Ecological facilitation hinders adaptation to climate change in a stressful environment

Many plants, in (semi-)arid ecosystems in particular, rely on so-called nurse plants for protection and growth, in a species interaction called ecological facilitation. However, it is not clear whether facilitation will protect the facilitated plant from extinction if the environmental conditions change, for example due to climate change. Here, we use an evolutionary model to study the impact of ecological facilitation on the adaptive potential of an annual plant facilitated by nurse shrubs under a changing climate, specifically, when the landscape becomes more arid. We find that two alternative strategies can arise: a stress-tolerant strategy, capable of surviving outside the facilitated patches as well as underneath shrubs, but at a fecundity cost; and a stress-sensitive strategy, with a higher reproductive output but confined to the facilitated patches. Under some conditions, these two strategies can coexist. The presence of the stress-tolerant strategy is key to preventing extinction when the climate causes more stress (drought). By running three different climate change scenarios (stress increase under the shrubs, whole-landscape deterioration and shrub-cover shrinkage), we find that a trade-off between fecundity and stress tolerance usually traps an initially stress-sensitive population into staying sensitive even as the facilitated patches recede under climate change. The population then continues to rely on facilitation, and is unable to evolve stress tolerance before it is too late and extinction is unavoidable. By contrast, an increase in stress in the facilitated areas, with or without an increase in stress outside of the facilitated areas, readily promotes adaptation to increasingly severe aridity. Thus, persistence of sheltered areas in a patchy landscape may prevent adaptation to the harsher surroundings, putting the population at risk of extinction in a changing climate.