Two paths to extinction: Effect of deteriorating environments on extinction time and distribution

Background/Question/Methods

In an era of rapid species extinctions and global warming, it is vital to understand the processes leading to population extinction, and how they are affected by non-stationary environmental changes. We used Daphnia magna, an aquatic ectotherm, as a model organism to study how temperature-driven environmental deterioration, propagated through population dynamics, affects the populations’ average extinction times and distributions. Three sets of twenty-four chambers of Daphnia (N₀ = 20) were subject to temperature regimes according to increasing, decreasing, and constant (control) treatments. We hypothesized that the treatment conditions would alter extinction risk differently: if one increased risk, the other would decrease it, resulting in a monotonic response in mean extinction time with respect to the direction of environmental change.

Results/Conclusions

Contrary to these predictions, analysis of the data with the Cox proportional hazards regression and analysis of variance (ANOVA) did not show a monotonic response in mean extinction time. Further investigation revealed that the presence of an unintentional factor, contamination of the chambers by algae, obscured the role of temperature change and directly influenced population decline. Accordingly, we took advantage of this unintentional treatment to investigate the role of temperature within a more complex network of causes. Importantly, temperature affected the shape of observed extinction time distributions in ways that can be quantified with respect to the difference among treatments in median extinction time and skew. Summarizing, the unexpected non-monotonicity of extinction time with respect to temperature occurred because treatments gave rise to extinction time distributions with qualitatively different shape properties. These results suggest that the effects of change in environmental parameters may be counter-intuitive and non-monotonic across even a small range of thermal conditions, a finding that warrants further investigation as non-stationarity in many environmental variables is expected to increase.