In Pennsylvania’s Susquehanna River, invasive flathead catfish have replaced native species as top predators, drastically changing the local aquatic food web. A new Exploratorium study by researchers Olivia C. Hodgson, Tyler Wagner, and Sydney Stark sheds light on this important subject. Their findings paint a concerning picture of the effect invasive species have caused on the river’s ecosystem. The study grabbed attention for its documentation of flathead catfish’s huge and fast age structure. It sheds light on their incredibly diverse diet, taking advantage of a wide variety of prey species.
Research on the implications of grasshopper swarming for ecology published in the journal Ecology on September 4. It compared invaded and uninvaded parts of the river. That concern motivated the study’s objective—to quantify the flathead catfish’s impact on species of conservation concern, such as mussels and darters. It sampled primarily channel catfish, smallmouth bass, crayfish, and minnows. Researchers collected 279 individual fish across 5 species and 64 individual crayfish for stable isotope analysis. These trendlines provide an unmistakable picture of the ecological devastation wrought by this predatory invader.
The Rise of Flathead Catfish
Flathead catfish have rapidly grown into apex predators throughout the Susquehanna River system. Their fast growth rate and large body sizes compared to native species lend these predators a decided competitive advantage. Hodgson says flatheads put on weight fast in this river system. Their potential body size and capacity to consume diverse prey types are immense.
To be able to compare, the researchers brought a diverse group of fish species. Their catch included 79 flathead catfish, 45 smallmouth bass, 113 channel catfish, and 42 minnows. These results indicate that flathead catfish are well suited for this context. As top predators, they are key players in determining the population dynamics of their prey.
Hodgson said these findings are an important indication of what’s to come. He underscored that because adult flathead catfish have few natural predators, they are able to exert a perniciously powerful effect on the ecosystem. That fish-finding capability puts native fish populations—a keystone species in the river—at long-term risk.
Impact on Native Species
As invasive flathead catfish settle into their new ecosystems as apex predators, native species are left with no other choice but to evolve or perish. This study indicated that resident species shift their diets to minimize competition and predation pressure from flathead catfish. Hodgson noticed that resident species are changing their diets. They do this in order to outcompete the invader and to avoid being eaten by it.
This study employed stable isotope analysis to reveal predatory relationships within the Susquehanna food web. It paired such patterns in habitats that were invaded and uninvaded. This rigorous approach gave the researchers key insights into how invasive species could upset the established interactions between native species. Hodgson added, “Stable isotope analysis was key to revealing important patterns in the Susquehanna food web. It allowed us to discover fascinating connections between species and, most importantly, it showed how invasive species could upset interactions and competition between native species.”
The study’s results support the “trophic disruption hypothesis.” This ecological theory posits that upon the addition of a new predator, existing prey species adapt by altering both their behaviors and diets. This change often causes long-term destabilization of ecosystems. Hodgson added that their research shows an invasive species can accomplish more than just the expected native suppression. It can profoundly change whole ecosystems, completely rewiring food webs and changing the flow of energy through entire ecosystems.
Methodology and Findings
The researchers formulated an extensive methodology to evaluate the ecological effects of invasive flathead catfish on the Susquehanna River. By looking at invaded versus uninvaded sections, they were able to quantify changes between all the most important fish species in the region and crayfish populations. The information gained from this study was pivotal to understanding competitive resource use and dietary shift in these native species.
Wagner, a scientist with the USGS Pennsylvania Cooperative Fish and Wildlife Research Unit Program and an affiliate professor of fisheries ecology at Penn State, emphasized the importance of understanding these dynamics: “We were able to infer resource use, helping us to better understand potential competition for resources and how this changes when flathead catfish become established.”
You can access the study’s DOI at 10.1002/ecy.70180. It gives you access to a wealth of additional information about their findings and methodologies if you want to dig deeper into this essential research.