Anthropogenic global change is increasing climatic variability, but it is not clear how this change will affect the long-term dynamics of wild populations. Classic ecological theory emphasizes that environmental variability should decrease long-term population growth. However, theory also shows that environmental variability can have neutral or positive long-term effects. The effect of variability depends on how climatic factors influence the natural logarithm of population growth rates (henceforth “population growth rate”). If the relationship between climate and population growth rate is linear, climatic variability has no long-term effect. However, when this relationship is nonlinear, climatic variability has non-neutral effects. In particular, concave or convex relationships result in climate variability respectively decreasing or increasing long-term population growth rate. Despite these clear theoretical expectations, assessing the frequency of these three scenarios across the tree of life requires relatively scarce long-term population data sets. We addressed this issue using long-term (10+ years) population data provided by the United States Long Term Ecological Research (LTER) network. We used any long-term data set, regardless of taxonomic group. We used a model selection approach to identify the best climatic predictors of population growth rate, and to establish whether these predictors had a quadratic (i.e. nonlinear) effect.
We found evidence for linear, nonlinear concave, and nonlinear convex relationship between climate covariates and population growth rate. The vast majority of relationships were equally divided between linear and nonlinear concave, while nonlinear convex relationships were rare. Only one taxonomic group showed a consistent response to climate: in aquatic arthropods, the relationship between population growth rate and climatic factors was predominantly concave. The commonness of nonlinear concave relationships supports the emphasis of classic ecological theory on the negative effects of environmental variability. However, the fact that linear relationships are equally as common as concave ones is surprising. This result indicates that in many species, an increase in climatic variability will have neutral effects on long-term population growth rate.