As the world’s population and its appetite for all manner of food grows, wildlife is paying a big price. By one estimate, agriculture accounts for almost 90% of deforestation.
What if a certain kind of farming could blunt the damage? People have touted small-scale farms that grow a broad array of crops and preserve patches of trees or shrubs as a kinder, gentler version of agriculture. But the long-term performance of such approaches isn’t always clear.
Now, research tracking Costa Rican birds for nearly two decades suggests that while nothing can take the place of forests, these smaller farms provide lasting benefits, acting as a second home for some forest-dwelling species.
“It isn’t that diversified farms can replace forests,” Nature Conservancy forest ecologist Nicholas Hendershot said in an email response to questions from Anthropocene. But “we also need to strategically think about how we manage farmlands for biodiversity outside of protected areas, because there are HUGE benefits for biodiversity.”
As a postdoctoral researcher at Stanford University, Hendershot was part of a team that tracked the fates of 429 tropical bird species in Costa Rican farms and forests between 2000 and 2017. Every year for 18 years, researchers visited 48 different places to count the number and types of birds living there. Some spots were in the forest. Some were in small farms averaging 3.3 hectares, roughly the size of six U.S.-style football fields. These farms usually had a patchwork of forest, leafy hedges and a mixture of crops such as coffee, pineapple, sugar cane, banana, melon and rice. They also checked for birds in larger farms averaging 65.5 hectares. These places usually grew a single crop with few trees.
The overall picture is that bird populations in all three types of landscapes were in flux. Whether it was forest, small farm or big, just 23% of species maintained steady numbers over the time of the research, the scientists reported Sept. 5 in the Proceedings of the National Academy of Sciences.
“Birds are kind of a proxy we use to track the health of ecosystems. And the birds we’re seeing today aren’t the same as we saw 18 to 20 years ago,” said Hendershot
But when the researchers delved into more details, they came up with a surprise. Intact forests witnessed a steeper drop in bird populations than did the diversified farms. Among the 164 species with changing numbers in the forest, 62% were in decline. And nearly half of those species were forest-dependent, insect-eating birds.
By contrast, roughly half of the fluctuating species were in decline in diversified farms. And on these farms, insect-eaters that prefer forests fared pretty well: 32 of these species saw population gains, while 19 experienced losses. The biggest losses were among birds commonly found on farms.
In other words, forest-associated birds seemed to be thriving more on these small-scale farms than they were in the forests.
A lot of it comes down to the needs of individual species, said Hendershot. Some birds, such as the nightingale wren, an anonymous-looking little brown bird with a high-pitched peeping call, have such a strong connection with forests that they weren’t found in farms at all. Their numbers were declining in the forests. Other species, however, seemed to adapt to life on the farm, and their numbers rose around diversified agriculture. That includes one bird listed as endangered by the International Union for the Conservation of Nature: the great green macaw.
It’s not clear why so many forest birds are declining inside forests. It’s possibly a delayed effect from past deforestation, the researchers suggested. The damage from nearby farms could be spilling over. A decline in forest insects could also be depriving the birds of food.
What is clear is that more intensive agriculture does not provide a refuge for forest birds. There, the winners were primarily invasive species, grain-eating birds, and birds with less specialized habitat or food requirements, such as the great kiskadee, a yellow-breasted songbird that will feed on seemingly anything it can fit in its beak: insects, lizards, mice, fish, berries, even baby birds.
The key insights is “not that diversified farms are suitable to replace forests. Forests contain a huge diversity unique species that are found nowhere else, and in this study, we found that this unique forest diversity is declining,” said Hendershot. “BUT we also found that, for many forest species that are more flexible with their habitat use, there was a real benefit of diversified farms relative to intensive farms.”
Hendershot, et. al. “Diversified farms bolster forest-bird populations despite ongoing declines in tropical forests.” Proceedings of the National Academy of Sciences. Sept. 5, 2023.