A new research paper concludes that the abundance of salmon in spawning streams determines the diversity and productivity of plants in surrounding forests.
Simon Fraser University biologists Morgan Hocking and John Reynolds have documented their findings in “Impacts of salmon on riparian plant diversity,” published in the journal Science.
It’s an extensive study of the interrelationship between salmon and forest ecosystems bordering 50 streams on the remote central coast of British Columbia, Canada.
Reynolds, the Tom Buell B.C. Leadership chair in Salmon Conservation, says their research resolves a question that previous studies have been unable to address: Do fluctuations in salmon populations, including those driven by human impacts, affect the biodiversity of terrestrial ecosystems?
“We now know that differences among streams in salmon populations translate into differences in the species of plants in adjacent forests. This can ultimately affect animals, insects and birds feeding on those plants,” says Reynolds.
“The impacts of salmon on plants are so radical that, even without knowing how many salmon spawn in specific streams, we can get a good idea by studying the surrounding plant life.”
After factoring in other variables such as stream size and slope, the scientists discovered that where salmon populations were high, overall plant diversity decreased. But the few species that dominated streamside forest communities, such as salmonberry, elderberry and stink current, were nitrogen-rich plants. They outcompeted other plant species for salmon nutrients.
In the study, plants with lower nutrient content, such as huckleberry, salal and blueberry, bordered streams with low salmon populations.
“We can now tie the strength of salmon populations to significant shifts in plant species and diversity,” Reynolds said.
“Changes in the abundance of salmon in the ocean evidently can have far-reaching impacts on the structure and functioning of terrestrial food webs adjacent to streams,” said Hocking. “We can also now predict that small streams with few alternative sources of nutrients will be most vulnerable to impacts of salmon declines.”
Hocking and Reynolds conducted their study in the Great Bear Rainforest, near the Heiltsuk First Nation village of Bella Bella. The region supports strong populations of bears, wolves and other natural salmon predators, which drop carcasses in forests beside the spawning streams.
They partnered with the Heiltsuk First Nation and the federal Department of Fisheries and Oceans to monitor the fish populations each fall in streams that ranged between having almost no fish to more than 100,000. This information was then matched to detailed botanical censuses, including more than 30,000 plant records and 6,000 trees.
Hocking is a Natural Sciences and Engineering Research Council postdoctoral fellow in SFU’s Earth to Ocean Research Group. He notes: “We now understand what ecosystems surrounding healthy streams with little human disturbances are like. Because we sampled across natural gradients in salmon populations and stream habitats we are able to predict how human activities that impact salmon such as climate change, overfishing and development will affect future forests.”
In the spring of 2007, Hocking and Reynolds led a group of 10 researchers aboard a sailboat owned by the Raincoast Conservation Foundation to the forests bordering 50 streams. They collected leaf samples to study their nutrient content, including the presence of “heavy” nitrogen, which is a signature of origin from salmon carcasses transported by bears and wolves into the forest.