New macroecological insights into functional constraints on mammalian geographical range size

Abstract

Understanding the determinants of variation in the extent of species distributions is a fundamental goal of ecology. The diversity of geographical range sizes (GRSs) in mammals spans 12 orders of magnitude. A long-standing macroecological model of this diversity holds that as body size increases, species are increasingly restricted to occupying larger GRS. Here, we show that the body size-GRS relationship is more complex than previously recognized. Our study reveals that the positive relationship between body size and GRS does not hold across the entire size range of mammals. Instead, there is a break point in the relationship around the modal mammal body size. For species smaller than the mode, GRS actually decreases with body size. We discuss mechanisms to account for these observations in the context of the energetics of body size. We also examine the possibility that the patterns are the result of a statistical artefact from combining two random, uni-modal, skewed distributions, but conclude that the patterns we describe are not artefactual. Our results redefine our view of the functional relationship between body size, energetics and GRS in mammals with implications for assessing vulnerability to extinction resulting from range size reductions driven by large-scale environmental change.

Figure 1.

Theoretical constraint space describing the functional relationship between the average body mass of individuals of a species and the extent of its geographical distribution. (a) The original constraint space proposed by Brown & Maurer [20]. (b) The modified constraint space proposed in this study. The red dotted line indicates the modal body size. Solid boundaries to the constraint space indicate absolute constraints; yellow dotted boundaries indicate probabilistic constraints. Positive and negative symbols indicate the direction of the relationship within the constraint space.

Figure 2.

Empirical relationship between body size and GRS in mammals. (a) Scatterplot of GRS versus mass. (b) Density plot of GRS versus mass. (c) Relationship between GRS and mass on either side of the modal mass. Lines are lines-of-best-fit from ordinary least-squares regression with 95% CIs about the fit. (d) The minimum observed GRS in body size classes on either side of the mode. Small mammals (less than 40 g) are in blue. Large mammals (more than 40 g) are in red. Points along the x-axis are plotted at the midpoint of each respective body size class. Dotted red lines in (a–c) indicate the modal mass for all mammals.

Figure 3.

Relationship between body size and GRS in different mammal Orders. Note the mean clade-specific mass increases from left to right. Dotted red lines indicate the modal mass for all mammals. (a) Soricomorpha, n = 175, r = –0.11; (b) Chiroptera, n = 683, r = –0.33*; (c) Rodentia, n = 1287, r = 0.02; (d) Primates, n = 259, r = 0.22*; (e) Carnivora, n = 210, r = 0.21*; (f) Artiodactyla, n = 196, r = 0.03. Significant correlations are indicated by an asterisk.