by Donna Wildearth
Published in the Winter 2019 issue of Darlingtonia
|This is an updated and revised version of an article about Douglas Tallamy and his work that appeared in the Winter 2019 issue of Darlingtonia. For more in-depth information, I highly recommend his books: Bringing Nature Home: How You Can Sustain Wildlife with Native Plants (2007) and Nature’s Best Hope: A New Approach to Conservation That Starts in Your Yard (2019). There are also several excellent videos of Tallamy on YouTube. Here’s the link to one of them: https://www.youtube.com/watch?v=qstt3davwWs|
Insects are making headlines—again. For years we have heard troubling reports of the crisis in honeybee populations, and there have been articles about the alarming decline of monarch butterflies (as much as 96% since 1976 in central and eastern U.S). But recent news stories are revealing a more widespread problem.
In September 2018 the Associated Press released an article entitled “Bye bye bugs? Scientists fear non-pest insects are declining” (Times-Standard, September 23, 2018). The study noted that scientists in the U.S., Canada, Costa Rica, Germany, Denmark, and Greenland who study beneficial flying insects such as bees, moths, butterflies, ladybugs, and fireflies are reporting declining populations. One study estimated a 14 percent decline in ladybugs in the United States and Canada from 1987 to 2006, while a 2017 study of 63 nature preserves in Germany found an 82 percent mid-summer decline in the number and weight of bugs compared to 27 years earlier.
On October 15 the Washington Post ran an article entitled “’Hyper alarming’ study shows massive insect loss,” detailing the results of a study in Puerto Rico where a sharp drop in the number of moths, butterflies, grasshoppers, and spiders was accompanied by steep declines in the number of insect-eating lizards, frogs, and birds. In Tallamy’s lecture he referred to a study that estimates invertebrate abundance worldwide has declined by 45% since 1974.
Should we care about this news? The answer is a resounding yes, because insects are the basis of the food web on land. And if insects are declining, multiple other organisms that eat insects are also at risk. Insects are highly nutritious, rich in protein and fat, and are an important component of the diet even for such animals as the red fox (25% of their diet) and black bear (23% of their diet)!
Tallamy’s lecture focused on an issue that many people aren’t aware of: the crucial importance of insects for birds. Ninety-six percent of terrestrial birds in this country raise their young on insects, primarily caterpillars. So if insect populations are declining, it isn’t surprising to learn that roughly one-third of the birds in this country are at risk of extinction. The 2016 State of the Birds report found 15 billion fewer birds breeding in the U.S. compared to 40 years ago.
Caterpillars are an essential food for baby birds. Caterpillars are large and soft compared to other insects, making them easy for nestlings to digest. Caterpillars are also higher in carotenoids than many other insects, and these carotenoids provide immune system support, antioxidants, and the nutrients that birds need to develop the bright colors of their feathers. A 1961 study found that Carolina chickadees require between 6,000 and 9,000 caterpillars to successfully raise one brood of young birds.
If insects are vital components of the food web, the next question is what plants produce the most insects, especially caterpillars? This seems to fly in the face of conventional gardening wisdom which regards most insects as pests. But the bottom line of current research is that if we want to encourage birds in our gardens, we need to grow plants that will serve as hosts to a variety of insects. And the plants that do this best are native plants.
Native plants harbor more insects because 90% of leaf-eating insects are specialists in terms of which plants they can eat. Plants don’t want to be eaten, so in order to overcome the chemical and physical defenses that plants have developed, these specialist insects need to have co-evolved with a particular lineage of plants for thousands of years. A good example of this is the monarch butterfly, which has a long evolutionary history with milkweeds that has enabled the butterfly to work out how to eat the plant without being poisoned by its toxic chemicals.
Tallamy and his students counted the numbers of caterpillars on a native oak tree and found 410 caterpillars from 19 species. In comparison, on a non-native Bradford pear, a popular ornamental, they found exactly one. In a recent study in a Washington, D.C. suburb one of Tallamy’s students found that, compared to native plant landscapes, yards dominated by non-native plants produced 75% fewer caterpillars. This study concluded that only yards with more than 70% native plant biomass can sustain chickadee populations.
Non-native plants are not evil in themselves—though some are certainly problematic in terms of invasiveness. They are simply plants that have not been here long enough to evolve a relationship with insect herbivores. And this is true even though some, like the ginkgo, have been grown on U.S. soil for 400 years.
We need to grow native plants, but not just any native plants. One of the most significant results of Tallamy’s research is the “keystone plant” concept—the discovery that some native plants have much more impact than others in terms of supporting caterpillars. His research has revealed that, across the country, only 5% of native plant genera (plural of genus) support 75% of caterpillars, and just 14% of native plants support 90% of caterpillars. These plants, primarily trees and shrubs, are what Tallamy calls “keystone” plants.
If you’re wondering what your local keystone plants are, visit the National Wildlife Federation website at www.nwf.org/NativePlantFinder/. Set your location by entering your zip code, and you will see trees, shrubs, flowers, and grasses native to your area ranked by the number of butterfly and moth species that use them as host plants for their caterpillars.
For Eureka (95501), for instance, the top ten native plants are: willows (which support 328 species of caterpillars); oaks (275); bitter cherry/choke cherry (262); aspen/cottonwood/poplar (230); beach pine (220 species); red alder/white alder (202); Oregon crabapple (155); bigleaf maple/vine maple/mountain maple (120); ceanothus (120); and grand fir/white fir (117). (Note that these numbers apply only to native plant species. Japanese maples, for instance, most likely support roughly half the number of caterpillars species listed.)
Finally for some good news: we can make a difference. In our own yards we can start to address the problem of declining insects, birds, bees, and butterflies by planting more natives, especially keystone plants; by reducing the area covered by lawn; and by creating wildlife corridors to connect existing stands of native plants. When we do so, we reap an added benefit—these creatures literally animate our gardens, providing another level of interest and beauty. And if you’re asking “Why would I want caterpillars on my plants?” Tallamy points out that birds eat most of the caterpillars before they get very large, usually before they damage the foliage to any noticeable extent.
In his words, “You can make a beautiful garden that also supports local food webs, sequesters carbon, improves your watershed, and helps pollinator populations all by yourself if you choose productive plants. And your contribution to local ecosystem function plays an important role in sustaining this planet.” Such gardens sustain us as well, enriching our lives with a stronger sense of place and the rewards of being attuned to natural processes and seasonal patterns.
Restoring Nature One Garden at a Time. Revised for formatting 2023.02.15