A framework for elucidating the temperature dependence of fitness
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.
Indeed, morphological traits linked to dispersal ability (for example, mobility through body size) are commonly conserved along the phylogeny, whereas several behavioural and ecological traits related to establishment success (for example, degree of habitat specialization should decrease habitat available for the species) are thought to be more evolutionarily labile.
Consequently, we expected to find little or no phylogenetic clustering of range expansion at the leading edge.
We demonstrate that these shifts are related to dissimilar mechanisms: whereas range retractions show some support for phylogenetic clustering due to a high level of conservatism in thermal safety margins, range expansions are underpinned by both evolutionarily conserved and labile traits, notably trophic position and life-history strategy, hence decreasing the strength of phylogenetic signal.Ectotherms are especially sensitive to climate change, because their physiology is directly influenced by temperature.Thermal preferences coincide with temperatures that maximize fitness, and individual performance rapidly drops outside the optimal range.Although this relationship seemed underlined by strong phylogenetic constraints existing on their thermal tolerances, the phylogenetic clustering of range retreat was further complicated by high variability in responses among species when considering the continuous rates of shifts, as outlined by the branches crossing within the phylogenetic traitgram (Fig. Interestingly, the trophic position appeared to be a strong predictor of shifts at the leading edge, with piscivorous species showing higher rates of expansion (Fig. These shifts were also strongly influenced by life-history strategies of species, which, contrarily to the trophic position, showed a weak phylogenetic signal (Fig. Species with high propagule pressure (that is, r-strategists, life-history PC2) displayed the greatest shifts without this being apparently influenced by phylogenetic constraints.Nonetheless, this relationship appeared further blurred by the influence of other characteristics, as outlined by the criss-crossing of colours along the internal branches of the tree (Fig. Finally, shifts were also related, albeit weakly, to mobility PC1 and TSMModel-averaged slope regression coefficients standardized to z-scores for phylogenetic generalized least squares relating shifts at (a) the lower and (b) the upper altitudinal limit to species traits for 32 freshwater fish species. High values of mobility PC1 and mobility PC2 indicate a greater mobility at larval and adult stages, respectively.
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.