g., peregrine falcons Falco peregrinus);
(iii) delays in sowing favours plant overgrowth which precludes nesting (e.g., skylark Alauda arvensis);
(iv) ploughing destroys nests in the breeding season (e.g., stone curlew Burhinus oedicnemus);
(v) land drainage and grassland management contracts the habitat for waders (e.g., lapwings Vanellus vanellus); and
(vi) the destruction of hedgerows, bush and forest patches delimiting the crops deprives some species of vital food (e.g., song thrushes Turdus philomelos), shelter from predators (e.g., curlews Numenius arquata), and nesting sites (e.g., turtle doves Streptopelia turtur, along with collapse of abandoned cottages important for species like barn owls Tyto alba).
Other causes have been identified; however, the overall picture, most unsurprisingly, suggests that practices that reduce habitat heterogeneity, clash with reproductive periods, and/or release pollutants and fertilisers to the environment at a large scale, are doomed to mute future springs.
1Hole, D. G. et al. Widespread local house-sparrow extinctions. Nature 418, 931-932, doi:10.1038/418931a (2002)
2Robinson, R. A., Siriwardena, G. M. & Crick, H. Q. P. Size and trends of the House Sparrow Passer domesticus population in Great Britain. Ibis 147, 552-562, doi:10.1111/j.1474-919x.2005.00427.x (2005)
3Wilson, J. D., Evans, A. D. & Grice, P. V. Bird conservation and agriculture: a pivotal moment? Ibis 152, 176-179, doi:10.1111/j.1474-919X.2009.00992.x (2010)
4Donald, P. F., Green, R. E. & Heath, M. F. Agricultural intensification and the collapse of Europe’s farmland bird populations. Proc. R. Soc. B-Biol. Sci. 268, 25-29, doi:10.1098/rspb.2000.1325 (2001)
5Donald, P. F., Sanderson, F. J., Burfield, I. J. & van Bommel, F. P. J. Further evidence of continent-wide impacts of agricultural intensification on European farmland birds, 1990-2000. Agriculture, Ecosystems & Environment 116, 189-196, doi:10.1016/j.agee.2006.02.007 (2006)
6Krebs, J. R., Wilson, J. D., Bradbury, R. B. & Siriwardena, G. M. The second Silent Spring? Nature 400, 611-612, doi:10.1038/23127 (1999)
7Flynn, D. F. B. et al. Loss of functional diversity under land use intensification across multiple taxa. Ecol. Lett. 12, 22-33, doi:10.1111/j.1461-0248.2008.01255.x (2009)
8Kleijn, D. et al. On the relationship between farmland biodiversity and land-use intensity in Europe. Proc. R. Soc. B-Biol. Sci. 276, 903-909, doi:10.1098/rspb.2008.1509 (2009)
9Green, R. E., Cornell, S. J., Scharlemann, J. P. W. & Balmford, A. Farming and the fate of wild nature. Science 307, 550-555, doi:10.1126/science.1106049 (2005)
10Godfray, H. C. J. et al. Food security: the challenge of feeding 9 billion people. Science 327, 812-818, doi:10.1126/science.1185383 (2010).
11Kleijn, D. & Sutherland, W. J. How effective are European agri-environment schemes in conserving and promoting biodiversity? J. Appl. Ecol. 40, 947-969, doi:10.1111/j.1365-2664.2003.00868.x (2003)
12Kleijn, D. et al. Mixed biodiversity benefits of agri-environment schemes in five European countries. Ecol. Lett. 9, 243-254, doi:10.1111/j.1461-0248.2005.00869.x (2006)
13Donald, P. F. et al. Response to Comment on “International Conservation Policy Delivers Benefits for Birds in Europe”. Science 319, 1042, doi:10.1126/science.1151939 (2008)
14Newton, I. The recent declines of farmland bird populations in Britain: an appraisal of causal factors and conservation actions. Ibis 146, 579-600, doi:10.1111/j.1474-919X.2004.00375.x (2004)