Bergmann's rule suggests that climate warming may lead to multiple consequences, such as (1) shrinking body size, (2) an increase in small-bodied species due to the extinction or population reduction of larger coexisting species or predators, and (3) a greater dominance of small-bodied species in local communities (Tan et al. 2021).

Climate and human activities may jointly influence the relevance and application of Bergmann's rule in conservation (Søren Faurby and Miguel 2016). On the IUCN (2022) Red List, threatened vertebrate species (i.e., Critically Endangered–CR, Endangered–EN, and Vulnerable–VU) are generally smaller in body size than non-threatened species (i.e., Near Threatened–NE and Least Concern–LC) (Supporting Information S1: Figures S2 and S3). However, in terms of proportions, a greater number of species with either very small or very large body sizes are threatened or at high risk of extinction (Supporting Information S1: Figure S3). More importantly, when each of the four classes of terrestrial vertebrates is analyzed separately, larger-bodied species are more likely to be threatened than smaller-bodied species among endothermic taxa (birds and mammals). In contrast, ectothermic species (reptiles and amphibians) show the opposite pattern (Figure 1). Overall, smaller-bodied species usually have smaller ranges and are mostly distributed in warmer or tropical regions. Body size and range size are considerably more variable among threatened species than among non-threatened species, especially for birds and mammals (Figure 2). The body size–range size relationship also indicates that the changes in body size and range size are more tightly coupled for threatened than for non-threatened species (Figure 2). Approximately 41% of all threatened terrestrial vertebrate species worldwide are amphibians, which have a small mean body size, followed by mammals (21%), with the largest mean body size (Supporting Information S1: Figure S4; see also Ripple et al. 2017). The extinctions of very large-bodied species (mostly mammals) in the recent past have been closely associated with increasing human activities (e.g., overkill and land use) (Munstermann et al. 2022; Ripple et al. 2019).

Following Bergmann's rule, global warming may cause some species, especially large-bodied species, to become smaller (Frӧhlich et al. 2023). Although there is a generally positive correlation between body size and range size, there are many exceptions, and the strength of this correlation varies among the four classes of terrestrial vertebrates. Large-bodied species with smaller ranges are more vulnerable to extinction, as they require more space to survive and reproduce (Brown and Maurer 1989; Shuai et al. 2021; Chen et al. 2019; Gaston and Blackburn 1996). As many large-bodied species occur at higher latitudes and altitudes (i.e., Bergmann's rule) (Gutiérrez-Pinto et al. 2014), a warming climate is expected to further reduce their range sizes. If an organism's body size can adapt quickly enough to climate warming (Herrera et al. 2022; Millien et al. 2006), latitude may become a useful predictor for assessing the applicability of Bergmann's rule, especially if warming is more pronounced at higher latitudes (Pörtner et al. 2022; Liu et al. 2023; He et al. 2023). Although efforts continue to identify the species or species groups at all taxonomic levels (superclass, class, order, family, genus) that follow Bergmann's rule, the rule may still offer useful insights into how the body size structure of regional assemblages or species groups (Figure 1) and species' distribution ranges might shift under changing climates.

The application of Bergmann's rule in conservation should also consider phylogenetics and species age, as there is a clear phylogenetic signal in both body size and threat status (Ripple et al. 2017; Shuai et al. 2021). Most endangered species belong to monotypic genera or genera with relatively few species (Mann–Whitney U = 46329, p < 0.001; Figure 3), which may reflect a degree of phylogenetic uniqueness. In addition, threatened species with smaller ranges tend to be older than the non-threatened species (Mann–Whitney U = 34464507.00, p = 0.002; based on species age data presented in Guo et al. 2024b), possibly due to progressive range contraction over time. Thus, whether a species is—or is likely to become—threatened may, to some degree, be predicted based on information from closely related taxa. However, whether phylogeny is incorporated into analyses of the body size−temperature (or latitude) relationship (Guo et al. 2024) and how this is addressed will help determine how Bergmann's rule should be applied to species conservation within certain taxonomic groups.

Changes in body size structure and distribution are closely linked to intra- and inter-trophic species interactions, such as competition and predator–prey interactions, through dynamic top-down and bottom-up forces or cascading ecological effects (Jochum et al. 2012; Ripple and Van Valkenburgh 2010). For example, increased primary productivity in certain habitats due to climate change could cause changes in both the behavior and body size of predators at higher trophic levels (Zhong et al. 2017). Species invasions and poleward range shifts of both native and introduced species lead to novel species assemblages (native plus exotic) and alterations in body size structure and distribution across regions worldwide (Treasure and Chown 2014). This is particularly true when more large-bodied species may shift toward higher latitudes or altitudes in response to climate warming (Treasure and Chown 2014). It remains uncertain whether these new latitudinal body size patterns will continue to follow Bergmann's rule once all (native and exotic) species are considered.

Species invasions and human-assisted migration could greatly alter body size and range size distributions at both regional and global scales (Treasure and Chown 2014). In particular, if human influence on animal body size varies across latitudes and regions over time, the relationship between temperature and body size—and thus the applicability of Bergmann's rule—may also change (Smith et al. 2019; Teplitsky et al. 2008; Ripple et al. 2017). As body size and range size are closely linked to extinction rate, human activities have profound implications for area-based conservation efforts (Guo et al. 2024a; Bonner 2007; Gaston and Blackburn 1996).