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Try telling raccoons that barking dogs don’t bite. The fear of large carnivores, such as dogs, can have knock-on effects throughout an ecosystem.

Predators don’t control populations of their prey just by killing them. They also paint what is termed a landscape of fear, inhibiting prey from feeding and turning parts of their habitat into no-go zones. Now it appears that this has far-reaching effects throughout the food web.

Domestic dogs are the main predators of raccoons on the Gulf Islands of British Columbia, Canada. and his colleagues at the University of Victoria in Canada wondered what would happen if they stoked the raccoons’ fear of dogs without increasing predation. They set up speakers along the shoreline on two islands and played either the calls of dogs, or of seals and sea lions, which also live here but are not a threat to raccoons.

The dog sounds cut the raccoons’ foraging time by 66 per cent over the course of a month. They also led to a rise in the abundance of crabs, fish and worms that raccoons feed on in the intertidal areas, and in turn, to a decline in numbers of those animals’ prey and competitors (Nature Communications, DOI: 10.1038/ncomms10698).

For red rock crabs, for example, this effect must be due to changes in their behaviour, since reproduction takes at least a year. The crabs move from subtidal into intertidal areas to forage, but avoid regions where they detect chemicals released by dead crabs. With raccoons foraging less, these chemical cues decreased and crabs entered the intertidal zone more.

It is the first experimental demonstration of how the effects of fear cascade through an ecosystem, says John Laundre at the University of California, Riverside. “We’re just beginning to understand these multilevel facets that can occur,” says Laundre, who originated the use of the phrase “landscape of fear” in ecology.

To understand how the presence of large carnivores keep herbivores and smaller predators in check and so protect biodiversity is critical, says Suraci. “We’ve succeeded in wiping out large carnivores in most of the globe, and we’re only now beginning to understand the ecological consequences,” he says. “The real message here is that we need large carnivores.”

Journal reference: Nature Communications, DOI:

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