Agriculture Is Reshaping the Avian Tree of Life
Evolutionarily distinct bird lineages are more likely to go extinct in farms than forests, but diversifying crops could curb the loss.
Sometimes a failed experiment is exactly what a scientist needs to propel her toward a big new idea. That’s where Gretchen Daily of Stanford University found herself in 1995 when her experiment to study fruit-eating birds in Costa Rica ended up attracting mostly killer bees instead. “That ended my project, and I had planned to be in the country for several months. I didn’t know what to do with myself,” she says. Daily took a walk to ruminate. “It’s easiest to look at birds in the farmland, but I hardly ever went out there. I’m wandering along, and it just hit me: Why am I not studying this stuff out here?” She began a decade-long collaboration with a local researcher, Jim Zook of Unión de Ornitólogos de Costa Rica, to survey birds across farms and forest. When the complete avian phylogeny was published by Yale’s Walter Jetz and his colleagues in 2012, Daily and Zook were able to connect that work with bird evolution and conservation. The culminating result, published in Science this past September, contains an important message: Industrial agriculture is reducing biodiversity in previously unappreciated ways.
Daily’s team, including Zook and doctoral student Luke Frishkoff, studied how much of the tree of life was supported in different habitats, using a measure known as phylogenetic diversity. This approach is different than traditional measures of biodiversity, such as simply counting number of species in an area, because it takes into account evolutionary history. “If you have a very simple community, say, two species, and both of them are species of blackbirds, it’s very intuitive that they don’t represent much of the entire tree of life,” Frishkoff says. “You have very little phylogenetic diversity. But if you have instead two species, and one is a blackbird and one is a tinamou [a chicken-like, forest-dwelling bird], they diverged around a hundred million years ago. Even though you have the same number of species, you’re representing much more of the tree of life.” Because phylogenetic diversity is linked to higher ecosystem productivity, stability, and diversity, it can be used to prioritize habitats for conservation.
Agriculture has a serious impact on phylogenetic diversity, favoring communities of relatively closely related species that do not encompass a large portion of the evolutionary tree. Older lineages, which are more evolutionarily distinct, are more likely to go locally extinct on farms. Although areas in monoculture, including those that grew sugarcane, melons, pineapples, coffee, and bananas, harbored a diversity of birds, phylogenetic diversity was 40 percent lower in these areas than in forest reserves. By contrast, small family farms with more diversified crops—growing about 30 different plants—were only 15 percent lower in phylogenetic diversity than forests, suggesting that different farming practices could provide habitat for a larger swath of birds.
The birds sustained in agricultural land, including pigeons, blackbirds, seedeaters, and swallows, tend to eat a wider variety of foods, especially seeds, and produce high numbers of eggs in a year. Daily thinks that this collection of birds in agricultural areas could represent a novel community in evolutionary history.
Drawing on 12 years of survey data, Daily and Zook knew whether each of 308 Costa Rican bird species they studied persisted in an area from one year to the next. They could predict the probability that a species would disappear from a particular habitat. For the birds that were in the top 10 percent of the most distinct, the probability of extirpation in monocultures was roughly double that of diversified farms. Specialized birds—for example parrots, antbirds that follow army ant colonies, and hummingbirds adapted to a particular type of flower—are the species most likely to be lost as agriculture expands. “In the past, studies have asked whether species position on the tree of life predicted whether they would be at critical risk of extinction,” Frishkoff says. “This study represents an advancement because it’s taking that idea to the ground, looking from year to year at what species are going locally extinct because of a very specific human impact on the environment.”
The authors recommend encouraging diversified agricultural systems to curb biodiversity loss. Such systems cultivate a wider variety of crops and livestock and in general include more trees, particularly fruit-bearing ones. A diversified approach is not necessarily less efficient, despite some farmers’ concerns. Daniel Karp, a postdoc at the University of California, Berkeley, and another coauthor on the paper, has shown in separate research that farms with more tree cover have more birds, which in turn eat more pests on coffee. “In order to have functioning ecosystems that provide pest control, pollination, and seed dispersal, you need numerous branches of the evolutionary tree of life present. If you only have one lineage that does one thing, you only benefit from that one particular service. But if you have many lineages, you have an integrated workforce doing these many different services for free,” says Frishkoff.
Daily points out that people will need to double food production over the next half century to meet the needs of the rapidly growing human population, but over the same period the planet is projected to lose about half of its species. “I came to this work thinking that the only option for protecting life on Earth while at the same time protecting food security was to keep nature in reserves, simply because none of it would survive out on farmland,” she explains. A dozen years in Costa Rica changed her mind. “We’re unlikely to get a big new reserve network, so instead we should move our attention to rural communities and more diversified farms.”—Katie L. Burke