Bringing Nature Home

PREFACE

Occasionally we encounter a concept so obvious and intuitive that we have never thought to articulate it, so close to our noses that we could not see it, so entangled with our everyday experiences that we did not recognize it. In this book, I address several such concepts. Primarily, the wild creatures we enjoy and would like to have in our lives will not be here in the future if we take away their food and the places they live. I examine how we threaten their survival by trading our wild lands for uncontrolled expansion. And I emphasize the obvious consequence of that trade: in too many areas of our country there is no place left for wildlife but in the landscapes and gardens we ourselves create.

I also introduce ideas that are perhaps not so obvious. All plants are not created equal, particularly in their ability to support wildlife. Most of our native plant-eaters are not able to eat alien plants, and we are replacing native plants with alien species at an alarming rate, especially in the suburban gardens on which our wildlife increasingly depends. My central message is that unless we restore native plants to our suburban ecosystems, the future of biodiversity in the United States is dim.

Fortunately, two points of optimism temper this gloomy prediction. First and foremost, it is not yet too late to save most of the plants and animals that sustain the ecosystems on which we ourselves depend. Second, restoring native plants to most human-dominated landscapes is relatively easy to do.

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.

I would like to thank my wife, Cindy, for keeping me on course in this and all other endeavors, as well as for her exceptional editorial skills and willingness to turn our property into a research station. For ideas, factual accuracy, advice, encouragement, and technical help, I also thank Tina Alban, Mary Ann Brown, Ed Bruno, Rick Darke, Vince D’Amico, Dale Hendricks, Bethany Plyler, Dot Plyler, and Jim Plyler, Kimberley Shropshire, Melinda Zoehrer, and all those I’ve met at conferences whose interest in these ideas has been my constant motivation.

CHAPTER ONE

largely (or entirely) ignored. For several reasons, however, the day of the native ornamental is drawing near; the message is finally beginning to be heard. If I were to ask a random group of gardeners to comment on the importance of native plants in their gardens, they would probably recount several arguments that have been made in recent years in favor of natives over alien ornamentals. They might describe the “sense of place” that is created by using plants that “belong” or the dangers of releasing yet another species of invasive alien to outcompete and smother native vegetation. They might recognize the costly wastefulness of lawns populated with alien grasses that demand high-nitrogen fertilizers, broad-leaf herbicides, and pollution-belching mowers. Or they might mention the imperative of rescuing endangered native plants from extinction. These are all well-documented reasons for the increasing popularity of growing native plants.

Owners of native nurseries are also finding it easier and easier to enumerate the benefits of their offerings. Native plants are well adapted to their particular ecological niche and so are often far less difficult to grow than species from other altitudes, latitudes, and habitats. After all, these plants evolved here and were growing just fine long before we laid our heavy hands on the landscape.

Most compelling to me, however, is the use of native species to create simplified vestiges of the ecosystems that once made this land such a rich source of life for its indigenous peoples and, later, for European colonists and their descendants. That most of our ecosystems are no longer rich is beyond debate, and today, most of the surviving remnants of the native flora that formed them have been finished off by development or invaded by alien plant species. Too many Oak Parks, Hickory Hills, and Fox Hollows—developments named, as the environmentalist Bill McKibben has noted, for the bit of nature they have just extirpated—have been built across the country. Although relatively small, strategically placed and connected patches of completely restored habitats might foster the survival of some of our wildlife, I will describe later why such habitat islands can only protect a tiny fraction of the species that once thrived in North America. With 300 million human souls already present in the United States and no national recognition of the limits of our land’s ability to support additional millions, we simply have not left enough intact habitat for most of our species to avoid extinction. All species need space in order to dodge the extinction bullet. So far we have not shared space very well with our fellow earthlings. In the following pages, I hope to convince you that, for our own good and certainly for the good of other species, we must do better. Native plants will play a disproportionately large role in our success.

The transition from alien ornamentals to native species will require a profound change in our perception of the landscaping value of native ornamentals. Europeans first fell in love with the exotic beauty of plants that evolved on other continents when the great explorers returned home with beautiful species no one had ever seen before. It quickly became fashionable and a signal of wealth and high status to landscape with alien ornamentals that no one else had access to. As the first foreign ornamentals became more common in the landscape, the motivation to seek new alien species increased. Even today, the drive to obtain unique species or cultivars is a primary factor governing how we select plants for our landscapes.

