Fear of large carnivores can have the same effect on prey as predation itself, and can also result in knock-on effects that cascade down the entire food web, finds a study of racoons in Nature Communications. This indicates that the influence of top-predators within an ecosystem is more far-reaching than previously thought.
The presence of top predators can have cascading effects within ecosystems by reducing the abundance of prey, which in turn allows other organisms to increase in abundance if this prey species feeds on, or competes with them. Fear and avoidance of predators may have functionally similar effects to predation if the animals that fear being eaten leave the local ecosystem, although how this fear affects the rest of the food web has been unclear until now.
Justin Suraci and colleagues investigate whether fear of dogs (a top predator) can produce these cascading effects in a food web composed of wild raccoon populations (often harassed or killed by dogs), which feed on crabs and fish along the shoreline of the Gulf Islands of British Columbia in Canada. By monitoring the behaviour of the raccoons when exposed to the recorded sounds of dogs barking over the course of one month, the authors find that the time raccoons spent foraging in their preferred intertidal areas was reduced by 66% under the effects of fear.
This reduction in foraging time was followed by a 97% increase in the abundance of shore crabs, an 81% increase in intertidal fish, and a 61% increase in red rock crabs - organisms upon which the raccoons feed. As a result, numbers of other species of invertebrates decreased, as they were outcompeted or eaten by the crabs that had been released from the predation pressure of raccoons, demonstrating that the effects of fear alone can be felt throughout entire food webs.
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