A murderous year 2006 E.coli An outbreak linked to spinach on a farm in central California has prompted growers to remove meadows and woods bordering their fields, fearing they could harbor disease-carrying wild animals. It turns out that this strategy could have done more harm than good.
Birds on California farms with nearby wild habitat were less likely to have disease-causing microbes in their droppings, while also feeding less on farmers’ crops, the scientists learned. The new findings underscore the potential benefits of farming alongside nature and how strategies to repel wildlife can have unintended consequences.
The research suggests “that agricultural landscapes with natural habitat tend to be good for conservation, farmers and public health,” said Daniel Karp, an ecologist at the University of California, Davis, whose lab led the works.
The 2006 incident sickened at least 204 people in 26 states and Canada, 104 of whom were hospitalized. Three people died. For a time, the United States Food and Drug Administration recommended against eating fresh spinach. Clues eventually led federal inspectors to a 2.8-acre field in San Benito County, an area dotted with farms just south of California’s Silicon Valley. There they found the stump of E.coli in the faeces of nearby cattle and wild pigs, as well as in the water of rivers.
The repercussions have extended far beyond the single farm, shaking up the fresh produce industry, especially in California. Among new food safety measures adopted in the wake of the outbreak, farmers have been urged to fence off fields and remove nearby strips of wild vegetation to reduce the presence of wild animals that could transmit disease. .
But Karp and his fellow scientists wondered if such measures really had the desired effect. To find out, they focused on a type of organism known to carry pathogenic bacteria: wild birds. Farmers often have an adversarial relationship with birds. For one, they feed on pesky crop-eating insects. On the other hand, they can feed on these same beneficial crops or insects.
To unravel the interplay of these different forces, the researchers visited 21 organic strawberry farms in Santa Cruz and Monterey counties, armed with delicate nets called mist nets. Between 2017 and 2019, scientists captured more than 1,000 birds from 55 species and collected faecal samples. In the lab, they sifted through DNA from faeces, looking for clues about the insects and plants they ate and the germs they carried.
At the same time, the researchers constructed maps of the farms and surrounding land, tracing the mosaic of crops and untended vegetation that they described as “semi-natural habitat”.
The results showed that what showed up in a bird’s gut depended in part on the landscape where it was captured. Birds captured from a dozen farms classified as having relatively little surrounding natural habitat were more likely to have strawberry DNA in their droppings, as well as Campylobactera bacteria that can cause diarrhea and vomiting in people, and, in rare cases, death, according to findings reported Feb. 22 in the Journal of Applied Ecology.
According to the researchers, the presence of a natural habitat emerged statistically as the most important factor when it came to determining whether the benefits of wild birds outweighed the harms. “Bird communities respond to changes in the landscape,” said lead author Elissa Olimpi, who was a postdoctoral fellow at UC Davis during the research. “As birds change in response to management, so do the costs and benefits they provide.”
While it’s unclear exactly why closer natural habitat might be a plus, it’s possible that these areas provide more attractive foods such as seeds and insects, tricking birds into ignoring strawberries. The most heavily exploited areas could also harbor communities of birds that are more likely to carry or transmit disease.
Among the different species, barn swallows got a “gold star”, in part thanks to their appetite for pests. Aerial acrobats catch flying insects and build mud nests under eaves. The American goldfinch, meanwhile, was on the other end of the spectrum.
The distribution between species suggests that farmers could take steps to encourage more beneficial birds. In addition to letting the land grow wild, people could leave barn swallow nests unmolested and build nest boxes with entrance holes small enough to let insectivorous birds in to build nests, while excluding larger, more pest birds such as starlings, the scientists said. “The best we can do is figure out how to leverage the benefits while reducing the harms,” Olimpi said. “Growers will tell you that you can’t keep birds off your farm.”
Olympia, and. Al.”Semi-natural habitat surrounding farms promotes multifunctionality of avian ecosystem services.” Journal of Applied Ecology. February 22, 2022.
picture by Catherine Kerlin,Birds, Strawberries, and Natural Habitat Study,” University of California, Davis