Top predators: shaping landscapes

In the absence of large predators, raccoons can have dramatic effects on birds, crabs and fish populations.

A raccoon perches on a branch in the intertidal zone in the middle on the night.

The large carnivores that inhabit BC’s coast – cougars, wolves and bears –engender deep respect from many coastal residents for their intelligence, their complex behaviours and their beauty.

However, these species are not just charismatic, they’re also critical components of coastal food webs, the top predators of an interconnected community that extends from the forests into the sea. Our native large carnivores help to maintain healthy ecosystems by keeping medium-sized “mesopredators” in check, preventing them from over-consuming smaller prey animals and thereby protecting biodiversity at many levels of the food chain.

Sadly, parts of our coast have lost their native populations of large carnivores, releasing mesopredators such as raccoons to sharply increase their impacts on a range of smaller animals. Raincoast’s research in the Gulf Islands, where large carnivores native to the islands have been extirpated, shows that raccoons have dramatic effects on both terrestrial and marine species, reducing the abundance of some bird species by almost 80% and reducing nearshore crab and fish populations by up to 95%. While an unfortunate example of the consequences of losing top predators, the absence of large carnivores in the Gulf Islands may help us to better understand just how important they are for the conservation of entire coastal ecosystems.

Raccoon foraging in the intertidal zone near Tofino

A raccoon  is captured by a field camera foraging in the intertidal zone at night.

Large carnivores control mesopredators by eating them, but that’s only part of the story. They also induce fear, and this fear can have far reaching effects. Raccoons and other mesopredators change their behaviour in the presence of large carnivores, spending more time hiding and less time foraging, which in turn benefits a diverse range of prey species. With the loss of large carnivores comes a loss of fear, and the dramatic impacts of raccoons in the Gulf Islands may be due in part to their fearlessness in the absence of predators, which allows much more time for eating.

Through a series of field experiments, Raincoast researchers have found that fear of large carnivores alone can indeed benefit raccoon prey and affect coastal food webs. Introducing just the sounds and smells of large carnivores caused lasting changes in raccoon behaviour, and these behavioural changes reversed the impacts of unrestricted raccoon foraging, leading to increases in crab and fish abundance. This new finding deepens our understanding of the ways in which large carnivores are connected to the landscapes (and seascapes) they inhabit, showing that the mere presence of these top predators is crucial to maintaining balanced food webs.

Two members of the Raincoast mesopredator field team

Lead researcher Justin Suraci (left) with  field crew doing quadrat sampling.

With this in mind, Raincoast’s Mesopredator Team has launched into its fourth field season with the goal of comparing mesopredator behaviour and food web impacts between habitats with and without large carnivores. This field season brings us to beautiful Clayoquot Sound on the west Coast of Vancouver Island, where healthy populations of cougars, wolves and bears persist, and where raccoons are not only less abundant, but are presumably much more wary as well. Work is underway to test whether our experiments in the Gulf Islands match the reality in Clayoquot Sound, where the presence of large carnivores may keep raccoon foraging in check. By using experiments and observations to demonstrate the tight connection between large carnivores and coastal food webs, we hope to underscore the urgency of protecting the habitats on which these charismatic predators depend.

For the coast,

Justin Suraci.

Help support Raincoast’s science and research programs.