People interested in ecologically sensitive landscaping are generally interested in native plants, but hardly anybody has a good definition of "native," nor any clear explanation of why nativeness is important.
This book, Bringing nature home, is the exception.
Natives, we often hear, are important because exotic plants can become invasive, or because they are not well-adapted to their new homes and so need extra water, extra fertilizer, and so on. Both are quite true, and both are very good reasons for avoiding exotic plants, but clearly they can't both be true of the same exotic plant species...explanations like these, and there are others, don't really get to the heart of the matter. They are reasons to avoid some exotic species, reasons to plant some natives, but they don't provide any cohesive justification for preferring natives categorically. These reasons do not help with questions such as are naturalized plants native? Are plants from another part of the same continent native? Are plants that were historically present on this exact spot before the climate zones moved still native?
Nobody has clear answers to these questions--except Dr. Douglas Tallamy.
Dr. Tallamy is an entomologist, and he therefor tends to see plant/insect interactions from a perspective the rest of us tend to miss--the insect's perspective. Some years ago, he noticed that exotic plants tend not to be chewed up by insects as badly as natives usually are--he wasn't the first person to notice this. In fact, one common explanation for how fast some exotic plants can grow is that the native insects don't know how to eat them. But Dr. Tallamy didn't look at the uneaten plants and think about the growth of vegetation; he looked at the plants the insects hadn't eaten and he thought about hungry insects.
Being a scientist, Dr. Tallamy didn't simply take his own word for it, of course; he conducted a series of experiments and found that, yes, native insects don't usually eat exotic plants. And yes, yards with a lot of exotic shrubs have fewer butterflies--and fewer songbirds--than yards landscaped with natives (songbirds mostly raise their chicks on insects, so no insects means no chicks).
The thing is, here on Planet Earth, all energy ultimately comes from the sun and is soaked up by plants. From plants, the energy goes through a series of animals, and makes the whole world go. Generally, the bulk of the plant-eaters are insects; insects are what a lot of other animals eat, either directly or indirectly. Planting a lot of plants insects can't eat is the same thing as shutting off the flow of energy from the sun.
Dr. Tallamy saw all this, and, alarmed, he started talking to everyone who would listen about landscaping with native plants. The people who listened asked if he could given them anything on the subject to read, so he wrote this book.
So, now that I've told you the moral of the tale, as it were (grow bird food: plant natives!), why do you need to read this book?
First, Dr. Tallamy delivers his message better than I do. He is one of the few writers I know of who can write well for both academic and popular audiences, and Bringing nature home actually follows all the major rules of good academic writing (clear, simple, and properly cited), while still being enjoyable to read--even if you're not an ecology geek. On the page (and in person), he is the very opposite of intimidating.
Second, there's other important stuff in the book; helpful plant lists and tables, beautiful pictures, and little hints at the mind-bending complexity of ecology. We are so used to thinking of good bugs and bad bugs, good plants and bad plants, and the real world doesn't work that way. For example, Dr. Tallamy describes finding a caterpillar eating one of his plants. He was on the verge of killing to protect the plant, when he noticed it had been parasitized by wasps--and the wasps had been parasitized by something else. If he killed all of the caterpillars, then the wasps would have nothing to eat and they would fly away. Then, when new caterpillars moved in (they always do), no wasps would be there to keep their numbers in check. But if nothing ate the wasps, the wasps would kill all the caterpillars themselves. So in order to protect his plants from the caterpillars, Dr. Tallamy not only had to actually have caterpillars in his garden (to provide food for caterpillar-killers), he also had to have caterpillar-killer-killers, to protect the caterpillars from the caterpillar killers, so enough caterpillars would be left to provide caterpillar-killer food.
This is how ecology works; there are no good guys and no bad guys, only a system of interactions that is only as stable as it is complex. Simplify the system--add exotic plants insects won't eat, remove insects that eat the plants, whatever it is, and something will go wrong.
See, this provides both a definition of "native," and a reason why native plants are important; a native organism is one that belongs to a complex web of relationships. If those relationships are missing, then it doesn't matter how long the plant is been here, and it doesn't matter how nearby its home is; it isn't native. And planting non-natives will jam the ecological web, like throwing rocks into the gears.
And of course, it's more complex than that. There are circumstances where exotic plants appear to be a good thing--places where exotics are the only known food of an endangered native butterfly, for example. See, insects don't read the scientific literature. They don't know what they're supposed to eat or not eat.
But wrapping ones' mind around this kind of complexity, and finding out a little more about what we do know, are more than enough reason to read this book. It could change the way you garden.
Tallamy, D.W. (2007). Bringing nature home: How native plants sustain wildlife in our gardens. Timber Press: China.