Beavers make wetland. Along the southern border of discontinuous permafrost in Canada, where Euan Nisbet and Mary Fowler have been sampling methane emissions from the Canadian forest there’s a reasonable argument that the majority of the water management is done by beavers. Canadian beavers cut down birch trees with trunks up to 50 cm or more across. They construct elaborate log dams, making lakes up to 10 ha or more,  which raise the water level so that more trees can be floated down towards the beaver lodge. Each beaver may bring a ton or more of digestible organic matter to the table in a year. The bark and twigs are eaten and converted to faeces that support anaerobic methanogens: in short, the whole shallow lake becomes a methane factory. Anecdotal evidence is that in an area where beavers were locally extinct, early air photography shows 2% wetland; a few decades later when beavers had returned, 11% was under water. Eurasian beavers are not so industrious as Canadian but fairly energetic nevertheless.

There’s an evolutionary tale here also. In 1670 the  “Company of Adventurers of England trading into Hudson’s Bay” was founded. Its purpose was to supply beaver coats and top hats to the gentlemen of England (it still exists and is now a major upmarket Canadian department store chain). Simultaneously, the French sent out their voyageurs. Mr. Rostbif and Monsieur Frog then battled for the beaver. By the 1930s, the beaver was almost extinct in North America, with only a few thousand left. An Englishman masquerading as a native American, ‘Grey Owl’, managed to save the beaver in one of its last habitats, spurring the foundation of Prince Albert National Park on the southern fringe of discontinuous permafrost. Some patches of permafrost may still exist in the park, but their days are numbered. The beaver battled back: how better than by emitting methane and warming the world? When did you last see a beaver-skin top hat, or a beaver coat, either in London or Paris, or indeed in the elegant department stores of ‘The Bay’? Now Canada has tens of millions of industrious wetland expanders making methane.

Professor Euan Nisbet, Royal Holloway University of London