Bringing Nature Home: How Native Plants Sustain Wildlife in Our Gardens by Douglas Tallamy has been next to my bed for a while. It's been out since 2007, so it's not exactly fresh off the presses. Slow starter that I am, I finished it only recently and thought I'd post a quick review and brief notes, since it has occasionally been the subject of controversy.
The bottom line is that I like and recommend the book. It's written in an approachable manner, with many fascinating color pictures that amplify on the text and the topic is timely given the increase of green thinking that is sweeping the nation.
The first half of the book is valuable to all gardeners. But starting around chapter 10, the book becomes much more focussed on east coast flora, and therefore of less interest to those outside that area. The opinions in the book are supported by citations from scientific literature and personal anecdote, though Tallamy admits that the data are incomplete and much work remains to be done. Nonetheless, he convincingly makes the case that extrapolation from the knowledge that we presently have suggests that urban and suburban gardeners can remediate the damage that human presence has had on the wild food web. (The food web is an extension of the grade school concept of the food chain, but with multiple redundant paths up and down. This concept and current understanding that we have of "keystone species" - species that are essential to preserving the food web, are covered early in his book. It's interesting to note that I'm confining this to a parenthetical side note and considering not even mentioning it at all because of its obviousness, but apparently until somewhat recently these concepts were not commonly understood.) The tone of the book is not preachy or histrionic. He makes his case calmly, with facts, anecdotes, and extrapolation: From Tallamy's point of view, it's not too late to do something that makes an essential difference.
Tallamy is an entomologist at the University of Delaware in Newark, so he takes a look at how humans have altered the historically longstanding food web as we've expanded into previously wild areas from the standpoint of disruption at the insect level. Looking one level up and down the food chain from herbivorous insects (by total tonnage the largest group of herbivores in the world and whose predation accounts for the most abundant source of food energy in the world) he pieces together an argument supporting the expanded use of natives as follows:
Tallamy takes as his definition of native plants as species "having a historical evolutionary relationship" with their environs. This definition does away with arguments, for example, that one can take a plant from anywhere in North America and plant it anywhere else, so long as the USDA hardiness zones are the same, and call it a native. By contrast, alien species do not have a historical evolutionary relationship with their environs and therefore do not participate completely in their local ecosystems. In fact, the nursery trade has made a virtue out of "pest free" plants, in other words, aliens which won't support significant amounts of indigenous insect life.
Because native insects have a common evolutionary history with native plants, the insects have a preferred diet of native plants and in some cases exist only on certain natives. Alien plants nourish far fewer (in number and in diversity) insects than natives, even those that have been here for 100s of years. (100s of years is quite short in terms of evolutionary adaptation.) Because insects feed on plants and in turn are fed upon by higher trophic levels (mostly birds), reducing the number of natives will proportionately reduce the number of higher higher trophic levels supported.
As remedy, Tallamy proposes that urban and suburban gardens favor native plants over aliens in order to take the place of the diverse woods and wildlands that they have displaced. As a corollary, we must actively guard (or garden) remaining "wild" areas to prevent intrusion of aliens. These steps, he argues, are essential if we are to preserve the next higher link in the food chain - birds, mostly, as well as other small mammals that eat insects.
In addition to native plant species, Tallamy recommends high diversity both in species and in structure. Structural complexity means having varied types of plants (grasses to trees and everything in between). This is logical from the standpoint of replicating the diversity of native habitats which provide for the habitat needs at many more cycles of life: The insect herbivores need both larval and adult food which are often completely different plants.
Finally, he points out that the beneficial insect predators that you want in your garden (such as ladybugs) need populations of prey insects (such as aphids) to sustain them or they will abandon your garden. Therefore the co-existence of small numbers of "pest" insects is necessary to ensure that the beneficial ones are there when you need them. Your garden won't be perfect, but he cites a study showing that even the most gardeners won't notice or perhaps care about 10% leaf damage, so there's the possibility of peaceful coexistence of a variety of insects, in balanced numbers, with peoples' requirements for nice looking plants. As for where to put these plants: He suggests shrinking your monoculture lawn and expanding your hedgerows to incorporate natives and increase diversity in a subtle way.
Here's an illustrative quote from the book on the topic of diversity.
In the East, the number one pest of ornamental gardens is the azalea lace bug (Raupp & Noland, 1984). This bug was introduced along with evergreen azaleas from Asia and now sucks the chlorophyll from alien azalea leaves wherever the plants are massed in a sunny setting, although the bugs won't touch our native azalea species. Why don't natural enemies, the insect predators and parasites native to North America, control this pest? Because the community structure of most of our gardens is far too simple to support the numbers and diversity of natural enemies required to keep the azalea lace bug in check. Picture a classic suburban foundation planting: a row of Asian azaleas along the front of the house, bookended by two arborvitaes. Just where are the natural enemies needed to control the lace bug supposed to come from? Ladybird beetles, assassin bugs, damsel bugs, and parasitic wasps can only live in a garden if there are enough types of prey available to support them at all stages of their life cycle. Because our classic suburban foundation planting is dominated by alien plants, the only insect available to support a community of predators is the azalea lace bug. When the lace bug population is small, which is the critical time for predators and parasites to prevent an outbreak, there simply is not enough [other] prey biomass in the garden to attract and support populations of natural enemies. And so the lace bug population explodes, the homeowner runs for the insecticide, and the goal of having an undamaged garden is lost.
One of the areas where Tallamy has come in for criticism is in his recommendation of trees as primary insect habitat. Of course, for habitats that didn't originally have a lot of trees, such as the California coastal prairie habitat originally around my house, this makes little sense. However, a reasonably careful reader of his book will note that in chapter 12 he writes, "I am also forced to slight western North America and focus on...eight states of the easter deciduous forest biome.... I restrict my discussion to this region because it is the only area for which we have done an exhaustive literature search for host plant relationships." So casual readers may misunderstand his writings, but the disclaimer is there for all to see, and the results are still quite interesting.
Working with moths and butterflies (Lepidoptera) as representative of all insect herbivores (apparently they represent over 50% of all insect herbivores in this country, so this is a good first choice) he then sets forth some interesting findings: Among woody plants, Oaks support the most Lepidoptera with 517 species, willows are next with 456, and Walnut, Beech, and Chestnut trees bring up the bottom of the list with 130, 126, and 125 each, respectively.
It's probably zeroth or first order correct to pull the tree genus' native to California directly from his list and assign them that relative rank in terms of insect value. However, there is no mention of the contributions of other plant types (groundcover, vines, shrubs, cactus, etc) so for areas of historically sparse woody plants, like California coastal scrub or prairie, there's no equivalent starting point to make most effective use of California's other plants.
Read an interview with Tallamy on Garden Rant.
A related link to BUGS - Biodiversity in Urban Gardens.