Empirical test on the relative climatic sensitivity between individuals of narrowly and broadly distributed species

Abstract

Climate change is already influencing global ecology, exacerbating human-induced biodiversity loss with potentially devastating results. A first step to addressing climate change impacts on conservation is to better understand how and to what extent species will be affected. Species with smaller geographic distributions are commonly perceived to be at highest risk of extinction. However, estimates of species vulnerability are frequently based on simplifying assumptions regarding climatic tolerance, usually arrived at through the use of models that associate mean spatial or temporal climate values with species distributions. Model estimated climatic tolerances may be improved by incorporating either finer spatial or temporal resolutions and/or additional distribution-limiting factors like dispersal, habitat connectivity, and species interactions. However, the underlying assumption that species-level climatic reflect individual tolerances can skew vulnerability estimates toward over- or underestimation. We use empirically derived fitness reaction norms of biomass and seed pod count to estimate the relative sensitivity of individuals of broadly distributed (BD) and narrowly distributed (ND) species across temperature and water gradients. Temperature and water treatments were based on local climate station data and IPCC projections of climatic change. On the basis of fitness reaction norms, we infer relative vulnerability to examine the assumption that ND species are relatively more vulnerable to climatic change than BD species. Study species included the BD Mimulus guttatus and Clarkia purpurea and the ND Mimulus nudatus and Clarkia gracilis ssp. tracyi. Compared to M. nudatus (ND), individuals of M. guttatus (BD) exhibited biomass responses that were significantly more sensitive to temperature and seed pod count responses that were significantly more sensitive to water and temperature. Conversely, compared to C. purpurea (BD), C. gracilis ssp. tracyi (ND) individuals exhibited biomass and seed pod count responses that were significantly more sensitive to temperature. In addition, we measured unexpected positive responses from both Clarkia species to increases in temperature. Our results support the idea that, when examined at a local scale, the size of a species' geographic distribution does not necessarily correlate to climate change vulnerability.