Sometimes untangling the complex web of ecological causes and effects brought on by invasive species can look a lot like a classic case of whodunit. Prof. Clifford E. Kraft, natural resources, collaborates with Cornell’s cooperative extension. His work removing smallmouth bass from a lake in the Adirondacks has given him a unique window into the mystery of restoring biodiversity where ecological villains have run amok.

Kraft, biological sciences ’75, pursued an M.S. and Ph.D. in oceanography and limnology at the University of Wisconsin, Madison, where he worked with the now famous invasive species Dreissena polymorpha, better known as Zebra Mussels. These pests had come over to America from Europe, carried in the ballast water of large ships. Zebra mussels menaced local mussel populations, water treatment plants and other great lakes water suppliers due to their pervasive crowding of pipes and waterways.

“I often see my work as an ecologist like that of a detective. You have incomplete pieces to a puzzle and you’re trying to put them together,” Kraft said of his current research. Smallmouth bass (Micropterus dolomieu) are a popular sport fish and as a result have been introduced into lakes and water systems where they are not native. Cornell operates a field station on Little Moose Lake in the Adirondacks where this invasive species had significantly reduced the population of native predators and prey.

It was here that Dr. Kraft started the research that provided the material for his presentation “When Efforts to Control Nuisance and Invasive Species Backfire.”

Beginning their research, Kraft and his team took the common approach to eradicating invasive species: simple mass removal.

“We were aided by the fact that smallmouth bass spawn around the shoreline in the spring, making them easier to catch,” Kraft said.

Electrofishing was used to extract the bass while leaving the other species of fish unharmed. The team passed an electric current through the water, stunning the fish and allowing the researchers to sort out the bass for removal while returning the native fish unharmed. This method was also useful for tracking the changes in native fish populations as the removal progressed.

Initial estimates of the smallmouth bass population in Little Moose Lake were around eight to ten thousand. Despite the massive amounts of bass removed each year, the population persisted.

“Removal began in 2000, after several years of pre-removal data collection on the lake … however, each year we were still catching large numbers of bass. Their size had decreased substantially, but numbers were still up,” Kraft explained.

Kraft turned to Elise Zipkin, grad, who had a strong mathematics background. Together with Zipkin and other members of the team, they constructed a mathematical model of the population response based on existing population theories about heavy harvesting. The results were surprising.

The removal efforts had drastically reduced the number of bass who were surviving to adulthood, freeing up resources and reducing predation, allowing the native fish populations to make a comeback. However, these efforts failed at reducing the actual number of bass in the lake. With fewer adults to compete for resources, the smallmouth bass population no longer suppressed its own growth. The result was a drastic increase in the number of new bass hatched each year.

Another interesting population response to the harvesting was regular oscillations in the fish population from year-to-year, which had been seen before during laboratory experiments involving beetles, but never in large animals living in wild populations like the smallmouth bass.

This meant that although they’d achieved their goal, it wasn’t a permanent solution. The team will have to continue to act as a check on the bass population year in and year out to maintain the increases in native species that they have seen.

“Brook Trout and Lake Trout were originally the main predators in Little Moose Lake,” Kraft said. With the prolific numbers of smallmouth bass, they could not compete as well for food sources and declined considerably in numbers. Now the trout are on the rise and their trophic position (how high they sit on the food chain) has increased significantly.

Thanks to the help of good theory, mathematical models and sound data analysis, Kraft has some good ideas about how to address the problem going forward. “We feel it might actually be better to leave more of the larger bass in the lake, rather than just removing everything,” Kraft said. More adult bass may improve the self-suppression of the population and help the overall management of the lake.

In applied departments like natural resources, Kraft believes it’s necessary to strike a balance between development and delivery.

“A large part of what we do centers around delivering solutions to people who come to us with ecological problems, but if we’re not giving equal time to developing good models grounded in theory, then before long we won’t have anything to deliver” he said.

“I think we’re story tellers. I think there’s something about telling stories that’s fundamentally human. As scientists and especially in the social sciences, we very much play the role of detectives,” Kraft said, recommending a favorite book, The Ecological Detective by Ray Hilborn and Marc Mangel. Although when it comes to scientific theories, Kraft does not think all stories are accurate, especially when data isn’t properly fitted to a working model. “I’m very suspicious of the stories people tell, especially scientific stories that aren’t grounded in specific theory,” he said. Like any detective, Kraft doesn’t make his conclusion until all of the evidence is in.