Most people have had pests in their house at some point or another, and the problem may have been so extreme that it required treatment of some sort--a trap or two, or perhaps even some sort of chemical. These sorts of measures can be effective at removing target species, but they may also have an impact on other animals that find themselves in the wrong place at the wrong time. One well-known example of this is the massive die-offs of large bird species, particularly vultures, that have resulted from the birds' unwittingly feeding on carcasses of poisoned pest species. Because nuisance animals are often considered a more interesting research topic for exterminators than for scientists, many pest systems are not understood in the sort of detail that helps managers develop good treatment plans, or lawmakers to write bills protecting innocent animal bystanders caught in the crossfire.
(Norway rat, Rattus norvegicus)
Two collaborators from the Western Ecology Division of the U.S. Environmental Protection Agency recently published results of research aimed at rectifying this situation. Their study focused on the use of anticoagulant chemicals in two urban/suburban areas in California. Anticoagulants are commonly used on mammalian targets such as rats and mice, but they may also impact non-target species such as squirrels, rabbits and other small mammals, as well as the avian and terrestrial predators that eat them. Other at-risk species include those that are threatened, protected, or regulated for any number of other reasons, including foxes, bobcats, coyotes, and mountain lions.
The researchers suspected that use of chemicals might be associated with both landscape factors, such as the proximity of "natural" habitat and the type/age of the buildings in which treatments were used, as well as socioeconomic factors. In order to investigate this, they distributed questionnaires to residents in the two study sites--southwestern Bakersfield and a suburban area near the Santa Monica Mountains National Recreation Area (SAMO)--in order to collect information allowing them to better understand how chemical usage patterns are related to mortality rates, and also to investigate landscape variables associated with the use of these products. The study sites were chosen after local biologists conducted necropsies and home range analyses of dead wildlife there; these examinations revealed that the animals had died from poison distributed in and around Bakersfield and SAMO.
(House mouse, Mus musculus)
While rats and mice were the most common target species at both sites, residents in Bakersfield also used poison to control for kit foxes, while residents in SAMO treated for small mammals such as squirrels, gophers, and rabbits, as well as for larger animals such as bobcats, coyotes, and mountain lions. Although you might think that older homes would be more permeable to pest species, control and chemical use did not vary with building age in either study area. Likewise, residents in both locations were more likely to treat for nuisance animals if they lived in single-family dwellings rather than apartment complexes, and in lower-density developments. The main difference between Bakersfield and SAMO chemical usage patterns was related to distance to open space. In the former, only squirrel control efforts were associated with this variable (with more treatment occurring nearer open spaces), while in the latter, distance to open space influenced control of the two main target species, rats and mice (with more treatment occurring closer to open spaces). These differences probably stem from the fact that Bakersfield is, in general, less densely urbanized than SAMO, and located much farther from large tracts of natural habitats; both of these variables influence availability of resources and the movements of small mammals.
Both study sites had control and/or chemical "hotspots," where large numbers of residents reported taking measures, chemical or otherwise, to reduce the incidence of pests. The single Bakersfield control hotspot was located near both a golf course and a construction site, and was close to otherwise open habitat. The two SAMO control hotspots overlapped with three chemical hotspots; all were near densely-vegetated wash channels close to open space, and one was associated with development of a new residential subdivision. Thus, it appears that pest mammals are likely to become particularly problematic to humans who live near "natural" areas and those located in habitats under construction.
(Kit foxes, Vulpes macrotis)
Cumulatively, the results indicate that landscape patterns impact the distribution of nuisance species, which, in turn, impacts use of chemicals and other control measures. Predictably, the most human-commensal species, mice and rats, were targeted fairly uniformly across the habitats. However, other species were only controlled for in pockets. The researchers hypothesize that this is because the presence of these animals is strongly related to both the presence of land cover (acting as shelter and facilitating safe movement between habitat patches) and patterns of land use (e.g., for feeding, breeding, or hiding). Survey respondents frequently associated the presence of some pest species with exotic vegetation--particularly fruiting trees that provide an attractive source of food for many animals. Future research will need to investigate the extent to which invasives influence habitat use by potential nuisance species--and to what extent this encourages nearby humans to use chemicals and other control measures.
Also intriguing is an apparent link between socioeconomic factors and the proximity of pests. Survey respondent income was linked to two variables (distance to open spaces, density of housing) that impacted chemical use. According to the authors, this suggests there is a "possible feedback loop of interacting ecological and social phenomena." Specifically, human habitat alteration creates appealing wildlife habitat, which draws animals in closer to humans; when these species become a nuisance, they are controlled for and, often, the damage they caused is repaired; because the original wildlife-attractive variables remain in place, more wildlife appear over time, and the cycle starts all over again. The researchers hope that the "hotspots" identified by their analyses can be used to pinpoint which urban areas are particularly at risk of entering this feedback loop. In these locations, landscapers and managers may need to get creative in order to find safe and effective ways to discourage pests from utilizing anthropogenic resources, while also ensuring that innocent bystanders don't get harmed in the process.
Morzillo, A.T., and Schwartz, M.D. 2012. Landscape characteristics affect animal control by urban residents. Ecosphere 2(11):1-16.
Thanks to the following websites for providing the images used in this post: