Bushland and farmland don’t always mix

Despite an analysis of the largest dataset of its kind, a simple landscape model for accurate pest control predictions on farms worldwide remains out of reach.

Natural habitat next to farms can improve crop yields by encouraging predators of crop pests – ladybugs thus protect soybeans in mid-west America, for instance, and songbirds keep down insect numbers in coffee plantations of Central America.

Conservation of natural habitat around cropping areas sounds like a sensible principle, and is widely adopted in agro-ecology. But a new study published in the journal PNAS argues against taking a “one size fits all approach”.

“There’s a widespread assumption among ecologists that when you have more natural habitat around farm fields you get more enemies of the crop pests, and that these enemies will control the pests and provide a benefit to growers,” says lead author Daniel Karp, from the UC Davis Department of Wildlife, Fish and Conservation Biology in the US.

In order to test this assumption, Karp and colleague Rebecca Chaplin-Kramer organised an international team of ecologists, economists, and practitioners at the National Socio-Environmental Synthesis Centre at the University of Maryland. The resulting dataset included 132 studies from more than 6,700 sites in 31 countries.

The findings revealed substantial variation in how pests, enemies, predation rates, crop damage and yields responded to landscape variables, including the presence of natural habitat. Overall, the effects of nearby natural habitat on cropland pests were inconclusive.

While many of the studies showed that surrounding natural habitat do indeed help farmers control unwanted insects, just as many showed negative effects on crop yields.

“This paper isn’t telling farmers to clear habitat by any means,” says Karp.

“There may be a lot of other benefits from natural habitat, such as pollination or carbon sequestration. But we need to be forthright about knowing when habitat conservation will be advantageous in terms of pests and when other means of pest control are needed.”

Large-scale pollination studies tend to report more consistently positive effects of natural habitat around farms – indeed, a key finding of a global quantitative synthesis of bee numbers and diversity in agricultural landscapes found that bee richness on cropping land benefited most from high‐quality surrounding land cover.

However, compared to relatively simple plant pollinator systems, there is greater diversity of organisms involved in pest control from multiple taxa – including birds, bats, beetles, flies and so on.

Despite the challenge, the authors believe the large dataset compiled by this study has the potential to become an open-access pest control database.

One of the ways forward is the inclusion of trait data, such as longevity, body size, and diet, in pest control models to help identify groups that respond in a similar way to landscape variables. This approach was used successfully in a study that examined tropical bird responses to land use change. The work found that short-lived, small, migratory, insectivorous habitat generalist bird species were more abundant than long-lived, large, resident fruit-eating birds in intensively used habitats.

The team, which included 153 co-authors from scientific institutions around the world, concludes that robust decision support tools are “needed to help farmers understand when habitat conservation represents a true win-win and when conservation activities will need to be carefully co-managed to reduce the risk of damaging pest outbreaks”.