Boosting beaver populations could have toxic consequences

A new study from CU Boulder raises concerns about the ecological impacts of reintroducing beavers on a large scale in the Western United States. The research reveals the potential for the spread of mercury-containing toxins in rivers and their surrounding habitats.

The study was presented at the 2023 American Geophysical Union meeting by Clifford Adamchak, a PhD student in the Department of Ecology and Evolutionary Biology.

“In a world where beavers are increasingly being seen as an effective means to achieve various conservation and restoration goals, there is a possibility that we would see an abnormally large flush of methylmercury if we were to reintroduce beavers in the Western U.S. on a larger scale,” said Adamchak. “So it is important to better understand the impacts of their activities.”   

Significance of beavers

Beavers are key players in ecosystem dynamics. Their ability to alter streamflow through dam construction creates rich wetland habitats. These changes have far-reaching effects, including water conservation and improvement in biodiversity.

The study was focused on the impact of beaver activities and their potential role in exacerbating the spread of methylmercury.

Environmental benefits

Beavers were widespread in North American streams before European settlement led to hunting and habitat loss. Their population dwindled, as well as their environmental benefits. 

Over time, beavers change the environment significantly. By damming up streams and trapping water in their ponds, they help to replenish groundwater supplies and maintain wetland habitats for other species, noted the researchers.

Beaver ponds can also ease the effects of drought and help to mitigate wildfires, which is increasingly significant in the face of climate change.

Mercury hotspots 

However, the study highlights a critical concern. Beaver ponds, which often lack oxygen, can become hotspots for bacteria producing mercury-containing neurotoxins. 

“A stream that flows smoothly with nothing stopping it would have very different biological chemical and geological processes than a stream that has cascading beaver dams and ponds,” said Adamchak. “Beaver activities also impact the surrounding landscape, because the animals forage for woody vegetation on land.” 

Human activities, such as coal burning and mining, emit mercury into the atmosphere, which eventually invades water bodies. Here, it transforms into methylmercury, a toxic compound that accumulates in organisms and travels up the food chain, posing health risks.

Key insights 

The research involved collecting over 300 water and sediment samples from beaver ponds in California and Colorado. Adamchak found low methylmercury levels in the water but high levels in the sediment, indicating potential accumulation in the ecosystem. 

This raises concerns about the spread of mercury-containing neurotoxins, especially as beavers move and abandon ponds, allowing vegetation in high methylmercury areas to grow and enter the food chain.

Since the research is still in its early stages, Adamchak said it’s unclear to what degree methylmercury can affect the wetland ecosystem as a result of beaver activities. 

More research is needed

Previous studies have shown that methylmercury concentrations in beaver ponds decrease with age, indicating a lesser potential impact. 

“That suggests beavers probably don’t have overwhelmingly negative effects on the ecosystem. But at this point, it’s very hard to say if more beaver activities are good or bad in terms of mercury levels,” said Adamchak.

Going forward, he plans to investigate the influence of pond age and seasonal variations on methylmercury levels.

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