Few forces so thoroughly obliterate varied natural habitat as urbanization. Paved streets account for almost one-third of the area of New York, rooftops even more. Yet wherever a bit of land is left alone for a while and wildlife is relatively unmolested, an abundance of adapting species may occur.
Because the urban environment is so inhospitable, even to humans, we are surprised by what can be sustained in the developed environment wherever the requirements of natural communities are met, whether by design or accident. In the midst of the busy industrial shipping corridor of Newark Bay, for example, over 29 percent of the entire colonial waterbird population along the Long Island-New York City Atlantic shoreline breeds on three artificial islands used as disposal sites for dredge spoil in the Arthur Kill, the river that separates New York from New Jersey. The waterway was the site of no fewer than ten oil spills between January and September 1990 and yet is home to snowy, great, and cattle egrets; black- and yellow-crowned night herons; little blue herons, and glossy ibises (The Trust for Public Land and New York City Audubon Society 1990). Recent monitoring, to evaluate the impact of an Exxon pipeline leak of millions of gallons of home heating oil, revealed unexpected populations of organisms surviving under conditions that would normally be lethal to individuals of that species accustomed to less-polluted environments. But most of these creatures are living at the very edge, in habitats that are largely unprotected. Without more effective conservation, they are extremely vulnerable. Even a minor added degradation could eliminate entire communities.
Effects of disturbance on wildlife mirror those previously described for vegetation: loss of habitat, changes in the structure and composition of the remaining habitat, changing population proportions, and the introduction of diseases and competing exotic species. Birds and other wildlife are excellent indicators of environmental degradation no matter how you look at the numbers. One-sixth of the bird species extant several hundred years ago globally are already lost, as a direct result of human activity (Wilson 1992). More than 1,000 of the world’s fewer-than-10,000 species of birds are threatened and 70 percent are in decline, according to Bird Life International of Cambridge, England. More than one-third of all frog species in the United States are declining in numbers. Nearly one-third of the world’s known mammal species are threatened with extinction, according to the World Conservation Union Red List. In only a few years, one scientist alone has collected over 20,000 bird carcasses that were killed when they struck the windows of a single highrise building in Texas. Imagine those losses multiplied by all the highrises found in migratory corridors. The Government Accounting Office has reported that activities harmful to wildlife, such as logging and military bombing, have occurred on 60 percent of the 91 million acres of the U.S. National Wildlife Refuge system (Chadwick 1996).
Fragmentation and local extinctions have dramatically changed the species composition of communities for many wildlife species, to the point that populations are critically altered. Lack of predators can mean overpopulation by a plant-eating species, and not just by deer, the most obvious example. In the New York Botanical Garden, for instance, squirrel density was measured at up to 25 per acre as compared to a forest level of 3 to 5. In Central Park squirrels seem to consume nearly every acorn and beechnut that is produced and any that are planted without extra protection. New seedlings are also vulnerable. The woodland planting crew in Central Park has observed squirrels digging up newly planted, two-year-old oak seedlings to get at the remaining bit of the original acorn still attached to the roots of the young tree.
Some species of wildlife are more vulnerable to predation in forest remnants. Fragmented forests, with their continuous and increased edges, provide greater access to prey than the unconnected gaps of a patchy forest. Introduced predators, in the form of cats and dogs, can number several per acre. Another companion of humans, the Norway rat, is also a predator in the landscape. People also prey on animals, even where hunting per se is not a factor. Reptiles and amphibians are especially hard hit. There are no snakes, toads, tortoises, or turtles at all in Central Park, except a few totally aquatic species that shelter safely under water. Birdwatchers in Central Park who had been following the progress of a nest of downy woodpeckers were dismayed to find that children had pulled over the tree stump so they could try to hand-rear the nestlings. Poaching is common, too, especially in larger protected landscapes. The National Park Service documents over 100 species of animals that are poached and almost as many plant species that are illegally harvested.
