The Eurasian beaver was brought back from near extinction and now thrives across Europe. But this conservation success story may have had at least one unintended and potentially harmful consequence. Scientists report in ACS’ journal Environmental Science & Technology that when beavers build new dams where no previous beaver colonies existed, downstream levels of toxic methylmercury rise, at least temporarily.
In the 19th century, beaver fur was in high demand, and hunters helped supply it. The animals’ numbers plummeted to as few as 1,200 in Europe, with no reported sightings in Sweden after the 1870s. Conservation measures allowed beaver populations to rebound. In Sweden alone, there are now about 130,000 — and many new dams.
While the structures are marvels of natural construction, they also change the chemistry of the water they’re in. They affect the sediments, water flow, oxygen content and temperature, creating conditions that help convert mercury into methylmercury. A few studies have suggested dams can boost levels of this form of mercury, which can cause developmental and neurological problems in animals and people. Oded Lavnoni, Frauke Ecke and colleagues wanted to take a closer look.
Over a two-year period, the researchers tested the water upstream and downstream of 12 dams. They found that methylmercury levels downstream of newly made dams were up to 3.5 times higher than in the upstream water. But dams that beavers reconstructed on abandoned beaver systems didn’t appear to affect methylmercury concentrations. The results suggest that protecting older dams could help reduce levels of methylmercury in areas colonized by the animals.