Climate change is a global phenomenon and we would expect it to have similar impacts on communities across large spatial scales. However, populations of wild species are driven up and down by many different processes and it can be difficult to isolate the impacts of just one process (such as climate change). Consequently, it is hard to show the extent of the impact of climate change on species, and even harder to show that it is having similar effects on communities in widely-separated areas.
In our new paper, we used distribution data for common breeding birds, together with climate data from the period during which the distribution data were collected, to determine the climatic conditions favoured by 145 species in Europe and 380 species in the USA. Knowing the conditions that each species favoured, we were able to use annual climate data from the period 1980-2010 to see whether the climate in each state of the continental USA, and 19 countries in Europe, had been getting more or less like that favoured by each species. This allowed us to calculate a "climate suitability index" (CST) for each species in each state or country that it occurs in, reflecting the trend in suitability over the 30 year period. CST was found to be an informative predictor of population trends over the same 30 year period.
For each state or country, we divided birds into 2 groups: those for which climate had been improving (the CST+ group) and those for which it had been deteriorating (the CST- group). We calculated composite abundance indices for these two groups, weighted by CST. Amalgamating these across states in the USA, or countries in Europe, we derived the composite abundance indices, at the subcontinental scale, of birds expected to be positively or negatively affected by climate change. The ratio of these composite indices shows the scale of climate change impacts. It is strongly positive and of a similar magnitude on the two continents. The equivalent interpretation is that, in Europe, if two species had similar abundances in 1980, the species favoured (to an average extent) by climate change would now be 40% more abundant than the species disadvantaged (to an average extent) by climate change. In the USA, we might expect an even larger divergence.
This work is important, because it isolates the impacts of climate change on two distinct and widely-separated communities. It shows that those impacts are of similar magnitude in the two areas, underlining the widespread nature of the phenomenon. The changes in community composition are large and will affect community stability and ecosystem service delivery. The resulting index of climate change impacts is clear and striking and should remind policy-makers of the commitments to reduce emissions to which they signed up at last year's climate conference in Paris; it might even encourage them to aim for more ambitious reductions!
The work also highlights what can be achieved as a result of the dedication and enthusiasm of thousands of volunteers who turn out annually to collect vital data on the distribution and abundance of birds across Europe and North America. It highlights the value of citizen science and should encourage more widespread monitoring of more taxa, if we are to document, explain and predict the consequences of large scale drivers of environmental change.
The paper is available here.