Oaks are our most essential native tree, according to entomologist Doug Tallamy, Ph.D. Because so many species rely on oaks for their survival, they’re considered keystone plants due to the pivotal role they play in the food chain.
Doug is a professor at the University of Delaware in the Department of Entomology and Wildlife Ecology. He studies the interactions between plants and insects, and what’s eating those insects. His latest book is “The Nature of Oaks: The Rich Ecology of Our Most Essential Native Trees, and his earlier works on the importance of conservation and native ecology include “Nature’s Best Hope” and “Bringing Nature Home.”
“Nature is not optional for humans,” Doug says. “We’re products of nature. We’re totally dependent on it. And insects are such a crucial part of the natural world that they’re essential. We can’t lose them.”
Doug wants people to learn that an oak is more than just a tree that drops leaves that they have to pick up — it is an entire community of living things that changes throughout the year. He wrote “The Nature of Oaks” to provide a roadmap of what’s happening on and in an oak any time of year.
For a full recap of my conversation with Doug, see the show notes from the original airing.
Oaks Are Diverse, Productive and in Need of Conservation
There are 435 species of oaks worldwide, all belonging to the Quercus genus, and 91 of them are found in the United States. Quercus provides food for more caterpillar species than any other genus of plants in North America — not to mention all the animals that eat the acorns that grow on oaks. That’s why he calls oaks keystone plants. If you take oaks out of the local food web, the food web collapses.
Doug’s research has found that oaks are the most productive trees in 84 percent of the counties in North America when it comes to producing food. He points out that of the insects that eat plants, 90% can only eat a few plant lineages — the plants that they co-evolved with and have specialized on.
There are 50% fewer oaks in eastern forests than there were 100 years ago. Logging and land clearing contribute to oak loss in addition to oak diseases, such as sudden oak death syndrome, bacterial leaf scorch and oak wilt. The Morton Arboretum in Illinois studied native U.S. oaks and found 28 of the 91 species are of conservation concern.
Why Oaks Are a Meal for Many Insects
One hypothesis to explain how oaks became so valuable to the food web concerns their chemical defenses. While other plants produce toxins or bitter compounds to make plants unappetizing, oaks only produce tannins. Tannins slow the absorption of proteins, which is a defense that’s much easier for insects to adapt to, Doug says.
Another hypothesis is that oaks have been around for so long that insects have had more time to adapt to them. Individual oaks are large and can live for several hundred years, which gives insects a lot of time to adapt.
Every plant lineage is protecting itself with a unique cocktail of chemical defenses. An insect species can’t adapt to all of those defenses, so it gets really good at circumventing the defenses of a small group of plants, Doug explains.
One example is the monarch butterfly, which can tolerate the cardiac glycosides and latex sap in milkweed plants. If there is no milkweed around for monarch butterflies to lay their eggs on, they will have no place at all to lay their eggs. They can’t just lay eggs on oaks instead, because monarch caterpillars haven’t evolved a way around the oaks’ defenses.
“Caterpillars transfer more energy from plants to other animals than any other type of creature,” Doug notes. When caterpillars don’t have native plants to eat, there won’t be caterpillars around to provide that energy to birds and other wildlife. A landscape without caterpillars is a dead-end for the food web.
Birds and Insects in Decline
A 2019 Cornell Lab or Ornithology study found that there are 3 billion fewer birds in North America than there were 50 years ago. In 2018, The New York Times wrote the headline “The Insect Apocalypse Is Here.” And back in 1987, naturalist and writer E.O. Wilson penned “The Little Things that Run the World” on the importance of the conservation of invertebrates and what it would mean if the world lost its insects.
Losing insects would mean losing most pollinators of flowering plants. Losing flowering plants would mean losing most of the energy that drives the food webs that support our animals — amphibians, reptiles, mammals, birds and many freshwater fish would disappear. Additionally, the loss of insect decomposers that recycle nutrients quickly would mean organic material would only break down slowly by bacteria and fungi.
These are the kinds of changes that humans wouldn’t survive, Doug says, comparing global insect decline to losing oxygen.
If you haven’t listened yet to my conversation with Doug Tallamy on why oaks are keystone plants, you can listen to this episode now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
Do you have a keystone plant on your property? Let us know in the comments below.
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“The Nature of Oaks: The Rich Ecology of Our Most Essential Native Trees” by Douglas W. Tallamy
“Nature’s Best Hope: A New Approach to Conservation That Starts in Your Yard” by Douglas W. Tallamy
“Bringing Nature Home: How You Can Sustain Wildlife with Native Plants” by Douglas W. Tallamy
“The Insect Apocalypse Is Here” from The New York Times
“The Little Things that Run the World” by E.O. Wilson
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