Warming up to changing trait frequencies: Rapid, climate change-induced shifts in population sex ratios along an elevation gradient

Background/Question/Methods

Population-level responses to climate change on generational time scales may consist of a combination of distribution changes and evolution. How these processes interact will have important consequences for the persistence of biodiversity: changes in trait frequencies may facilitate or dampen the ability of a species range to track abiotic habitat shifts. We examined how population allele frequencies have responded to climatic change in the dioecious montane herb Valeriana edulis (Valerianaceae) using population sex ratio as a proxy for sex-determination allele frequency. First, we surveyed population sex ratios over a ~2 km elevation gradient to ask whether sex ratio covaries spatially over a climatic gradient. Second, we tested whether local climate has changed and documented the response of flowering phenology, an important life history trait. Finally, we revisited populations for which sex ratios were measured 3.5 decades prior to ask whether current sex ratios reflect change in a direction consistent with predictions under climate change. 

Results/Conclusions

We provide the first evidence for a clinal shift in the frequency of a genetically controlled trait attributable to climate change: males have increased in relative abundance across the elevational range of V. edulis. This shift is consistent with 1) a clinal pattern of higher male relative abundance at hotter, drier low elevations, 2) an upward movement of low elevation climate as evidenced by an increase in temperature and a reduction in the availability of water from snowmelt, and 3) shifts in other important traits such as flowering phenology. Translated to an elevation index of upward movement, population sex ratio is moving upward at 160 m/decade, suggesting that trait frequencies may shift much faster than species ranges move. Because the relative frequency of traits may affect population growth rates as in the case of mate limitation under biased sex ratios, the role of trait frequency changes in promoting or constraining species range movement offers a conceptual frontier and management challenge to global change biology.