JEFF COX, APR 7, 2020,Horticulture online
A few decades ago, I wrote Landscaping with Nature (Rodale Press, 1991), a book about gathering inspiration for home landscaping by going into wild nature and noting what was aesthetically pleasing, then finding ways to bring those inspirations home. I didn’t know it at the time, but I only had half the story.
It takes an incredible 6,000-9,000 caterpillars to raise one clutch of chickadees.
The other half is to create those natural home landscape designs with native plants. There are so many good reasons to do so, chief among them is the lifeline that native plants throw to native fauna, especially insects and birds. A true champion of this notion is Douglas Tallamy, a professor in the Department of Entomology and Wildlife Ecology at the University of Delaware. “You might think we gardeners would value plants for what they do. Instead, we value them for what they look like,” he says. He has worked to educate us on the merits of native plants and conservation that starts in our gardens through his books Bringing Nature Home (Timber Press, 2009) and Nature's Best Hope (Timber Press, 2020).
Most of us know some of the things that our plants do: produce oxygen, build topsoil, prevent erosion and flooding, sequester carbon dioxide, buffer extreme weather, clean our water and shade our houses. But it’s their ability to turn sunlight into food for all of earth’s creatures that’s supremely important, especially in the context of local ecologies.
One summer, Professor Tallamy did a simple experiment. He counted the number of caterpillars on a native white oak in his yard and compared it to the number of caterpillars he found on a nearby ornamental Bradford pear, an Asian native. “I found 410 caterpillars on the white oak comprising 19 different species, and only one—an inchworm—on the Bradford pear,” he said.
Why such a huge difference? Native insects have co-evolved with native plants. To avoid predation, plants load their tissues with nasty insect-repellant chemicals, but the native insects have developed ways to de-fang those chemicals, usually with enzymes. The Bradford pear is a relative newcomer, and there are no insects that have yet evolved the ability to eat it—except maybe that inchworm.
“In the past,” he says, “we thought this was a good thing. After all, Asian ornamentals are planted to look pretty, and we certainly didn’t want insects eating them. We were happy with our perfect pears, burning bushes, Japanese barberries, golden rain trees, crape myrtles and all the other foreign ornamentals.”
Then he pointed out the ecological cost. “If you have a pair of nesting chickadees, watch what they bring to the nest to feed their hatchlings: mostly caterpillars. It takes an incredible 6,000 to 9,000 caterpillars to raise one clutch of chickadees.
“What we plant in our landscapes determines what can live in our landscapes. An American yard dominated by Asian ornamentals doesn’t produce nearly the quantity and diversity of insects needed for birds to reproduce. We have 50 percent fewer birds than 40 years ago, and some 230 species of North American birds are at risk of extinction,” he said, citing the 2014 State of the Birds Report.
“By the way,” Professor Tallamy says, “you might assume that my oak was riddled with unsightly caterpillar holes, but not so. Since birds eat most of the caterpillars before they get very large, from 10 feet away the oak looked as perfect as the Bradford pear.”
He adds that since almost all native insects have specialized relationships with native plants, planting non-natives reduces biodiversity. For example, very few insects other than the juniper hairstreak butterfly can eat the tissues of the eastern red cedar without dying. So if we don’t include cedars in our landscapes, we lose the hairstreak. “And the only host for the great fritillary butterfly is the native violet,” he points out. “When violets are mowed down, we lose the fritillaries. And if we lose the insects, including spiders and moths, we lose amphibians, bats and rodents. Even the fox eats insects—25 percent of his diet is insects.”
In his book written with Rick Darke, Living Landscapes: Designing for Beauty and Biodiversity in the Home Garden, Professor Tallamy tells the story of the Atala butterfly, a native of South Florida that once thrived on its sole host plant, Zamia pumila, a native cycad. The butterfly disappeared as the cycad was harvested to near extinction to make starch from its roots. But in the mid-1970s, landscape designers rediscovered it as a valuable evergreen that could take drought and heat. As it started showing up in more and more South Florida yards, the Atala butterfly returned.
“Maybe it had been harbored in the Everglades or somewhere, but adding that single plant brought the butterfly back,” said Professor Tallamy.
Horticulture columnist Jeff Cox writes from his home in northern California.
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