Drivers of local extinction risk in alpine plants under warming climate

Abstract

The scarcity of local plant extinctions following recent climate change has been explained by demographic inertia and lags in the displacement of resident species by novel species, generating an 'extinction debt'. We established a transplant experiment to disentangle the contribution of these processes to the local extinction risk of four alpine plants in the Swiss Alps. Projected population growth (λ) derived from integral projection models was reduced by 0.07/°C of warming on average, whereas novel species additionally decreased λ by 0.15 across warming levels. Effects of novel species on predicted extinction time were greatest at warming < 2 °C for two species. Projected population declines under both warming and with novel species were primarily driven by increased mortality. Our results suggest that extinction debt can be explained by a combination of demographic inertia and lags in novel species establishment, with the latter being particularly important for some species under low levels of warming.

Keywords: Climate change; competition; demography; elevation gradient; extinction risk; integral projection models; novel species; population growth rate; population-dynamics; transplant experiment.

Fig. 1. Transplant experiment design.

Turfs from 2200 m containing plants and soil were transplanted to three lower elevation sites (1950, 1750, 1400 m) to simulate climate warming of, respectively, 1.65, 3.05 and 4.85 °C average growing season (May-September) temperatures during the experiment (2017-2019; inset) (a). By planting individuals of four alpine plants into the high elevation community turfs at the lower elevation sites, we tested the response to climate warming while individuals continue to interact with high elevation species currently found at 2200 m. At each of the low elevation sites, turfs containing local communities were also excavated and planted back into the site from which they came (b). By planting individuals into the low elevation communities, we tested their response to climate warming when interacting with novel species from low elevation. Individuals were transplanted into high elevation turfs and soil at 2200 m to evaluate the performance of individuals in their current environment (c)

Fig. 2. Effects of warming and novel species on predicted population growth rate (λ).

Estimates of λ from IPMs of four focal alpine species when growing with current or novel species across a temperature gradient simulated by transplantation from a high elevation (2200 m) to three lower elevation sites. Elevations are expressed as degrees of warming following transplantation (see Fig. 1). The dashed horizontal lines represent zero population growth (i.e. replacement). Error bars indicate 95% bias corrected confidence intervals based on 5000 bootstrapped λ values. Significant differences (*) between treatments were obtained from the pairwise comparisons of bootstrapped λ values (see Methods). n = 50 individuals per site/treatment/species/year before mortality

Fig. 3. Estimates of extinction time under warming with current or novel species.

Estimated time to extinction based on a population growth model (see Methods) assuming initial population sizes of 49 000, 147 000, 51 000 and 228 000 for P. alpina, A. alpestris, T. badium and C. scheuchzeri respectively. Predicted extinction times are shown for observed λ values (points) for focal plants experiencing different levels of climate warming and interacting either with current or novel species. Significant differences (*) were obtained from pairwise comparisons of 5000 predicted times to extinction obtained bootstrapped λ values. Trendlines indicate predicted extinction times from fitted values of linear regressions of λ on warming between 1.65 and 5 °C. Note the logarithmic scale of the y-axes

Fig. 4. Relative Life Table Response Experiment (LTRE) contributions of each vital rate to Δλ under warming and with novel species.

The figure shows average responses (± SE) to the surrounding community identities across warming treatments, with the relative LTRE contribution for each level of warming (elevation) shown by symbols