Bringing Nature Home: How You Can Sustain Wildlife with Native Plants

by Douglas W. Tallamy

My central message is that unless we restore native plants to our suburban ecosystems, the future of biodiversity in the United States is dim.

Although I do suggest approaches here and there, this is not a how-to book; there are many other fine references on how to select and grow natives in different parts of the country. Nor is this a book about landscaping per se. I am not posing as a landscape architect, and I am not skilled in landscape design. I am simply proposing a justification for the liberal use of native plants in the landscape that has not yet been clearly articulated. I hope the reasoning presented in this book is logical and convincing, and maybe even entertaining.

most insect herbivores can only eat plants with which they share an evolutionary history.

If you turn the clock forward to the point at which this equilibrium has been reached, you will find that the number of species that will survive human habitat destruction is a simple percentage of the amount of habitat we leave undisturbed, a 1:1 correspondence. For example, if we take 50 percent of the land in the United States for our own use, we will end up with 50 percent of the species that originally inhabited this country. If we usurp 80 percent of the land, we will lose 80 percent of the species.

Unless we modify the places we live, work, and play to meet not only our own needs but the needs of other species as well, nearly all species of wildlife native to the United States will disappear forever. This is not speculation. It is a prediction backed by decades of research on species-area relationships by ecologists who know of what they speak. And the extinction of our plants and animals is not a scenario lost in the distant future. It is playing out across the country and the planet as I write. Our preserves and national parks are not adequate to prevent the predicted loss of species, and we have run out of the space required to make them big enough. For conservationists, and indeed for anyone who celebrates life on earth, this is perhaps the direst possible consequence of the human enterprise.

In a diverse ecosystem, many species perform similar tasks. Penstemon flowers, for instance, might be visited by three species of bumblebees, five species of moths, and one hummingbird species. If one or two of those pollinators disappear, the plants will still be pollinated and make viable seeds. The rodents that eat those seeds will still have food, as will the screech owls that eat the rodents. Redundancy in pollinators will save the day. But if the ecosystem is depauperate in pollinating species, the loss of one pollinator might quickly lead to the local extinction of the penstemon population. This would not only hurt the rodents and the screech owls, but it would also lead to the extinction of all insects that eat only penstemon leaves. These losses would then reduce the food available for all insectivores that included those insect specialists in their own diets. If this food reduction were large enough, insectivorous birds that bred in the area would not be able to feed their young, and they too would be lost from the ecosystem.

Dozens of common plants would make us quite sick if we were to eat them (Kingsbury 1964). Black cherry leaves, pokeweed leaves, and deadly nightshade all fall into the “poisonous” category for people. The tannins in oak leaves would bind up all of the protein we ingested, eventually starving us to death. Even many of the crops we do eat originally had toxic chemicals in their leaves before we removed them through plant-breeding programs that have been in progress for thousands of years. When we first domesticated lima beans, we discovered the hard way that if we don’t boil the beans, we risk a lethal dose of cyanide. The cucurbitacin triterpenes in cucumber leaves would kill us in short order if we could choke them down. And we all know the consequences of consuming nicotine from tobacco or opiates from poppies. In most cases, though, we have no trouble avoiding leaves containing such compounds because they taste so bad. Our lettuce has to be young and fresh or it gets too bitter for our fussy taste buds. We have all eaten cucurbitacins in minute quantities in the occasional bitter cucumber, but we don’t make that mistake twice in a row.

insects suffer the same constraints. They can only tolerate a narrow group of chemicals to which they have been repeatedly exposed over thousands of generations. All other chemicals taste bad to insect herbivores and signal that the insect is on the wrong host. Specialists do not have the option of eating any plant in the neighborhood. Nor do they have the option of quickly adapting to the alien plants that have replaced their native hosts. Evolution simply does not work that fast. If an insect’s hosts are not present, it won’t be either.

