The Urban Naturalistby Steven D. Garber
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This informative, useful field guide reveals the amazing biodiversity within city and suburban landscapes, including trees, insects and other invertebrates, fish, amphibians, reptiles, birds, and mammals. The author explains why these organisms live in cities and how they survive, offers tips on which species to look for, and shares hundreds of fascinating facts.
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The Urban Naturalist
By Steven D. Garber, Jerome Lo
Dover Publications, Inc.Copyright © 1987 Steven D. Garber
All rights reserved.
Just as oceans and forests and prairies are natural systems, cities and suburbs are also natural. While oceans have had hundreds of millions of years to evolve a network of interacting organisms, humans are relatively new to the planet. Cities are even more recent, the oldest having developed only about 5,000 years ago. We city inhabitants are animals who affect our surroundings in a natural manner, like any other creature. Other species have transformed the parts of the world in which they lived, but we have made by far the greatest impact.
When plants first moved from water and colonized the land, when the first vertebrates moved out of water and inhabited terrestrial environments, and when vertebrates evolved the capacity to lay shelled eggs on land, each innovation caused a major, irreversible revolution that transformed the world. The most recent transformation has resulted from the rapid growth of cities and suburbs, with entire regions greatly affected by humans.
Urban ecosystems are still in an early stage of development. For years our cities have been, and still are, built with only human needs in mind. Yet everywhere we go, we bring a similar set of variables, create a similar environment, plant the same trees, attract the same insects and rodents, and have many of the same birds. We take much of this for granted, without realizing that many of the species that live with us have not always been here. Starlings and house sparrows, for example, are abundant in every city in America, but they have been on this continent for only about 100 years.
New species continually move in, gain a foothold, and add to the diversity and complexity of the urban landscape. It is too early to predict with certainty which species will eventually adapt to, and thrive in, these new settings, but educated projections can be made. Opportunities exist for certain species in urban environments that do not exist in rural environments. Cities offer food sources, places to live, wide open spaces, species to parasitize, species to prey upon, trees to nest in, buildings to roost on, warm evening skies to catch insects in, warm polluted waters to seek refuge in, rich effluents that support aquatic ecosystems, underground sewers to breed in, and cavernous subway tunnels and hundreds of thousands of miles of pipes to move about in.
Cities have their own variables: buildings, pavement, scattered green areas, street trees, gardens, and house plants. These affect the ecosystem, as do the thousands of people who walk in the parks and compact the soil, making it difficult for rains to be absorbed, creating problems with runoff and erosion. Nutrients that flow into low areas and collect in city ponds feed the plankton and the algae so the water clarity decreases and blocks out sunlight. The plants die, fall to the bottom, and rot. The bacteria, while consuming the dead plants, also consume the oxygen, creating a deficit that is difficult for gill-breathing animals to tolerate. As a result, city ponds are generally inhabited only by species adapted to living in such an extreme environment. These aquatic ecosystems may exhibit less species diversity, that is, contain fewer numbers of species, than similar areas found outside the cities, but that's because it takes years for those species that can live in these new, isolated, metropolitan islands to arrive and become established. More often than not, they are brought here, sometimes unintentionally.
When natural areas are cleared to establish an urban landscape, most of the woodland, grassland, or desert species that once lived there will be unable to adapt to the changed environment. They must leave or die. Only a few remain and survive. One survivor may be the cicada. Years after a city has been built up in what was once a woodland, we still find cicadas each summer. These cicadas are probably descendants of populations that lived there before the city was built. Their numbers may have diminished, but in some urban parks you'd never know it. In other parks, however, where the trees have been cut and the ground has been paved, the cicadas haven't had a chance.
Many populations of species continue to survive in cities after their natural environment has been radically altered. Some may even benefit from the changes because of reduced competition from other species. Painted turtles and snapping turtles survive in many freshwater environments, sometimes even in brackish water, after other turtles are long gone. Pumpkinseeds and bluegills—two species of freshwater fish—actually experience population explosions after their habitat becomes urbanized. Some pioneer tree species, those that move into open areas, thrive in vacant lots after an area has been cleared. They would have been significantly less numerous if the city had not been built because they do poorly among stands of mature trees. And many species of roadside plants, which never had roadsides to grow along before the people moved in, experience windfall increases in new places to live. Many of these plants thrive on alkaline soils, which are relatively scarce in the wild, while some are among the more successful species in cities because of the lime that leaches from the concrete, affecting the surrounding soil.
Other opportunities exist for species that can move in from surrounding areas and exploit the changed environment. For instance, chimney swifts and barn swallows, which nest in hollow trees, will sometimes nest in man-made structures instead. Because there are many more buildings than there ever were hollow trees, these species may actually increase in numbers as an area becomes more urban.
