THE UNIVERSITY OF VICTORIA
OTHER WILDLIFE AND PLANTS BENEFIT
Bears are key recyclers in forest ecosystem, researcher finds
by Valerie Shore
A black bear about to scoop
Valerie Shore photos
Bears may play a much more profound role in the health and diversity of B.C.'s coastal forests than previously thought, says a University of Victoria biologist.
Studies of the interaction between bears and spawning salmon by Dr. Tom Reimchen have demonstrated that by carrying large numbers of dying or dead salmon into the woods to eat, black bears may be responsible for a massive transfer of essential nutrients from the streams into the forests.
"This potentially represents a major component to species diversity and productivity in forests adjacent to salmon streams," he says.
Reimchen became intrigued with the bear/salmon relationship while conducting a long-term study of stickleback fish in the Queen Charlotte Islands. "To me, salmon are like the wildebeest of the Serengeti," he says. "They've sequestered carbon and energy from elsewhere and arrive in huge numbers where, in a short period of time, dozens of other species depend on their abundance."
As he witnessed the spawning spectacle, Reimchen wondered what effect natural fluctuations in salmon numbers &emdash; and high harvest rates from commercial fishing &emdash; might be having on forest ecosystems.
In 1992, he set out to find some answers. As a study site, he selected a stream flowing into Bag Harbour, a small bay in Gwaii Haanas park in the southern Queen Charlottes, where about 5,800 chum salmon spawn every fall. He surveyed the number of salmon predators &emdash; such as black bears, eagles and seals &emdash; and inspected the salmon leftovers. "When you walk through the forest there are fish remains everywhere," says Reimchen. "These are salmon that were taken out of the stream and into the forest by bears, and partially eaten."
Conveniently, bears won't eat the hard lower jawbone of a salmon, so he was able to count the number of carcasses in the forest &emdash; sometimes located as far as 100 metres away from the stream. He found that about 80 per cent of the salmon that entered the stream ended up on the forest floor.
Reimchen returned to Bag Harbour the following year and got similar results. This time, he also assessed the reproductive condition of carcasses, and found that at least 70 per cent of females were spawned out and 80 per cent of males had spawned at least twice. "In other words, the effects on the next generation of salmon are small," he says.
More significantly, the eight black bears in the area each took about 1,600 kilograms of salmon into the forest &emdash; for a total of 5,200 kg &emdash; and ate about half. The remnants were used by scavengers, including a large number of insects.
"The bears are a major entry of nutrients into the forest that has not been recognized before," says Reimchen.
But the cycle doesn't end there. As the fish decay, nutrients such as nitrogen &emdash; a natural fertilizer &emdash; seep into the soil. "When you're walking through the forest and see not only salmon carcasses strewn everywhere, but also bear droppings, you can't help but suspect that there are nutrients being used by forest plants as well," says Reimchen. He calculates that rotting salmon contribute about 120 kg of nitrogen per hectare at Bag Harbour. "That's very close to the level of fertilizer used by professional foresters," he notes.
According to fishery statistics, Bag Harbour's chum salmon run peaked at 35,000 fish in 1947. Perhaps three to four times as much nitrogen was going into the forest back then. It raises questions about the long-term health of forest/salmon stream systems where bear predation is negligible due to human activity, such as Goldstream Provincial Park near Victoria.
Still, trees do grow where there are no salmon, so Reimchen's next goal is to find proof of a link between vegetation growth and salmon numbers. He was recently awarded a $110,000 grant from the Suzuki Foundation to trace salmon nutrients throughout the forest ecosystem using a form of nitrogen found in high levels in salmon. His team includes graduate student Deanna Mathewson, who is tracking this "salmon nitrogen" in vegetation, and postdotcotral colleague Dr. Jonathan Moran, who is researching the nitrogen content of soil.
Wood samples will be taken from Bag Harbour, Princess Royal Island and Clayoquot Sound.
"If we could detect this nitrogen signature in trees, then we could go ring by ring and look at the relative contribution of salmon year by year, possibly going back centuries," says Reimchen. "It might provide us with a direct estimate of how many salmon there were in past centuries, which in turn, would tell us about current ecological processes in our coastal forests."
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