Scientists from MIT have made an intriguing discovery regarding the influence of river erosion on promoting biodiversity in regions with minimal tectonic activity. By focusing their study on the Tennessee River Basin, they found that erosion played a crucial role in diversifying populations of a specific fish species, suggesting that other species might also be affected similarly. This breakthrough emphasizes the significance of geology in preserving biodiversity.
Shedding Light on Biodiversity Hotspots in Geologically Stable Areas
If we could observe the process of species evolution across the world over millions of years, we would notice a concentration of biodiversity in regions experiencing tectonic activities. Landscapes that undergo significant shifts, such as the Himalayan and Andean mountains, are particularly rich in diverse flora and fauna, as the changing environments gradually separate and diversify species.
However, biodiversity can also thrive in geologically calm areas that have not experienced substantial tectonic movements for extended periods. An excellent example of this is the Appalachian Mountains, where freshwater biodiversity is remarkably abundant despite the lack of recent tectonic activity spanning millions of years.
The Role of River Erosion in Shaping Species Diversity
In a recent study conducted at MIT, researchers uncovered a geological process that influences species diversity in regions with low tectonic activity. The team published their findings in the journal Science, focusing on the southern Appalachians and specifically the Tennessee River Basin, renowned for its vast array of freshwater fishes. Through their research, they discovered that as rivers erode through various types of rocks in the region, the evolving landscape leads to the greenfin darter fish species colonizing different tributaries within the river network. Over time, these isolated populations underwent genetic differentiation, forming distinct lineages.
The team hypothesizes that erosion has played a pivotal role in driving the diversification of the greenfin darter. Despite the outward similarities among the separate populations, which share the characteristic green-tinged fins, they exhibit significant differences in their genetic composition. Currently, these populations are classified as a single species. However, the researchers predict that given enough time, these distinct lineages will eventually develop into separate species.
Maya Stokes, a former MIT graduate student, and co-author of the study, states, “If erosion continues to shape the landscape over an extended period, I believe these distinct lineages will evolve into different species.”
The Changing Landscape and the Greenfin Darter’s Evolution
The Tennessee River Basin’s transforming landscape led to the greenfin darter fish colonizing various tributaries. Subsequently, these separate populations developed into genetically distinct lineages. The researchers noticed a strong correlation between the greenfin darter’s habitats and the types of rocks present in those areas. Specifically, the southern half of the Tennessee River Basin, where the species is abundant, primarily consists of metamorphic rock. In contrast, the fish is not found in the sedimentary rock-dominated northern half.
Furthermore, the team observed that rivers flowing through metamorphic rock formations tend to be steeper and narrower, generating increased turbulence—an environmental characteristic preferred by greenfin darters. This prompted the researchers to explore whether the distribution of greenfin darter habitats resulted from the changing landscape due to erosion gradually exposing different rock types.
To test this hypothesis, the scientists developed a model to simulate landscape evolution as rivers erode various rock types. They input information about the current rock types in the Tennessee River Basin and ran the simulation backward to depict the region’s appearance millions of years ago when metamorphic rock was more exposed. They then observed how the exposure of metamorphic rock decreased over time. Notably, they paid particular attention to the points where connections between tributaries intersected non-metamorphic rock, thereby preventing fish movement between those tributaries. Comparing this timeline of blocking events with the phylogenetic tree of diverging greenfin darters, they found striking similarities. The emergence of separate lineages in the fish corresponded to the time when their respective tributaries became isolated.
Stokes asserts, “This suggests that erosion, occurring through different rock layers, caused the isolation of different greenfin darter populations, leading to lineage diversification.”
This groundbreaking study provides compelling evidence of a subtle yet significant mechanism for speciation in geologically stable regions. Josh Roering, an Earth sciences professor at the University of Oregon not involved in the study, comments, “Stokes and Perron have revealed some of the intimate connections between aquatic species and geology that may be much more common than we realize.”
Reference: Stokes, M. F., Kim, D., Gallen, S. F., Benavides, E., Keck, B. P., Wood, J., … Perron, J. T. (2023). Erosion of heterogeneous rock drives diversification of Appalachian fishes. Science, eadd9791. doi:10.1126/science.add9791
