Birds find sanctuary in old-growth neighborhoods

Violet sabrewing hummingbird in Costa Rica. Photo credit: Oregon State University, United States.
3 April 2018

BONN, Germany (Landscape News) — For rare tropical birds, old-growth forest matters more than we might think, according to a study carried out by scientists from the Oregon State University’s College of Forestry.

The study, which took place in Costa Rica’s southerly Coto Brus region, found that the amount of primary forest, but not secondary growth, increased the probability of seeing rare tropical bird species during their surveys. The results add an important new piece of useful information for conservationists.

The scientists recently reported their findings in the Journal of Applied Ecology.

“Until now, combined effects of changes in forest composition, disturbance and configuration on the conservation value of Human-Modified Tropical Landscapes remained unclear, which critically restricts conservation planning,” the paper states.

“We show that future policy interventions such as (the U.N. program) REDD+ (reducing emissions from deforestation and forest degradation) will have to go beyond simply the maintenance of forest cover to prioritize old-growth forest conservation, and take into account landscape configuration. Although small forest remnants, which are among the most abundant tropical landscape elements, can harbor bird species found in pristine forests, this only occurs given a high proportion of proximal old-growth forest.”

“Our study is not there to show that re-forestation doesn’t help,” explained Urs Kormann, lead author of the study. “We don’t want to discourage people from that. Our data suggests that it really takes a lot of time before secondary forest might reach the value of true primary forest.”

The scientists looked at bird populations in 49 forest fragments, ranging in size from less than a hectare to over 1,000 hectares. Deforestation for agriculture had wiped out about 70 per cent of the tropical forest in the area, which, according to the paper, is representative of human-modified forest landscapes in many Latin American regions with moderately intensive land use.

“Basically what we saw if we only look at how many forest specialist species we find during our surveys it didn’t really change in the large fragments, depending on how much old growth we had around a plot,” Kormann said. “In the small fragments, however, it was really a large issue.”

In smaller patches of secondary forest with no primary forest nearby, the incidence of tropical forest specialists and insect-eating birds declined by half. “You’ll see lots of birds, but always the same set of a few generalist species. You won’t typically find the specialists or rare species,” he said.

Added Adam Hadley, a co-author of the study: “If you have old-growth forest around, those birds can colonize or use small patches. If small patches of forest are totally isolated, those little populations may just go extinct. They can’t maintain small local populations.”

By providing evidence not only on community shifts and diversity within segments (alpha-diversity) but also diversity among segments in the larger landscape (beta-diversity) the scientists see their work as a groundbreaking argument for the importance of retaining as much old-growth forest as possible.

“It’s not just the forest cover,” said Kormann. “If you have a lot of secondary forests in your neighborhood it doesn’t really do the job. It really needs to be old-growth forest. That’s the message.”