My epiphany

Although I chose entomology as a profession, I understand the thrill of growing an exotic plant for the first time. When I was in graduate school at the University of Maryland, I took a course in woody landscape plants from the noted horticulturist Robert Baker. He introduced me to the world of ornamental horticulture and the many alien species with landscape value. I left that course with an intense desire to plant as many of the species I had just learned about as possible. The only thing that slowed me down a bit was that I had no place to plant them. Still, I gathered seeds from many of the ornamentals on the University of Maryland campus, germinated them in the greenhouse, and planted the seedlings all over the yards of my parents and relatives. Among other things, my parents got a Japanese hardy orange, and I bestowed the gift of Paulownia trees, of all things, on unsuspecting Uncle George. I now find it ironic that, at the same time Robert Baker was turning me on to alien ornamentals, I was taking courses about plant-insect interactions. These were the courses that explained why most insect herbivores can only eat plants with which they share an evolutionary history. All of the information I needed to realize that covering the land with alien plant species might not be such a good idea had been neatly and simultaneously placed in my lap during those months in graduate school, but it was 20 years before I made the connection: our native insects will not be able to survive on alien plant species.

In 2000, my wife and I moved to 10 acres in southeastern Pennsylvania. The area had been farmed for centuries before being subdivided and sold to people like us who wanted a quiet rural setting close to work. We got our rural setting—sort of—but it was anything but the slice of nature we were seeking. Like many “open spaces” in this country, at least 35 percent of the vegetation on our property (yes, I measured it) consisted of aggressive plant species from other continents that were rapidly replacing what native plants we did have. We quickly agreed to make it a family goal to rid the property of alien plants and to replace them with the forest species that had evolved within the eastern deciduous biome over many millions of years. This rather optimistic and, I admit, peculiar use of our spare time has put us in intimate contact with the plants on our property, both alien and native, and with the wildlife that depends upon those plants.

had obviously supplied many insects with food. This was alarming because it suggested a consequence of the alien invasion occurring all over North America that neither I—nor anyone else, I discovered, after checking the scientific literature—had considered. If our native insect fauna cannot, or will not, use alien plants for food, then insect populations in areas with many alien plants will be smaller than insect populations in areas with all natives. This may sound like a gardener’s dream: a land without insects! But because so many animals depend partially or entirely on insect protein for food, a land without insects is a land without most forms of higher life (Wilson 1987). Even the most incorrigible antienvironmentalist would be hard pressed to make an attractive case for such sterility. Pure anthropocentrists should be alarmed as well, since the terrestrial ecosystems on which we humans all depend for our own continued existence would cease to function without our six-legged friends.

But does the pattern of leaf damage I noticed in my backyard hold true? Does it occur elsewhere? If alien plants do reduce insect populations, by how much do they do so? Do aliens exclude all insect herbivores or just some?

bird and butterfly lovers among us, to fight invasive aliens as if it really matters and to reevaluate our centuries-old love affair with alien ornamentals. Beyond providing a challenge to ecologically minded gardeners, I will also explain how gardening with natives can create plantings that will stay beautiful and in balance without the use of pesticides. Gardening with natives is no longer just a peripheral option favored by vegetarians and erstwhile hippies. It is an important part of a paradigm shift in our shaky relationship with the planet that sustains us—one that mainstream gardeners can no longer afford to ignore.

CHAPTER TWO

determining taste), greater numbers of plant species mean more opportunities for animals to obtain their energy without interfering with one another. That is, plant diversity creates niches to which animals adapt over evolutionary time. This is why we hear so much about the incredible animal diversity of the tropics. There are so many different types of animals in tropical ecosystems because plant diversity is so high there. For example, a single hectare (2.47 acres) of Amazonian rainforest in Ecuador can support as many as 473 species of trees (Valencia, Balslev & Paz y Mino 1994), whereas there are only 134 species of trees in all of Pennsylvania (Rhoads & Block 2005). So if we want to create ecosystems with a diversity of animal species, we first have to encourage a healthy diversity of plants.

