The altitudinal frontier in avian climate impact research

Climate change has already had profound effects on biodiversity in a range of habitats across the globe (Parmesan & Yohe 2003), including significant impacts on the distribution, abundance, productivity and survival of a range of bird species (e.g. Crick 2004, Gregory et al. 2009). However, evidence for effects in certain habitats and regions, including those that may be expected to experience the most profound impacts (e.g. EEA 2010, Harris et al. 2011), is limited, largely due to logistical and economic constraints. High mountains are likely to be particularly vulnerable to climate change: the rate of warming in mountains has been twice the global average (Brunetti et al. 2009) and increasing temperatures, higher snow lines and lower snowfall, evident over the past century, are expected to continue (EEA 2010). The biodiversity of these habitats includes many specialists and species of conservation concern that are vulnerable due to large climatic variation over small spatial scales (e.g. on average, a 100-m increase in altitude equates to a )0.5 C change in temperature; Moser et al. 2010). A likely consequence of climate change in mountain regions is that vegetation zones will shift upwards, a phenomenon already widely evident in changing plant communities (e.g. Pauli et al. 2007) and treelines (Harsch et al. 2009), the latter being especially relevant to birds. This could result in the loss or increasing fragmentation and isolation of high alpine habitats. There is evidence of altitudinal shifts in bird distributions (e.g. Pounds et al. 2005, Tryjanowski et al. 2005, Harris et al. 2011) but less evidence of shifts in species occurring in habitats above the treeline (Popy et al. 2010, Maggini et al. 2011), and the patterns are not universal (e.g. Archaux 2004). The evidence for altitudinal range shifts in high alpine environments (which we define here as high-altitude mountain habitats at and above the natural treeline) is therefore weak at best, although other evidence suggests that birds in highaltitude habitats may respond to climate change in several ways (Beale et al. 2006, Bech et al. 2009, PearceHiggins et al. 2010). Whilst there are some schemes that include some monitoring of high alpine bird populations (e.g. the Swiss scheme, Monitoring Häufige Brutvögel (MHB); Kéry & Schmid 2006), in comparison with avian communities in other habitats, the status and trends of high alpine bird populations are poorly known (EEA 2010), due largely to the logistical difficulties of carrying out rigorous monitoring in such challenging environments. This makes an assessment of the impacts of climate change on high alpine birds problematic, yet these are likely to be among the most vulnerable groups (EEA 2010). There is no doubt that further research on even the basic ecology of high alpine species is needed, first to quantify current distributions, secondly to establish baselines to assess future changes and thirdly to assess finescale species–habitat associations. However, there is also a need to understand better the demographic mechanisms that underpin distributions and that may therefore dictate future altitudinal shifts. Such studies could also identify those species whose alpine distributions are truly (directly or indirectly) dictated by climate as opposed to other confounding factors (e.g. changes in grazing regimes; Laiolo et al. 2004, Garcia et al. 2008). The need for an understanding of demographic mechanisms in climate change research in general (e.g. Pearce-Higgins et al. 2011), and in relation to upslope shifts in forested habitats (Harris et al. 2011), has been highlighted, but there are issues unique to high alpine environments that need to be addressed if we are to detect strong evidence for climate-induced range shifts, and to make predictions about how bird communities will respond under future climate scenarios. Our aim here is to highlight these issues for high-altitude mountain habitats.