Trophic Cascades

The Unexpected Effects

written byMatt Davies Co-Founder, Mossy Earth

Matt Davies

Trophic cascades take place when predators at the apex of a food chain alter the numbers not just of their own prey, but also of the species with which they have no direct connection. Their impacts cascade down the food web, in some cases radically transforming the ecosystem and surrounding landscape.

Vultures & Rabies

One of the strangest trophic cascade effects is the correlation between the decimation of India’s vultures and an epidemic outbreak of rabies. In a relatively short period, vulture populations dwindled due to the use of an anti-inflammatory drug administered to cattle called diclofenac. The drug was fatal to vultures when they consumed carcasses containing traces of it. As the vulture populations collapsed, the carrion they ate, was instead consumed by feral dogs, which correspondingly boosted their numbers. Dog bites are said to be the cause of 95% of the deaths from rabies in India, and the rising number of feral dogs has meant more people are likely to be infected by the disease.

A pair of Griffon vultures on a craggy cliff in the Douro valley, Portugal
A pair of Griffon vultures look on for carrion

Beavers, Salmon & Flood Prevention

In northern Europe, salmon that live in beaver dams are generally much bigger than salmon in other parts of the rivers. The dams built by beavers provide salmon with shelter they cannot find elsewhere, and so the juvenile salmon grow faster and become healthier and stronger. The total weight of all the creatures living in the water may be between two and five times greater in beaver ponds than in undammed waterways.

In addition to improving salmon stocks, the beaver plays an unwittingly yet vital role in flood prevention. So much so that environmental agencies are now mimicking beaver behaviour by putting woody debris back into streams and rivers to slow the water flow. Such an approach to flood management is simple and effective but very expensive – while beavers could be doing it for nothing!

Beavers provide salmon with shelter they cannot find elsewhere, which is just one of the positive tropic cascade effects beavers can have.
Beavers provide salmon with shelter they cannot find elsewhere.

Lynx & Lyme’s Disease

Though the majestic lynx has not yet been reintroduced into Scotland, there are a growing number of advocates for its return. An introduction of lynx would help curb numbers of roe and other small deer species, which currently overrun much of the Scottish Highlands.

Yet an even more surprising trophic cascade effect is lynx suppressing deer numbers would be a corresponding decline in cases of Lyme’s disease. Spread to humans by deer ticks, Lyme’s disease is a debilitating and sometimes incurable illness, which according to an article on Lyme’s disease in Health Spectator has increased in England, Wales and Scotland over the last ten years. There could be no better time to bring back the lynx!

A lynx in a forest. There are a growing number of advocates for the return of the lynx to Scotland for the positive trophic cascade effects they can have on an ecosystem.
There are a growing number of advocates for the return of the lynx to Scotland.

Wild Boars & Bluebells

Every springtime bluebells transform the UK’s woodlands, and it’s these uninterrupted carpets of intense blues and purples that have made British woods so famous. However during a recent visit to Knepp Wildland in Sussex, I learnt an inconvenient truth that a monoculture of any species (including bluebells) is an indication, not of the wealth of the ecosystem, but its poverty!

If you go to Knepp Wildland in spring, which is as close to being an undisturbed ecosystem in the UK, you won’t find woodland floors carpeted in bluebells. Instead, you’ll find a whole host of wildflowers jostling one another in an explosion of colour. Such vibrancy and variety demonstrate just how much the rest of Britain’s woodlands are missing, and the extent to which wild boar or in the case of Knepp, Tamworth pigs transform their environment.

Trophic Cascades are not just synonymous with large carnivores. - The free-roaming Tamworth pigs at Knepp and the wild boars of northern Europe can live alongside bluebells, but not only bluebells. By rooting and grubbing on the woodland floor, they create small wetlands in their wallows, and habitats for a fascinating and vibrant host of flora and fauna, and ultimately a much richer ecosystem, albeit with fewer bluebells.

A bluebell woodland in England.
A carpet of bluebells can indicate a lack of biodiversity Photo by: Alex Wrigley @ www.clickandlearnphotography.com

Trophic Cascades & Climate Mitigation

Trophic cascades can in certain circumstances help to combat and mitigate the impacts of climate change by restoring healthy ecosystem services. For example, where herbivores exist at extremely high densities, excessive browsing is limiting the ability of woodlands to regenerate, thereby inhibiting their ability to sequester carbon. Returning predators to an area can therefore help woodlands regenerate and lead to more carbon being captured in the trees and soils. Similarly, an absence of marine predators, specifically whales could lead to increased greenhouse gases. Whales play a crucial role in ocean carbon sequestration by circulating essential nutrients from the bottom of the ocean to the surface, where they are feed on phytoplankton and other carbon sequestering flora. By conserving these species, we can protect their vital place in the ocean’s food web and secure their role in capturing carbon and fighting global climate change.

Two whales breaching the surface of the ocean. Similarly, an absence of marine predators, specifically whales could lead to increased greenhouse gases.
An absence of marine predators, specifically whales could lead to increased greenhouse gases.

Wolves & Soil Fertility

The infamous trophic cascade effect of wolves of Yellowstone, kept elk populations in check as well as influencing their grazing patterns, which inadvertently rewilded riverbanks. The resulting regrowth attracted greater numbers of songbirds, larger birds of prey, and enabled populations of salmon, beaver and bison to increase.

And if that wasn’t enough, the wolves unwittingly re-engineered the soil composition. As the numbers of salmon rose, which are caught by bears, otters, eagles and ospreys, as did the amount of salmon carcasses dragged onto land and eaten. The carcasses, rich in nutrients, were spread along the riverbanks in the animals’ dung. It’s said that up to 16% of the nitrogen in the leaves of trees within 500m of a salmon stream came from the sea.

Two Iberian wolves play together. Wolves epitomise rewilding and trophic cascades.

The last word in ignorance is the man who says of an animal or plant: What good is it?

Aldo Leopold, an influential American scientist and conservationist

Sources & further reading

Peer Reviewed Research Section
  1. Loss of large predators has caused widespread disruption of ecosystems - Science DailyExternal link
  2. Making space for rewilding: creating an enabling policy enviornment - Oxford University Research ArchiveExternal linkIcon Peer Review
  3. Flood management and nature – can rewilding help? - ECOS JournalExternal linkIcon Peer Review
  4. Wolves influence elk movements: behavior shapes a trophic cascade in Yellowstone National Park - Wiley.comExternal linkIcon Peer Review
  5. What determines the strength of a trophic cascade? - WileyExternal linkIcon Peer Review

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