The native tree selection guide presents data from Professor Tallamy on the number of caterpillar species (Lepidoptera) hosted by native trees (see “what is the biodiversity indicator?”).
Below, Dr. Tallamy answers questions about how and how well these data indicate the relative ecological contributions of various native trees.
Doug Tallamy is well-known to many Takoma Park residents through his New York Times best-selling books (The Nature of Oaks, Nature’s Best Hope, and Bringing Nature Home), and his 2016 lecture at our city auditorium (Bringing Nature Home to Takoma Park). He teaches in the Department of Entomology and Wildlife Ecology, University of Delaware
Are caterpillar host records recorded at the plant genus level a good index for the caterpillar productivity of all of the species in a genus?
A fair question. Let’s look at why we record host records at the genus level.
Host plant records are scattered throughout the literature and many of them are quite old, sometimes reaching as far back as the late 1800s. It was (and still is) commonplace for natural historians and scientists to record host plants at the genus level. Comments like “Eats oaks” or “eats maples” are far more common than” eats white oak” or “red maple.” This is not just laziness on the part of the recorder. If a caterpillar is commonly collected on white oak, red oak, pin oak, shingle oak, post oak, burr oak, etc., it is far easier to say “eats oaks”, than to list every oak species it has been found on. Specific host records do occur in the literature, but they are far less common; if we only use them in creating our productivity index, we would lose more than half of the host records that actually exist.
Are there serious trade-offs when we record hosts only at the genus level?
One possibility is that summing all the host records over the entire genus in a particular region inflates the number of caterpillar species any one member of that genus can support.
The other possibility, and the assumption we have made, is that if a caterpillar is adapted to eating one member of a genus, it will have the adaptations required to eat all members of the genus. There are good reasons to make this assumption. Plants defend themselves against caterpillars with phytochemicals that are usually traits of every species in a genus. For example, oaks protect themselves with tannins. All oaks have tannins, so if a caterpillar species discovers through evolutionary time how to counter tannins as a defense, it would then be able to eat any oak that relies on tannins for defense. We are sure that there is variation in the number of caterpillar species that various species in a genus supports, but there is good evidence that it is minor compared to the variance among genera in records of caterpillar use.
For example, oaks support 557 species of caterpillars in the mid-Atlantic region. Tulip poplars support only 21. You might argue that there are 20 species of oaks in the mid-Atlantic region and only 1 species of tulip tree and that any one of the 20 oak species may only support 27 species of caterpillars (divide 20 into 557). This may be theoretically possible, but one would think that by now, a great deal of host specificity would have been noticed in oaks. In fact, there are no records of host specific caterpillar associations within the genus Quercus.
Supporting the broad use of oaks by caterpillars is the leaf damage caterpillars leave behind. Two summers ago, one of my students compared total leaf area consumed on 16 different species of oaks in southeast Pennsylvania. He found no significant differences in the amount eaten among 14 oak species. Two species, willow oak and water oak, had slightly less damage than the other species, but they are the two species planted north of their natural range and so beyond the reach of caterpillars that would normally eat them in the south.
But why look at caterpillars at all? Why not measure host use by beetles, or plant hoppers?
We have two good reasons for this. First, host records for caterpillars are far and away the best among any insect herbivores. Host records for beetles are particularly bad because the larvae of so many beetle species feed on roots or within plant parts and are rarely encountered except as adults. The second reason is that caterpillars really are more important within food webs than other insects. They are transferring more energy from plants to other animals than any other plant eater (Janzen 1988). So, the number of caterpillar species a plant genus supports is a good index of its value, not just to caterpillars, but also to all the birds and other creatures that eat caterpillars.
I have made my arguments using oaks as an example, but they apply to all of the plants in an area. The number of caterpillar species recorded on a plant genus is the best indicator we have of the ecological value of that plant to local food webs. One might worry that this host index will undervalue good native plants, but before we had this index, we were overvaluing many natives in terms of their contribution to food webs. The reality is that all natives do not contribute equally. Therefore, it is important to include the best contributors in our landscapes, not exclusively but as the foundation plantings of our landscapes.
For more information on Professor Tallamy’s work, visit Home Grown National Park.
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