Aim: Differences in realized and fundamental thermal niches can reveal how temperature constrains species distributions. Because climate-induced range shifts assume that temperature influences distribution limits (i.e., climate equilibrium assumption), understanding the factors that determine the realized thermal niche of species is critical for understanding their climate tracking abilities.

Location: Thermal niches were evaluated for two scales: Globally (n = 95 species) and across the Great Plains – Rocky Mountain region, U.S.A (n = 28 species)

Taxon: Marine and freshwater fishes

Results: First, freshwater species were not more likely than marine species to underfill their latitudinal cold-edge limit, despite restricted dispersal in freshwater environments from biogeographic barriers. Second, biological traits of dispersal ability were not associated with patterns of overfilling or underfilling along latitudinal gradients, aside from migratory species being more likely to overfill their cold-edge limits. Third, similar biological traits were not associated with freshwater fishes overfilling or underfilling their cold-edge, elevation limit with respect to their cold-edge latitudinal limit. Fourth, 78% of freshwater fish species (22 of 28) were observed underfilling their elevation limit relative to their latitudinal limit.

Main conclusions: Our results reveal that the climate equilibrium assumption is not satisfied for half of marine and freshwater fishes along thermal-spatial gradients and is largely unrelated to dispersal and movement. Freshwater fishes appear particularly limited in their distribution potential along elevation gradients, given the strong patterns of elevation underfilling.

We used a combination of realized and fundamental cold limit, thermal niche data to investigate how fish species overfill or underfill their distribution limit along altitudinal and latitudinal gradients in the context of four hypotheses about movement. We provide the excel datafiles, the associated R code, a citation list for where fundamental thermal niche data was acquired for each species, and a READ ME document to explain the datasets.