On a June day in 2001, not long after he bought the property, Tallamy, an entomologist at the University of Delaware, was walking his land when he noticed something that struck him as unusual. Before he bought it, most of it had been kept in hay, but at that point it hadn’t been mowed in three years and “was overgrown with autumn olive and Oriental bittersweet in a tangle so thick you couldn’t walk. The first thing I had to do was cut trails,” Tallamy recalls. And walking through his woods on the newly cut trails, what he noticed was what was missing: caterpillars.

No caterpillars on the Oriental bittersweet, the multiflora rose, the Japanese honeysuckle, on the burning bush that lined his neighbor’s driveway. All around him plants were in a riot of photosynthesis, converting the energy of sunlight into sugars and proteins and fats that were going uneaten. A loss, and not just for him as a professional entomologist. Insects—“the little things that run the world,” as the naturalist E.O. Wilson called them—are at the heart of the food web, the main way nature converts plant protoplasm into animal life. If Tallamy were a chickadee—a bird whose nestlings may consume between 6,000 and 9,000 caterpillars before they fledge, all foraged within a 150-foot radius of the nest—he would have found it hard going in these woods.

Tallamy knew, in a general sense, why that was. The plants he was walking among were mostly introduced exotics, brought to America either accidentally in cargo or intentionally for landscaping or crops. Then they escaped into the wild, outcompeting their native counterparts, meeting the definition of an “invasive” species. By and large, plants can tolerate a wide range of environmental conditions. But insects tend to be specialists, feeding on and pollinating a narrow spectrum of plant life, sometimes just a single species. “Ninety percent of the insects that eat plants can develop and reproduce only on the plants with which they share an evolutionary history,” Tallamy says. In the competition to eat, and to avoid being eaten, plants have developed various chemical and morphological defenses—toxins, sticky sap, rough bark, waxy cuticles—and insects have evolved ways to get around them. But as a rule, insect strategies don’t work well against species they have never encountered. That’s true of even closely related species—imported Norway maples versus native sugar maples, for instance. Tallamy has found that within the same genus, introduced plant species provide on average 68 percent less food for insects than natives. Hence, a plant that in its native habitat might support dozens or hundreds of species of insects, birds and mammals may go virtually uneaten in a new ecosystem. Pennsylvania, for example.

mk, with your rain garden and all, I thought you'd like this.