Credits

Created by Virginia Pellington, Christina Prehn, and Robert Copus, Explore Natural Communities Interns Summer 2016, NatureServe. 

Music: Autumn Day, by Kevin MacLeod (incompetech.com). Licensed under Creative Commons: Attribution 3.0.

Sounds: Ocean Waves, recorded by Mike Koenig (soundbible.com). Licensed under Creative Commons: Attribution 3.0.

Photo: Bedrock Terrace Oak - Hickory Forest, by 2016 Summer Interns. Licensed under Creative Commons: Attribution 4.0. 

References:
Brent W. Steury, Natural Resources Program manager at George Washington Memorial Parkway, National Park Service. Personal Communication 2016.
The River and the Rocks:The Geologic Story of Great Falls and the Potomac River Gorge
Ephemeral Pools (Potholes)
Pothole Ecology - Ephemeral Pools
Viburnum rafinesquianum (Downy Arrrow-wood)

Transcript

Podcast time: 3:40 minutes

If you haven't been to Great Falls Park, it’s worth going. Hiking along the River Trail, you can see the raging Potomac rapids slamming against the massive rocks anchored below the waters. As the trail parallels the River, it leads you along the top of the Potomac Gorge, where you can seek solace in the quiet cover of the trees high above the water. The trail cuts between two rare natural communities. On one side are sunny rock outcrops and stunted pine trees along the rim of the gorge—called the Riverside Outcrop Woodland. On the other side is a forest set back a little farther from the River. This natural community is known as the Bedrock Terrace Oak – Hickory Forest, and it’s not found anywhere else in the world. It’s critically imperiled. Hickory (Carya spp.), ash  and oak (Quercus spp.) trees, a few shrubs, and grasses grow here on nearly level terraces of bedrock. 

In this forest in late April/early May, you may find beautiful puffs of white flowers blooming on downy arrowwood (Viburnum rafinesquianum). This shrub thrives in the scant soil and partial shade of this community. The flowers mature into dark blue, rounded, berry-like fruit—prized by birds. These berries may look appealing, but their taste is not appetizing to humans, so leave them for the birds! 

When you’re walking along the River Trail beside the Bedrock Terrace Oak – Hickory Forest, look for the occasional pothole in the flat rock surfaces. These bowl-shaped indentations in the rock vary in size and depth, and were sculpted by rocks caught in eddies during flood events. They are further weathered and eroded by water and wind. These small rounded craters collect rainwater and become little temporary pools, also known as vernal pools. You wouldn’t think that the small amount of stagnant water in these pools is an ideal place for life to thrive. However, these potholes host some of the coolest, tiny communities, with species that are able to survive in harsh conditions. The surrounding trees provide partial shade, but even so, the water in these pools can evaporate fairly quickly, and the organisms that live there have to be well adapted.

Some species like frogs simply escape to larger pools of water when the smaller temporary pools dry up, but for others, their whole world is one pothole. How do they survive in these vernal pools? Tiny snails have a calcium-hardened shell with a "trap door," called an operculum, that they can close tightly over the opening to retain moisture, keeping them consistently hydrated even during dry periods. The branchiopods, clam shrimp and the rare fairy shrimp, complete their entire life cycle in a matter of weeks. Before they die, they lay tiny, hardy eggs, smaller than a grain of sand. These eggs are classified as “drought tolerators,” and they are able to lose up to 92% of their water for long periods of time and still survive. When the next rain fills up their pothole again, they rehydrate, develop, hatch, and grow to complete their life cycle again.

The tiny, dynamic potholes are a great indicator of how healthy the surrounding environment is. Since the pools are small, they cannot dilute acids like rivers or oceans can, so acid rain can more quickly destroy these micro-communities. If air pollution levels were to rise, it would be a threat to all the natural communities in this park, and these miniature worlds filled with super adapters might disappear completely.

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