Alien species fly on the wings of ducks and other waterbirds

ring-billed gull

Seeds can survive a trip through the digestive tract of birds such as the ring-billed gull. Because of that, waterbirds can transport alien species to new homes where they may cause trouble.

T. Siegfried

A few years ago, researchers in Wisconsin were trying to figure out how invasive species were spreading between lakes in the state. A survey of 450 lakes had turned up plenty of worrisome species, such as zebra mussels and spiny water fleas. But to better manage those plants and animals, scientists and managers needed to know how they were moving around. So researchers looked closely at where the invaders lived and saw that lakes with boat access were often home to invasive species, while remote lakes were not. People and their boats — not birds or other wildlife — must be the ones transporting the troublesome species, the researchers concluded.

That the Wisconsin researchers even considered ducks and other waterbirds as potential spreaders of invasive species was somewhat out of the ordinary. Despite the fact that scientists as far back as Charles Darwin considered birds as potential vectors in the spread of aquatic plants and invertebrates, waterbirds have been left out of most recent work on invasive species, and that could undermine attempts at management, contend the authors of two reviews in Diversity and Distributions.

In the first review, Chevonne Reynolds of the University of Cape Town in South Africa and colleagues searched for studies that examined the dispersal of alien species by waterbirds and came up with only 14. It wasn’t a lot, but the studies showed that birds could transport nonnative plants and invertebrates to new homes, both on their bodies and inside them.

Andy Green of the Spanish National Research Council in Sevilla also tackled the subject, finding 22 additional studies and a total of 79 alien plant species and eight alien invertebrates that could be transported from one place to another via various waterbirds, including ducks, geese, gulls and albatross. “Aliens are usually dispersed after being ingested or becoming attached to plumage, bills or feet,” he notes, “but waterbirds also disperse alien species when making nests [or] when preying on other vectors such as fish.” One study even found nonnative barnacles stuck to tracking bands that scientists had attached to gulls.

Darwin and other 19th- and early-20th-century scientists looked to birds as potential spreaders of species around the globe. In modern times, humans have become the main perpetrator when it comes to species dispersal, moving them from one place to another, often accidentally, to new homes where they can cause huge problems. Most current studies, Green notes, focus on the consequences of this human activity rather than “unaided” pathways of species spread, such as by bird. That could prevent natural landscape managers from fully comprehending what they’re up against since even the limited amount of research shows that waterbirds can play a “vital role” in spreading alien plants and animals.

And even if waterbirds aren’t playing a role in spreading invasive species in a particular region, that’s good information to have, too. In Wisconsin, for instance, now managers know that boaters are largely responsible for moving species around. So the next time an angry boat owner complains about having to clean his craft before moving it to another lake — and sneers that ducks don’t have to wash between landings — there’s solid evidence for why the practice is necessary. 

Sarah Zielinski is the Editor, Print at Science News Explores. She has a B.A. in biology from Cornell University and an M.A. in journalism from New York University. She writes about ecology, plants and animals.

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