OPINION: The threat of alien plant species on our forest ecosystem

By Larry Stowe

Are you planning a spring landscaping project? Before you head to your local nursery or garden center to drool over the alluring photographs of trees, shrubs and perennials, please reflect on the wildlife of our state.

The human species has done terrible things to the forest ecology in Connecticut and most other states. We have replaced most of our woodlands with roads, parking lots, lawns and agricultural fields, leaving a fragmented forest ecosystem. We have introduced pests from Eurasia that have virtually eliminated chestnuts, elms and ashes. Finally, in what may be the coup de grace to our forests, we have introduced, and continue to adorn our yards with, alien plant species.

Species like burning bush, Japanese barberry, bittersweet, autumn olive and privet grow well in Connecticut as well as in their Eurasian lands of origin, and they can be appealing near our houses. All suffer little or no damage from herbivorous insects, and all attract birds with their berries. These last two properties, which seem at first like great advantages, spell ecological disaster for our Connecticut forests.

The seeds of these species can pass through the digestive systems of birds unharmed, and birds have spread them throughout our state. Once planted, they grow without any significant predation. Virtually all plants have defensive compounds in their tissues that protect them against most herbivores. However, over millions of years, certain insects have evolved a tolerance for specific defensive compounds and have become specialists.

A monarch butterfly caterpillar, for example, can eat the somewhat toxic leaves of milkweed without harm, and it won’t eat anything else. Pearl crescent butterflies lay their eggs on asters and black-eyed Susans; the spicebush swallowtail caterpillar eats only… guess what! ... spicebush and its relatives. There are doubtless many insect species that can eat our invasive species, too, but they are back in Eurasia!

Having escaped their insect predators, invasive species tend to grow more quickly, tolerate shade better, and produce more seeds than similar native species. They therefore colonize rapidly any wind throws or man-made clearings and displace native species. Tyler Mill Preserve, Bertini Park, the Farmington Canal Linear Park and the forest near Hubbard Park are a few local examples of natural areas in which invasives have made significant inroads. At this point it will take a huge effort to halt their expansion.

The call to address this problem has been given new urgency by Douglas Tallamy, an entomologist at the University of Delaware. In his book, “Nature’s Best Hope,” Tallamy considers the effect of invasive plants on the ecosystem as a whole. In his research, he compiled the results of many studies in order to determine the number of insect species supported by various plant species.

Native plant species supported hundreds of insect species, with oaks topping the list and cherries, willows, goldenrods and asters contributing importantly. Alien species supported a handful at most. As forests and grasslands are replaced by agricultural fields and by the lawns and alien plants of suburbia, the insect populations plummet. This may not seem like a great loss at first, but it’s important to realize that, globally, insects constitute about half of all animal biomass!

Many of our birds, reptiles, amphibians, fish and mammals rely on insects, or on animals that eat insects, for survival. The World Wildlife Fund estimates that, globally, we have lost about a third of these animal groups in the last 50 years, and our landscaping practices are partly to blame. To the great majority of insect species, and to the animals that depend on them, our alien-bordered lawns are unproductive green deserts.

Tallamy encourages us to support and enjoy natural ecosystems by bringing them close to home rather than confining them to scattered parks and preserves. He envisions landscaping with limited lawn (perhaps only paths!) and with native species to create a less fragmented “Homegrown National Park,” in which our native plants and animals can find their niche. In Connecticut, native shrubs like winterberry holly, red twig dogwood, chokeberry, elderberry and mountain laurel, mixed with perennials like goldenrod, aster, butterfly weed, black-eyed Susan and Joe Pye weed, can provide both beauty and sustenance for wildlife.

As we plan our next landscaping project, let’s remember our beleaguered forests and …er… go native!

Wallingford resident Larry Stowe has a doctorate in plant ecology from the University of Chicago. He was an assistant professor in the botany department at UMass Amherst for a few years, then spent 35 years teaching at Choate Rosemary Hall, retiring in 2017.


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