The Nature of Oaks

A Book Report

“Caterpillars are built from the energy and nutrients stored in the leaves they have eaten. In effect, caterpillars are repurposed leaves that can walk.” —Doug Tallamy, PhD

Dell J. Hood

In the past 20 years, Dr. Douglas Tallamy, professor of entomology at the University of Delaware, has become one of the most popular authors and speakers for garden and horticultural clubs, suburban homeowner associations, and native plant groups. His common-sense proposals for how millions of individual landowners can contribute to improving local and small-scale environmental conservation have struck responsive chords across the country. His 2007 book, Bringing Nature Home, has become one of the most influential works of the past fifty years on ecology and the role of humans in preserving natural diversity.

A recent book by Dr. Tallamy—The Nature of Oaks: The Rich Ecology of our Most Essential Native Trees, published in 2021—may not have gained as much attention as that work, but I found it fully as interesting and informative, and its focus on North American oaks makes it particularly relevant to our Central Texas situation, where several oak species may form the largest component of the tree canopy. Although many of the observations he reports were made on oaks in southeastern Pennsylvania where he lives, his focus is on the entire ecosystem created by every oak species in the U.S. As common as live oaks and Texas red oaks may appear to us today, his concern about the wide range of environmental and human-created pressures that oaks face everywhere makes this work important no matter where you live. He notes that the percentage of oaks in eastern forests has fallen from 55% before European colonization to 25% today; one analysis has found that 28 of the 91 oak species in North America may soon disappear from the wild completely.

This majestic oak provides shade and an inviting spot for visitors and families to rest and play as they walk between the parking lot and “The Well” at Jacob’s Well Natural Area in Wimberley, Texas. Live oaks, such as this one, are the icons of our Central Texas landscape. Photo: courtesy Betsy Cross

The book is organized into twelve short chapters according to the months of the year, from October through September. For each month, Tallamy reviews just a few of the amazingly large numbers of insect, microinvertebrate, and bird and animal species that depend on oaks, either totally or partially. Much of his discussion covers the caterpillars of moths and butterflies—many more species than we find in the handbooks on these insects. There are also gall wasps, weevils, ants, cicadas, leaf hoppers and leaf miners, and walking sticks to be observed, all participants in the web of year-round consumers of oak leaves or twigs, or consumers of those that are. It is amazing how gall wasps, “among the most heavily parasitized group of animals on earth,” have evolved their reproduction to avoid or prevent their arthropod enemies from killing and consuming their larvae growing in the many forms of galls the host oaks produce in response to their enzymes.

Oak leaf galls are formed when an insect, such as a tiny wasp, “…lays an egg just under the surface of a stem or leaf. A gall is formed because the tree is stimulated by the presence of the egg and eventually the hatched larvae to grow a structure around the wounded area. Galls are very common in oaks and are not considered harmful to the health of trees.”
Source: https://www.wildflower.org/expert/show.php?id=7873 Photo: courtesy Betsy Cross

Tallamy begins this fascinating exploration with October and a discussion of the phenomenon of masting, when some oaks produce prodigious crops of acorns all at the same time. He examines the explanations for this evolutionary development, the behaviors of the birds and animals that consume acorns, and how it affects their reproduction patterns.

A male Summer Tanager forages for caterpillars in the oaks at Jacob’s Well, in Wimberley, Texas. Photo: courtesy Betsy Cross

For April, May, June, and July, Tallamy describes more caterpillar species than you’ve ever heard of. With hard data produced by some of his graduate students, he makes clear how crucial the oaks are to them, and how they are, in turn, to the success of the dozens of bird species that rely almost exclusively on them to feed their nestlings. “Caterpillars are built from the energy and nutrients stored in the leaves they have eaten. In effect, caterpillars are repurposed leaves that can walk.”

The discussion for August summarizes the larger services that oak trees provide to their ecosystems: moderating the impact of rains on the soil beneath them, thereby reducing surface runoff and promoting soil stability; acting as carbon storage systems for decades and centuries; and moderating the air temperature wherever they provide shade from the summer heat.

He even notes the importance of the leaf litter under oak trees, which supports an array of microscopic life forms that still have not been fully identified and described. These are the food sources needed by all the critters in the next higher trophic level, and so on up the chain to the birds, amphibians, reptiles, and mammals we associate with a woodland system. Whenever the leaf litter is removed, for appearance’s sake or convenience, that particular food chain is destroyed, members of uncounted species are lost, and ultimately, even the health of the oak above it can be adversely affected.

Gulf coast toads love to burrow in the leaf litter of our oak trees. Photo: courtesy Betsy Cross

Tallamy illustrates the book with exquisite photographs of many of the small and ultra-small species he describes, plus varieties of oak acorns and oak galls. Clearly and crisply written, this book will add to every Master Naturalist’s appreciation and concern for our own community of oaks and their unequaled importance in our environment.

About the Cover…

Maltese Star-Thistle