BY MARTHA GOODWIN
Predation fear experienced by wildlife can shape the ecosystem, researchers have argued in a recent study, featured in the journal Nature Communications on Wednesday, February 24.
The experiment was conducted by a team of experts led by Liana Zanette, associate professor of conservation and behavioral ecology at the Western University, in Ontario, Canada.
The purpose was to test a prior hypothesis, according to which predation fear is sufficient when it comes to influencing the behavior of certain animals, even when apex predators are actually absent.
The theory initially emerged in the 1990’s, when it was decided that gray wolves should once again inhabit Yellowstone National Park, following 7 decades during which they had been removed from this landscape.
It was discovered that after a while the elk population suffered a significant decline, allegedly because these mammals became too apprehensive due to the close proximity of the gray wolves.
Instead of grazing vegetation and mating, these ruminants were too preoccupied with observing their surroundings, so as to detect the presence of potential predators as soon as possible.
As a result, they started producing much fewer offspring, and grew more frail and vulnerable, the deer population that eventually perished being much higher than the number that actually fell victim to gray wolves.
As a consequence, since there were fewer elk feeding on grass, shrubs, leaves and bark from aspen trees and willows, the vegetation flourished.
That enabled beavers to thrive, given that they had much more wood to eat and build nests with. Also, the number of songbirds grew significantly, as more trees emerged. Basically, the entire ecosystem was dramatically altered, seemingly just because of the menacing presence of the gray wolves.
Some scientists have challenged this theory, claiming that actually the elk population dwindled due to other factors, such as drought and predators like bears and humans.
However, more recent research has once again supported the importance of predation fear in altering wildlife population patterns.
For example, in 2011 it was revealed that the number of hatchlings produced by song sparrows can be curbed by around 40% just by installing speakers broadcasting sounds made by owls, hawks and other birds of prey.
The findings held true even when the sparrows’ nests were guarded by fences and mesh which would’ve made it impossible for predators to ever reach them.
Similarly, in 2012, Dror Hawlena, senior lecturer in ecology and evolution at the Hebrew University of Jerusalem, proved that spiders whose mouths have been glued shut can terrorize grasshoppers.
Basically, being near their predators, can heighten the leaping insects’ stress levels, causing them to change their diet patterns. Even after they die, their remains have much lower levels of nitrogen, soil microbes therefore having a harder time releasing decomposing enzymes.
Now, this new study has once again investigated the effects of predation fear on wildlife distribution and behavior.
The focus was on unusually fearless raccoons that populate the Gulf Islands, situated south of Vancouver. As a whole, raccoons tend to be nocturnal and wary of open spaces, preferring thick forests where they can easily hide away from predators.
However, in the Gulf Islands, raccoons are incredibly reckless, foraging for food in broad daylight, showing little preoccupation for what happens around them, even when choosing grounds with sparse vegetation.
That’s because in this area the species’ key predators, such as wolves, pumas and bears, have already been depleted, and the only danger is posed by domestic dogs.
The setting seemed ideal for Liana Zanette’s trial, which involved setting up speakers in the trees located on two beaches from the Gulf Islands.
On one of the coastlines, recordings of howling dogs were played, meant to induce predation fear among the raccoons.
On the other beach, sounds made by sea lions or seals were broadcast instead, based on the premise that the supposed proximity of these animals will have no effect on the raccoons’ feeding practices and overall behavior.
It was discovered that the “landscape of fear” weaved just by evoking the presence of an apex predator is indeed a valid concept.
On the beach where barking sounds had been transmitted, raccoons reduced the amount of time they dedicated to food searches by approximately 66%.
As a direct consequence, after just a month had passed, the number of worms found in the area increased by 59%, the red rock crab population climbed by 61%, and a similar upward trend was also reported among rock pool fish, whose numbers soared by 81%.
Meanwhile, as red rock crabs became more widespread, the number of periwinkle snails that these crustaceans feed on was greatly reduced, and so was the population of staghorn sculpin fish, which vie with the crabs for the same prey.
The findings clearly show that predation fear does indeed influence ecosystems significantly, and suggest that by triggering the depletion of many apex predators, human beings have also caused numerous other species to be severely perturbed, in ways we are yet to fully comprehend.