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| Triple Falls - an incredibly popular destination along western North Carolina's Little River |
If you drive through almost any portion of the southern and central Appalachians, you're bound to be bombarded with advertisements pitching the region's natural features as tourism attractions to boost the mountains' economy. Hiking trails through densely-forested wilderness areas are one major draw for visitors, as are long-ranging views from high-elevation outcrops and treeless mountain "balds." Waterfalls, however, are arguably Appalachia's main draw for tourism. Some of the region's most popular hiking trails and sightseeing destinations are built around waterfalls, and some entire regions within the mountains - such as the Highlands Plateau surrounding the towns of Highlands, Cashiers, and Brevard, North Carolina - have made the high abundance of waterfalls in these regions a central point for tourism marketing.
So why is Appalachia, specifically, such a hotspot for waterfalls and cascades? Elevation and slope (the steepness of the mountains' hills) are two major drivers of the region's abundance of falls, as streams that lose more elevation over a shorter distance tend to, by the their very nature, be predisposed to sharp drops. Geology, however, forms another cause for the region's scenic cascades. Much of southern Appalachia - particularly the region's easternmost fringe near the Blue Ridge - are made of "metamorphic" rocks, or rocks that have been changed into highly resistant rock types through the combined effects of heat and pressure.
Metamorphic rocks, such as granite and gneiss, tend to be very resistant to the weathering properties of wind and water, unlike less-resistant rocks elsewhere in the region which tend to easily fragment or wear when exposed to an erosive force. This means that when streams and rivers flow across outcrops and escarpments of metamorphic rock types, the more resistant rocks tend to not wear away easily under the erosive action of moving water - causing steep drops over these deposits to their base below. Some of the best examples of how geology, elevation, and slope can combine to create spectacular falls can be found in the Jocassee Gorges region of North and South Carolina. Waterfalls here tend not only be high but contain the flow from entire rivers as they drop from the Blue Ridge to the foothills below, with Whitewater Falls (pictured here) being one spectacular example.
So why are waterfalls being mentioned on a website about evolution? The answer can be found not in waterfalls themselves but in the organisms that live within the streams that flow throughout the region. For almost all aquatic organisms, waterfalls form a barrier to dispersal - put simply, a "wall" over which organisms cannot move. Fish are one classic example of organisms that are especially susceptible to waterfalls as dispersal barriers. Here in Appalachia, for example, our native brook trout - a species of high conservation concern - is often restricted to stream reaches above waterfalls that act as barriers to introduced (or "stocked") rainbow or brown trout that would otherwise wipe out populations of native fish.
Dispersal barriers, though, are important not just because they restrict the movement of individuals across a landscape. Instead, these individuals also carry their own unique genotype, or a combination of various forms of genes that can vary from organism to organism and between populations. When organisms move from one place to another - when there is no significant barrier to dispersal - they not only move themselves physically but also transport their genes which can be spread through reproduction, a process called gene flow. Gene flow itself is a major evolutionary force, allowing new combinations of genes to move between populations with the movement of individuals. When a barrier such as a waterfall is present, however, migration and gene flow become limited, often to great evolutionary consequence.
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| Image in public domain (Wikimedia Commons) |
A case study from Appalachia
The aforementioned brook trout (Salvelinus fontinalis) forms a fascinating case study for why gene flow and barriers restricting gene flow are so important to the evolution of taxa worldwide. As already mentioned, the brook trout is a species of conservation concern across Appalachia, largely due to declines caused by a slew of factors from predation by stocked fish species to acid rain, sedimentation in waterways, and climate change. Current focus in the region is on restoring this native species, but bringing lost populations back and saving existing ones first requires understanding how various natural and anthropogenic (manmade) stream features influence the genetic continuity of populations. As a general rule, populations need to have and maintain high genetic diversity - many different forms of varying genes - in order to adapt to changing environmental conditions and be buffered from the effects of population declines. Understanding gene flow and its consequences on individual populations throughout Appalachian streams is therefore one large step in solving the puzzle of bringing brook trout back from the brink.
Within the past several years, biologists working in Connecticut borrowed approaches from evolutionary biology and population genetics (the study of genetic profiles within and among populations of an individual species) to examine patterns of gene flow in native brook trout populations spread across two headwater stream systems. Fish from both stream networks were sampled for tissue, which then had DNA extracted and sequenced in the lab from each individual. From there, biologists created a table of genetic distances (a measure of genetic differentiation between all possible pairs of individuals) and combined these distances with a number of stream features to determine which features specifically drive patterns in genetic distance. Waterfalls were included in this dataset, along with the distance between sites where individuals were sampled, stream temperatures, stream gradient (the "steepness" of the stream), and the number of intersecting tributaries between pairs of sampled fish.
In addition to analyses investigating various molecular aspects of trout in these stream networks, a type of statistical analysis called a Mantel test was then used to determine which stream features best explain patterns of genetic distance between sampled trout. Besides indicating that trout showed a pattern called "isolation by distance" - a common pattern in which individuals spaced father apart tend to be more genetically different - these tests indicated that seasonal waterfall barriers greatly reduced gene flow and led to increased genetic differentiation between sampled fish. Beyond that, biologists found that the presence of a major waterfall in one of their stream networks may be responsible for this stream network having lower overall genetic diversity than the other stream system...all because fish from downstream would be unable to migrate to headwater regions and bring in new combinations of genes when a waterfall barrier is present, ultimately keeping genetic diversity lower than in a waterfall-less stream.
All of this matters for brook trout because understanding how to reintroduce and manage brook trout populations first requires understanding how gene flow and other evolutionary processes might act in regional streams. Streams that are highly fragmented by natural or manmade barriers such as waterfalls or road culverts may keep non-native rainbow and brown trout out, but these same barriers may also restrict gene flow and lower genetic diversity relative to more continuous stream environments. And since maintaining gene flow and keeping genetic diversity high are two keys to evolutionarily "healthy" populations, weighing the influence of such barriers is a key in restoring our native trout, in addition to complicating factors such as stream temperatures, flow rates, and stream network complexity identified by additional studies throughout the region.
Beyond trout and even beyond Appalachia, though, gene flow is still a major influence on evolutionary dynamics of organisms.Waterfalls act as barriers to many species worldwide, as do rivers and mountain ranges to many terrestrial species. Understanding how the landscape itself structures genetic diversity is just one key to uncovering how evolutionary processes act in nature - a process that is especially important here in the southern mountains.
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See it for yourself - major waterfalls in Appalachia
- Whitewater Falls (major waterfall in the Jocassee Gorges of North and South Carolina)
- Fall Creek Falls (dramatic falls in Tennessee)
- Little River Falls (centerpiece of Alabama's Little River Canyon National Preserve)
- Cascades (popular waterfall in Virginia's Ridge and Valley)
- Cumberland Falls (powerful falls on Kentucky's Cumberland River)
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Relevant Sources from the Scientific Literature
Kanno Y, Vokoun JC, & Letcher BH (2011). Fine-scale population structure and riverscape genetics of brook trout (Salvelinus fontinalis) distributed continuously along headwater channel networks. Molecular ecology, 20 (18), 3711-29 PMID: 21819470
Petty, J. T. & Merriam, E. P. (2012) Brook Trout Restoration. Nature Education Knowledge 3(7):17
Pettya, J. Todd, Jeff L. Hansbargerac, Brock M. Huntsman & Patricia M. Mazik (2012). Brook Trout Movement in Response to Temperature, Flow, and Thermal Refugia within a Complex Appalachian Riverscape Transactions of the American Fisheries Society, 141 (4), 1060-1073 DOI: 10.1080/00028487.2012.681102
