The University of Alberta study, published Tuesday in the journal Nature Communications, suggests that forests with a moderate level of disturbance actually have more plant species.
But lead author Stephen Mayor adds that new biodiversity comes mostly from invasive species and quickly reaches a tipping point.
"The forest can tolerate some level of disturbance without an enormous negative effect on biodiversity. But there really is a limit to how much intrusion by human land use it can tolerate and eventually we do really lose a lot of our native flora."
Mayor and his colleagues wanted to test a theory that suggests forest biodiversity increases when parts of it are disturbed because the disturbed areas increase the variety of habitats.
"The more variety on the landscape you have, the more species you have."
Although most previous studies have failed to support that theory, Mayor guessed that was because they didn't look at a broad enough area. He and his team decided to consider the entire northern half of Alberta, one of the most hotly contested landscapes in the country — home to farms, ranches, oilsands mines, forestry cutblocks and national parks.
"The scale is unprecedented," Mayor said.
Zooming in on 242 one-hectare plots, the scientists cross-referenced disturbances revealed by satellite and aerial photos with surveys of the plant species found within each.
They concluded that, on a regional basis, biodiversity actually increases as more disturbances are scratched into the landscape. But once that disturbance reaches a level of 47.5 per cent, biodiversity starts to crumble.
"If you're creating different environments, you can get more species," Mayor said. "The critical thing is that it only works up to a point, so the more disturbance you get past that 50 per cent range, you start losing more species."
As well, the nature of the forest starts to change. Invasive species start to take over.
"The composition of our flora will change with more disturbance," said Mayor. "We will be losing more native species and seeing a lot more weedy exotics that might not necessarily be preferable.
"Native species from boreal Alberta started declining quite quickly after about 40 per cent (disturbance). But invasive species — exotic weeds and things like that — were increasing right from the start."
The research holds important lessons for a province — and a country — striving to balance a resource-heavy economy with environmental preservation, said Mayor.
"Forests are quite resilient to changes that we're putting on the landscape," he said.
Alberta recently instituted a land-use plan for northeastern Alberta, a region that includes both the oilsands and vanishing caribou herds. Another plan for southern Alberta around the South Saskatchewan River basin is in the works. Similar plans are expected for the entire province.