Why insects are essential

The second trophic level comprises all the animals that eat plants: the herbivores, or phytophages. In our neck of the woods, the most familiar and apparent herbivores are white-tailed deer, rabbits, and groundhogs. My wife and I were reminded of the strict herbivory of beavers when one showed up in our neighbors’ pond and made meals of their birch and willow trees. Other common vertebrates, such as chipmunks, squirrels, mice, raccoons, box turtles, and of course humans, include plants in their diets but are not restricted to them. Many of these omnivorous creatures are relatively large, and most fall into the category of what have been termed “charismatic mega-fauna.” It may be a surprise that when it comes to transferring energy from the first trophic level (plants) to the predators, parasites, and omnivores in other trophic levels, these charismatic vertebrates are relatively unimportant. What, then, do most animals in higher trophic levels rely on to pass on the energy held within the plant? Insects!

I cannot overemphasize how important insect herbivores are to the health of all terrestrial ecosystems. Worldwide, 37 percent of animal species are herbivorous insects (Wiess & Berenbaum 1988). These species are collectively very good at converting plant tissue of all types to insect tissue, and as a consequence they also excel at providing food—in the form of themselves—for other species. In fact, a large percentage of the world’s fauna depends entirely on insects to access the energy stored in plants (Wilson 1987). Birds are a particularly good example of such organisms. If you count all of the terrestrial bird species in North America that rely on insects and other arthropods (typically, the spiders that eat insects) to feed their young, you would find that figure to be about 96 percent (Dickinson 1999)—in other words, nearly all of them.

***

It is increasingly clear, as we shall see, that much of our wildlife will not be able to survive unless food, shelter, and nest sites can be found in suburban habitats. Let’s focus on the first of these essential resources: food. Because food for all animals starts with the energy harnessed by plants, the plants we grow in our gardens have the critical role of sustaining, directly or indirectly, all of the animals with which we share our living spaces. The degree to which the plants in our gardens succeed in this regard will determine the diversity and numbers of wildlife that can survive in managed landscapes. And because it is we who decide what plants will grow in our gardens, the responsibility for our nation’s biodiversity lies largely with us. Which animals will make it and which will not? We help make this decision every time we plant or remove something from our yards.

Unfortunately, because we have been so slow to recognize the unprecedented importance of suburban gardens for the preservation of wildlife, gardeners across the nation have been caught off guard. We have proceeded with garden design as we always have, with no knowledge of the new role our gardens play—and, alas, it shows. All too often the first step in the suburbanization of an area is to bulldoze the plant assemblages native to our neighborhoods and then to replace them with large manicured lawns bordered by a relatively few species of popular ornamentals from other continents. Throughout suburbia, we have decimated the native plant diversity that historically supported our favorite birds and mammals.

CHAPTER THREE

lawn. (Those neighbors would eventually pay me two dollars a week to mow that new lawn.)

that depend on those habitats off to make do someplace else. This is partly because no one is going to choose a pollywog over a human if presented with such a choice, and partly because, until recently, there always has been someplace else for nature to thrive.

But no longer. We can no longer safely relegate nature to our parks and preserves, assured that it will be there for us when we need it. We can no longer replace the native vegetation in our neighborhoods with foreign plants and remain confident that our native species will survive somewhere else. We can no longer rely on local natural areas to supply food and shelter to the birds, mammals, reptiles, and amphibians of North America.

Why we can no longer take nature’s persistence for granted becomes evident when we consider how much of the landscape we have taken for our own use. We humans have co-opted such a large percentage of natural areas that, in far too much of the country, there are no undisturbed habitats left (Rosenzweig 2003). What’s more, plants from other continents have now invaded the tiny remnants of the great ecosystems that once sustained our biodiversity (Mooney & Hobbs 2000). As we take more and more space for our own use, we are consigning the animals that used to live there to “natural” areas in which many of the plants that constitute the vital first trophic level are no longer “natural”: they are alien species that have either been planted deliberately or have escaped from our gardens, ports, or rail yards. There simply are not enough native plants left in the “wild”—that is, not enough undisturbed habitat remaining in the United States—to support the diversity of wildlife most of us would like to see survive into the distant future.