The changed microclimate of cities also affects plants and animals. Most cities are warmer than the surrounding regions because tar, concrete, and stone absorb heat and then slowly radiate that heat when the temperature drops. This allows some species to flower earlier, lay their eggs earlier, or store more food during the summer, changes so significant that some will do markedly better in cities than in the country. Other species will extend their ranges, moving from one city to another.
During recent years several southern birds and mammals, such as armadillos, opossums, mockingbirds, and cardinals, have been extending their ranges north in this way. The reasons aren't always clear, but the range extensions are well documented. Other species moved from the Great Plains into regions where forests have been logged. These include loggerhead shrikes, brown-headed cowbirds, cliff swallows, and dickcissels.
Some species accompany people wherever they go, arriving in a region once it has been greatly affected by humans. People bring along their pets such as dogs and cats, while many other species are brought along inadvertently, such as the book lice that live in old books and the silverfish and roaches that live in human dwellings. And of course rats and mice live in just about every city. Other species that commonly accompany people are the beetles that live in our stored grain, and the plants that have escaped from our gardens. Some insects from other countries come in on fruits and vegetables. Worms, centipedes, and millipedes may arrive in imported flowerpots. New species are constantly arriving; occasionally an alien becomes established in its new environment.
Some of these new species are seldom noticed; others become a tremendous problem. If a species has few competitors and predators, its numbers may increase dangerously. Or a plant disease may run rampant in a new area. For example, a disease that attacks chestnut trees did little damage in Europe where the trees had built up a resistance. When the disease traveled to the United States where the trees had no resistance, most of our chestnuts were dead in a matter of decades. A disease that attacks elms also managed to hop a ride to North America. It infected American elms, a favorite urban tree, and we lost hundreds of thousands of beautiful, mature trees.
If a crate of parakeets breaks when being unloaded at an airport, all the birds may fly off, soon to become established in the surrounding region. Or rabbits from the West may escape and settle in the East. Fish may be released and survive in local waters. These things tend to happen with greater frequency in places where many people live; as a result, new species are introduced into urban areas all the time. Over the years, many of the same species repeatedly colonize other cities; thus urban ecosystems around the world slowly increase in complexity while their floras and faunas survive and even flourish.
In recent years, some cities and countries have begun to try to reverse the degradation of the urban environment. People are trying to clean the air and the water, create parks and improve those already in place, plant trees, and maintain green patches. It is possible to utilize natural processes and ecological principles to revitalize degraded habitats. Intervention can take the form of attracting, introducing, or reintroducing specific species that can add to the quality of an urban environment. More effort needs to be expended to make our cities more livable, for us as well as for other organisms. One way or another, species are going to move in and live by our sides. We can make it easier for them to survive and flourish, and in doing so, we will create a better environment for ourselves.CHAPTER 2
GRASSES AND WILDFLOWERS
Plants include fungi, algae, grasses, sedges, rushes, forbs, vines, shrubs, and trees. Since a full discussion of all plant forms that appear in cities would fill several volumes, I will deal only with those species of grasses, wildflowers, and trees that are very well represented in U.S. cities. In this chapter I'll discuss the most frequently seen urban grasses first, then talk about some important wildflowers in more detail. Trees are the subject of Chapter 3.
Of the 250,000 species of plants, over 10,000 are grasses. They include many of the economically most important plants. Species such as barley (Hordeum vulgare), bamboo (Bambusa spp., Dendrocalamus spp., and allied genera), corn (Zea mays), millet (Setaria italica), oats (Avena sativa), rice (Oryza sativa), rye (Secale cereale), sorghum (Sorghum vulgare), sugarcane (Saccharum officinarum), and wheat (Triticum spp.) are some of the most useful plants in the world. Once some 42 percent of the land area of the earth used to be grasses, that has now declined to about 25 percent. Grasses grow well in rich soils, but many species will thrive in less favorable locations. Some, in fact, flourish along roadsides or grow in the cracks in sidewalks and in other urban places where nothing else will grow.
Most of the prairie in much of the Midwest was once covered with grasses, legumes (plants in the pea family, Leguminosae), and forbs (flowering plants that aren't grasses, aren't grasslike, and aren't woody). In the East, the colonists cleared the forests and planted English grasses to feed their cattle, species such as timothy (Phleum pratense), a coarse grass with cylindrical spikes; Kentucky bluegrass (Poa pratensis); and fescue (Festuca spp.). These grasses still grow in cultivated hay meadows, but are a diminishing part of our landscape. When the settlers moved farther west, clearing the forest as they went, western prairie grasses and other associated species moved east, taking advantage of the new open habitat. In the East, where many of the meadows have been left unmowed and where no cattle graze, the species composition seems to favor, along with many of the wildflowers, some of the coarser grasses, such as the little bluestem (Andropogon scoparius ). The next step for these old fields would be the invasion of woody plants, but this can be averted by having controlled burns. The dried grass readily burns off, hardly singeing the soil, leaving the extensive root systems unharmed. The bark of the woody species burns, destroying the cambium layer or exposing it to insects and pathogens that kill the trees, allowing the grasses to grow back even more vigorously afterward.
