Does metabolism constrain bird and mammal ranges and predict shifts in response to climate change?

Abstract

Mechanistic approaches for predicting the ranges of endotherms are needed to forecast their responses to environmental change. We test whether physiological constraints on maximum metabolic rate and the factor by which endotherms can elevate their metabolism (metabolic expansibility) influence cold range limits for mammal and bird species. We examine metabolic expansibility at the cold range boundary (MECRB) and whether species’ traits can predict variability in MECRB and then use MECRB as an initial approach to project range shifts for 210 mammal and 61 bird species. We find evidence for metabolic constraints: the distributions of metabolic expansibility at the cold range boundary peak at similar values for birds (2.7) and mammals (3.2). The right skewed distributions suggest some species have adapted to elevate or evade metabolic constraints. Mammals exhibit greater skew than birds, consistent with their diverse thermoregulatory adaptations and behaviors. Mammal and bird species that are small and occupy low trophic levels exhibit high levels of MECRB. Mammals with high MECRB tend to hibernate or use torpor. Predicted metabolic rates at the cold range boundaries represent large energetic expenditures (>50% of maximum metabolic rates). We project species to shift their cold range boundaries poleward by an average of 3.9° latitude by 2070 if metabolic constraints remain constant. Our analysis suggests that metabolic constraints provide a viable mechanism for initial projections of the cold range boundaries for endotherms. However, errors and approximations in estimating metabolic constraints (e.g., acclimation responses) and evasion of these constraints (e.g., torpor/hibernation, microclimate selection) highlight the need for more detailed, taxa‐specific mechanistic models. Even coarse considerations of metabolism will likely lead to improved predictions over exclusively considering thermal tolerance for endotherms.

Trait‐mediated filtering drives contrasting patterns of species richness and functional diversity across montane bird assemblages

“…First, CCA ordination and regression models revealed that variations in vegetation variables were divided into two niche-axes, representing the frequency of large trees/PC1 and dense understorey shrubs and herbs/ PC2 (Figure 3; Table S5 in Appendix S1). Buckley, Khaliq, Swanson, and Hof (2018) also suggested that metabolic constraints provide a viable mechanism for initial projections of the cold range boundaries for birds and mammals, and highlighted the need for taxa-specific mechanistic models. In addition, while variation in mountain climate has been related to species richness of plants and ectothermic animals through the 'physiological tolerance hypothesis', it directly influences community composition of endotherms such as birds mostly under stressful climatic conditions (Ferger, Schleuning, TA B L E 2 Correlation coefficients between single trait metrics and elevation as well as phylogenetic structure indices (SES MPD) for 16 assemblages across elevation Hemp, Howell, & Böhning-Gaese, 2014).…”

An oversimplification of physiological principles leads to flawed macroecological analyses

“…The inverse assumption that T b can be used to predict metabolic rate is also problematic. For example, a difference in estimate of only 1°C (the black line vs. the gray line on the left panel of Figure ) for the T b of the four‐toed hedgehog ( Atelerix albiventris ) would lead to a difference in the metabolic rate at the cold range boundary (MR CRB ) described by Buckley et al () of ~12%.…”

“…In light of the rapidly changing climate, there is an urgent need to develop a mechanistic understanding of how physiological functioning mediates ecological patterns. Recently, there has been a spate of papers using analyses that scale up from a standard physiological model, the Scholander–Irving model, to make predictions about range constraints on endothermic vertebrates (Buckley, Khaliq, Swanson, & Hof, ; Fristoe et al, ; Khaliq, Böhning‐Gaese, Prinzinger, Pfenninger, & Hof, ; Khaliq, Hof, Prinzinger, Böhning‐Gaese, & Pfenninger, ). Here, we argue that oversimplifications of the Scholander–Irving model and the use of questionable datasets lead to questionable macrophysiological analyses.…”

“…Demonstration of the inherent limitations of using body temperature ( T b ) and the lower critical temperature ( T lc ) of the thermal neutral zone to calculate thermal conductance (C) when T b and T lc are poorly defined. Buckley et al () analyzed metabolic expansibility, which is the metabolic rate at the range boundary (MR RB ) divided by basal metabolic rate (BMR). MR RB , and therefore ME, changes drastically depending on assumptions of T b and T lc used to calculate C. Data are for a rock mouse ( Chaetodipus intermedius , left; Bradley et al, ) and a hedgehog ( Atelerix albiventris , right; McNab, ).…”

“…This error stems from the assumption of a single, unchanging T lc . Two of us (JGB and DLL) independently judged the quality of the T b and T lc included in the first 20 mammal species included by Buckley et al () (excluding species with partial data and including only one species from a genus; Table ). Of those 20 datasets, we classified five as appropriate for inclusion in the analysis (i.e., data generally followed the Scholander–Irving model, and T b variation was relatively low).…”