The drivers of diversity

To understand why we need to restore the ecological integrity of suburbia in order to prevent the extinction of most of our plants and animals, we must first understand what creates and maintains diversity. If we look at species diversity around the world, the first thing we notice is that the number of species in a given area depends on the size of the area (Rosenzweig 2003). Large continents have more species than small continents, and continents have many more species than islands. There are two reasons for this relationship between area and species number. First, the rate at which new species are created is higher in large areas than in small areas. This is because the primary cause of speciation, the geographic isolation of some individuals from other individuals of the same species, is far more likely to occur in large places than in small places (Mayr 1942). Second, the rate at which species go extinct is slower on large land masses than on small ones (Dobson 1996). The huge size of continents, for example, allows species to occupy large ranges, reducing the likelihood that a hurricane, cold snap, drought, or volcano will kill all members of a species. We learned long ago from Robert MacArthur and E. O. Wilson (1967) that the number of species existing in any one place represents a balance—an equilibrium, as they put it—that is reached over time between the rate at which new species arrive through speciation and immigration and the rate at which existing species disappear through extinction and emigration. And so, because species form faster and disappear slower on continents, we find the greatest number of species on these huge land masses.

Habitat fragmentation equals extinction

Problems for biodiversity in North America started when humans began destroying the diverse forests and grasslands that once covered the continent in order to plant crops and create living spaces that resemble the savannah parkland in which our species feels most comfortable. We did not systematically start at one end of the continent and wipe out everything as we proceeded. Instead, we left islands of suitable habitat in which most of the plants and animals that survive today found refuge. At first, these habitat patches were relatively large, but today they are miniscule, far too small to sustain populations of most living things for very long. For example, as of 2002, the once contiguous forest cover of Delaware had been reduced to 23 percent of its original size; 46 percent of the small woodlots that remain are less than 10 acres in size. It is curious that the news media have drawn our attention to the loss of tropical forests yet have been silent when it comes to how we have devastated our own forests here in the temperate zone. Only 15 percent of the Amazonian basin has been logged, whereas well over 70 percent of the forests along our eastern seaboard are gone (Brown 2006). We have reduced the enormous land mass that, over millions of years, created the rich biodiversity we can still see today in this country to tiny habitat islands. And therein lies the problem. Tiny habitat islands have high rates of species extinction and emigration and low rates of speciation and immigration.

Immigration? Just where are animals supposed to immigrate from? There are no large, pristine, and productive areas left to provide a pool of individuals for immigration to our habitat islands. We have only what ecologists call “sink” habitats: isolated locales that constantly lose individuals to death and emigration. As the “extinction debt” of the habitat islands we have created comes due, we will end up with far fewer plant and animal species. Fortunately, extinction takes a while, but because we have already fragmented the continent into habitat islands, we have set the clock ticking for our biodiversity and time is running out.

An excellent example of how habitat fragmentation leads to local extinction over time is currently being documented on Barro Colorado Island, an island in the middle of the Panama Canal (Robinson 1999). In 1914, this patch of tropical forest was the top of a small mountain rather than an island and was surrounded by hundreds of square miles of undisturbed habitat. When the canal was completed, the Chagres River was dammed, and the rising waters of what is now called Gatun Lake turned the mountaintop into an island isolated from the adjacent forest. Barro Colorado Island may not seem that isolated to us; the distance to the nearest shore in most places is less than a mile. But to most of its animals and plants, it may as well be in the middle of the Atlantic Ocean. From the start, its isolation from the surrounding forest began to take a toll on the number and kinds of animals that would survive on the island. Because most forest animals are reluctant to leave their habitat, the island’s woodland creatures were cut off from any interactions with the mainland. They would not swim or fly to find food or mates or to disperse to new territories. Even though the island spreads over 3700 acres, a sizable chunk of real estate compared to most of the habitat islands we have created in North America, it is too small to sustain the populations of many of its original inhabitants. Sixty-five species of birds have disappeared from the island since 1914, and many others are on the brink of extinction. In all the years since Barro Colorado Island was cut off from the mainland, the extinction debt has only been partially paid. No one knows how long it will take before the island community reaches its final equilibrium number of species balanced between extinction and immigration. What is clear, however, is that the number of species that can be sustained on the island is a fraction of what existed on that same acreage at the beginning of the 20th century.

