Historical demography along a climatic gradient: Generating predictions of population responses to climate change in the montane dioecious herb Valeriana edulis

Background/Question/Methods

Climate change is unambiguously altering species traits and ecologies.  However, there is a disjunction between retrospective studies, showing patterns of response to past climate change, and studies that manipulate climate to infer response to future change.  While the former is ecologically realistic, it often lacks a clear demonstration of underlying mechanism, and while the latter provides mechanistic detail, such studies are usually simplified to manipulations of single climatic variables.  Populations of the montane herb Valeriana edulis (Valerianaceae) vary in sex ratio both clinally over a climatic gradient (25% shift over 1800 m of elevation) and in response to the past 35 years of warming (pace of 20% shift by 2100).  The similarities between these patterns – an increase in the relative abundance of males in warmer, drier conditions – suggest that they are likely underlain by parallel mechanisms.  Using a space-for-time substitution and unpublished demographic data from the 1970’s, we sought to determine the mechanisms underlying this population sex ratio variation.  We compared demographic processes among 4 populations varying in sex ratio by 15% along 700 m subset of the elevation gradient. With sex- and size-specific vital rates (growth, survival, and reproduction) from each population, we constructed matrix population models. We then used sensitivity analyses and life table response experiments to ask: 1) how do demographic statistics (population growth rate and equilibrium sex ratio) vary over the elevation gradient, 2) which vital rates are responsible for these differences, 3) to what degree is climate-induced population change driven by uniform vs. sex-specific responses, and 4) how will ongoing climate change affect the persistence and structure of these populations?

Results/Conclusions

We found clinal variation among the four populations in equilibrium sex ratio, but not population growth rate.  While vital rates did not vary clinally, males and females consistently differed in these rates and the magnitude of this difference increased with elevation.  For example, one important variable driving changes in equilibrium sex ratio was sexual dimorphism in survival rates; at low elevation males survived better than females, but this reversed at high elevation.  Such variation within population and its consequence for among population variation in structure imply that similar mechanisms may underlie changes in sex ratio over time.  Furthermore, identifying the mechanisms underlying population changes allows for the evaluation of management strategies to buffer the impact of climate change.