Weak effect of climate variability on coexistence in a sagebrush steppe community.

Climate variability, which is expected to increase in the future, can promote coexistence through a mechanism called the storage effect. Currently, we have little understanding of how the importance of the storage effect varies among ecosystems. We tested for the three conditions of the storage effect in a sagebrush steppe plant community in Idaho (USA) by combining long-term observational data with statistical models. The four sagebrush steppe species that we studied satisfied the first two conditions of the storage effect: a long-lived life stage and species-specific responses to the environment. But the critical third condition, environment-competition covariance, was very weak in this community. While the direction of the covariance was consistent with a stabilizing effect of variability (stronger competition in more favorable years), its magnitude was small, reflecting low temporal variability in both competition and species responses to the environment. Consistent with this result, simulations of species population growth rates when rare showed that climate variability had no consistent stabilizing effect on coexistence. This case study provides an important reminder that species-specific responses to the environment are not sufficient for coexistence via the storage effect. Instead, the magnitude of temporal variability in species performance also plays an important role. Comparison of our results with those from a similar study in Kansas mixed prairie suggests that temporal variability in species performance may reflect both the strength of environmental variability as well as life history strategies.