By Douglas W. Tallamy
Bird lovers are united by their love of birds.
A profound observation, to be sure, but in most cases what bird lovers really love is seeing birds. The diversity of birds in most places is just large enough to trigger our innate human desire to categorize and organize information. That is, we love to list all the birds we can see, wherever we go. And these days many of us are no longer satisfied with just recording what we have seen in our notebooks. Now we need to photograph each species we see.
But birders everywhere are also united by the unhappy realization that birds, especially many species once common in the United States, are disappearing. The unscientific, anecdotal impressions that have needled us for years were confirmed in 2010 when a large collaborative study commissioned by George W. Bush issued its alarming report (http://www.stateofthebirds.org/). One-third of North American bird species are rapidly declining, threatened or endangered. My late father-in-law saw this coming over a decade ago. A passionate lister himself, he expressed the feelings of many of his peers when he said, " I want to see the birds before they disappear."
There is nothing inevitable about the demise of our birds. Their decline is not mysterious, and we are not scrambling for answers about what we can do about it. We know exactly why there are fewer birds each year. Our birds are in trouble because we have not shared our human-dominated spaces with them: the places in which we live, work, and farm. We havenít shared our spaces because we havenít thought we needed to. We thought that everything our birds needed to be happy and healthy was abundant somewhere else.
What we havenít thought much about was our ever-expanding human footprint. Suddenly, we are living, working, farming and mining just about everywhere. Satellite imagery has enabled scientists to measure exactly how much of the U.S. we have
taken to a greater or lesser extent for our own use, and it turns out to be an astounding 95% of the lower 48 states. Nearly everywhere, birds must eek out a living in landscapes that have been designed for our convenience and our current perception of aesthetics. Unfortunately, such landscapes are profoundly different from those in which our birds evolved.
Focus for a moment on your yard, or the landscapes in your neighborhood. Think about the landscapes you drive by on the way to work, and about the corporate landscape surrounding your work place. Unless you live in a very special place, the landscapes you have just pictured are dominated by large stretches of lawn. In many cases the lawn is devoid of any other plants, but in others it will be dotted here and there by ornamental plants: plants we value for their flowers, their fall color, their shape.
You have just pictured two of the most harmful features of todayís landscapes. First, they contain just a tiny fraction of the plants that once grew there. Whatís more, the plants we do put in our landscapes are largely from someplace else. Because we all want very much to "fit in with the neighbors," we have planted our yards exactly like our neighbors have planted their yards, and we have done that everywhere. Copycat landscapes prevail from coast to coast, as if we had come to a nationwide consensus on a short list of acceptable landscape plants. And because there has been little status in growing native plants, there are very few natives on that list.
In short, we have demoted plants from their role as the foundation of our ecosystems to mere ornaments for our entertainment.
What we present to our birds, then, both during the breeding season and migration, is a matrix of urban, suburban and exurban landscapes in which most native plant communities have been replaced by sparse plantings of Asian ornamentals.
The unmanaged patches of "natural" areas that remain are anything but natural. Instead, they are heavily invaded by exotics -- again, mostly from Asia. Over 3,400 species of non-native invasive plants are out-competing native plants in every ecosystem in North America. Where did these plants come from? Many herbaceous invasives were introduced through agricultural practices, but 85% of our woody invasives are escapees from our gardens.
Today we are asking our birds to survive and reproduce in landscapes with far fewer native plants than existed in the past. We are asking a lot. In fact, we are asking the impossible. Most of our terrestrial birds, 96% to be exact, rear their young on insects and spiders. And since insects are also what spiders eat, the reproductive success of our birds depends on how many insects are in their environment. Insects - -not seeds, not berries - - but insects. No wonder: Insect protein is the best there is, and insects are also loaded with fat bodies, high-energy organs that enable baby birds to grow at extraordinary rates.
