If you have ever played Jenga, you know how important a strong foundation is. Remove the top block, and your wooden tower might shake, but it won’t fall. Remove a bottom block, and your tower will plummet.

In ecosystems, each trophic level, or position in a food web, is a “bottom block.” Whether it is a microscopic plankton or a twenty foot long goblin shark, removing any species will have a series of negative effects on other trophic levels. This phenomenon is known as a trophic cascade. 

Although trophic cascades arise from population shifts in a single species, the implications can be so monumental that they affect the abiotic landscape.  Take the feral hog for instance. These omnivorous animals thrive in a range of habitats. One such habitat is the salt marsh, an ecosystem that is especially resistant to climate change and provides a wide breadth of ecosystem services. Yet, these benefits are being challenged by the destructive foraging behaviors of feral hogs. 

Above: A sounder of feral hogs wade through a salt marsh. Image courtesy of Springer Nature.

In their attempts to feed on mussel mounds, feral hogs trample through the salt marsh, leaving a wide wake of destruction marked by hog feces, ripped up cordgrass plants, and shattered mussels shells. These mussels have an important facultative relationship with the cordgrass in the marshes; while mussels shield the cordgrass from drought, attract burrowing crabs, and facilitate enhanced soil richness, the cordgrass builds the ideal environment for mussel growth by providing shade and settlement substrate. Thus, by excessively feeding on mussels, feral hogs threaten the mutualism between cordgrasses and mussels, destabilizing the overall ecosystem and increasing its vulnerability to climate change.

While one hog may not seem to make much of a dent, a whole sounder (a group of hogs) can wreak havoc. In fact, one study by researchers from our very own Nicholas School of Environment found that in some areas, hogs caused a 50% reduction in cordgrass biomass, as well as a slower overall post-drought recovery rate.

This complex interaction is just one example of how top-rank predators can affect the environment in a fashion appropriately dubbed the “top-down” effect.

From Freshwater to Saltwater: Paalman’s Journey to Marine Science

Above: Tiffany Paalman and Dr. Brian Silliman. Image courtesy of Dr. Brian Silliman. 

As a second year Ph.D student in Dr. Brian Silliman’s lab at the Nicholas School of Environment, Tiffany Paalman is currently unravelling similar mechanisms of disturbance by marine megafauna in an even more dynamic setting: coastal ecosystems. 

Although Paalman is currently diving deep into becoming a marine science expert, she wasn’t always surrounded by the ocean. Growing up next to the massive Lake Michigan in Green Bay, Wisconsin, Paalman was much more familiar with the freshwater ecosystems that populated her backyard. However, by the time she got her degree in animal biology at the University of Wisconsin–Green Bay, Paalman knew she wanted a change of pace. Equipped with unique interactions, unpredictable patterns, and rich species diversity, marine science was the perfect answer.

Part of what attracted Paalman to marine science was how impactful marine science is. As the ocean covers around 71% of Earth’s surface, provides billions of jobs, and houses more than half of all animals on Earth, understanding marine science means understanding the world. Unlike other fields, which may be more limited to scientific aspects, marine science spans across a multitude of topics. Disciplines such as sociology and political science intersect with marine science to create meaningful international policies, while mechanical engineering contributes valuable field research tools such as drones and submersibles. “There’s a lot of ways to get involved with marine science without necessarily having to only be a marine scientist,” said Paalman. 

It is at Duke’s marine science community where Paalman finds both the freedom to collaborate across many different fields within the realm of marine science and the opportunity to conduct her science in the water. 

Above: A video of a past year’s travel course in Belize. Image courtesy of The Nicholas School of the Environment.

While Paalman has the opportunity to explore a coastal shoreline in North Carolina at the Duke University Marine Lab, global outreach initiatives have also allowed Paalman to venture to places like Belize as a teaching assistant for travel courses. “I think that it's really exciting, all the people you get to meet, all the cultures you get to see and learn about is cool, beyond just the science,” said Paalman. 

Above: Paalman with a field crew in Hoop Pole Creek, Atlantic Beach, NC. Image courtesy of Tiffany Paalman (Marine Science and Conservation, MSC, Program ‘30).

Molding the Landscape: The Impacts of Stingrays on Coastal Ecosystems

With aspirations of becoming a general ecologist, Paalman seeks to investigate “the role of predators on the structure, biodiversity, and resiliency of temperate oyster reefs.” Her interest in these delicate yet vital ecosystems began in June of 2024, when she was helping another graduate student with a project on the North Carolina coast.

Above: A sole stingray glides through a sandbank surrounded by pits. Image courtesy of the Scientific American. 

While out in the water, she observed numerous peculiar sand pits that resembled miniature underwater craters. After asking fellow graduate students and consulting the world’s best assistant, Google, Paalman found out that these pits were formed by the foraging and burying behavior of stingrays. While individual stingrays only make a few pits, collectively, many stingrays can drastically shift the landscape of a coastal ecosystem. In fact, researchers from the University of Newcastle in Australia found that over the course of seven days, the inhabitant stingrays of Australia’s Brisbane estuary made 1,090 sand pits and transported almost 80 tons of sand in just two percent of their feeding area!

Dugongs (sea manatees) and horseshoe crabs are also known to create similar pits. This behavior has been scientifically linked to changes in seagrass edge expansion and other coastal ecosystem shifts. However, stingrays remain an elusive species. Especially within the NC coastal area, the fundamental trophic impacts of their foraging behavior and the significance of stingray-made sand pits have yet to be delineated. As Paalman puts it, “They are finicky because they move around a lot.”

‍Above: An open plot with a stingray foraging pit next to an oyster reef. Image courtesy of Tiffany Paalman (MSC Program ‘30).

Scientific interest in oyster reefs has grown significantly due to their fragility, commercial importance and role in stabilizing coastal ecosystems. Destructive fishing techniques, such as dredging, strip away entire oyster colonies, while climate change continues to push the limits of what these foundational species can endure. Due to these combined effects, 85% of oyster reefs in the past century have been degraded. With such an impressive array of ecosystem services and an equally alarming plethora of anthropogenic threats, Paalman found that “There are more questions you can ask about [oyster reefs] all the time, enough to go down a wormhole.”

‍Above: Hundreds of stingray foraging pits lie near the edges of an oyster reef. Image courtesy of Tiffany Paalman (MSC Program ‘30).