Richard C. Bogren, Stouffer, Philip C.
News Release Distributed 06/22/11
During a 25-year period, many bird species in Brazilian rainforest fragments that were isolated by deforestation disappeared and then reappeared according to a research paper published June 22 in PLoS One, an online, peer-reviewed journal.
Although species loss following habitat conversion can be inferred, long-term observations are necessary to accurately identify the fate of bird populations, said Philip Stouffer, an ornithologist with the LSU AgCenter and lead author of the paper “Understory bird communities in Amazonian rainforest fragments: Species turnover through 25 years post-isolation in recovering landscapes.”
Stouffer’s research, funded for the past five years by a grant from the National Science Foundation and conducted in cooperation with Projeto Dinâmica Biológica de Fragmentos Florestais, Instituto Nacional de Pesquisas da Amazônia and the Smithsonian Tropical Research Institute in Manaus, Brazil, shows bird species began reappearing following a 10-year hiatus.
Stouffer and his colleagues – Erik Johnson, who was Stouffer’s graduate student and is now with the National Audubon Society, Richard O. Bierregaard at the University of North Carolina, Charlotte, and Thomas E. Lovejoy with the Heinz Center in Washington, D.C. – measured bird populations in 11 forest fragments ranging from about 2.5 acres to 250 acres in the Amazon rainforest near Manaus, Brazil.
Bierregaard and Lovejoy set up the project in the 1970s, and Bierregaard managed it until 1993. Stouffer joined the team in 1991.
When the project began, bird populations were measured using established mist-net protocols before the forests were cut. During the first year after cutting, the bird species disappeared in what the researchers call “localized extinction,” meaning a species has disappeared from a particular area.
The area is fragmented in “cookie cutter chunks” as a result of government policies that encouraged use of the land – mostly for cattle – but required landowners to leave a portion of the area uncleared.
Bird populations were measured before the deforestation process began and then again in 1985, 1992, 2000 and 2007. During the first 10 years, birds abandoned the fragments and became “extinct.” Then during the past 20 years, many species have come back, but others have gone extinct or remained extinct.
Agriculture has diminished, and although some of the fragments are deteriorating, the matrix between the fragments and nearby forests is recovering into forest, Stouffer said. “Early on, the small fragments lost most of their understory birds, and the area that was cut had no forest birds at all.”
Between the time the forest fragments were created and 2007 when the most recent measurements were taken, all fragments lost bird species, Stouffer said. Losses ranged from below 10 percent in the largest fragments to around 70 percent in the smallest fragments.
Analysis of individual time intervals revealed that the 2007 result was not because of gradual species loss beginning at isolation; both extinction and colonization occurred in every time interval. In the last two samples – taken in 2000 and 2007 – extinction and colonization were approximately balanced.
The extinction process started with bird species leaving or dying out. Now, they’re coming back. “A handful of species have ‘gone extinct,’ but many more species are in flux,” Stouffer said. “They come and go. Some of the areas have 20 to 25 years of forest regrowth.”
The project measured only understory, resident birds and not those that live in the forest canopy or may migrate. “We don’t know the actual demography of the birds,” Stouffer said. But the counts include estimates of uncounted birds.
“We’ve been looking at the rate of extinction and colonization,” Stouffer said. “Our samples are snapshots in time. And they show that forest fragments have potential to recover their biodiversity if they’re imbedded in a landscape that can recover. They’re not doomed.”
The research shows how birds exist within a human-modified environment and the effects of allowing a forest to regenerate, Stouffer said. “We can consider a balance of abandoned and returned forests because within a 20-year window, birds will begin to treat the fragments as continuous forest.”
Landscape dynamics must be considered as second-growth structure and overall forest cover contribute to processes in fragments, Stouffer said. Using a method that accounts for imperfect detection, they estimated extinction and colonization based on standardized mist-net surveys within discreet time intervals – one to two pre-isolation samples and four to five post-isolation samples.
“Of the 101 species netted before isolation, we detected 97 in at least one fragment in 2007,” Stouffer said. “Although a small subset of species is extremely vulnerable to fragmentation and predictably goes extinct in fragments, developing second growth in the matrix around fragments encourages recolonization in our landscapes.”
Species richness in these fragments now reflects local turnover, not long-term attrition of species. We expect that similar processes could be operating in other fragmented systems that show unexpectedly low extinction.
“By combining one of the first controlled fragmentation experiments in tropical forests with the opportunity for long-term observation, this study provides verification that local extinction is accompanied by continual recolonization, dependent on habitat size,” said Saran Twombly, program director in NSF’s Division of Environmental Biology, which funded the research. “The results bolster island biogeography theory in one of the most diverse regions on the planet.”