Gardeners love to thumb through nursery catalogs and decide which new flowering trees, shrubs, and other ornamental plants they will add to their yard in the spring. In most cases, unless they are an invasive species that can escape into the wild, non-native trees and shrubs are considered colorful additions to the urban landscape. But a new study in the journal Proceedings of the National Academy of Science suggests there’s a big downside. Planting too many non-native woody plants in a yard can drastically reduce insect populations and in turn impact the breeding success of insectivorous birds. 

Lead author Desirée Narango, a research associate at the Smithsonian’s Migratory Bird Center and current visiting researcher at the City University of New York, says that ecologists have known from past studies that non-native woody plants aren’t great for insects. 

“Non-native plants have different chemical compounds, so they might not be able to be eaten by certain leaf-eating insects. They also leaf out and flower at different times; they have a different phenology,” she says. “We already knew they could impact insect diversity, the number of species using these plants, but we didn’t know how that might impact the animals that rely on insects for food. A chickadee doesn’t necessarily care what kind of insects are on a tree; it just cares if there’s enough to eat and survive and reproduce.”

Narango and her colleagues worked with an ongoing project called Neighborhood Nestwatch that has been running in the Washington, D.C., area since 2000. The team placed 159 nest boxes in yards around D.C., then monitored the success of Carolina chickadee pairs that moved in over the next three years. They also assessed the biomass percentage of native and non-native trees in a 50-meter radius from the boxes, which is roughly the size of a chickadee territory, and sampled the trees to assess the quantity of insects and caterpillars on native and non-native trees. 

What they found is that landscaping matters. When the percentage of non-native trees exceeded about 30 percent of the biomass, the breeding success of chickadees plummeted below replacement levels, meaning that over time yards with lots of exotic trees were population sinks, reducing the number of chickadees. The only reason Carolina chickadees still bounce around such suburban yards is because birds produced in more successful neighborhoods or parks recolonize the non-native landscapes. 

The finding was not too surprising for Narango, who, along with coauthor and well-known conservationist and entomologist Douglas Tallamy, published a paper last year in the journal Biological Conservation showing just how much more productive native trees were compared to non-natives. The chickadees, though they are omnivores eating any bug, seed, or berry they can find, rely on energy-packed caterpillars to feed their chicks in the spring, as do many other bird species—even primary seed eaters. In that study, the team found 20 or more caterpillar species clinging to native species like oaks, cherries, and maples. Related non-native species had a less diverse collection of bugs, and some introduced species had just one species or none. In fact, according to Tallamy, native oak trees can support at least 534 species of moths and butterflies alone while cherries, willows, and birch trees support over 400 each.  

The importance of planting natives is magnified by the fact that over 80 percent of land in the U.S. is in private hands. “These novel, artificial suburban landscapes are found across the country,” Narango says in a statement. “But a gingko that you plant in D.C. and a gingko that you plant in L.A. are doing the same thing for bird conservation—nothing.” 

The study isn’t chickadee-specific either. Because the little birds are one of the few that will breed in nest boxes, they are used as a stand-in for all the species that rely on insects, like warblers and vireos. “Chickadees are a good proxy for insectivorous birds,” Narango says. “Migratory species may not be breeding in your yard, but they are moving through and using these yards as habitat.” 

Other recent research shows the bugs—and in turn the larger species that rely on them—need our help. A study looking at insect populations in nature reserves in agricultural areas across Germany indicate that insect abundance has dropped by a staggering 75 percent in the last 30 years. Another study released earlier this month shows that insect biomass in Puerto Rico’s Luquillo rainforest has also crashed in the last 40 years, falling by a similar amount, likely caused by climate change. The study found that the loss of insects led to a more than 50 percent reduction in anole lizards in the forest and harmed bird species like the Puerto Rican tody, which has diminished by 90 percent in the same time period.

In urban and suburban areas, protecting wildlife that relies on insects means choosing and planting the right tree. The gardening industry, which is in a constant search for new exotic species and varieties of trees to sell, has made it more tempting for homeowners to plant a unique specimen rather than a traditional hickory or cherry tree. Still, U.S. homeowners and landowners are pretty fond of native trees. Native oaks, maples, dogwoods, redbuds, basswood, and others are still popular choices. But as diseases like oak wilt and pests like the emerald ash borer decimate native tree populations, cities and homeowners are opting for non-native trees with fewer diseases or predators.  

Narango says once homeowners know the potential impacts of the trees they plant, they’re usually supportive and begin to see their yard as an important part of the ecosystem. 

“A lot of these homeowners, when they think about backyard birds, think about cardinals and robins and chickadees,” she says. “But for a short period during the year we have this tremendous influx of birds that are moving north to the boreal forest or south to Central and South America. They may only be in your yard for three to seven days throughout the year, but they could potentially be the most important days of their life, because we know they use these urban areas extensively for habitat during migration. By providing habitat for chickadees, you’re providing habitat for birds you may not even know about.”