Urban habitats

The structural complexity of cities includes features that provide harborage and food for arthropods and other animals.

Parks, recreation areas, and other greenspace have natural habitats for vertebrates and invertebrates; the system of storm water and sewer pipes provides artificial habitats for other animals. Garbage collection points and landfills are consistent features of urban environments around the world, and these sites provide habitats for arthropods, rodents, and pest birds. Livestock agriculture in the form of poultry egg and meat production, feedlots for swine, and beef cattle often interface with residential and commercial land.

Parks, greenspace, and gardens

Many cities have been designed to include space for large and small parks, peripheral green belts, or forested areas along small streams and rivers. These areas break the monotony of residential and commercial buildings, influence local temperature and humidity, and provide neighborhoods with an open recreation site. Early in the development of cities in the USA and Europe large tracts of land were set aside for parks: New York's Central and Prospect Park, and Hyde Park in London are examples of this planned and dedicated space. Once established and integrated into the landscape and seasonal activities, they become an important part of the urban environment.

Cities can have two classes ofopen areas or greenspace: those that have been intentionally established as parks or recreation plots, and the unplanned sites of vacant lots and roadways. In the former, the diversity of plants and animals may be limited, and these sites are somewhat influenced by use patterns of people and domestic pets. Vacant lots, backyards, roadway median strips, and the rights-of-way of railroads and other roads may have a great variety ofplants and animals. Modern highway and expressway systems that enter or circle urban areas often have broad medians and shoulders, and these may be planted with turfgrass, wildflowers, trees and shrubs. These narrow strips ofland often have a large and diverse invertebrate fauna.

Accompanying the recent phenomenon of urban sprawl and expanding suburbs has been the increase in household flower gardens. Despite the conditions of urban high-rise buildings and a concrete and asphalt substrate, urban gardens are flourishing in many regions. Although gardens have been a feature in European cities since the 1760s, the availability of potted plants and exotic species have made it a personal pastime with psychological and economic benefits. An urban or suburban landscape of trees, shrubs, or flowers adds economic value to property: in some cases an increase of 12-30% can be achieved. However, the widespread popularity of household and public gardens can also be accompanied by some health hazards. Whether native or exotic plant species are used, urban gardens may provide food, habitat, or harborage for invertebrate disease-vectors and their vertebrate hosts. Urban wildlife, such as rabbits (Sylvilagus), deer (Odocoileus), chipmunks (Tamias), mice (Peromyscus), and voles (Microtus), feed on a variety of garden plants and seeds, and populations often become large and difficult to control or even manage. Their pest status is based on damage to garden plants, nesting habits, and serving as hosts for ticks and other blood-sucking insect vectors. Increases in Lyme disease and Rocky Mountain spotted fever in eastern USA may be attributed to deer and rabbit populations.

Sanitary sewers and storm sewers

An essential urban infrastructure is the network of underground pipes that remove waste water from toilets and kitchens, and storm water runoff. Many of the urban sewers and storm drains constructed in the 1700s and 1800s are still in use, and in some cities they have been extended or connected to recently developed networks. This elaborate drainage system is hidden from view, and perhaps from the realization that it often provides food and harborage for mosquitoes, cockroaches, rats, and other invertebrate and vertebrate pests. The availability ofresources and uniform environmental conditions often results in year-round pest populations in these underground pipes.

Urban areas may have different systems for handling household waste water and for removing surface or storm water. A combined system brings together household waste and surface runoff water into one network of pipes and delivers the combined discharge to a centralized sewage treatment facility. Some cities have a system which diverts household waste and storm water to separate pipes. Those pipes carrying only surface water discharge at various points into natural watercourses, and the waste water is directed to a sewage-treatment facility. The separate system diverts the majority of surface water to storm sewers, but some of it may be combined with sewage and treated before being released. While both methods can provide harborage and other resources for pests, the combined waste water system is most likely to support pest populations, because of the food contained in the toilet and kitchen refuse.

The storm water drainage system of pipes carries away large amounts ofwater that may otherwise accumulate on roads and streets following excessive rain or snow. Water from streets and sidewalks flows into the underground network of pipes through inlets and catch basins positioned along the curb and street corners. Inlets are covered by a grate and connected to a catch basin before leading to a drainpipe. A catch basin is usually a rectangular storage box located under the street. It is designed to trap street debris before it enters and obstructs the flow of water into drainpipes. Not until water reaches a certain height in the catch basin does it flow into the major storm drain. Because of their construction and underground location, catch basins often retain water for long periods. The combination of organic matter and standing water in a dark and protected location provides a breeding site for several species of mosquitoes. These sites also provide a source of food for cockroaches and rats. Similar conditions are present in some of the underground mass-transit systems and shopping areas in major cities ofthe world.