Recognizing that the whole system represents the greatest value, not just some of the components, is the most important concept in land management. Planting and managing appropriately to benefit native wildlife by necessity means restoring and managing for the full spectrum of native plant communities, not just wetland or rare species. Many efforts to manage for single species of wildlife, especially game populations, have had serious negative impacts on native communities of both plants and animals. People still widely recommend and disseminate many invasive, alien plant species on the grounds that they benefit wildlife when in fact they may actually benefit a few species at the expense of many others. Well-meaning managers planted autumn olive, a highly invasive small tree, throughout wildlife management areas in New Jersey and other mid-Atlantic states to increase the numbers of small gamebirds for hunters, without adequate regard for its effect on other species in the system. More than a third of native streamside birds avoid areas of olive because it supports few insects and has very hard wood with few cavities. The olive can fix nitrogen directly from the atmosphere and compete effectively with native species, establishing in shade under native cottonwoods and willows and growing up to replace them completely. Efforts at exotics control may have similar consequences. To cite just one example, an exotic parasitic wasp deliberately introduced to control the exotic gypsy moth is now preying upon three native moth species that are federally listed as endangered.
Some animal species, like invasive plants, have naturalized at the expense of indigenous wildlife. Some introductions have been accidental, such as the escape of a pair of gypsy moths from a professor in Montpelier, Vermont; others have been deliberate, such as the release of starlings into Central Park as part of a larger effort to introduce all the animals and plants that were named in the works of William Shakespeare into the United States. One of the most familiar introductions is the ring-necked pheasant, a bird from China that most of us remember from childhood and think of as native, having never seen the turkey that preceded it. Many states still pen-raise and release pheasants for hunters, such as in New Hampshire’s “put and release” program.
Invasions are not just by land. For instance, efforts to stock fish, such as brown trout, for sportfishing have been enormously successful with the license-purchasing public but have had detrimental impacts on native fish, such as the brook trout, which they have displaced. Warmer streamwaters and reduced corridor vegetation already have reduced suitable brook trout habitat; continued stocking where they persist increases pressure on these and other native species.
Wildlife management has sometimes affected native wildlife in ways that make their behavior more like that of an introduced species. For example, when the practice of using live decoys was banned, game farms released thousands of Canada geese that had been raised for this purpose and had lost the habit of migration after generations of captivity. The birds readily adapted to farm ponds and pastures and then spread rapidly. Today they are equally at home on turf and in retention ponds and have proliferated with increased development of corporate office parks and golf courses to such an extent that they are often considered pests. Similar changes may have already occurred to the farm-raised native turkeys that are now being released throughout the East. New Jersey, for instance, now boasts over 10,000 wild turkeys. You can easily catch a glimpse of this once rarely seen forest bird seeming very much at home in the hedgerows and cornfields of the rural landscape.
Remnant natural areas provide habitat conditions for wildlife that are very different from those of the former forest. One aspect is food type and value, which change when vegetation is altered in composition. The nonnative shrub honeysuckles and multiflora rose, spread so widely by the birds and other animals that feed on their sweet fruits, are of less benefit to other species, especially those dependent on the high-lipid, that is, high-fat, fruits of native forest species such as dogwood or viburnum that the invaders have replaced. Migratory songbirds, for example, require lipids to sustain them on their journeys. The native spicebush berry is 35 percent lipids; the multiflora rosehip is 40 percent sugar and only 10 percent lipids. As nonnative species expand throughout a region replacing high-lipid native species, important food sources for migratory birds decrease — yet another reason that restoring indigenous plant species is so important.
Ultimately, the preservation of wildlife diversity will depend on our management of the largest preserves where the whole of the food chain can be represented, from wolves, bears, and bobcats to the smallest soil creatures. The fragmented landscape, no matter how well managed, cannot functionally replace larger sites, which hold out the most opportunities for preservation and survival to the largest number of species.
Nevertheless, fragments are still very important not only as greenways but also as refuges in densely populated areas. For migratory birds, many of whose populations have dropped precipitously in the last century, the distance between sizable fragments is crucial. The loss of stopover sites along their migratory routes may be as serious as loss of breeding or overwintering habitat. A good example is Central Park, which provides at least temporary food and shelter to more than 260 species of birds. This relatively degraded landscape, considered a birdwatching hot spot, is one of the few remaining links on an increasingly tenuous chain of seminatural areas connecting the megalopolitan corridors.
We are not just losing animal species with loss of habitat; we are losing plant species as we lose animal species. Plants and animals have coevolved over millennia, and plant reproduction is inextricably tied to wildlife — for pollination, seed transport, seed scarification, planting, and regulating competition. Where wildlife is impoverished, many plants have no means of effective reproduction and survival. We must include the interdependencies of plants and wildlife in our planning of habitat preserves. We cannot any longer separate one from the other in our thinking and our management practices. This will be no easy task. Managing to improve conditions for native wildlife in landscape fragments will place some of the greatest demands on our skills and ingenuity.