Besotted by beavers …

Dubh_loch_dam

The beaver dam at Dubh Loch, Knapdale, Argyll, photographed in April 2013.

Beavers are back in the news. An article in today’s Independent reports that Natural England have granted a licence to Devon Wildlife Trust to manage a population of beavers that had escaped from a wildlife park.   This overturned a decision by the Department for Environment, Food and Rural Affairs which argued that beavers are an invasive non-native species and that the Devon population should, therefore, be trapped and returned to a zoo. As I wrote in 2013 (see “In pursuit of beavers …”), beavers do challenge our preconceptions of what is “natural” and I can also report two other recent studies that show how beavers can change both the structure and function of freshwater ecosystems.

The first is a study co-authored by my sometime colleague Nigel Willby on the Scottish Beaver Trial at Knapdale in Argyll (the locations I wrote about last year) which showed changes in the aquatic vegetation due to direct grazing by the beavers, as well as by the water level changes brought about by dam-building. A few species (Schoenoplectus lacustris and Cladium mariscus in particular) decreased due to grazing but the site where the water level rose due to dam building saw an increase in both the diversity and heterogeneity of aquatic vegetation. This, in turn, had beneficial effects on invertebrate diversity in the area.   Or, more accurately, the long-term decline and eventual extinction of beavers due to human pressures altered the balance of plants and animals and the Knapdale trials are giving us an insight into the natural state of the area 200-300 years ago. Paradoxically, our ideas of “natural” are largely shaped by our own expectations, as I commented in a post about Himalayan Balsam last year (see “The future is pink …”)

The second study looks at the wider consequences of beaver activities.   Beaver dams create large areas of standing waters in places that once would have been terrestrial or semi-terrestrial habitats with a stream running through them.   The sediments of these standing waters do not have as much access to oxygen compared to their pre-beaver condition and, consequently, organisms capable of anaerobic respiration can thrive. Respiration in the presence of oxygen produces carbon dioxide as an end-product; however, anaerobic respiration can produce methane which is a far more potent greenhouse gas than carbon dioxide.   A team from the University of Saskatoon in Canada have done some calculations and shown that the resurgence of beavers worldwide is now contributing 200 times more methane to the atmosphere than was the case in 1900.

Like all broad-scale modelling studies, the calculations are based on several assumptions and extrapolations, meaning that we need to treat the figures with caution. However, it is useful because it broadens our understanding of how beavers change landscapes from the visually obvious (less club rush, for example) to the more subtle.   And, once again, it challenges our current understandings. The reality is that beaver-mediated methane production underwent a “blip” between the 17th and 20th centuries and is now returning, slowly, to values that our medieval ancestors (those with gas chromatographs, at least) would have recognised.   Even today, greenhouse gas budgets are dominated by natural sources; though this is no cause for complacency: the absence of beavers in effect created a little extra “headroom” into which we pumped the by-products of our addiction to fossil fuels.

References

Whitfield, C.J., Baulch, H.M., Chun, K.P. & Westbrook, C.J. (2014). Beaver-mediated methane emission: the effect of population growth in Eurasia and the Americas. Ambio 44: 7-15.

Willby, N.W., Perfect, C. & Law, A. (2014). The Scottish Beaver Trial: Monitoring of aquatic vegetation and associated features of the Knapdale lochs 2008-2013, final report.   Scottish Natural Heritage Commissioned Report No. 688.

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