Of Bears, Berries, and Hoofed Invaders
There are few images more fearsome than that of an angry bear, with teeth bared and claws flashing. It is intriguing, then, to consider a community in which a population of mighty ursids was driven to extinction by a diminutive prey species: the deer. Who knew Bambi had a bad streak?
This surprising story hails from reseach conducted by Dr. Steeve Côté on Anticosti Island. This 8,000 sq km island is about 35 km off the stretch of coastal Quebec that is home to the Mingan Archipelago National Park Reserve. Anticosti used to be home to a healthy population of black bears ( Ursus americanus ), according to records stretching back to the seventeenth century. In the early twentieth century, however, bear populations started to evaporate like mist over the Labrador coast. Hunting had not increased, no serious weather events had occurred. So why, after so many centuries, were the bears suddenly failing to thrive on Anticosti?
Bears, despite their reptuations as fierce carnivores, are actually widely omnivorous foragers that easily settle for the equivalent of wild fruit salads, taking in enormous quantities of berries and other fruits in order to plump up for hibernation. The bears of Anticosti were especially frugivorous, due to lack of nut and seed sources on the island, and the paucity of fish compared to the salmon runs exploited by many mainland bears.
Therein was the Achilles heel for these carnivores. In 1896, some ostensibly well-intentioned yet gravely short-sighted humans decided to translocate about 200 white-tailed deer ( Odocoileus virginianus ) to Anticosti. Because bears are not significant predators of deer, mostly feeding on them only during the first couple of months of life, and the population ran amuck, skyrocketing to unbelievable densities. After just 70 years, population estimates ranged from 60,000 to 120,000 deer on the small island.
So, we had a sudden invasion of hoofed aliens onto the island. But how do timid, skittish ungulates succeed in decimating a population of large carnivores?
In a broad approach, there are two forms of ecological competition. Interference competition is the classic, duel-style clash over food or territory that makes for breath-taking footage on nature documentaries. The less sexy but endlessly fascinating counterpart is exploitation competition, in which one species causes declines in another by simply using resources faster and more efficiently, creating hardship for its competitor without having to directly confront it.
So, as the deer population exploded on Anticosti island, the burgeoning herds of ungulates chomped down on every edible bit of vegetation they could find. And this included the shrubs upon which black bears depended for their yearly bouts of berry gluttony. Côté and a team of students surveyed 420 plots, 10 square meters each, in 14 different locations on Anticosti and failed to identify any twigs or stems from the main berry-producing shrubs that used to grow on Anticosti in abundance. They calculated that the berry availability is 235 times lower than the minimum requirement necessary for a black bear to maintain healthy body mass. The deer, without ever laying a hoof on their unfortunate bear neighbors, were able to defeat them through force of appetite alone. In under 75 years, the bears were considered to be extirpated from the island.
Over-browsing by deer has become a common and much-reported problem, with the story of the Yellowstone ecosystem often being used as a prime example of how reintroduction of carnivores can restore balance to an ecological community that has been overrun by herbivores. Yet in those cases, extirpation of carnivores allowed deer populations to rise above carrying capacity, with severe repercussions for the rest of the community. On Anticosti, however, deer were introduced to an island with a healthy population of carnivores, yet their quick reproduction and unstoppable browsing habits still allowed them to overwhelm the resource on the island to such a degree that the berry-munching bears did not stand a chance.
Of course, once the island is completely browsed down, the deer populations will probably crash as well, showing that although in this case the meek did inherit the land, it is a tenuous hold at best.
COTE, S. (2005). Extirpation of a Large Black Bear Population by Introduced White-Tailed Deer Conservation Biology, 19 (5), 1668-1671 DOI: 10.1111/j.1523-1739.2005.00252.x