The cascade effect: wolves and rivers, bees and Ireland
If it’s hard to see how a wolf can change a river, it’s easier to see how fewer bees might be bad for crops. Both show how small changes can cause big ripples
Trophic cascades: a wolf in Yellowstone National Park. Photograph: Norbert Rosing/National Geographic/Getty
If you’re interested in biodiversity you might have seen George Monbiot’s short film about the reintroduction of wolves to Yellowstone National Park, in the United States. The video, which is called How Wolves Change Rivers, and comes from a Ted Talk that the British environmentalist gave in 2013, tells the story of a trophic cascade, an ecological process that starts with the interruption of an ecosystem and results in a diverse range of knock-on effects on flora and fauna, many of them unforeseen. The reintroduction of wolves to Yellowstone in 1995, after an absence of 70 years, is the classic example.
Monbiot explains how, despite our efforts to control them, deer had overgrazed almost the entire park. Bushes had been eaten down to the point at which they withered away, and grasslands were picked bare. Introducing a few wolves meant that they would kill a few deer, but the big impact was to deter the deer from vast areas of the park, such as valleys and ravines, where they might be trapped in the event of a wolf suddenly appearing.
So the trees started to grow again, bare valley sides becoming forests of aspen and willow. This in turn was followed by the appearance of many more birds. Beavers also started to return, creating dams in the rivers and building refuges for otters and muskrats and fish, among other animals.
Because the wolves killed coyotes, the rabbit and mouse populations increased, attracting hawks, weasels and badgers. Ravens and bald eagles became more common, too, as did bears – which could feed on berries in the regrowing shrubs.
What fascinated Monbiot especially was the way the recovering vegetation stabilised river banks, reducing flooding and creating more deepwater pools and channels. The reintroduction of wolves changed not just the ecology, he concluded, but also the landscape.
Although the speed and scope of some of the changes he attributed to wolves have been challenged, notably by the field biologist Arthur Middleton in the New York Times, everyone seems to agree that the reintroduction of wolves was positive and necessary.
Trophic cascades as dramatically successful as the Yellowstone example are hard to find in Ireland, but scientists are also having successes.
Kate McAney of the Vincent Wildlife Trust says, for example, that reintroducing bats can be particularly beneficial for humans, as they eat the midges that can be such a pest on rural summer evenings. “That’s a great example of where encouraging bats can be helpful to people.”
Efforts to reintroduce the golden eagle have been more challenging. The first of 50 young birds was released in Glenveagh National Park, in Co Donegal, in 2001, but they have struggled to overcome ecological conditions that mean it’s hard for them to find enough prey to catch and eat.
A more widespread problem is that of the decline of the honeybee – described as our smallest and most indispensable farm worker – which has been dying in worrying numbers. Almost half of the insects in some colonies have died in recent winters, causing particular concern given that bees are responsible for the pollination of more than 30 per cent of global food crops.
Last week the European Parliament hosted an annual two-day meeting about how best to protect and propagate bee colonies. Bees Caring for Europeans, Europeans Caring for Bees heard that the extinction rate ranges from 9 per cent to 30 per cent across the continent, and up to 60 per cent of bee species may now be endangered.
The Independent MEP Marian Harkin points out that European agricultural policy is putting a greater emphasis on areas such as the enhancement of biodiversity, the protection of the environment, and the encouragement of bees and other pollinators. She urges farmers to take a positive approach to delivering on this “public-good dimension” of the common agricultural policy.
Harkin also says that, given the commercial and environmental value of bees, insects and butterflies, the European Parliament should back “the precautionary principal in relation to harmful chemicals which can damage bee colonies”.
The Republic’s farming organisations should, she adds, follow the example of the Ulster Farmers’ Union by taking part in the All Ireland Pollinator Plan, which seeks to protect bees and other insects and to enhance their habitats.
That plan’s participants might be hoping to have as much of an effect on the bee population as the return of the pine marten has had on Ireland’s squirrels.
Ireland is home to both red and grey squirrels, but only the reds are native. Greys are said to have been introduced in 1911, at Castle Forbes, in Co Longford, when the duke of Bedford gave six pairs as a wedding present to a daughter of the Forbes family. Released on the estate, they mated – since when grey squirrels have outcompeted Ireland’s red squirrels, not least by carrying a pox that quickly kills the reds but rarely kills the greys. Grey squirrels also damage the bark and wood of trees.
People used to kill pine martens because of the damage they could do to flocks of chickens and pheasants, among other prey. Their ability to climb fences and jump into chicken runs made martens a target of farmers and gun clubs.
But then, in 1976, the Wildlife Act made pine martens a protected species, which helped them to recover, according to Dr Colin Lawton, a zoologist at NUI Galway, who with his colleague Dr Emma Sheehy reported the first known population crash of invasive gray squirrels in a paper that appeared in Biodiversity and Conservation in March 2014. The banning of the agricultural poison strychnine is also likely to have helped.
Now red squirrels appear to be quickly recovering in places where the greys have disappeared, Lawton told Science magazine this month. “It gives me faith that the red squirrel has a future,” he says.
It could be that grey squirrels are easier than red squirrels for pine martens to catch, as they look for beech nuts and acorns on the ground, whereas “reds tend to stay in trees and nibble cones”, according to Lawton.
It’s another example of the effects of a trophic cascade.