An Indicator of the Impact of Climate Change on North American Bird Populations

The value of biodiversity for human welfare is becoming clearer, and for this reason there is increasing interest in monitoring the state of biodiversity and the pressures upon it. A recent study produced a biodiversity indicator showing that the pressure of climate change on bird populations in Europe has increased over the last 20 years (Gregory et al., 2009). In North America, climate change effects on distributions and phenology have been documented for various taxa, especially the Aves. However, evidence of population declines resulting from climate change is comparatively limited. Here, I produce species distribution models based on climate for 380 bird species, all with information available on their population trends across the USA. Following Gregory et al., I make predictions using these models based on past and future climate in the same region. From these I produce two metrics indicating how I expect these species to be affected by climate change. By comparing population indices for those species expected to be positively vs. those expected to be negatively affected by climate change, I derive Climatic Impact Indicators (CIIs) for North American birds. These summarize how the population level impacts of climate change, both positive and negative, have varied over the past 40 years. Much like the indicator for European birds, these indicators show an overall increase in climatic impacts on populations during a period of climatic warming. Furthermore, when indicators are downscaled to the state level around 80% of states exhibit an upwards trend in climatic impacts. I highlight that further work is needed to optimize the method used to produce a CII, and to determine what influences the slope of a CII. Nevertheless, the results presented here are strikingly similar to those seen across Europe, indicating that climatic impacts on populations may have increased across the Northern Hemisphere.