Exceptional body size–extinction risk relations shed new light on the freshwater biodiversity crisis

In PNAS, Ripple et al. (1) show that the smallest species of reptiles, amphibians, and bony fishes suffer unusually high extinction risks given what we know about general body size–extinction risk relationships (2). The results of this study have important implications, considering that most studies exploring the community- or ecosystem-level consequences of nonrandom biodiversity loss focus on the well-known, pronounced extinction risk for the largest species (2). For example, recent analyses show that extinctions and population declines of marine mammals, seabirds, terrestrial animals, and anadromous fish have caused major loss of large-scale nutrient-cycling functions, affecting many ecosystems across the globe (3). In contrast, little is known about how small species with restricted range sizes contribute to community stability or ecosystem functions, although a recent study suggests they might be crucial for those functions that are most vulnerable (4), thus emphasizing the need for increased conservation efforts toward cryptic and small species.

We want to emphasize here that, based on existing evidence, the pattern reported by Ripple et al. (1) will also be key to understanding the current biodiversity crisis that hits freshwater species and ecosystems at disproportionally high levels (5, 6). Notably, the observation that it is particularly in freshwaters, where the smallest species are among the most threatened ones, has been made before in a global analysis of fishes (7). Also, in accordance with the results presented by Ripple et al. (1), a recent analysis of endangered frogs, who also depend on freshwater habitats, has shown that their traits are highly threat-dependent. In particular, large species are threatened by human consumption, whereas small species are threatened by the pet trade (8). Hence, general allometric scaling rules based on a particular set of life-history traits (e.g., small species have a higher fecundity) that “protect” the majority of small species do not apply for freshwater species, nor do the traits typically assigned to species with a fast or slow life history (8, 9).

Similarly, the allometric scaling of dispersal limitation could work fundamentally differently in the dendritic networks of rivers and streams or the fragmented, insular characteristics of lake systems compared with more continuous marine and terrestrial ecosystems. Data to test these relationships are scarce, but together with the above-outlined differences in life-history traits, they may be key to understanding the contributing factors causing freshwater biodiversity to be more threatened than their terrestrial or marine counterparts (6).

In the light of recent global change and associated redistribution of species occurrences, these relationships become even more important, while at the same time the specific and distressing conditions of freshwater biodiversity are often ignored. For example, a recent review of global species’ redistributions in response to climate change only included one specific example from freshwater systems [i.e., cyanobacteria blooms (10)]. The unique position and imperilment of freshwater biodiversity, however, needs much better acknowledgment. In conjunction with size-based approaches that are able to integrate information across geographical and taxonomic boundaries, we might be able to understand and ultimately solve this biodiversity crisis.