For At Least 50 Years, Bird Population Kept on Falling in Tropical Rainforest

For 44 years, bird numbers in a Panamanian rainforest have steadily dropped. According to a new study done by the University of Illinois, 70 percent of understory bird species disappeared in the forest between 1977 and 2020. And the great majority have been reduced by half or more.

Almost 5 Decades of Decline

(Photo : Photo by Matt Cardy/Getty Images)

Many of these are species that would be expected to thrive in a 22,000-hectare national park with no significant land-use change in at least 50 years, according to Henry Pollock, a postdoctoral researcher in the University of Illinois' Department of Natural Resources and Environmental Sciences (NRES) and the study's lead author. "It was somewhat unexpected."

Jeff Brawn describes it as "concerning." Brawn is a co-author of the paper and holds the Levenick Chair in Sustainability at NRES. He's also spent more than 30 years studying birds at the research site, Parque Nacional Soberana.

"This is one of the longest studies of its sort in the Neotropics, if not the longest," Brawn adds. "Of course, it's only one park; we can't necessarily apply the same logic to the entire region and declare the sky falling, but it's alarming."

Pollock claims that the extinction of birds in any environment might jeopardize the ecosystem's stability. These birds are important seed dispersers, pollinators, and bug eaters in the Neotropics. Fewer birds might risk tree reproduction and regeneration, affecting the forest's overall structure, following large bird decreases in the past.

However, the researchers have yet to investigate the consequences or the underlying reasons. First and foremost, Pollock, Brawn, and their colleagues concentrated on compiling the data.

Research that Started in the 70s

Jim Karr, a former University of Illinois faculty member who is now an emeritus faculty member at the University of Washington, started a twice-yearly bird sampling operation in 1977. During the wet and dry seasons, team members put up mist nets to capture birds traveling through the research location. Mist nets trap birds delicately, allowing researchers to pick them out with care. The birds are then identified, measured, and banded before being released unhurt into the forest.

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At-Risk Species

The researchers collected almost 15,000 distinct birds spanning roughly 150 species for 43 years and 84,000 sampling hours and obtained enough data to follow 57. The researchers found that 40 species, or 70%, had declined, and 35 species had lost at least half of their original populations. Only two species grew in number: a hummingbird and puffbird.

"We'd catch 10 or 15 of numerous species at the start of the research in 1977, and by 2020, we'd be down to five or six individuals for several species," Pollock adds.

The researchers noticed reductions in three major kinds of birds:

Common forest birds.
Species that move seasonally between altitudes.
An "edge" species specializing in transition zones between open and closed-canopy forests.

According to Brawn, the reduction of common species is the most concerning.

"The main conclusion is that these are birds that should be doing well in that area, but they aren't, for whatever reason. We were taken aback."

The decreases in the other two categories were less pronounced. Birds that travel to high elevations require some degree of forest connectedness, but Panama's forest, like most other countries, has been progressively fragmented in recent decades.

Edge Species Having it Worst

Edge species took the worst of the damage, with most of them decreasing by 90% or more. Pollock and Brawn, on the other hand, were not surprised. The extinction of edge species increased their confidence in their findings. That's because a paved access road ran through the property 40 years ago. It generated the ideal edge habitat for birds that prefer forest canopy openings. However, the road ceased to be maintained over time and degenerated into a narrow gravel road, with the forest canopy filling in overhead.

According to Pollock, the fact that edge species vanished along with the road isn't a cause for concern. It demonstrates what we should expect when forests mature and lose successional habitats.

The researchers are hesitant to extrapolate their findings beyond their research site, citing the lack of such sampling initiatives in the tropics.

According to Pollock, this is "essentially the only window we have right now into what's going on in tropical bird populations." The findings raise the issue of whether this is occurring across the area, but we are unable to provide an answer. Instead, our research emphasizes the scarcity of data in the tropics and the importance of long-term investigations.

Biodiversity's Importance

The study wasn't intended to explain why forest birds are diminishing, but the researchers have some ideas they'd like to pursue. Climate change may be linked to changes in rainfall, food supply, and reproductive rates, among other things.

Regardless of the cause, the researchers showed a strong desire to discover it.

The Neotropics are home to about half of the world's birds, but we don't precisely grasp their population trends. As a result, I believe it's critical to do additional ecological research to identify patterns and processes of decrease in these populations, according to Brawn. "And we have to do it right now."