Geologic History Explains Freshwater Fish Species Richness Across the Conterminous USA

Aim Freshwater fishes comprise over 20% of vertebrate biodiversity despite occupying < 1% of the Earth's surface. However, species richness differs substantially among river basins. Fundamentally, richness patterns can be explained by spatial variation in diversification rates, evolutionary time and habitat capacities, which are in turn shaped by landscape change over geologic timescales. To test how geologic disturbances have influenced the accumulation of freshwater fish biodiversity, we hypothesized species richness would be (1) ordered by regional geologic history, (2) associated with high or intermediate river capture rates, (3) higher in assemblages with older evolutionary origins and (4) positively associated with stream size. Time Period 2008–2019. Location Conterminous United States (USA). Major Taxa Freshwater fishes. Methods We analysed native species richness from a spatially representative survey of 5321 fish assemblages at 3609 sites. Geologic history was determined from surrogates of tectonic activity, glaciation, sea levels and river capture over the last 66 million years, which were paired with previously published evolutionary time estimates. Hypotheses were tested with spatial linear models. Results All hypotheses were at least partially supported. (1) Rank-order richness matched hypothesized effects of geologic disturbances on evolutionary time and diversification rates. (2) Richness peaked in lowlands with high putative river capture rates. (3) Richness increased with evolutionary time at broad scales, but this relationship was weak and influenced by non-teleost taxa. (4) Richness largely increased with stream size. Overall, the tectonically active western USA exhibited lower richness, weaker effects of stream size and a greater share of young lineages compared to the more geologically stable eastern USA, especially unglaciated lowlands within the Mississippi Basin. Main Conclusions We demonstrate that deep-time processes leave a persistent mark on fish species richness. Thus, accounting for geologic history can improve assessments of freshwater biodiversity and biological condition in the USA and beyond.

Impact/Purpose

Freshwater fishes are among the most diverse vertebrate groups, which support valuable fisheries and ecosystem services across the United States. Fish are threatened by human activities, but inferring large-scale impacts of specific stressors is challenging because fish biodiversity is highly variable across multiple spatial scales and has been shaped by natural processes over tens of millions of years. Therefore, accounting for natural sources of variability in assessment models could produce more accurate biodiversity baselines and better identify causes of biodiversity loss. In this study, we analyzed freshwater fish species richness from the EPA National Rivers and Streams Assessment (NRSA), which we linked to novel proxies of geologic history (tectonic activity, glaciation, sea level fluctuations, river capture frequency) and species evolutionary origination times. We found that richness (the number of fish species) was highest in tectonically stable regions east of the Rocky Mountains, especially in unglaciated lowlands within the Mississippi Basin and Coastal Plain. In contrast, richness was lowest in tectonically active western regions, which were dominated by species with young evolutionary origins. This work demonstrates that differences in the geologic stability of river basins and habitats left a persistent mark on fish biodiversity in the United States. Therefore, incorporating geologic history into existing models of NRSA fish assemblage condition should produce better estimates of expected biodiversity under reference conditions, and support future models of fish habitat quality that incorporate other relevant metrics. In addition, NRSA data can be used to target restoration activities and evaluate their success. NRSA evaluates compliance with the Clean Water Act, Section 1254, which requires assessment and protection of the physical, chemical, and biological quality of freshwaters. As such, our work supports EPA Administrator Zeldin’s Initiative to give every American access to clean air, land, and water (Pillar 1), through water resource surveys and assessments in partnership with states, tribes, and federal agencies outside EPA (Pillar 3).

Citation

Gallagher, B., E. Santos, M. Dumelle, Phil Kaufmann, AND Joe Ebersole. Geologic History Explains Freshwater Fish Species Richness Across the Conterminous USA. Wiley InterScience, Silver Spring, MD, 34(10):e70139, (2025). [DOI: 10.1111/geb.70139]

DOI: Geologic History Explains Freshwater Fish Species Richness Across the Conterminous USA