So far I have described well-accepted hypotheses about how evolution has molded most insect herbivores to become “specialists”: insects that are adapted to find, eat, digest, and survive on plant lineages that produce particular types of phytochemicals. I have said little, though, about the species that are not specialists on particular host plants, about 10 percent of the insect herbivores in a given ecosystem. In contrast to specialists, “generalist” insects have evolved the ability to eat several types of plants. Toxicologists have learned that one of the adaptations that permit some insects to generalize on many host plants is the ability to produce very powerful gut enzymes called mixed-function oxidases (Brattsten, Wilkinson & Eisner 1977). These enzymes are so good at detoxifying different classes of plant-defensive chemicals that generalists can eat lots of unrelated plants with no measurable reduction in their ability to grow and reproduce.

When I first became interested in the impact of alien plants on our nation’s biodiversity, I was surprised to learn that the terms “native” and “alien” were controversial. Somewhat naively, I thought that when a plant found in China is sold in the United States as an ornamental, it can be classified as an “alien” without much debate. I was wrong. Recently a good friend insisted that if a plant has been in North America long enough, it can be considered a native, regardless of its evolutionary origins. Even more problematic is achieving consensus on the definition of a “native” plant. The broadest definition is also the most commonly employed: a native is any plant that historically grew in North America. Some people recognize that it’s a stretch to call a plant adapted to a California desert a native of New Jersey, but these same people happily consider all plants that grow east of the Mississippi as native to any area in the East. Maps of U.S. hardiness zones are often brought out to justify the “nativeness” of one species or another. By this reasoning, a plant adapted to zone 6 in Tennessee will serve as a fine native in the areas of Pennsylvania with a zone 6 climate.

So far I have focused on one detriment associated with the overuse of alien ornamentals in suburban gardens: the cost to wildlife as native food sources disappear. In my view, this is reason enough to increase the percentage of natives in our gardens. But there is another serious cost that arises when we garden with aliens that we can no longer ethically ignore. When gardeners support the market for plants from other countries, they encourage the introduction of alien stock to North America, with two serious consequences. First, despite our best efforts to bring only “clean” plants into this country, we continually introduce harmful diseases and insects that evolved on other continents along with our beloved alien ornamentals. The problems stemming from the introduction of an unwanted disease or insect pest are obvious. Less obvious (in fact, downright controversial) is the suggestion that gardening with alien ornamentals places our native ecosystems at risk of destruction by yet another invasive plant species.

It is hard to overemphasize the impact that the loss of American chestnuts has had on deciduous forest ecosystems. Castanea dentata was the primary nut producer of eastern forests, dwarfing the contributions of oaks, beeches, and hickories as wildlife food sources. Squirrels, chipmunks, deer, elk, black bears, turkeys, passenger pigeons, doves, blue jays, and mice were just some of the animals that depended on copious quantities of chestnuts to make it through long winters. Equally important but more poorly documented was the role American chestnuts played in producing insects that supported huge populations of songbirds. Castanea dentata is a member of the plant family Fagaceae, which supports hundreds of species of caterpillars and other insects eaten by birds. The niche vacated by chestnut trees was slowly filled with other tree species (Cech 1986), and our forests appear today as though nothing had happened—unless you compare the density of wildlife that forests support today with what they used to support. Such a study is now impossible, not only because the chestnut is gone—along with an unknowable number of insect specialists that ate only the chestnut—but also because our forests are now so fragmented. Nevertheless, because none of the species that replaced the chestnut is as productive as the chestnut (Diamond et al. 2000), I predict that such a comparison would show smaller populations of both birds and mammals in today’s eastern forests. For example, tulip tree ...

Exacerbating the problem is our love of huge lawns. Japanese beetle larvae develop primarily on grass roots. With up to 40 million acres of North America covered in lawns (Milesi et al. 2005), there is no shortage of food for the beetles’ young. Every year billions of adults emerge from our lawns to skeletonize our plants. One obvious way to fight this scourge is to limit the amount of food we provide for these little buggers: reduce the size of your lawn!

