A framework for elucidating the temperature dependence of fitness
1) of the extent to which species characteristics were phylogenetically related to range shifts at the contracting (trailing edge) and expanding (leading edge) limits.We show that species shifts at both range limits are related to different combinations of traits and exhibit different phylogenetic correlation patterns, bringing novel evidence that divergent mechanisms underlie how species are responding to current climate changes by shifting their ranges.Phylogenetic signal indices and tests in rates of range shifts and species traits for 32 freshwater fish species calculated over 100 trees selected at random from the post-burn-in MCMC search of the parameter space during phylogenetic reconstruction: (a) Pagel’s λ, (b) Blomberg’s K and (c) Abouheif’s CSimilarly, species characteristics displayed different levels of phylogenetic signal, with the three indices leading to consistent results (Fig. For instance, traits related to morphological attributes (for example, mobility PC1) or trophic position showed strong phylogenetic clustering, while those related to life-history characteristics and specialization (for example, niche breadth) were more evolutionary labile.Although thermal tolerances per se showed a strong phylogenetic signal, with λ values for T of 0.76 (±0.02 s.d.) and 0.63 (±0.10 s.d.), respectively, measures of thermal safety margins appeared to be less linked to phylogenetic relatedness, especially when considering the optimal spawning temperature (TSM relative to climate means) have suffered more from local extinctions at their trailing edge than species with higher thermal safety margins.Overall, our findings suggest that trait-based approaches coupled with phylogenetic information may offer a simple way to help predict species vulnerability to future climate change. Our conclusions regarding the significance of phylogenetic clustering in shifters versus non-shifters were robust to variation in tree topology as we found only slight variation in the D values and associated tests over the 100 trees selected at random from the posterior distribution of trees (lower limit: D=0.28±0.03 s.d., P=0.007–0.029, permutation test, n=1,000; upper limit: D=1.22±0.04 s.d., P=0.633−0.823, permutation test, n=1,000).
Throughout their evolutionary (that is, phylogenetic) history, species have been continuously exposed to climate fluctuations.Idiosyncratic responses of species to climate change are underpinned by the interplay between three dimensions of climate change vulnerability, namely exposure (the extent to which the species thermal habitat changes), intrinsic sensitivity (for example, due to physiological limits or trophic specialization) and resilience capacity (species ability to avoid negative impacts of climate change through dispersal and/or microevolutionary or phenotypic changes).Conservatism of range shifts at the leading and trailing edges of distribution can be expected to be driven by the degree of phylogenetic clustering of traits involved in responding to climate change.The thermal safety margins, describing the warming tolerance of organisms relative to their current thermal habitat, should thus be the principal trait driving extinctions or shifts to cooler habitats.Additional traits underlying the ability of species to respond to climate change impacts (that is, resilience ability) may, however, delay extinction at the trailing edge or impede colonization at the leading edge, although acting in opposite directions.
Nevertheless, propagule pressure (that is, the quantity and frequency of dispersing individuals) may enable species to temporarily compensate for local extinctions caused by adverse climatic conditions.