Contrasting changes in bird abundance and diversity (1971-2010)

Bird monitoring data analysis

We analysed long-term bird monitoring data in, to our knowledge, the largest comparative assessment to date of temporal changes in multiple biodiversity measures, including metrics of abundance, taxonomic diversity, functional diversity and phylogenetic diversity. We used a large-scale dataset covering 40 monitoring years from the North American Breeding Bird Survey (BBS) and calculated 12 abundance and diversity metrics based on 5-year average abundances of 519 species for each of 768 monitoring routes.

Trends in bird assemblages

We found that the majority of the biodiversity metrics increased or remained constant over the study period, whereas the overall abundance of birds showed a pronounced decrease. This was primarily driven by declines of the most abundant species. These results highlight how stable or even increasing metrics of taxonomic, functional, or phylogenetic diversity may occur in parallel with substantial losses of individuals.

We further found that patterns of change differed among subgroups of species, with both abundance and diversity increasing for woodland birds and decreasing for species breeding in grasslands. Finally, we identified that three key metrics were sufficient to capture the major part of the variation in the changes: overall abundance, richness, and proportional abundance.

Conclusions and implications

The contrasting changes between abundance and diversity and among the breeding habitat groups underscore the relevance of a multifaceted approach to quantify biodiversity change and highlight the importance of population abundance as an essential biodiversity variable.

Photograph by ZJN Steinmann