We found a consistent and important change in the composition of wader assemblages over 33 years towards species more dependent on high temperatures. This temporal trend was robust to the removal of 75 per cent of the species from the initial pool and similar when estimated with presence–absence data. These results demonstrate an important reassembly of the wader assemblages at large spatio-temporal scale on their wintering grounds.
Focusing on wintering wader assemblages provides new insights on climate change impact on bird assemblages. Not only do changes in the temperature in the breeding grounds affect the composition of bird assemblages [9
], but the migratory behaviours of long-distance migratory species may also be affected by changes in temperatures in their wintering grounds.
Such changes in assemblage composition may result from different, albeit non-exclusive, mechanisms. First, MacLean et al
] suggested that, following climate warming, young waders (that have not yet become established in favoured wintering sites) could tend to reduce their migration routes and to winter further north in contrast to adults that are site faithful to their traditional wintering grounds [13
]. We can expect that these young birds predominantly belong to species with high STIs that do not have to winter in the southernmost sites anymore to thrive during very cold winters. Second, we can expect that hot dwellers (with high STIs), which are potentially more sensitive to cold temperatures, benefit more than cold dwellers from midler winters, by reducing their winter mortality to a greater extent than cold dwellers. Third, some cold dwellers (all generations pooled), which used to winter in northern France in the past, may have shifted northwards recently, and now tend to winter even further north (e.g. in the past, many oystercatchers wintered in France when tidal flats of the Wadden Sea froze over [14
]). These three mechanisms could contribute to a faster relative increase in hot dweller abundance and presence in local wader assemblages, and therefore to an increase in CTI. Finally, the increase in temperature may have both direct impacts on waders (e.g. on their physiology) and indirect impacts such as changes in the abundance, distribution or accessibility of macrobenthic preys.
Maclean et al
] found a northward shift for seven wader species in Northwest Europe that was five times slower than our result. The northwards shift we found of ca
20 km yr−1
however, is only based on a latitudinal gradient between the southernmost and northernmost monitored French estuaries that represent a tiny part of the wader wintering range, which stretches from the Baltic Sea to tropical Africa.
This study further suggests that CTI is a suitable indicator for assessing the response of local animal assemblages to climate change. Moreover, measuring the change in assemblage composition can reveal trends that are masked at the species level. For instance, almost all wader species are increasing in France (electronic supplementary material, figure S3) and seem to be progressively shifting northwards in Europe [5
]. However, our results suggest that different species have different dynamics within communities, which results in important changes in local assemblage compositions. In the future, because waders are among the main predators of the benthic compartment, this change in wader assemblages may have serious consequences for estuarine functions in Western Europe.