Metabolic ecology and habitat stability explain the disproportionately high species richness in standing waters

The Metabolic Theory of Ecology (MTE) conceptualizes that temperature is the primary driver of species richness, a pattern well supported in terrestrial taxa but less certain for freshwater organisms. Limited global-scale evidence and frequent violations of MTEs assumptions, particularly the stationarity of body size and abundance, further obscure its applicability. In freshwater systems, body size and abundance are tightly linked to dispersal and range size, which differ markedly between running-water (lotic) and standing-water (lentic) species, as proposed by the Habitat-Stability-Dispersal Hypothesis (HSDH). Adaptations to habitat stability may therefore generate distinct biogeographical trait patterns and modify richness-temperature relationships predicted by MTE. Utilizing comprehensive global functional, phylogenetic, and distributional data on dragonfly and damselfly species (83%) and habitat information (46%), we tested MTE predictions for lentic versus lotic species. Lotic species richness followed MTE expectations (slope: -0.469) more closely than lentic species richness (slope: -0.283). The proportion of lentic species in an assemblage was the strongest predictor of deviation in the species richness-temperature relationship (R2 = 38%). Assemblages dominated by lentic species clustered in climatically unstable regions and mainly including smaller-bodied species with larger ranges. Phylogenetic comparative analysis shows a strong phylogenetic signal in habitat preference, with the most species rich and northernly distributed families comprising predominately lentic species. Our findings suggest that adaptations to habitat stability facilitated the colonization and persistence of lentic species in harsh and fluctuating climates both past and present causing largely divergent species richness patterns of lentic and lotic odonates. Integrating HSDH-related traits (body and range size) not only substantially improves the explanatory power of the MTE, but also reveals a trait syndrome with broad implications for the biogeography and climate change responses of freshwater communities.