Changes in Danish bird communities over four decades of climate and land-use change

Our understanding of how human activities impact biodiversity comes largely from space-for-time substitutions. However, spatial gradients are a poor surrogate for changes through time as they do not account for dynamic processes such as delayed extinction debts. Here we contribute towards filling this research gap by assessing the trajectories of local avian assemblages over 40+ years of climate and land-use change. Using four decades of volunteer observations in Denmark we investigated long-term trends of local bird richness, community structure, function, abundance, and biomass to better understand their anthropogenic drivers. Between 1976 and 2020, volunteers recorded ~ 2.4 million birds at 378 routes spanning a median of 15 years (range: 10–44). At the local level, we found a restructuring of bird communities over time (6% change per decade) and declines in abundance (−7% per decade), but stability in biomass, functional diversity, and spatial turnover. Local species richness showed a shallow decline on average. These results provide evidence that temporal turnover and loss of individuals are the most prominent features of recent ecological change in these communities. We found that the rate of local warming was positively associated with trends of species richness and functional diversity, suggesting a potential redistribution of warm-adapted species. Meanwhile, communities that were becoming more spatially homogenous were associated with urban and farmland areas. In space, environmental changes are often distinct and recognisable, e.g. between forest and farmland. Through time, however, changes can be infrequent, gradual, and non-linear. Despite these challenges, our results illustrate the power of spatially replicated, long-term biodiversity monitoring programs for detecting the trends and attributing drivers of local biodiversity change.