Survival of climate warming through niche shifts: Evidence from frogs on tropical islands

How many species will Earth lose to climate change?

Climate change may be an important threat to global biodiversity, potentially leading to the extinction of numerous species. But how many? There have been various attempts to answer this question, sometimes yielding strikingly different estimates. Here, we review these estimates, assess their disagreements and methodology, and explore how we might reach better estimates. Large-scale studies have estimated the extinction of ~1% of sampled species up to ~70%, even when using the same approach (species distribution models; SDMs). Nevertheless, worst-case estimates often converge near 20%-30% species loss, and many differences shrink when using similar assumptions. We perform a new review of recent SDM studies, which show ~17% loss of species to climate change under worst-case scenarios. However, this review shows that many SDM studies are biased by excluding the most vulnerable species (those known from few localities), which may lead to underestimating global species loss. Conversely, our analyses of recent climate change responses show that a fundamental assumption of SDM studies, that species' climatic niches do not change over time, may be frequently violated. For example, we find mean rates of positive thermal niche change across species of ~0.02°C/year. Yet, these rates may still be slower than projected climate change by ~3-4 fold. Finally, we explore how global extinction levels can be estimated by combining group-specific estimates of species loss with recent group-specific projections of global species richness (including cryptic insect species). These preliminary estimates tentatively forecast climate-related extinction of 14%-32% of macroscopic species in the next ~50 years, potentially including 3-6 million (or more) animal and plant species, even under intermediate climate change scenarios.

Genetic Load and Adaptive Potential of a Recovered Avian Species that Narrowly Avoided Extinction

High genetic diversity is a good predictor of long-term population viability, yet some species persevere despite having low genetic diversity. Here we study the genomic erosion of the Seychelles paradise flycatcher (Terpsiphone corvina), a species that narrowly avoided extinction after having declined to 28 individuals in the 1960s. The species recovered unassisted to over 250 individuals in the 1990s and was downlisted from Critically Endangered to Vulnerable in the International Union for the Conservation of Nature Red List in 2020. By comparing historical, prebottleneck (130+ years old) and modern genomes, we uncovered a 10-fold loss of genetic diversity. Highly deleterious mutations were partly purged during the bottleneck, but mildly deleterious mutations accumulated. The genome shows signs of historical inbreeding during the bottleneck in the 1960s, but low levels of recent inbreeding after demographic recovery. Computer simulations suggest that the species long-term small Ne reduced the masked genetic load and made the species more resilient to inbreeding and extinction. However, the reduction in genetic diversity due to the chronically small Ne and the severe bottleneck is likely to have reduced the species adaptive potential to face environmental change, which together with a higher load, compromises its long-term population viability. Thus, small ancestral Ne offers short-term bottleneck resilience but hampers long-term adaptability to environmental shifts. In light of rapid global rates of population decline, our work shows that species can continue to suffer the effect of their decline even after recovery, highlighting the importance of considering genomic erosion and computer modeling in conservation assessments.