Just the sound of dogs was enough to keep raccoons from foraging in their favorite crabbing spots along the shoreline of British Columbia. For some prey species, the fear of carnivores has the same effect as predation itself, and these effects can be felt throughout the entire food web – from raccoons to crabs to the fish that crabs eat. The findings, published in Nature Communications this week, highlight the importance of protecting and reintroducing large carnivore populations.
Within ecosystems, the presence of top predators can reduce the abundance of prey, which then increases the numbers of organisms that the prey species either feeds on or competes with. Previous work suggested that fear and avoidance of predators may have functionally similar effects to actual predation if the prey animals leave the local ecosystem because of it. Though until now, exactly how this fear affects the rest of the food web has been unclear.
A team led by University of Victoria’s Justin Suraci wanted to see if the fear of dogs (a large carnivore) produces these cascading effects in a food web comprised of wild, free-living raccoons (a mesocarnivore) and the crabs and fish they eat on four Gulf Islands: Coal, Portland, Wallace, and Penelakut. Many native large predators known to hunt or harass raccoons – wolves, cougars, and black bears – were wiped out on these islands by humans in the last century, leaving “fearless” raccoons to forage freely, devastating local plant and small animal populations.
Over the course of a month, the team monitored the behavior of local raccoon populations while playing 10-second recordings of dogs barking using speakers hanging in trees (see video below). They also played recordings of the vocalizations of non-predators – harbor seals and Steller sea lions – for another month. The influence of top predators is more far-reaching than researchers thought: The effects of fear alone were felt throughout entire food webs.
Wolf on the shoreline in Clayoquot Sound, British Columbia. Where native large carnivores persist, shorelines can be risky habitats for raccoons. Justin Suraci
The raccoons’ immediate responses to playbacks of dogs were to either abandon foraging entirely by leaving the intertidal area or reduce foraging and increase their vigilance. These same reactions persisted throughout the month-long playback, and overall, the raccoons spent 66 percent less time foraging.
Furthermore, this reduction in raccoon foraging time was followed by a 97 percent increase in the abundance of intertidal crabs, a 61 percent increase in subtidal red rock crabs, and an 81 percent increase in intertidal fish, based on trapping and mark-recapture experiments. Some invertebrates species that weren’t preyed on by raccoons suffered low survival rates: Staghorn sculpins and periwinkle snails were outcompeted or eaten by red rock crabs newly released from the predation pressure created by raccoons.
Previous attempts to manage unchecked populations of small predators were often ineffective. “The reason for this is that humans can't actually replicate the role of large carnivores,” Suraci explains to IFLScience. “These top predators kill some prey, analogous to our hunting and trapping programs, but they also instill fear with their mere presence, changing the behavior of entire prey populations.”
Raccoon hears playback of dogs. Justin Suraci