Commentary: Unravelling the mechanisms linking climate change, agriculture and avian population declines

During the last decade or more, much research has attempted to predict the likely consequences of climate change for biodiversity. Many studies have focused on modelling species distributions as a function of climate (climate envelope modelling), and then used future climate projections to model the potential change in the distribution of that species were it closely to track the change in climate. This approach has usefully highlighted the potential magnitude of the conservation problem that may result from climate change (Erasmus et al. 2002, Thomas et al. 2004, Harrison et al. 2006) and identified species traits associated with projected vulnerability to change based on the relative size and overlap of current and projected future ranges (Huntley et al. 2007, 2008, Jetz et al. 2007, La Sorte & Jetz 2010). However, it provides little information about the likely mechanisms by which such changes will occur, and yet this mechanistic information is required to develop appropriate management actions. Identifying and quantifying the mechanisms through which climate change will influence population change is therefore one of the major challenges we currently face. For most species, the main impacts of climate change are likely to operate indirectly, through alterations in key factors such as prey availability (e.g. Frederiksen et al. 2006, Pearce-Higggins et al. 2010), predation risk (e.g. Martin 2001) and disease (e.g. Cattadori et al. 2005). Human land-use is also heavily influenced by climatic conditions, and many indirect effects of climate change are therefore likely to operate through enforced and adaptive changes in agriculture, forestry and other sectors. The potential power of such land-use change to drive population changes is clearly illustrated by consequences of agricultural intensification for European open-country birds (Donald et al. 2001). Changing climatic conditions are very likely to influence agricultural management and drive land-use change, which may increase the pressure on vulnerable bird populations, but these mechanistic links have rarely been examined. In this issue of Ibis, Kleijn et al. (2010) describe how the Black-tailed Godwit Limosa limosa, a rapidly declining bird of intensively managed farmland in the Netherlands, was first affected by agricultural intensification and, more recently, by interactions between climate change and agriculture. This case study neatly demonstrates that climate change does not act in isolation, but interacts with a range of other processes to produce potentially complex consequences for species (e.g. Mustin et al. 2007). This has two important consequences for ecologists. First, whilst broad patterns from climate envelope models indicate the potential direction and magnitude of shifts in species distributions (Green et al. 2008, Gregory et al. 2009), projections for individual species may have a high degree of uncertainty associated with them, particularly if they are based upon generic bioclimatic variables (Heikkinen et al. 2006) or exclude other potential drivers of change (Anderson et al. 2009). This means that, secondly, species-specific studies of the likely mechanisms by which climate change will impact upon populations can provide more detailed information about how individual species are likely to respond to climate change (e.g. Pearce-Higggins et al. 2010), and thus inform adaptive management. Ornithologists have been at the forefront of developing mechanistic models of population-level responses to environmental change (Sutherland 1996, Norris 2004). Identifying the routes through which climate change is likely to influence these processes, as in the Kleijn et al. study, provides a basis both for incorporation of climate change impacts into these models and for the development of potential management solutions to adapt to the effects of climate change. We have long known the problems that Black-tailed Godwits in the Netherlands face as a result of agricultural intensification. This species breeds primarily on lowland grasslands that receive a high amount of fertilization, and are subject to frequent mowing or grazing (Kleijn et al. 2001). Mowing results in the loss of nests and young chicks (Kruk et al. 1997), whilst chicks that avoid direct mortality may also suffer reduced survival as a result of increased predation and reduced food availability (Schekkerman & Beintema 2007, Schekkerman et al. 2009). The severe impact of changes in mowing dates on productivity of Black-tailed Godwits and other meadow birds in Western Europe led to the development of agri-environment initiatives ranging from postponement of farming operations to direct protection of nests. However, these measures have not been sufficient *Corresponding author. Email: james.pearce-higgins@bto.org