The word "succession" refers to how an ecosystem returns after a disruption or natural disaster: does the environment return to its original species composition, or does it change?
"A forest that sees a wildfire is the classic case of succession," says Kayla Christianson, a former NC State graduate student and first author of a paper detailing the study. "As the city recovers from the burn, it follows a consistent growth pattern, beginning with grasses and progressing to trees and a mature forest. The seed bank under the surface, which helps the population regenerate regardless of when the disturbance occurs, is largely responsible for this stable order of succession."
Marine fouling populations, on the other hand, do not follow the same patterns as terrestrial habitats. Invertebrate filter-feeding animals such as barnacles and mussels settle on hard substrates such as docks, pilings, and ship hulls to form marine fouling habitats. Fouling species include soft-bodied tunicates like the invasive C. oblonga.
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Invasive C. Oblonga
"Since various species of larvae are present at different times of the year, what the habitat looks like after a disruption depends on when it happens," says David Eggleston, a professor of environmental, earth, and atmospheric sciences at North Carolina State University and the research's corresponding author. "Disturbances increase population diversity of aquatic ecosystems."
Invasion in the North Carolina Coast
Eggleston and Christianson decided to see how the invasive species C. oblonga would impact succession in a fouling climate, given its recent arrival and spread along the North Carolina coast.
Christianson and Eggleston replicated succession studies conducted in Beaufort, North Carolina, 50 years ago. From May 2017 to September 2018, they submerged terra cotta settlement plates along the docks every four weeks and observed the formed societies.
22 separate fouling organisms were found on the plates over the course of the analysis. When C. oblonga settled in each instance, however, it quickly pushed out the other animals. The researchers were able to look at succession following a natural disturbance during the study time, which included an unusually cold winter and landfall from Hurricane Florence in September 2018. While the cold and the hurricane were effective in removing C. oblonga, it soon came back and squeezed out other habitats, resulting in a loss in species diversity.
"We find that in the absence of invasive species, succession theory holds true, but not in the presence of invasive species," Eggleston says. "Regardless of whether a disturbance occurs, the population still reverts back to the invasive species with these fouling species."
"C. oblonga has only been around since 2015, but it seems to have no natural predators, crowds out native plants, and has established itself." We'll have to keep an eye on this species to make sure it doesn't damage our local shellfish industry."
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