There are estimated to be around 30 million beavers across North America. As a keystone species, beavers create richer ecosystems around them. By building dams, they hold water down on its way through the habitat – retaining the flow during times of drought and slowing it down during heavy rains and floods. This creates habitats for other endangered species such as toads and songbirds, while beaver ponds create nurseries for juvenile salmon. However, a study by the American Society of Agronomy says beavers are doing something more: they now represent a relatively new and substantial sink for watershed nitrogen.
Found in agricultural fertilisers, nitrogen is often introduced to streams by runoff. Once in the water system, it has been known to cause eutrophication in coastal waters. This is where a sudden increase in nutrients can cause blooms of algae. As the algae dies and decomposes, oxygen is consumed from bottom waters, creating ‘dead zones’ for fish.
However, Arthur Gold, Professor of Natural Resources Science at the University of Rhode Island and co-author of the study, has noticed that where there are beavers, there is less excess nitrogen in the watershed. Thanks to a bacterium present in the soil of beaver ponds, five to 45 per cent of nitrogen in the water can be removed, the study found. These bacteria, protected in the beaver ponds, were able to transform nitrogen into nitrogen gas.
‘This means the nitrogen is no longer stored within the stream or pond and thus, can no longer degrade downstream water quality,’ Gold tells Geographical. ‘However some of the nitrate is not transformed to gas, but instead is stored in organic soils.’
‘The beavers make the dams, and those dams create the conditions to remove the nitrogen from the water.’
As beaver populations increase and spread into mixed-use watersheds nearer to agriculture, their dams can be found in sites with elevated nitrate levels. These dams and ponds are often considered a nuisance and destroyed, with the beavers relocated to somewhere more rural. However, a destroyed dam can release the stored nitrogen back into the ecosystem. ‘When the dam is destroyed, we expect that the organic soil deposits that have built up in the bottom of the beaver ponds will slowly be transformed by the plant and microbial community,’ says Gold. ‘This can release nitrogen back into the watershed.’