Human transportation of exotic species across natural barriers to their dispersal has altered dramatically the structure and function of natural ecosystems across the globe (Samways 1995, A. Suarez et al. 1998, Wallner 1996). Examples include the devastation of island vegetation by pigs and goats introduced intentionally by explorers; destruction of grasslands globally by domesticated, often introduced, livestock; disruption of aquatic communities by introduced amphibians, fish, and mollusks (e.g., African clawed frog and zebra mussel in North America); and disruption of grassland and forest communities by introduced plants (e.g., spotted knapweed in North America), mammals (e.g., rabbits in Australia), reptiles (brown tree snake in Oceania), insects (e.g., gypsy moth in North America, the European wood wasp, Sirex noctulio, in Australia), and pathogens (e.g., chestnut blight and white pine blister rust in North America, Dutch elm disease in North America and Europe, pinewood nematode in Japan). Exotic species, espe cially of insects, can be found in virtually all "natural" ecosystems on all continents. Many herbivorous insects and mites have arrived on agricultural or forestry products and become plant pests in agroecosystems or forests. Some herbivorous and predaceous arthropods have been introduced intentionally for biological control of exotic weeds or plant pests (e.g., Croft 1990, Kogan 1998, McEvoy et al. 1991). Despite evaluation efforts, these biological control agents, especially arthropod predators, compete with native species and have the potential to colonize native hosts related to the exotic host and develop new biotypes. Indigenous herbivore species also can colonize exotic hosts and develop new biotypes (D. Strong et al. 1984), with unknown consequences for long-term population dynamics and community structure. Samways et al. (1996) found that different invertebrate assemblages were found on exotic vegetation, compared to indigenous vegetation, in South Africa.
Urban areas represent increasingly large and interconnected patches on regional landscapes and are particularly important ports for the spread of exotic species into surrounding ecosystems. Urban centers are the origin or destination for commercial transport of a wide variety of materials, including forest and agricultural products. Urban areas are characterized by a wide variety of exotic species, especially ornamental plants and their associated exotic insects and pathogens. Exotic or native ornamental species usually are stressed by soil compaction, air and water pollutants, elevated urban temperatures, etc. Arriving exotics often have little difficulty finding suitable hosts and becoming established in urban centers and subsequently spreading into surrounding ecosystems.
Road systems connecting urban centers and penetrating natural ecosystems represent major corridors that facilitate spread of exotic species. Roadsides usually are highly disturbed by road maintenance, other human activities, and air pollution from vehicles and provide suitable habitat for a variety of invasive species. Gypsy moth is particularly capable of spreading via human transportation (of pupae or egg masses attached to vehicles, outdoor equipment, or commercial products) between urban centers. Stiles and Jones (1998) demonstrated that population distribution of the red imported fire ant, Solenopsis invicta, was significantly affected by width and disturbance frequency of road and powerline corridors through forests in the southeastern United States (Fig. 7.9). Mound densities were significantly highest along dirt roads not covered by forest canopy and lowest along roads covered by forest canopy. Powerline and graveled or paved roads not covered by forest canopy supported intermediate densities of mounds. These trends suggest that canopy openings of intermediate width and high disturbance frequency are most conducive to fire ant colonization.
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