Much is known about coffee plantations’ effects on Costa Rica’s agricultural landscape, but their impacts on nature–specifically tropical bird biodiversity–have largely remained a mystery until now.
While some coffee plantations are able to host a surprising number of bird species, they can’t maintain bird biodiversity, according to a new paper published in Proceedings of the National Academy of Sciences.
“This work was designed to see how we can integrate nature into the vast regions devoted to agriculture,” said study coauthor and Stanford Biology professor Gretchen Daily.
“It’s two sides of a coin,” said study lead author and University of Utah ornithologist Çağan Şekercioğlu. “These tropical agricultural habitats that are mostly deforested still maintain large numbers of tropical species. But the flip side of the coin is, in the long term, most of these species are still declining.”
See Şekercioğlu’s photos of Costa Rica birds, including the rare turquoise cotinga, at https://unews.utah.edu/costa-rica-coffee-birds/
Over 11 field seasons, between 1999 and 2010, Şekercioğlu trekked through the forests and coffee fields of Costa Rica, painstakingly banding individual birds. Such long-term studies are rare, partially because of the research timeline of a typical graduate student. Doctoral students in biology may be able to collect four years of data before compiling their research into a dissertation to complete their degree. But Şekercioğlu wanted more than four years. To get the long-term trends he wanted, he needed at least six years. Ten would be even better.
With the blessing of his doctoral advisory committee at Stanford, Şekercioğlu set out for Costa Rica in 1999 for the first field season. Between 1999 and 2010, he and his team set up stations in both protected forests and in coffee plantations where they extended nearly invisible nets. The “mist nets” allowed the researchers to temporarily capture birds and affix an ankle band, or check for an existing band, before releasing the bird. Banding allows each bird to be individually identifiable—an essential element of fine-grained population surveys. “These are small, often secretive songbirds we’re talking about,” Şekercioğlu says. “You need the bird banding. It’s labor intensive, but there’s really no substitute.”
After earning his doctorate in 2003, Şekercioğlu remained at Stanford, first as a postdoctoral scholar and then as a senior research scientist, to keep the Costa Rica project going. Over the years, with the support of his co-authors including Gretchen Daily and Paul Ehrlich, Stanford Woods Institute for the Environment Senior Fellow and Bing Professor of Population Studies, he and his colleagues mist netted 57,255 birds from 265 species in 19 different study sites. In 2009, after securing a position at the University of Utah, he wrapped up his field work in Costa Rica before moving to Salt Lake City in 2010 to found the Biodiversity and Conservation Ecology Lab.
Diversity can be deceiving
In this new paper, the authors present their results. First, some definitions of the land types in the study:
The results of the study are a mix of bad news and good news.
The bad news: over the study period, 61% more bird populations declined than remained stable or grew, and this trend held in all study sites except those in the large La Amistad International Park.
“If you really want to conserve the native tropical forest bird community, there’s really no replacement for big forest reserves like La Amistad,” Şekercioğlu said. “You really need at least 1,000 hectares (3.86 square miles) of native primary forest.”
The good news: coffee plantations host nearly as many bird species as forests, with 230 species recorded in forest sites and 185 recorded in coffee sites.
But just looking at the number of species can hide some bad news—species richness isn’t always descriptive of how diverse an ecosystem really is, or a habitat’s value for conservation.
“These plantations get spillover species from the nearby intact rainforest,” Şekercioğlu said. “That adds to the richness.”
Other species present in the coffee plantations include those that prefer open habitat as well as those who can inhabit a variety of habitats. “Their populations are not declining,” he added. “In fact, many have globally increasing populations. But a lot of the species missing from these coffee plantations are the most sensitive specialist species. Any study of the conservation value of a habitat has to look at the details of the community composition and not just the sheer number of species detected.”
The coffee plantations redeem themselves, somewhat, with a little bit of good news to end on. The researchers found that shaded plantations (again, not shade coffee, just shaded—to be clear) with an average of about 13% tree cover hosted double the number of forest specialist birds and half the number of open-habitat birds compared with plantations with only an average of 7% tree cover. In other words, a small increase in tree cover results in a significant increase in bird habitat.
Are shaded coffee plantations an answer to the threat of deforestation? “No” Şekercioğlu said. “In the long term, by themselves, they are not enough to preserve tropical biodiversity. We need a combination of agricultural areas with as much tree cover as possible and extensive primary forest reserves.”
Despite the labor-intensive nature of a bird banding study and the relative rarity of such a long-term project, an undertaking like this is necessary to get a more accurate picture of bird population compositions and trends. “These days it would be very hard to do a study like this,” Şekercioğlu says. A “snapshot study” might have concluded, for example, that the number of bird species in the Las Cruces Forest Reserve indicated a healthy forest, when in fact Şekercioğlu’s study found more than three-fifths of the bird species in decline.
“But there is hope,” said Daily. “Even modest increases in tree cover—along streams and on steeper slopes—can greatly improve the chances for birds and other wildlife, while benefitting people with services of pest control, crop pollination, water purification and flood protection, as well as the stunning beauty of birds in flight.”
The study’s coauthors were Chase D. Mendenhall of the Carnegie Museum of Natural History, Federico Oviedo-Brenes of the Organization for Tropical Studies, Joshua J. Horns of the University of Utah and Paul R. Ehrlich and Gretchen C. Daily of Stanford University.
The study was supported by grants from the Moore Family Foundation, the National Geographic Society, the Wildlife Conservation Society and the Winslow Foundation.
Gretchen Daily is the co-founder and faculty director of the Stanford Natural Capital Project, Bing Professor in Environmental Science at the school of Humanities and Sciences, and a Senior Fellow at the Stanford Woods Institute for the Environment.
This story is modified from the original University of Utah press release.
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