Blog by Prof Richard Gregory, Head of Species Monitoring and Research, RSPB Centre for Conservation Science and Centre for Biodiversity & Environment Research, University College London

Accelerating climate change is recognised as one of the major global threats to humanity and nature. It threatens to undermine our food and water supplies, and bring extreme weather with increased and damaging temperatures, flooding, inundation and sea level rise. That threat extends to nature with the potential for disruptive and harmful impacts on species and communities.

Yet our understanding of the impacts of climate change on biodiversity and of the consequences for species’ distributions and populations is poor and we have much to learn. The science in this field of research is in its infancy. For example, to date, the evidence for beneficial impacts of climate change on populations is thought to be stronger and more plentiful than that for negative impacts, which feels odd, though few studies have been able to investigate this disparity, and many have wondered whether this might be an artefact. Linked to this, we’d expect a range of ecological factors to affect population responses to climatic change, but again, our empirical knowledge is poor.

Our newly published paper tackled both these questions head on using two amazing datasets from European and North American birds. Specifically, we examined the strength of the relationship between species-specific population changes and climate suitability trends (CST) using 30-year datasets of population change for 525 breeding bird species in Europe and the USA.  A positive correlation across species between population change and climate suitability would represent strong evidence for the contemporary impacts of climate change.  So, we addressed these two issues, the apparent difference in response between species expected to benefit or be negatively impacted by climate change, and the role of ecological traits in influencing the relationship of population change to climate suitability trend.

Our results were striking (Fig. 1), as we show that climate change has affected common and widespread bird populations across Europe and the US over the last three decades; and these are not hypothesized possible changes, this is what has been observed and recorded.  Importantly, we found no statistical support for a difference in the strength of the relationship between population trend and climate suitability change between those species expected to benefit from climate change and those expected to be adversely impacted.  Thus, failing to support previous reports that the positive effects of climate change have been more pronounced. 

In addition, we show that species’ responses to changing climatic suitability vary with ecological traits to a degree, particularly breeding habitat preference and body size; although in the latter case, patterns differed between Europe and the US for reasons that are unclear.  Taken together, our results indicate that ecological traits and habitat associations affect the strength of bird responses to changing climate, but that their impact may not always be consistent, even between continents. 

One can also imagine a long list of reasons as to why long-distance migrant birds might be disadvantaged by climate change compared to other groups, for example, by ‘phenological mismatch’ whereby the timing of nesting in migrant birds becomes out of step with their main prey and chick food.  Surprisingly, however, migratory behaviour was not found to be important in affecting the strength of the relationship between population trend and climate suitability on either continent.  

 

Fig. 1 The effect of climate suitability (CST) on bird population trends (r) for Europe and the USA. Dotted lines represent continent-specific regression models of r on CST from analyses presented by Stephens et al. (2016). Solid lines are those predicted by continent-specific piecewise regression models with a breakpoint at CST = 0 (see Table 1 in the paper for model coefficients). All models also included the random effects of species and state. Plotted points are mean values of population trend and CST (± 95% confidence intervals, which are very small for CST) for species-state combinations binned according to their CST value for each continent (species states sorted by descending CST value and grouped into three bins containing approximately equal numbers of CST- species states and three bins containing approximately equal numbers of CST+ species-states: CST- = 284 and 1263 per bin, CST+ = 278 and 991 per bin for Europe and the USA respectively.)

Note also that the population trend versus CST relationship was slightly less positive for CST+ species in Europe than the other groups tested (Fig. 1), indicating that this group may be less able to take advantage of an increasingly suitable climate. This may reflect differences in the underlying trajectories of avian abundance between the two continents, with average trends in Europe predominantly downwards since 1980, whilst US bird populations have on average remained relatively more stable. Alternatively, CST+ species may not be responding as positively in Europe because they are following a pattern of increasing climate suitability into an ‘ecological trap’, due to changing land use and intensity patterns. This could be the case if pressures other than climate change, such as harmful agricultural policies were particularly prevalent in Europe compared to the US, but this is conjecture.

To conclude, we found the relationship between bird population trend and climate suitability is positive across an entire taxonomic bird group and across two continents using data over a thirty-year period. We found no evidence that species favoured by climate change warming have responded more strongly than those disadvantaged, in contrast to previous work. There is clear evidence that climate change is causing widespread population change in birds, both quantifiable increases and declines in equal measure.

The long-term consequences of climate warming on bird populations and their ranges, and on biodiversity more broadly, remains uncertain and is an urgent priority for research like our own, that combines observational data with statistical modelling in different species groups.

Full reference: Mason LR Green RE Howard C Stephens PA Willis SG Aunins A Brotons L Chodkiewicz T Chylareck P Escandell V Foppen RPB Herrando S Husby M Jiguet F Kålås JA Lindstrom A Massimino D Moshøj C Nellis R Paquet J-Y Reif J Sirkiä PM Szép T Tellini Florenzano G Teufelbauer N Trautmann S van Strien A van Turnhout CAM Voříšek P & Gregory RD (2019) Population responses of bird populations to climate change on two continents vary with species’ ecological traits but not with direction of change in climate suitability. Climatic Change. https://doi.org/10.1007/s10584-019-02549-9.

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