Geologic history explains freshwater fish species richness across the conterminous USA

Freshwater fishes comprise over 20% of extant vertebrate biodiversity despite occupying <1% of the Earth’s surface area, while species richness across river basins differs by several orders of magnitude. Explanations for these patterns often invoke spatial variation in origination times, diversification rates, and local species capacities, as well as how these macroevolutionary processes are shaped by landscape change over geologic timescales. However, most large-scale fish biodiversity studies have focused on species pools aggregated across entire river basins, even though species richness and geologic history can vary substantially among sites within basins. In this study, we analyzed freshwater fish species richness from a spatially representative survey of streams and rivers across the conterminous United States, which provided 5,321 observations from 3,609 unique sites and sampled 466 native species. We hypothesized that species richness would exhibit predictable patterns based on regional geologic history and would be positively associated with species origination times. To test these hypotheses, we used spatial linear models to relate species richness to proxies of geologic history (tectonic activity, glaciation, sea level fluctuations, river capture frequency), species capacity, and an independent phylogenetic origination time dataset. Results often, but not always, supported our initial hypotheses. Within tectonically stable regions east of the Rocky Mountains, richness was highest in unglaciated lowlands as predicted, but was lower than expected in unglaciated highlands and higher than expected in glaciated regions. Richness was lowest in tectonically active western regions, which largely contained assemblages dominated by species with young evolutionary origins. This study demonstrates that large-scale and deep-time processes can leave a persistent mark on species richness, which could conceivably extend to other aspects of community assembly. Moreover, the geologic history proxies developed in this study should support more accurate assessments of aquatic biodiversity and biological condition in the United States and beyond.

Impact/Purpose

Freshwater fishes are among the most diverse vertebrate groups but are threatened by human activities such as dam construction, land use change and habitat alteration. However, inferring large-scale impacts of specific stressors is challenging because fish biodiversity is highly variable across multiple spatial scales, and much of this variability has been shaped by natural processes over tens of millions of years, which could lead to inaccurate estimates of human impacts. In this study, we analyzed freshwater fish species richness from the EPA National Rivers and Streams Assessment (NRSA) and linked these data to regional geologic history (tectonic activity, glaciation, sea level fluctuations, river capture frequency) and an independent dataset of species evolutionary origination time. We found that richness was highest in tectonically stable regions east of the Rocky Mountains, especially in unglaciated lowland regions. However, richness was lower than expected in unglaciated highlands and higher than expected in glaciated regions. Richness was lowest in tectonically active western regions, which largely contained assemblages dominated by species with young evolutionary origins. This work demonstrates that large-scale, deep-time processes can leave a persistent mark on fish biodiversity. Therefore, incorporating geologic history into biological assessment models can produce better estimates of expected species richness under reference conditions, and support future models of fish habitat quality that incorporate hydrology, physical habitat, and other relevant metrics. Furthermore, this work demonstrates that NRSA is likely among best fish biodiversity datasets in the world, and may be an underappreciated resource for a wide array of basic and applied research questions.

Citation

Gallagher, B., E. Miller, Phil Kaufmann, AND Joe Ebersole. Geologic history explains freshwater fish species richness across the conterminous USA. Oregon State University Monday Morning Meeting, Dept of Fisheries, Wildlife &amp; Conservation Science, Corvallis, OR, January 27, 2025.