Lest you think that Japanese beetles are another exceptional case that would have been detected in time had they been imported on nursery stock today, think again. Three additional turf pests, all closely related to the Japanese beetle and all introduced accidentally on nursery stock after the Japanese beetle, have established themselves in the East and are beginning their westward expansion. Exomala orientalis, the oriental beetle (Cowles 2003), and Maladera castanea, the Asiatic garden beetle (Nielsen 1989), both look a lot like the Japanese beetle, although neither is as colorful. Rhizotrogus majalis, the European chafer, not to be confused with the native rose chafer (Nielson 1992), more closely resembles a small June beetle. As with the Japanese beetle, these three species entered the country unnoticed, despite our best intentions to screen them out.

That alien ornamentals are particularly good at outcompeting native vegetation should not come as a surprise. As we’ve seen, the species we import for the ornamental trade are not a random subset of plants that have evolved on other continents but instead are attractive plants selected specifically because they are particularly well defended against insects and pathogens (Tallamy 2004). How many times have you bought a plant that is advertised as being “pest free”? A plant that is “pest free” is inherently unpalatable to insects and often is not susceptible to local diseases. Because such plants do not pass the energy they capture from the sun up the food chain, they do not become functioning members of the ecosystem in which they are planted. What’s more, if one escapes from your garden, it has a competitive edge over native plants that are eaten by a host of native insects and are infected by disease organisms, precisely because it is not burdened by such attacks.

Many gardeners vigorously oppose the notion that their beloved garden plants can and do escape from cultivation. After all, the Amur honeysuckle that they tucked in the ground as a seedling and then nurtured for years is still there, right where they put it. What most gardeners do not see is the local mockingbird or migrating warbler swoop down, pluck a berry from the bush, and then fly off. The berry, of course, surrounds a seed that is genetically programmed to germinate after it has passed through the gut of a bird. In time, the mockingbird will perch somewhere nearby, perhaps in a neighbor’s garden, or more likely, on the edge of the nearby woodlot, and relieve itself of the load of alien seeds in its gut. If any of those seeds germinate the following spring, the escape from your garden is complete. The migrating warbler is even more efficient at dispersing seeds. It doesn’t wait around while it processes its meal of alien berries. Instead, the warbler resumes its journey south shortly after its meal of berries and may be several hundred miles away by the time the seed is defecated.

Aliens that do not produce animal-dispersed fruits or nuts rely on the wind to carry their seeds. A Norway maple produces thousands of seeds each spring, each of which bears a winglike structure that sends the seed into a helicopter spin as it descends to the ground. If there is even a slight breeze blowing when the seed detaches from the tree, it can travel a dozen yards or more before it hits the ground. Who knows how far wind-dispersed seeds can go when a strong wind blows? One spring my wife found three Japanese maple seedlings growing on our property. After searching the nearest housing development, we found what was probably the parent tree—more than half a mile away! Some aliens produce so many seeds that the sheer volume of seeds ensures that some will move considerable distances from the parent plant. Purple loosestrife is an excellent example. A single mature plant can release over 2.7 million seeds every year of its long life. Think of the seeds produced in a wetland choked by thousands of purple loosestrife plants! These billions of seeds remain viable in the soil for at least two years and can form a seed bank of immense size. Similar seed characteristics have enabled aliens like garlic mustard to invade nearly every forested acre in the eastern deciduous biome.

Because it is one of the few nonleguminous plants that fix nitrogen, autumn olive does well in poor soils and quickly colonizes fields and disturbed sites. Though it reaches a height of only 20 feet, autumn olive is a bushy multi-stemmed tree that shades out all native competitors. Monocultures of autumn and Russian olive are a common sight in the United States. Like oriental bittersweet, the foliage of autumn olive is inedible for almost all native insect herbivores. A field rich in goldenrod, Joe-Pye weed, boneset, milkweed, black-eyed Susan, and dozens of other productive perennials supplies copious amounts of insect biomass for birds to rear their young. After it has been invaded by autumn or Russian olive, that same field is nearly sterile.

