Not all exotic species need to be controlled, researcher says

Credit: Radboud University

Invasive species often get a bad reputation for harming the environment, but not all of them are harmful.

Ecologist Pim Lemmers argues in his Ph.D. thesis that some non-native species do not need to be controlled. He will defend his thesis at Radboud University on May 30.

One notorious invasive species is the red swamp crayfish.

“It really is the worst,” says Lemmers. This crayfish walks on land, destroys aquatic plants, and digs into banks, harming water quality. It’s a significant problem.

In contrast, the spinycheek crayfish is causing fewer issues in the Netherlands. Although it did transmit crayfish plague in the past, it does not burrow into banks like the red swamp crayfish.

Lemmers’ research focused on various exotic species—animals that do not naturally occur in the Netherlands—and their ecological and socio-economic impacts.

He conducted extensive lab and fieldwork, using a special fishing net and waders to explore the Meuse, the Rhine, and their tributaries.

To catch the fish, Lemmers used electro-fishing, an electrified net that attracts fish. “As soon as you switch off the current, the fish immediately swim away,” he explains.

He counted, measured, and identified the fish, and took some specimens to the lab to examine the ratio of stable isotopes of nitrogen and carbon in their muscle tissue. This analysis helped determine what the fish were eating and whether exotic species were causing harm by consuming the same food as native species.

Lemmers found that some invasive species do harm native ones. For example, the bullhead, a fish native to the Meuse, has nearly disappeared due to competition from the invasive round goby. The round goby is a particularly aggressive fish that quickly finds food and displaces other fish from their hiding places.

However, not all exotic species cause problems. The Cottus rhenanus, a similar fish to the bullhead, has been doing well since the 1990s due to improved water quality. The round goby has not yet invaded its habitat, showing that not all invasives have the same impact.

Another example is the vimba bream, which originally lived in the Danube but has spread to the Netherlands through various canals. It thrives here without troubling native fish.

Lemmers emphasizes that not all exotic species have negative impacts. Some can even be beneficial. For example, the zander was once considered an exotic species but is now well-established in the Netherlands and plays an important role in commercial fishing.

Lemmers’ research provides a valuable risk analysis of exotic species, helping the government make informed decisions about managing them.

His findings suggest that while some invasive species need to be controlled, others can coexist with native species or even offer benefits. This balanced approach can lead to more effective and nuanced management of exotic species.