The Thames: One of the World's Most Invaded Rivers

You might call us invasive reporters in England, transplants from America looking for a few good stories in the UK. While we’re minding our p’s and q’s, London is dealing with an entirely different breed of North American invaders, and they’re quickly filling up the city’s largest river.

We’re talking invasive planets, fish, insects, birds, and mammals establishing themselves in the River Thames. We caught up with Michelle Jackson, an aquatic ecologist who last year studied the species of the Thames. The good news is that the river has a rich diversity of life. The bad news is that many of those species aren’t supposed to be here.

Over the past two centuries, nearly a hundred species have been introduced into the Thames, either up stream or directly within the London city limits. The vast majority are fish and plants—species like the fathead minnow that was discovered in 2002 and has spread a bacterial disease to the river’s trout and eels. Or floating pennywort, a plant first seen in the early 90’s that can grow over the entire river channel and suffocate the ecosystem. The common guppy was even spotted in the Thames in 1963.

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The fathead minnow (right), one of the Thames’ invaders, first seen in 2002. Photo courtesy GBNNSS

A good chunk of the species—40 percent to be exact—came from North America. What’s surprising about them, though, is that many of the introductions were intentional, brought over for river stocking or ornamental purposes. Jackson said that London being a main shipping port hasn’t helped. In fact human activity has made it worse. “It’s definitely accelerated,” Jackson told us. “From 1800 to 1820, you get one new species every decade. Now you see a new one every year.”

Of course ecosystems change over time and new species are often introduced into new environments. But what makes invasives more concerning is the impact they have on the broader ecosystem that’s already established. In the U.S., we’ve heard lots about Asian Carp that have spread between rivers in the Midwest and threaten to disrupt the entire food chain in the Great Lakes. The Thames in London appears to be facing a much broader problem, not just from a single offender, but compounded by how invasives interact with each other.

So what can be done? Environmentalists in London have tried to get serious, removing some of the swimming species before they breed and uprooting the plants before they reach too deep. In the past few years, the rate of introduction of invasives has started to slow. One reason why: The economy matters a lot in England (and everywhere, of course, but certainly in the land of pounds and pence). One study last year showed that every year, invasive species end up costing the £1.7 billion ($2.6 billion). So much for just small fish and plants.