Southern species expand northwards
Mediterranean species that reach their northern distribution limit in Switzerland have increased and extended their ranges to the north since 1993–1996. In contrast, central and northern European species whose southern range limit lies in Switzerland appear to be decreasing. Climate change is presumably a driving force behind these trends.
The large-scale distribution of a species, also called its range, is often determined by the climate and therefore by types of vegetation that dominate large areas (e.g. deciduous forests, mountain ranges). Climate change has brought the range shifts of plants and animals to the attention of scientists. Models that account for climate change predict that the current ranges of many European birds will shift to the north or northeast.
Southern species on the rise in Switzerland
In order to verify whether range shifts have occurred in Switzerland using the atlas data, we selected 17 species whose European range limit runs through Switzerland. We only included species whose centre of distribution in Switzerland lies below 900 m. The sample includes nine «northern» species whose main European breeding ranges lie in central and northern Europe, and eight «southern» species with core ranges in southern Europe. For each species, we calculated the centre of distribution in Switzerland during 2013–2016, i.e. the mean geographical location of occupied atlas squares (10 × 10 km) and compared it with 1993–1996.
Results showed that all of the eight «southern» species had expanded their distribution in Switzerland between 1993–1996 and 2013–2016; in other words, they were found in a greater number of atlas squares than 20 years ago. In the case of six species, the centre of distribution shifted to the north or northeast. The average northward shift of the eight «southern» species was 9.4 km. Four of the nine «northern» species expanded their range in Switzerland; five species experienced a contraction. There was no clear pattern in terms of the direction of the shifts either. The ranges of Grey-faced Woodpecker and Icterine Warbler show a distinct northward shift, in accordance with the predictions; the range of Rook, a species that is increasing in number, moved to the southeast, that of White-backed Woodpecker to the southwest.
Only few «northern» species exhibited a substantial range contraction. This finding may be related to the fact that extinctions are a lengthy process, while it takes only a few individuals to colonise new territory.
Similar trends elsewhere
Due to its small size, Switzerland does not lend itself well to studies of large-scale range shifts. Moreover, the Alps produce a steep climate gradient independent of geographical location on the north-south axis, which may mask large-scale range shifts.
A distinct shift of southern species to the north, but no or only minor shifts of northern species to the north, has been observed elsewhere too. In Finland, southern species are spreading northwards by about 1.2 km per year, while the distribution of northern species is shifting at half that pace. In the UK, the range margins of southerly-distributed species have moved north by 18.9 km in 20 years, while no shift was observed in the case of northern species.
Is climate change the cause?
Many studies show that range shifts and changes in climate occur in parallel, suggesting that climate warming is a substantial contributor to range shifts. Accordingly, models based on climate factors predict considerable range contractions for several species in Switzerland. Studies have been able to show that bird populations evolve in accordance with climate-related predictions in Switzerland as well. The Swiss Bird Index SBI® «Climate Change», for instance, summarises the population trends of 20 species that, according to predictions, will be most affected by climate warming. The climate index reveals a picture that resembles the observed changes in distribution: those species expected to benefit from the warming climate have indeed increased in number, while the suspected «losers» have not (yet) responded with declines. The results also show that range shifts can rarely be explained with climate change alone, but are influenced by other factors such as habitat changes or conservation measures.
Long-term prospects look bleak
Range shifts are expected to become more pronounced in the future. But they are hard to predict, because besides the rising temperatures, other factors play a role that can also be related to climate, such as rainfall during the chick-rearing period, extreme weather events, increasing aridity, the delayed effects of changes in vegetation (e.g. forests) and anthropogenic habitat changes.
In the long run, the birds most at risk in Switzerland are the «northern» species and cold-weather specialists as well as wetland birds. At the European level, models predict an average range shift of 335 km for 409 species by 2050 due to climate change and land-use changes. Of these 409 species, 71 % are expected to experience range contractions.
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