In suburban areas, by far the most widespread grassland is the lawn. Left alone, lawns would become meadows, but social pressure keeps them carefully manicured. The social mores are sufficiently powerful to stimulate homeowners to spend more time, energy, and expense maintaining their lawns than it takes to maintain equal areas of corn or tobacco. The total acreage of American lawns amounts to 20 million acres, almost 2 percent of the entire country. In each part of the country these weed-free lawns usually harbor at least another 30 species of plants and 100 species of insects. As many as 10 to 15 species of birds often inhabit these communities, as well as several associated species of mammals. The importance of these habitats should not be underestimated.
Another grassland found near many coastal urban areas is the salt marsh. Only about one-half of America's original salt marshes survive. The others have been filled in and used for other purposes. However, the existing marshes are now recognized for their major ecological contributions, and many of them are protected by law. Of course, developers will often try to circumvent the laws, but with the recent proliferation of grass-roots environmental organizations, it usually takes a considerable fight to overtly destroy a salt marsh today. Many developers have learned that the occasional victory is rarely worth the expense and bad publicity.
The lush green salt marshes are flooded twice each day with the incoming tides. The salt-tolerant species of cordgrass in the genus Spartina tend to dominate these habitats.
Another well-known grass is Phragmites, the common reed, which is often found along the drier edges of the salt marshes. Until recently called Phragmites communis but now called Phragmites australis, this species is found around the world, in temperate climates. It grows in a wide range of habitats and is very much on the increase in urban environments. These reeds create a thick, tall mass of growth along the edges of marshes, especially those that have been disturbed. They often reach 6 to 9 feet (2 to 3 m) in height, creating a dense barrier that looks like a fence, which is what the Greek word phragma means.
Because this plant rapidly invades disturbed areas, doing especially well in low, wet regions, it finds many opportunities for growth in cities and suburbs, where the land is often disturbed. Phragmites moves in and outcompetes the other vegetation, creating what appears to be a monoculture, or a stand of just one species. Actually if you walk through a stand of Phragmites and carefully evaluate its value in terms of other species living there, you will find that the Phragmites may radically alter a habitat once it has moved in, but it doesn't totally destroy the habitat. And in cities, where disturbed areas are often replaced by meadows of Phragmites, the beauty is unassailable. The cover for birds and other species is also valuable, and these meadows are better than no meadows. This species is not everyone's favorite. Some people consider Phragmites opportunistic, aggressive, invasive, and impossible to get rid of. But the grass is to be commended for its ability to move in and take advantage of the damage humans leave in their wake. If we don't want Phragmites, we should treat the soil with more care.
Once the land is disturbed, it's usually only a matter of time before Phragmites moves in to fill the void. Its large, feathery tops contain small, umbrellalike seeds that fill the air in the fall. Most people never notice the seeds, but on a good and windy day they are absolutely everywhere, millions blowing in from nearby Phragmites meadows, which may be miles away. Those that land on the water wash to shore where they germinate; those that land on suitable soil grow and in time make their presence known.
Some of the more remote, wild urban parks have large stands of Phragmites, and fires are common among the dead, dry standing canes. Most of these fires are thought to be a result of arson. Like it or not, arsonists may be as hard to control as Phragmites, so the frequent burns in urban stands of Phragmites might as well be viewed as a natural part of the ecosystem. Some of the fires can be traced to lightning; the dead vegetation seems to lend itself to ignition, as if waiting for lightning to strike. Some researchers have even suggested that the dead material may be capable of self-ignition, but that has never been shown. At any rate, these fires appear to be beneficial, as evidenced by the rapid increase in biomass (weight of living material, usually expressed as dry weight per unit area) after a fire. The fires seem to burn off the old, dead biomass, creating new hollows in the old wetlands and rejuvenating the habitat for the Phragmites and other wetland-dwelling species. Without such fires, the rapid buildup of dead canes would fill in the low areas, creating higher, drier ground where Phragmites would eventually be replaced by highland vegetation. So the fires tend to preserve the Phragmites populations.
Excerpted from The Urban Naturalist by Steven D. Garber, Jerome Lo. Copyright © 1987 Steven D. Garber. Excerpted by permission of Dover Publications, Inc..
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