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76 3.4 Influence of Roads on approach. Such an approach is not possible or feasible in the Small Mammals time available for this project. However, permeability can be assessed and species responses to roaded landscapes can be Introduction measured. The null hypothesis that the research team tested Highways have the potential to affect the abundance and is that indirect effects, taken as a whole, have little significant distribution of small mammals. Differences in the density of effect on animal population response. Significant was defined many small mammals have been reported when road verges as greater than 10% deviation, after background variation has have been compared to the habitats beyond them.2,1,19 This been taken into account. The first level predicted responses density difference may be due to structural or vegetative dif- were an expected species' presence or absence, composition, ferences in habitat, water runoff, or the additional impact of and relative abundance to change at increasing distances noise, vibration, deposition of road salt or other chemicals, or from the road if habitat quality was reduced, if habitat frag- differential rates of predation between the verge and adjacent mentation was increased, if there were edge buffer effects, if land. Highways may also act as barriers or partial barriers to there was habitat disconnectedness, and if there were barrier movement.186,143,153,103,49,107,170,36 Such barriers may indirectly effects. lead to population impacts due to the reduced probability of Assessment of causality to a specific indirect effect was not genetic flow and demographic "rescue" (inflow of animals to possible or practical within the time schedule and funding counter local extirpations caused by random events) for small available. The summation of the effects, however, was simple populations. Direct mortality of small mammals on the high- to document. Animal response near roads could be compared way surface186 appears to have variable effects on population with a control response to a non-roaded area. The term "re- density2 as well as demographic changes such as the dispro- sponse" means the difference in the number of small mam- portionate loss of sex or age classes that tend to disperse. mal species diversity and their relative abundance. Jaeger While highways have been well established as contributing to et al.133 explained that roads and traffic can affect the persis- such impacts,186,143,153,103,49,107,170,36 to what extent is not entirely tence of animal populations in four distinct ways: (1) a de- clear. Questions remain as to what impact highways, includ- crease in habitat amount and quality, (2) increased mortal- ing traffic volume, have on the diversity and density of species ity, (3) barrier effects that prevent animals from accessing found in the dry forested ecosystems typical of much of the resources across the road, resulting in (4) fragmented and mountainous region of western North America, to what ex- subdivided populations. tent the effects extend beyond the highway, and if the impacts The small-mammal research in Utah and British Colum- are due to the highway specifically or to the presence of a dis- bia allowed the impact of roads on habitat quality for small turbed ROW generally. mammals to be assessed at varying distances from the road. Both direct effects (animal mortality) and indirect effects If habitat quality declined due to road traffic, the research influence animal response to the roaded landscape. Direct ef- team expected a decline in the numbers and relative abun- fects such as actual road kills, impact all species, but collisions dance of small-mammal species nearer to roads. To investi- with larger wildlife species (deer, elk, moose, caribou, and gate this question, the research team compared the relative large carnivores) pose the most risk to driver safety and result abundance of small mammals at varying distances from a in higher automobile damage and human injury. Knapp major interstate highway in Utah and a two-lane highway and ( showed that for the high-voltage transmission-line ROW in British Columbia. five-state Upper Midwest (Illinois, Iowa, Michigan, Wisconsin, These locations allowed the research team to compare the ef- and Minnesota), 121,584 deer-vehicle collisions caused over fects of two very different types of roads while simultaneously $206.6 million in vehicle damages, but more important, re- addressing the effect of distance from the ROWs. sulted in 35 human deaths and 4,666 injuries from 2003 to For this field effort, the research team selected sites in west- 2004. Direct effects are on the rise, and so are the costs to cit- ern British Columbia and in the Intermountain Region of izens. Indirect effects of roads on wildlife putatively are as im- Utah to determine if any general response of small, terrestrial portant to ecological communities as are direct effects such as vertebrates exists for arid and mesic sites. There is tremen- mortality. The most commonly reported indirect effects in- dous variation across the North American continent in terms clude (1) loss of habitat, (2) reduction of habitat quality, (3) of vegetation cover, topography, levels of urban develop- fragmentation of once "more continuous" habitat with asso- ment, land use practices, road density, and traffic volume, as ciated increases in edge density and edge buffer effects, (4) well as differences in the typical species diversity, richness, habitat disconnectedness, and (5) barrier effects. One com- and abundance in local areas. Yet, it was impossible to cap- plication is that the landscape consequences from indirect ture that entire variation in one study. Nevertheless, this is effects are interrelated suggesting that parsing out the contri- the case with most ecological studies, and there is an impera- bution of each effect will take a long-term experimental tive to capture the basic ecological responses and apply those