Impact of climate change on bird populations

Climate change impact indicator for European birds

Note: How to read the graph: The indicator demonstrates the impact of climate change on widespread bird populations has increased strongly in the past twenty years. The Climatic Impact Indicator (CII) measures the divergence between the population trends of bird species projected to expand their range and those predicted to shrink their range due to climate change.

Data source:

• An Indicator of the Impact of Climatic Change on European Bird Populations provided by PLoS ONE

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Weighted population trend of species predicted to lose range in response to climatic change (92 species)

Note: How to read the graph: The weighted population index of species expected to lose in range due to climatic change has decreased by 20% since 1989.

Data source:

Gregory, R. D.; Willis, S. G.; Jiguet, F.; VoYíaek, P.; KlvaHová, A.; van Strien, A.; Huntley, B.; Collingham, Y. C.; Couvet, D. and Green, R. E., 2009. 'An Indicator of the Impact of Climatic Change on European Bird Populations'. PLoS ONE 4(3): e4678. doi:10.1371/journal.pone.0004678.

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Weighted population index of species predicted to gain range in response to climatic change (30 species)

Note: How to read the graph: The weighted population index of species expected to gain in range due to climatic change has increased by over 30 % since 1989.

Data source:

Gregory, R. D.; Willis, S. G.; Jiguet, F.; VoYíaek, P.; KlvaHová, A.; van Strien, A.; Huntley, B.; Collingham, Y. C.; Couvet, D. and Green, R. E., 2009. 'An Indicator of the Impact of Climatic Change on European Bird Populations'. PLoS ONE 4(3): e4678. doi:10.1371/journal.pone.0004678.

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The Climatic Impact Indicator (CII) measures the divergence between the population trends of bird species projected to expand their range and those predicted to shrink their range due to climatic change. The indicator is based on a combination of observed population trends monitored from 122 common bird species in 20 European countries over 26 years, and projected potential shrinkage or expansion of range size for each of these species at the last part of this century (2070 - 2099), derived from climatic envelope models. The ensemble in this case is the average climate envelope forecast based on six differing future scenarios.

As with any biological data, there is annual variation and statistical noise around the observed trend in the CII. However, the general trend of the CII is clearly upwards indicating that climatic change is having an increasing impact on bird populations. Where the trend is downwards, this means that the impact of climate warming on bird populations is being overridden by other pressures in the environment; these could be man-made pressures, or natural ones, such as cold winter weather. The CII demonstrates unequivocally and for the first time that climatic change has affected bird populations at a European scale. It shows conformity between observed population trends and modelled projections of how each species should respond to climatic warming.

The CII fell in the 1980s reflecting the influence of cold winter weather events during this time (especially around 1980 and 1985 when such events significantly increased mortality in small birds) combined with other known drivers, such as land use change and agricultural intensification, which acted to depress bird populations. The indicator shows no signal of climatic warming until approximately 1986. The stable temperatures in the early 1980s represent the end of a period of stable annual average temperature in Europe that began around 1950.

From the late 1980s, however, the CII shows the effects of climatic warming on bird population trends, similar to that predicted by the climatic envelope models; and the impacts have increased roughly linearly to date. The number of bird species whose populations are observed to be negatively impacted by climatic change is three times larger than those observed to be positively affected by climate warming in this set of widespread European land birds. The CII has increased rapidly in the past twenty years, coinciding with a period recognised by climatologists as a time of rapid observed climatic warming in Europe.

A closer analysis reveals that the sub-indicator figures, which lie behind the construction of the CII (Figures 2 and 3), show differing fortunes for those species whose ranges are predicted to be negatively impacted by climatic warming compared to those positively impacted. While many European land birds show signs of decline in response to warming (92 species predicted to shrink in range size, reflected in sub-indicator figure 2), some bird populations have responded positively to climatic change and have increased in number (the 30 species predicted to gain range size, reflected in sub-indicator figure 3). This is likely to have led, and will increasingly lead, to changes in species composition at a regional scale. We can only speculate as to the potential correlation between such changes in bird populations and ecosystem function and resilience. It is suggested that increasing climatic effects might alter ecosystem functioning and resilience.

The effects of climate change for some migratory bird species may be most severe outside their European range and a comprehensive response would need to be effective beyond European territory.

NOTES

The indicator is based on the combination of two data sets:
(i) Population trend data on 122 common and widespread bird species for any part of the period between 1980 and 2005 in 20 European countries (from the Pan-European Common Bird Monitoring Scheme: PECBMS). See Gregory et al. (2005, 2008);
(ii) Climatic envelope model projections for each of the 122 species for the simulated future between 2070 and 2099 showing an expanding potential geographical range or a decreasing potential geographical range. These are based on an ensemble forecast built on three General Circulation Models and two IPCC SRES emissions scenarios. See Gregory et al. (2009), Huntley et al. (2007, 2008).

METHODS IN BRIEF

The CII is calculated in two steps. First, the 122 bird species were divided into those for which the ensemble climatic envelope model projection indicated an increase in a potential geographical range (30 species: sub-indicator figure 3) and those with projected decreases in their geographical range (92 species: sub-indicator figure 2). For each of the two groups of species, a multi-species population index (proceeding from population indices for individual species) was then calculated, with the weight of the contribution of each species to the index being based on the modelled projected change in a potential range extent. Extreme projections of the range increase, or loss, for individual species, thus, have a greater influence on the line. In simple terms, population trends displayed by birds predicted, in the models, to be significantly affected by climatic changes (either negatively or positively) register a strong influence over the direction of the lines shown in the subindicator figures.
In the second step, the CII itself is calculated for a given year. It is done as a ratio between the index for species whose potential range is projected to increase (30 species, reflected in sub-indicator figure 3) and the index for the species whose geographical range is projected to decrease (92 species, reflected in sub-indicator figure 2). The two lines have equal weighting in the indicator. The methodology developed here is equally applicable to any other species group where equivalent information is available. For full methods and discussion, see Gregory et al. (2009).

FURTHER INFORMATION

• European Bird Census Council: www.ebcc.info.