Keys to Life: Response Plans for Threatened Ecosystems

By Joseph Schreder

What if you knew the best way to respond to the stresses of your everyday life? Your life would be more successful and productive. The stakes are even higher for the environment. If we could develop the most advantageous response plan to things such as hurricanes, wildfires, and extensive pollution, the organisms that keep our planet rich with biodiversity would be protected. Food webs show the relationships of all organisms within an ecosystem and can display the consequences of changes to the environment. Currently, research is being done by Dr. F. Joel Fodrie of the University of North Carolina Institute of Marine Sciences to better understand these webs, which will allow scientists to formulate the optimal response to environmental stress. One of the most prominent examples of an environmental stressor in the United States was 2010’s Deepwater Horizon oil spill, in which 5 million barrels of crude oil leaked into the Gulf of Mexico. The 5 million barrels of oil spread to over 100 kilometers of shoreline, with the marshes of Louisiana absorbing the brunt of the damage. Dr. Fodrie studies coastal biological oceanography with a focus on both trophic interactions in estuarine communities and the connectivity of marine populations and ecosystems. He conducted a study of the food webs in the Gulf of Mexico, and how they were affected by the tragic Deepwater Horizon oil spill.1 Disastrous accidents like this rarely occur, so as a marine scientist, Fodrie was interested in the possible effects that oil would have on the salt marsh ecosystem.2 To better understand the implications of the spill on local ecosystems, Dr. Fodrie began the study by constructing a model of the salt marsh food web located on the coast of Louisiana with a focus on one of the most heavily polluted areas: the Barataria Bay region. A food web is essentially a series of interdependent food chains, so there are many

Image courtesy of Wikimedia Commons

Figure 1. Oil Spill Area. Image courtesy of BBC News.

links between organisms. For the purposes of the study, links were only established if organisms were directly observed in a predator-prey relationship, whether observed on site, experimentally, or through analysis of stomach contents. Data from over 120 studies was analyzed to accurately represent the food web. Overall, the web has 52 organisms with a total of 376 links between them.3 To produce an accurate food web, Dr. Fodrie needed to understand which species were the most sensitive to oil. He determined this by analyzing 37 different studies that documented the impacts of the oil spill on the populations of the 52 species in the food web. These pieces of literature were either published papers or field-based studies relevant to the effects of oil on the organisms. The purpose of this analysis was to create a baseline of oil sensitivity to compare to the results gathered by Fodrie and his team. If no studies existed for a particular species, the team used data from the

Dr. Joel Fodrie in the salt marsh effects of other oil spills or that of laboratory experiments. From the publications, every species involved was given an oil sensitivity index score. If no effect was observed or that species sustained a population increase, a 0 was recorded. Taxa that were minimally affected by the oil and recovered quickly received a 1 and taxa that experienced slow recoveries were given a score of 2. These results were then utilized to determine the consequences that each species’ resilience to oil has on the complex food web of the salt marsh ecosystem. This was broken into four categories: critically sensitive, critically resilient, sensitive with few food web effects, and uncertain consequences for the food web.3 Critically sensitive taxa were denoted by species that received high oil sensitivity indexes and were depended on by other species in the ecosystem. This impairs the food web, as organisms such as gulls and terns are killed by the oiling, destabilizing the ecosystem. Wading birds such as herons, egrets, and ibises were also categorized as critically sensitive. Additionally, bottlenose dolphins and terrestrial plants were categorized as critically sensitive, as they perform versatile functions in the salt marshes. Even with the extensive research and data, some questions are left unanswered. While blue crabs were ranked as the single most crucial species to the existence of the food web, their moderate sensitivity to oil and unclear population response makes it extremely difficult to pinpoint the consequences of the oil spill. The number of adult blue crabs in the Gulf of Mexico following the oil spill remained unchanged. The abundance of blue crabs in the early stages of life decreased, but studies were not able to conclude that it was a definite result of the Deepwater Horizon oil spill.3 The research provides considerable progress for understanding the higher-level trophic interactions in salt marsh food webs. However, the Gulf of Mexico is teeming with biodiversity, so studying the effects of an oil spill on all of the species involved in the ecosystem and food web is exceptionally challenging. The team was unable to come up with oil sensitivity index scores for a few salt marsh species, including reptiles and omnivorous terrestrial mammals. Reptiles included snakes, alligators, and terrapins. The land mammals that the team was unable to study were coyotes and raccoons. Researchers like Fodrie normally repeat experiments over and over to confirm results, but Fodrie acknowledged the fortuitous aspect of his study. As the oil spill occurred “emergency room science” was conducted, and “everyone scrambled to collect data that would soon disappear”.2 According to Dr. Fodrie, the constructed model of the Gulf of Mexico salt marsh food web in relation to the relative oil sensitivity will allow scientists to predict responses to future oil spills in areas similar to that of the Deepwater Horizon oil spill. His study provides a valuable template for future research on trophic interactions, whether they be in salt marshes or completely different ecosystems. The framework for evaluating the role of each individual species in the food web as they respond to environmental stressors will facilitate more studies in the future. This method provides scientists with the data they need to properly respond to environmental stressors capable of decimating entire ecosystems anywhere in the world. When scientists know what the effects of an environmental stressor will be before it occurs, they can determine the best possible response plan to help ecosystems recover based on important species and their resistance.

Figure 2. Oil Sensitivity to Food Web Importance Matrix. Image courtesy of ESA Journals.

References

1. F. Joel Fodrie. UNC Marine Sciences. https://marine.unc. edu/people/faculty/f-joel-fodrie/ (accessed February 3, 2020). 2. Interview with F. Joel Fodrie, Ph.D. 01/30/2020. 3. Able, K; Christian, R; Fodrie, J; Jensen, O; Johnson J; López-Duarte, P; Martin, C; McCan, M; Olin, J; Polito, M; Roberts, B; Ziegler, S. Key taxa in food web responses to stressors: the Deepwater Horizon oil spill. 2017, eCollection