Nonnative Plants: Ecological Traps?

Cedar waxwing feeding on white mulberry
A cedar waxwing feasts on white mulberry in a Michigan backyard. Photo by John E. Heintz, Jr.
When I moved into my first house several years ago, it was also the first time I’d ever had my own yard—and with a double lot located on a corner, it was a substantial yard indeed. A lover of lush gardens, I quickly began replacing the property’s lawn with a variety of plants: bulbs, herbs, perennials, vines and shrubs as well as (way too many) trees. In addition, I left alone most of the “volunteers” that sprouted in the yard on their own.

Among them, perhaps my favorite was a white mulberry, Morus alba, a fast-growing tree that is native to northern China. Every spring, this tree—which seemed to double in size each year—would be covered with berries that lured in an amazing assortment of resident and seasonal birds, from chatty mockingbirds and catbirds to colorful Baltimore orioles and scarlet tanagers to large flocks of ravenous cedar waxwings—all in the middle of metropolitan Washington, D.C.

Meanwhile, at my job as senior editor for NWF’s National Wildlife magazine, I began to learn about the benefits to birds and other wildlife of cultivating native rather than nonnative plants as well as the work NWF does to combat invasive species. As my knowledge grew, I stopped planting new exotic species altogether, particularly once I registered my property as an NWF Certified Wildlife Habitat ® site. That prized white mulberry, though, I left in peace. After all, the tree did not appear to be invasive, and many of the birds that were feasting on its berries were migratory species. I was surely helping these birds by providing an abundant and nutritious food during their long, arduous journeys north.

But recently, after working with Virginia writer John Carey on a story just published in the magazine, I’m no longer so sure about that decision. In the article, “Ecological Traps,” Carey describes research conducted by Ohio State University ecologist Amanda Rodewald on the potential effects on northern cardinals of feeding and nesting in nonnative Asian honeysuckle. Over the course of six years, Rodewald and her students meticulously monitored the fates of 888 northern cardinal nests in honeysuckle and other plants in central Ohio, observing each nest at least every few days during the breeding seasons. “It was a ton of work,” says the scientist.

Northern Cardinals
A male northern cardinal courts his mate in a Texas backyard. Photo by Howard Cheek.
The researchers’ surprising—and alarming—discovery was that cardinals nesting in Asian honeysuckle reared 20 percent fewer young than did birds nesting in native plants. The reason? Increased predation by raccoons, crows, hawks and other animals on cardinal nests in the exotic plants.

And there was more bad news. As another part of the study, Rodewald’s team collected feathers from 280 male cardinals, then photographed them and used computer software to measure each feather’s shade of red and color intensity. Normally, males with the brightest feathers are the most fit because they’ve competed successfully for foods rich both in nutrients and the pigments that make feathers red. But in urban and suburban areas where cardinals feed in honeysuckle, that connection is lost because the nonnative berries contain abundant pigments but fewer nutrients than do native berries. The result: Nonnative honeysuckle “reduces the value of plumage brightness as a sign of male quality,” Rodewald says.

I wondered: What unexpected effects might nonnative mulberries be having on the cardinals, waxwings and other birds that feed in my yard? Fortunately, white mulberry is a short-lived as well as fast-growing species, so the tree is beginning to die on its own. And these days, whenever I notice a nonnative volunteer sprouting on my property, I quickly pull it up, roots and all.

Certify Your Garden as a Wildlife HabitatFind out which plants are native to your region, then learn how to help birds and other wildlife by making your yard a Certified Wildlife Habitat ® site.