But where do insects come from? They come from plants, and the more plants there are, the more insects there are -- not just insects that eat plants directly but also the thousands of species of insect predators and parasitoids that eat insect herbivores. Unfortunately, our landscaping paradigm has thrown a monkey wrench into the relationship between plants and insects, because most of our local insects cannot eat plants that evolved outside of their food webs.
All plants defend themselves from insects by packing their leaves with toxic phytochemicals. Typically, only insects that have specialized enzymes and behaviors for circumventing the chemical defense of a particular plant species are able to eat that plant. It takes most insects long periods of evolutionary exposure to a particular plant lineage to develop such adaptations. Ecological time scales simply are not long enough. So, when we replace the native plants to which our insects are adapted with Asian ornamentals such as Bradford pear, Zelkova, forsythia, crape myrtle, and burning bush, our local insects have nothing to eat, and both they and the food webs they support disappear.
The good news is that by increasing the number and diversity of native plants in our yards, we can quickly restore the complex food webs that provide all of the insect protein our birds need to reproduce. If we do that in enough places, suburbia will become a refuge rather than no manís land for birds.
Does this mean that we must sacrifice beautiful landscapes in order to feed our birds? Not at all! There are many misconceptions about using native species as landscape plants, but one of the most pervasive is the fear that natives will be defoliated by the very insects we are trying to attract with them. After all, thatís one of the reasons "pest free" plants from Asia and Europe appeared to be the logical choice. No gardener wants her favorite plantings to be riddled with insect damage. It may seem paradoxical, but planting natives that are part of local food webs is the best way to prevent insect outbreaks.
It is true that native plants attract more species of insect herbivores than non-native ornamentals -- 15 times more species, by some measures. What we must remember, however, is that all of those insects attract a diversity of predators, parasites and diseases that keep their populations in check. To have a diverse community of natural enemies present in your yard at all times, you must have a diversity of prey available at all times. When one prey species becomes too uncommon to support a predator, other species will be present for it to eat and will therefore prevent the predator from leaving the area.
The key to controlling insect outbreaks is to nip them in the bud. This can only happen if natural enemies are ready to pounce whenever an insect becomes too numerous. Fortunately, birds are among our best insect-eaters; a pair of bluebirds, for example, will bring up to 300 caterpillars back to the nest each day that they are feeding young!
We run into trouble when we landscape with plants that support very few herbivores, because then there usually is not enough food to keep insect predators and parasitoids, as well as hungry birds, nearby. When there is an outbreak of one of the many insects we have imported along with our Asian ornamentals -- insects like the Japanese beetle or the Euonymus scale -- there are not enough natural enemies to control them. This helps explain why as much as four times more pesticide by weight is applied to suburban landscapes than to the agricultural landscape in the U.S.
Landscaping with native plants is not just for bird-lovers; itís for human lovers too. If we create a world that does not support a diversity of birds, we will have created a world that will not be able to sustain humans for very long.
Although we donít act like it, humans need healthy ecosystems as much as everything else needs them. It is ecosystems that produce life-giving ecosystem services like oxygen production, water purification, weather moderation, carbon dioxide sequestration, and the breakdown of our trash, and it is biodiversity that runs our ecosystem.
Birds are superb indicator species of ecosystem health. Most are predators, and some are top predators that cannot exist unless a complex food web that creates their food also exists. And complex food webs can only be sustained in stable, productive ecosystems. If we have disrupted ecosystem function to the point where our birds disappear, we have also threatened our own life support systems. Something to think about, to be sure.
Douglas W. Tallamy, author of "Bringing Nature Home - How You Can Sustain Wildlife with Native Plants," prepared this article for Bird City Wisconsin, and also supplied the photos that accompany it.
Tallamy is currently Professor and Chair of the Department of Entomology and Wildlife Ecology at the University of Delaware in Newark, Del., where he has written more than 65 research articles and has taught insect taxonomy, behavioral ecology, and other subjects. Chief among his research goals is to better understand the many ways insects interact with plants and how such interactions determine the diversity of animal communities.
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