Growing disequilibria in species distributions and assemblage diversity until the end of the 21st‑century

Etienne Lalechère , Jonathan Lenoir , Ronan Marrec, Franz Essl, Torbjørn Ergon

Biodiversity forecasts almost universally assume that species are in equilibrium with contemporary environmental conditions. The consequences of this assumption are rarely tested, yet it may systematically bias inference and prediction whenever past natural or anthropogenic disturbances leave legacy effects keeping biodiversity in disequilibrium. Here we test how accounting for such legacy effects alters estimated species’ niches, and projected biodiversity trends, using a general framework quantifying species niche under disequilibrium dynamics.
Using a hypervolume-based approach, we quantified the realised niche of 84 European forest breeding birds and compared two niche models: one equilibrium model using only forest cover and climatic conditions that are contemporary to bird species distribution data vs. one disequilibrium model that also incorporates past trajectories of change in forest cover and climate. We then projected the future distribution of all bird species across Europe (under two coupled climate and land‑use scenarios) by using stacked range maps to derive the α, β, and γ components of taxonomic, functional, and phylogenetic diversity, towards the end of the 21st century.
Accounting for past environmental trajectories to quantify ‘the disequilibrium niche’, we revealed a dramatically expanded realized niche space (by a factor of 8.1), indicating a substantial niche truncation by equilibrium-based models. Despite broadly consistent average temporal trends in projected bird diversity under equilibrium and disequilibrium assumptions, this apparent agreement masks pronounced and increasingly divergent regional differences which are projected to increase towards the end of the 21st century.
Our study shows how violations of the equilibrium assumption can reshape biodiversity projections. Unveiling the hidden legacy of past forest cover and climate on the current distribution of forest breeding birds allowed us to provide the first spatially explicit projections of how future biodiversity disequilibria may increase under accelerating climate and land-use change. This calls for including disequilibrium dynamics in biodiversity forecasting.