Urban ecosystems have been altered in many ways. Much of the land is covered with buildings, sidewalks, and streets, which reduced the surface available for plants, ponds, and creeks, and increases the amount of impervious surfaces. This impervious surface does not "hold water" well and this urban ecosystems are more prone to both flooding after rains and droughts in dry periods. Animals and plants that can survive in these altered ecotones usually do so by being more reproductively successful than other species.
Urban ecosystems are complex mixtures of native and non-native organisms. As nurseries have made it possible to purchase plants from around the world, the introduction of non-native, potentially invasive plants has increased. Many animals have been imported as pets or have moved into urban areas because they can adapt to these human inhabited environments. These non-native (imported) organisms compete with the native Northwestern plants and animals for space and other resources. Many imported organisms adapted to northwestern ecosystems and now they are more successful than the native species. Examples of these non-native species include the House Sparrow, European Starling, Asian slugs, Norway rat, eastern gray squirrel, American opossums, nutria, English Ivy, Morning Glory, Himalayan Blackberry, Japanese Knotweed and the Common Dandelion. These introduced organisms now outnumber native species in many urban and suburban areas.
Animals and plants that survive in urban and suburban areas are biologically described as tolerant generalists. Tolerance refers to their ability to withstand a wide variety of environment conditions, for example: wide temperature ranges, variable rainfall, high to low humidity, poor soil conditions, etc. Generalist refers to an animal that can adapt to diverse conditions as opposed to a specialist that is adapted to one specific niche .
Non-native species found in urban areas have replaced many native species because they have successfully adapted to a wider variety of niches . For example, Rock Doves (Pigeons) will eat anything from birdseed to bread. They are frequently seen in Seattle parks eating picnic leftovers or other edible garbage. Rock Doves have also adapted to urban settings by nesting on the roof or ledges on buildings, in garages or in abandoned buildings. These non-native birds have fewer predators than the native birds. Recently many cities have encouraged Peregrine Falcons to nest on skyscrapers because these Endangered raptors eat birds exclusively. A children's book , Stewart: The Skyscraper Falcon by Linda Birman, documents a year in the life of a pair of Peregrine Falcons that nested and raised three eyasses (baby falcons) on the roof of the Washington Mutual Tower. These Falcons dined on Rock Doves from Pioneer Square.
Another non-native animal that flourishes in the Northwest is the American opossum ( Didelphis virginiana ). The opossum is the only marsupial in the United States. The American opossum is native to the Eastern United States. The opossum may have migrated north from California where it is known to have been introduced. Opossums are nocturnal animals with a keen sense of smell. Their diet consists of insects, fruit, invertebrates, mammals, reptiles, grain, birds and bird eggs. In urban areas they also eat dog food, birdseed and garbage. Some Northwesterners with pet doors have been awakened by opossums that have come into their homes to eat dog and cat food or garbage.
Opossums have the pouch that is characteristic of marsupials . The female produces eggs, which are fertilized in the uterus. The embryos develop in the uterus for about 13 days. The tiny young leave the uterus and crawl into their mother's pouch. Although 25 young are born every two months only about 7 complete their development. At this reproductive rate a pair of opossums could produce 42 surviving offspring every year. No wonder there is an urban legend that suggests that there are more opossums in Seattle than people.
Eastern gray squirrels (Sciurus caroliniensis hypophaeus) have adapted to urban areas in the Northwest. Unlike the native squirrels (Douglas squirrel and western gray squirrel) the eastern gray squirrel thrives in cities. This squirrel is considered a pest by many gardeners because its diet includes planted seeds, buds of all kinds of plants, roots, bulbs, and tubers of ornamental plants. The Douglas squirrel or chickaree (Tamiasciurus douglasii) and the western gray squirrel (Sciurus griseus griseus) are abundant in rural areas throughout the Northwest.
The list of non-native plants that have adapted and become problems in the Northwest is extensive. You may already be aware of some of these species from your own yards and gardens. Himalayan Blackberry, English Ivy, Scot's Broom, Purple Loosestrife, Bamboo, Japanese Knotweed, and Canary Reed Grass are all examples of plants that are quite successfully out competing native plants. As the invasive, non-natives take over an area they turn a once diverse landscape into a monoculture which in turn negatively affects the health of the area. In general, a habitat is more able to handle a changing climate when there is biodiversity in the community. Many groups and individuals work tirelessly to combat the problem, but unfortunately these invasive, non-native plants are still being sold in plant stores throughout the area and are quite popular. (Ivy for instance is one of the most popular plants sold in stores, but can be devastating once it gets into natural habitats.)
What can you do? Start by limiting your plant purchases to native plants as much as possible, and if you must plant non-natives research first to find out which are the least damaging.
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