'Biodiversity magic pills' – Why Cities Need Beavers

Rewilding is about restoring the intimate relationships between animals and their habitats that have been disrupted by human societies so that wildlife can rebound naturally. Reintroducing keystone species is key to this process. It’s hard to find a better illustration of this than the relationship between the once nationally-extinct Eurasian beaver and Britain’s fastest declining mammal, the water vole.

‘Beavers are like a biodiversity magic pill. You put them into a wetland and they catalyse some incredible processes. They introduce that chaos and mess and openness and destruction and death and rot and decay. Mess is what nature actually thrives in. We’re going to see an explosion of life here.’ - Sean McCormack

Sean McCormack is the force of nature behind London’s first urban beaver reintroduction - the Ealing Beaver Project. A few years ago, while the urban park was being prepared for the beavers’ arrival, Sean showed me around and explained how the loss of the beaver in Britain and the modification of waterways over centuries have put water voles at serious risk.

‘Water voles are an indicator species of the health of aquatic ecosystems. They want to have open, sunny bank-side habitats with lots of aquatic vegetation. They feed on lots and lots of different plant species in the waterside habitat.’, Sean says.

‘They’ve been a victim of modern times and what we’ve done with our river systems. We have canalised our rivers into straight channels in a misguided effort to alleviate flooding. We have tidied those messy margins along rivers. We’ve drained the land in those complex fen and marshland systems that water voles would have thrived in. We’ve lost a load of our wetlands. And we’ve polluted our waterways as well.’

The final nail in the coffin for water voles came from the fur farming industry. As far back as the 1960s, American mink had been escaping from fur farms and establishing wild populations across much of England and Wales. This is a classic case of invasive species: an exotic species brought to a new place by humans, which has unforeseen ecological impacts.

‘North American mink naturally would predate on musk rats, which are a very similar animal [to water voles]’, Sean explains.

‘Our water voles have not evolved alongside mink and don’t have good evasion tactics for mink. And when we’ve put them into the straight lines of our man-made watercourses, mink has a very good opportunity to go up and down a bank system on one side of a river and down the next on the opposite side of a river and clear out the entire water vole population. Mink are stockpilers. A female mink with a litter of young will literally go up and down a bank system and cache every single water vole she kills in a day.’

Due to this combination of habitat loss and mink predation it’s estimated that water vole populations have dropped by 97% since the 1900s.

I can see that just talking about this pains Sean. But it’s also his biggest motivator. During the pandemic, he used his free time to survey for local water vole populations in Ealing and hatch plans for their comeback. The more he researched, the more he realised that beavers were the missing piece of the puzzle.

‘A water vole reintroduction project [in Ealing] would require us to be doing huge amounts of habitat management, coppicing trees and opening up the canopy on a lot of our overshaded stretches of river. And I thought that’s crazy because that’s an impossible task to maintain for the next 20 years. What could do that for us? Beavers!’

Beavers are the quintessential ecological engineer. Mother Nature has gifted them with a unique hatred of flowing water and beavers work tirelessly, felling trees and creating dams, to slow the flow. This causes flowing water to pool and expand, creating rich wetlands with micro-habitats that benefit countless invertebrate, amphibian and rodent species.

Sean is hopeful that once the beavers are established in Paradise Fields, water voles are just one of the species that stand a chance of bucking the national trend of declining numbers.

What’s more, Sean tells me, there’s good evidence that water voles are more resilient to mink predation in the complex habitats created by beavers compared to the simplified watercourses most often found in Britain. Even if American mink still pose a threat in the future, beavers may inadvertently give water voles a fighting chance of evading the predators.

This appreciation of the interconnectedness of natural systems is a key tenet of rewilding. In contrast to traditional nature conservation, which tends to target specific species and actively manage habitat for them, rewilding takes a more systemic approach. Rewilding is fundamentally about recovering lost natural processes so ecosystems can be self-sustaining and humans can take step away from actively managing landscapes. Sean calls it ‘nature-led nature recovery.’

And this is just one of the complex interactions between species that can be found in beaver wetlands. Within just three years, the urban beaver site in Ealing, aptly named ‘Paradise Fields’ has seen a boost in biodiversity. The site has attracted eight new species of bird, two new bat species and one new breeding species of amphibian since the beavers arrived.

As nature’s flood engineers, the beaver family has also been storing and slowing down rainwater on-site, which previous rushed into surrounding neighbourhoods. Since they’ve taken up residence in Paradise Fields, the local London Underground station has remained dry for three winters in a row after decades of seasonal flooding.

If we truly embrace rewilding in cities, the biodiversity recovery could be exponential.

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The quotes from Sean McCormack are from his interview on the Green Urbanist Podcast, available here: Sean McCormack - Rewilding Urban London