It's rare that a new (to me) ecological concept related to gardening and wildlife hits me upside the head, but I treasure it when it does. Reading Douglas W. Tallamy's new book, Bringing Nature Home, brought me just such a moment.
For years I have believed that gardening with natives was important, but I have allowed myself to become less single-minded about that belief, especially in recent years. I couldn't answer people's questions about why, for example, a native berry-producing shrub was better than an exotic berry-producing shrub if the birds liked to eat both types of berries equally. Instinctively I felt the native was better, but I couldn't produce the underlying logic to share with others.
Tallamy nails my missing logic in this book.
Every time a plant is described as "pest free", the nurseryman is saying that leaf-eating insects (and often other herbivores) won't eat it, usually because they truly can't eat it due to its chemical makeup. This means that the particular plant in question - almost always an exotic introduced from another country or from a different area of this country - is taking space, using nutrients from the soil, capturing sunlight...and adding little or nothing to the local food chains. In essence, it is a sterile placeholder, taking the place of other, native plants that can and do support diverse, healthy populations of native animals. Because the exotic plant is not getting eaten or controlled in any way, it can easily outcompete the locally palatable native vegetation, so when the exotic escapes into the wild, there is nothing holding back its takeover of local habitats and sterilizing them.
So right now I can practically hear the question going through your head, "Why would I possibly WANT insects to eat my plants? Won't that kill them?"
The simple answer is, "No, it won't kill them. Native plants evolved along with native herbivores so that both balance each other. Then, too, having native predators feeding on the native herbivores further keeps that balance in place."
Insects are one of the primary ways that the energy captured by plants gets moved up the food chain. They are the primary herbivores of many, probably most, plants; then they become the food for primary predators, from other insects to amphibians to birds.
If you love to attract birds to your yard, it's important to know that the energy and nutrients for bird reproduction are almost entirely based upon insect food. Berries in the fall won't do birds any good if there haven't been any birds produced earlier in the year because there were no insects available to provide the food and energy for eggs and young.
I can hear another question, "Japanese beetles are eating my roses to stubs. How can you say that insects won't kill my plants?"
Japanese beetles aren't native insects, and they are not acting "naturally" for our native ecosystems. Many major pest outbreaks are actually caused by alien insects or diseases introduced accidentally into our country. Like the alien plants they often come in on, these alien pests have no natural enemies and ravage unchecked throughout the countryside. Chestnut blight, Dutch elm disease and now sudden oak wilt are all examples of alien disease organisms; (Asian) azalea lace bug, soybean aphid and Japanese beetles are examples of devastating alien insects.
I've just hit the highlights of Tallamy's book here. He describes the concepts in much greater depth than I have here, plus he provides lots of interesting examples. He has also included a wonderful section about the different types of insects, including some fascinating and fun insect life histories. If you love to garden, especially if you love to attract wildlife to your yard, I highly recommend that you read this book.