Citation: Jerin, Tasnuba & Phillips, Jonathan. Biogeomorphic keystones and equivalents: examples from a bedrock stream. Earth Surface Processes and Landforms 45, 1877-1894 (2020). DOI: 10.1002/esp.4853
What’s it mean to be key?
Keystone species are those which really help keep their ecosystems to look the way they do. Were a keystone species to be lost, its absence would send ripples through the rest of the ecosystem, whereas non-keystone species might not be noticed nearly as much were they to disappear. When wolves were reintroduced into Yellowstone National Park for example, they sent cascading effects through the rest of the park, affecting their prey animals, the plants those animals fed on, and other plants and animals not directly linked to wolves. This concept of a keystone species, however, has typically only been applied to the biological aspects of ecosystems despite both that these systems existing among non-biological environments, and that organisms are already known to be able to influence their environments.
PhD candidate Tasnuba Jerin and Dr. Phillips of the University of Kentucky propose the concept of biogeomorphic species. These species, unlike regular keystone species, have a strong impact on local geomorphic processes (like soil erosion or stream meandering), landforms, or the properties of the materials making up their environments. They lay out the details of the concept and provide examples of biogeomorphic keystone trees at work in Kentucky.
Sycamores and chinquapin oaks: breaking, retaining, creating
Biogeomorphic (BGM) keystone species as a concept cannot be considered in a vacuum. Like with keystone species and ecosystem engineers- organisms that actively shape parts of their environments- BGM keystone species are context dependent. The same species in one location may not exhibit the same sort of influence as it would in another, and that influence can change over time in the same location. While other organisms tend to influence their environments in small ways, whether through moving rocks in a stream or fortifying soil, BGM keystone species exhibit wide influence that cascades through the system.
Tasnuba and Dr. Phillips conducted surveys exploring the concept in the Shawnee Run, a tributary of the Kentucky River in the central part the state. A bedrock-controlled stream, it behaves differently than most encountered streams with beds of substrate, making it easier to identify the influence of organisms on stones in the stream. Bedrock streams are not well understood, so the two researchers gathered data on what the physical and biological conditions within the stream were like. While doing so, they also gathered evidence of biogeomorphic processes: signs that there might be BGM keystone species responsible for the conditions they found.
After running their collected data through statistical analyses and considering the specific natures of the organisms and environment they sampled, Tasnuba and Dr. Phillips reported on the presence of two potential BGM keystone species: the American sycamore and the chinquapin oak. They found that these trees had three strong biogeomorphic influences on the stream: they formed a specific type of pool, a specific type of island, and root banks.
Pools, the slow and deep parts of streams home to unique conditions and organisms, can form in a variety of ways. Some clues, like the growth of nearby trees and how deep the stream is next to those trees, point to some pools being formed because of the presence of trees. Young sycamores, for example, don’t do well when their roots are submerged, so finding healthy, intact roots below the water line in a pool is evidence the tree came first. They are in a unique position among the local trees to grow their roots into bedrock and help create a pool, at which point they are old enough to withstand the water. Root banks may form, acting as a biological equivalent of bedrock, a slab of highly resistant material forming the stream’s bank, with the added benefit of offering a more complex living space for many organisms.
Water resistance also comes into play with island formation. Sycamores and chinquapin oaks were found to be the dominant trees on the stream’s islands and its floodplains. Learning the tree populations on and off the islands were approximately the same age, the researchers determined the islands were left behind from stream erosion and held in place by the trees instead of being formed from accumulating sediments with trees later growing on them: the trees allowed the islands to form!
There are stories to be told in the landscape
It is a complex story to learn from individual pieces, but the evidence Tasnuba and Dr. Phillips gathered point to a more fluid tale. Some trees might grow their roots into bedrock, contributing to faster erosion, and crumble away a stream bank- until they themselves form the bank- and help to form a pool, a unique stream environment. Should the stream begin eroding new areas, these established trees can hold onto their surrounding material, creating islands, again increasing physical diversity in the stream. These significant physical changes to the environment have important implications for the organisms living there, opening access to new habitat and resources.
While many other types of trees or plants can do some of these things, in this system it’s the sycamore and chinquapin oak that seem to have the most profound, sweeping effects. It’s that disproportionate effect that makes them their system’s biogeomorphic keystone species.
Ecology is a field of science defined by its highly complex web of interactions between organisms and their environment, weaving together aspects of biology, geology, hydrology, chemistry, atmospheric science, and more. Even an individual idea, like that of a keystone or biogeomorphic keystone species, can itself be a highly complex. That, however, is the nature of our world, and it’s a real treat to piece together how it all works.