Area effects are no less devastating on habitat islands—forests surrounded by developments rather than by water—than they are on real islands. The creatures that depend on the resources found only in forests cannot make a go of it among manicured lawns crisscrossed by paved roads. Just as on islands surrounded by water, the loss of species from habitat islands takes time, but it inevitably happens. For example, Ashdown Forest in Sussex, England, the inspirational setting of A. A. Milne’s Winnie-the-Pooh, has become entirely isolated by development. Since the 1920s, when Milne was writing his famous children’s stories, the forest has lost 47 plant species (Marren 2001).

What do Barro Colorado Island and Ashdown Forest tell us about the number of species we can expect to survive habitat fragmentation in the United States? You might think this would be impossible to predict accurately, but studies by Michael Rosenzweig and his students at the University of Arizona (1995, 2003) have shown that such predictions are as simple as can be. As we have just seen, species diversity is a function of the area of suitable habitat that is available for plants and animals, as well as the time it takes to reach species equilibrium in that habitat. 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. So the mystery is gone. We now know exactly how our actions are going to affect biodiversity if we continue on our present course.

The simple relationship between species survival and habitat area gives real urgency to the next question. How much land have we already taken? This is such a big country. Surely there is plenty of undisturbed habitat out there . . . somewhere. Our perception has always been that, no matter how many subdivisions we build, or how much land we put to the plow, or how many roads we construct, there will always be plenty of undisturbed space left. As long as we believe this, there is no reason to reevaluate our use of the land. This complacent view is not one held only by a few old-timers who remember the open, undisturbed spaces of their youth. I once asked one of my 20-year-old students how much land he thought humans had taken in the United States. Bless his optimistic heart: without hesitation he estimated that we have modified just 20 percent of the land for our own use. But we need not guess at this critical figure. Global Information Systems and satellite images now allow us to make very precise measurements of land use anywhere in the world.

We have taken it all

Let’s start by looking at where we live. According to the USDA Economic Research Service, over 60 percent of the U.S. population lived in rural areas in 1900. This number had decreased to 36 percent by 1950 and sank to a mere 17.4 percent in 2000. Today, almost 83 percent of our 300 million people live either in cities or in sprawling suburbs (U.S. Census Bureau 2005). By 1986, we had converted over 69 million acres to managed urban and suburban landscapes (Grey & Deneke 1986). That is an area 53 times the size of Delaware. In Pennsylvania, my home state, less than 1 percent of the land can still be considered “wild” (Coleman 2003). According to the USDA’s Natural Resource Conservation Service, 2 million acres—an area the size of Yellow-stone National Park—were lost to development nationally each year between 1982 and 1997, and the rate of development has been accelerating (McKinney 2002). Incredibly, suburbia in some areas of the country has increased 5909 percent since 1960 (Hayden 2004).

Unfortunately, that’s not the end of the grim news. We have paved at least 4 million linear miles of public roads in this country. (Hayden 2004). Add parking lots, driveways, and other paved surfaces to our streets and highways, and you have 43,480 square miles of blacktop smeared over the lower 48 states (Elvidge et al. 2004)—an area five and a half times the size of New Jersey. Regrettably, our rate of paving is increasing. Impervious surface increased more than 40 percent in the Chesapeake Bay watershed between 1990 and 2000 (They paved paradise, 2004).

Second only to paving in its impact on biodiversity is our love affair with sterile lawns. Christina Milesi has estimated that we have converted between 32 and 40 million acres—as much as 62,500 square miles—to suburban lawns in this country (Milesi et al. 2005). That is an area more than eight times the size of New Jersey dedicated to alien grasses! For further confirmation of our urbanization of North America, glance at the composite image derived from hundreds of photos taken by the U.S. Defense Meteorological Satellites Program (DMSP) in the mid-1990s. Each spot of light represents a town, city, or major metropolitan center, all of which are surrounded by miles and miles of suburbia. Huge areas east of the Mississippi have been completely “developed.”