Decomposing abundance change to recruitment and loss: analysis of the North-American avifauna

Species richness is the most commonly used metric to assess biodiversity crisis, but fluctuations in species number start with fluctuations in the number of individuals (i.e. abundance). Population abundances are known to be globally plummeting with, e.g., three billion fewer birds in the US compared to the 70’s. However, assessing population decline doesn’t give insight on the dynamic of the ecological processes driving abundance change, namely losses and recruitments of individuals.

Here, we address this gap by decomposing the abundance change into processes of loss and recruitment using the North American Breeding Birds Survey dataset together with the hierarchical Dail-Madsen model accounting for imperfect detection. Using data on 564 bird species over 35 years (1987-2021) in 1033 local time series across North America, we also assess the change in growth rate and decompose it into processes of recruitment rate and loss rate. Doing so, we show which mechanisms are responsible for the decline of the avifauna all across North-America. We also show that the decline of growth rate is mainly due to an increase of individual loss rate at the North-American scale, and we show the spatial heterogeneity of those processes by mapping them.

Disentangling the processes responsible for population change (i.e. loss and recruitment of individuals, and their respective rates) is critical to provide insights into the ecological mechanisms driving those changes. By providing maps of those processes all across North-America, we hope that efficient conservation measures can be taken by targeting either the recruitment or loss and thus, try to bend the curve of population decline.