Solid waste disposal and landfills

Collection and disposal of solid waste is important to human health and the daily operation of a city. Waste produced by households and commercial sources is collected and transferred to a landfill, a site dedicated and specifically managed for waste disposal. It may be close to the city or carried to a distant location. Municipal solid waste originates from daily activity in households, hotels, hospitals and health care facilities, and restaurants, and it contains 10-50% wet and putrescible organic material. The high organic content is a potential food resource and harborage for insects, pest birds, and rodents. The utility this material has to these pests is influenced by the techniques used for collection, and the short- and long-term disposal.

Open refuse sites may be the primary method for collecting the garbage from small communities or neighborhoods in some parts of the world. These sites are usually exposed, three-walled bins, large metal containers, or simply a vacant plot of land. Depending on the size of the areas served, there may be one or more of them in a neighborhood. Although this method leaves organic refuse vulnerable to pest infestation, concentrating household refuse in designated sites enables efficient removal and is better than uncollected garbage in the street. Depending on climate and seasonal temperatures, frequency of collection, and the organic content, open public refuse sites supportlarge infestations offlies and rodents, and often attract birds, dogs, cats, goats, and other animals. Rodents and flies may establish long-term populations at these sites, and move from there to forage in or infest surrounding buildings. Fly maggots within the garbage at the time of collection may be removed from the population; full-grown larvae leave the refuse to pupate and avoid collection, and remain to reinfest. Hotand dry weather can reduce the attractiveness of refuse piles to flies, and hot and wet weather may extend it.

Galvanized steel or plastic containers with lids are typically used to hold household dry and putrescible material. Ideally, the garbage is secure until emptied into a collection vehicle, but lids on garbage containers may not completely prevent entry of rodents, flies, wasps, and other insects. Various species of flies caninfestthese containers: fruitflies access openings that are 1-2 mm wide and adult blow flies are capable of moving through openings 3.2 mm wide. Holes or cracks in the bottom of containers allow full-grown maggots to leave, or large blow fly maggots may climb the inside surface ofmetal containers to find a suitable pupation site outside. In some cities, 60% of the garbage containers may be infested with fly larvae. Rodents gnaw small holes in the bottom and sides of plastic containers, and leave them accessible to further attack. The lids of garbage containers are often not used and garbage is exposed. Daily or weekly garbage collection is partly a function of climate and the local authorities. Long collection intervals, combined with putrescible waste, loose-fitting lids, and damaged containers often result in pest problems.

Many of the large cities of the world rely on a local landfill to take their daily garbage; these sites are usually originally established at the periphery of the city. Landfill sites must be easily accessible and large to accommodate the quantity of solid waste and other material a city produces in the course of 10-15 years. For disposal in most large metropolitan landfills, garbage is first taken to a transfer site where it is emptied from the collection vehicle and loaded into a compactor or incinerator to reduce the volume. It is then transported to the landfill, which may be local or a long distance away. Key to the successful operation of transfer stations is the rapid processing of refuse. Regardless of their efficiency, transfer stations often attract flies, rodents, and pest birds, and their presence can cause problems in surrounding neighborhoods.

Compacted or loose garbage at the landfill is usually covered to reduce odor and the attraction it has to various pests. Soil is commonly used for cover, and the thickness of the layer is important to fly control. Cover soil that is less than about 150 mm is not sufficient to prevent fly emergence completely. House fly adults are capable of moving to the surface from beneath 250 mm of soil, and blowflies and flesh flies are known to emerge from feeding sites 450 mm within compacted refuse. When soil is unavailable or the costs for it are high, other materials, such as paper pulp, fragmented plastic, sand, woven geotextiles, and plastic sheets may be used. In direct sunlight plastic sheets create in the underlying refuse a microclimate with temperatures high enough to prevent fly development. However, these sheets may interfere with rainwater percolation and natural compaction, and trap landfill gases.

The house fly and local species of blow flies are the most common insects at urban landfills around the world. At landfills, these flies may breed continuously through the year, but with decreased numbers in the cold months. Crickets and cockroaches, including the German cockroach, can become established atlandfills, depending on local conditions. Infestations of cockroaches have been linked to buried lots of household material that came to the landfill infested. Once at the site, populations were maintained by the available food and only limited compaction to provide harborage. The pest bird species varies according to location, but the most common are gulls, crows, starlings, and kites. They rarely nest at the site, but usually include the landfill within their foraging territory. The brown rat is common in landfills around the world. Large vertebrates, such as foxes, feral dogs, and goats also regularly occur.

There may be few stable habitats directly on the landfill to support vertebrate populations; most pest species only move to the landfill for feeding and have established nests offsite. Although there is a continuous source of garbage, the working face for dumping changes and there is regular (day and night) disturbance by workers and vehicles. Sudden disturbance of house fly, cricket, and cockroach populations can result in the dispersal of large numbers to areas surrounding the landfill. House flies and blow flies are capable of traveling 1-3 km from infested sites, and cockroaches can move across a varied landscape to building perimeters. Large numbers of seagulls at landfills can disrupt the operation of compaction and earth-moving equipment and spread disease. Feces from gulls at landfill sites have been shown to contain human pathogenic bacteria, such as Escherichia colt 0157. Landfill gulls have the potential of transporting such bacteria to farm and urban sites.

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