Their study shows that as much as 10 percent of the foliage in a garden can be damaged by insects before the average gardener even notices. This is exciting news. It tells us that most gardeners do not have a zero tolerance of insects in the garden and that maybe, just maybe, the populations of insects that create 10 percent damage levels might be large enough to support communities of natural enemies so diverse and numerous that the foliage damage levels never exceed 10 percent. I believe this utopia will be easier to achieve when most plants in our gardens are native.

Of the 4 million or so insect species on earth (to put things in perspective, there are only about 9500 species of birds), a mere 1 percent interact with humans in negative ways. The other 99 percent of the insect species pollinate plants, return the nutrients tied up in dead plants and animals to the soil, keep populations of insect herbivores in check, aerate and enrich the soil, and as I keep stressing, provide food either directly or indirectly for most other animals. John Losey and Mace Vaughan (2006) have valued the ecosystem services provided by insects at $57 billion each year. As E. O. Wilson (1987) has pointed out, insects have done fine on this earth without humans and would continue to do so in our absence. If insects were to disappear, however, our own extinction would not be far behind. It may be hard to admit, but we need healthy insect populations to ensure our own survival.

You can enjoy the fascinating world of insects if you give them the food they need to reach maturity and reproduce. The most common way gardeners attempt to connect with insects is by planting for butterflies. It is a noble idea, and exactly the sort of thing I hope to promote with this book. Sadly, the execution of this enterprise is so often directed by misinformation that we end up with fewer butterflies than we started with. When designing a butterfly garden, you need two types of plants: species that provide nectar for adults, and species that are host plants for butterfly larvae. Most people focus only on the plants that produce nectar. Even worse, they often turn to alien plants that are promoted as being good for butterflies, the most popular of which, hands down, is the butterfly bush (Buddleja species). Planting butterfly bush in your garden will provide attractive nectar for adult butterflies, but not one species of butterfly in North America can use buddleias as larval host plants. To have butterflies, we need to make butterflies. Butterflies used to reproduce on the native plants that grew in our yards before the plants were bulldozed and replaced with lawn. To have butterflies in our future, we need to replace those lost host plants, no if’s, and’s, or but’s. If we do not, butterfly populations will continue to decline with every new house that is built. Instead of building a butterfly garden with aliens that will make no new butterflies, use a native species...

Coneflowers and black-eyed susans (Rudbeckia species) also wear two hats in the butterfly garden. Along with their attractive floral display and nectar, rudbeckias support the reproduction of dozens of species of Lepidoptera, including the pearl crescent (Phyciodes tharos), silvery checkerspot (Chlosyne nycteis), and wavy-lined emerald (Synchlora aerata). Buttonbush (Cephalanthus occidentalis) is another of my favorites for the butterfly garden. Its ball-shaped flowers capture the eye, it does well in wet areas, butterflies fight to gain access to its nectar, and it serves as a host plant for 18 species of Lepidoptera in my neck of the woods. These include the ethereal promethea moth (Callosamia promethea), the hydrangea sphinx (Darapsa versicolor), and the saddleback caterpillar (Acharia stimulea). What better way to introduce your kids to the wonders of nature than to have them watch a female promethea moth emerge from her cocoon, inflate her wings, and without moving from her cocoon, release a powerful pheromone into the air, attracting a male of her species within minutes for coupling?

We lose much when we remove leaf litter because it provides so many free services for us: free mulch, free fertilizer, free weed control, and free soil amendments. Litter also provides habitat for many of the arthropod predators that help keep garden communities ecologically balanced. Above all, a deep bed of leaf litter acts like a sponge, soaking up enormous quantities of water during downpours. Without litter, rainwater typically flows off our properties and into the gutters, flooding streams, rivers, and occasionally our homes. When the rain stops, leaf litter that has been allowed to accumulate slowly releases its moisture, keeping the plants and trees in your garden well hydrated, even during dry periods. Bare ground (or lawn) does none of this.