Spatial scale influences relationships between indices of organisms’ thermal tolerance

Many thermal vulnerability indices potentially describe organismal responses to climate. However, the inter-relationships among indices and the effects of body size and spatial scale are mostly unknown. Existing literature on relationships between indices remains unclear, e.g. different hypotheses predict no, positive or negative relationship between critical thermal minimum and maximum (CT_min and CT_max). We used phylogenetic corrected analysis to determine relationships between CT_min, CT_max, thermal breadth (=CT_max-CT_min), warming tolerance (=CT_max-annual mean habitat temperature), warming capacity (=CT_max-annual maximum habitat temperature) and organism size in 121 species of freshwater insects (Ephemeroptera, Plecoptera and Trichoptera) from temperate and tropical streams distributed along elevation gradients in eastern Australia. When data were collated across all sites (i.e. both elevation gradients), CT_min and CT_max were positively related, indicating a trade-off between tolerating high and low temperatures at this multi-region spatial scale. However, within each gradient, these indices were uncorrelated, indicating that tolerance to high temperatures had no effect on tolerance to low temperatures and vice versa at this within-region scale. All pairs of variables, except CT_max and body size, were related (either positively or negatively) in one or more of the three datasets. Inconsistent relationships across sites in both the temperate and the tropical gradients occurred in 43 % of variable pairs. Hypotheses about the direction of relationships between pairs of variables were supported consistently in 50 % of the contrasts. While the variables examined are mostly related to each other, spatial scale is important in determining the nature of that relationship.