Does an interstate highway act as a barrier to the movement of Peromyscus maniculatus?
Michael W. Archer, Department of Biology, University of Nebraska Kearney, Kearney, Nebraska 68849-1140, USA
Mentor: Dr. Joseph T. Springer, Professor, Department of Biology, University of Nebraska at Kearney, Nebraska 68849-1140, USA. Email: springerj@unk.edu)
To Cite This Paper:
Archer, M. W. 2003. Does an interstate highway act as a barrier to the movement of Peromyscus maniculatus? Research Thesis, Department of Biology, University of Nebraska at Kearney, Kearney, Nebraska.
ABSTRACT
We trapped Peromyscus maniculatus (deer mouse) along U.S. Interstate-80 to test the theory that an interstate highway acts as a barrier to their movement. Traps were set at three different sites along Interstate-80. Twenty traps were set at each site with 10 traps on the north and 10 traps on the south. Deer mice caught were then marked with a permanent marker on the inside of the ear (red for all deer mice captured on the north and black for those captured on the south), and released to see if recapture would be on the opposite side. As predicted, no mice were recorded crossing the interstate. We recorded 80 captures; with 29 of those being recaptures at Site 1. Site 2 recorded 82 captures, with 23 of those being recaptured. At Site 3 there was a total of 80 captures, with 21 of those being recaptured. A Chi-squared analysis of the data (P < 0.0005) allowed rejection of the null hypothesis: deer mice can and do freely cross an interstate highway. Key Words: deer mouse, dispersal barrier, interstate highway, Peromyscus maniculatus.
Numerous studies have been conducted that examine the impact of roadways on mammalian habitat. Previous studies have examined the use of road right-of-ways by small mammals (Adams & Geis 1983) and of small mammal use of a highway median as habitat (Adams 1984). Meunier et al. (1999) studied the corridor function that roadsides provide for small mammals. They found that the road, acting as a barrier, facilitated a corridor for small mammals much like the findings of roadway barriers for arthropods (Mader et al. 1990). Roadside prevalence does not always lead to a decrease in species richness when compared to surrounding habitat; in fact it has been found that in certain habitats the exact opposite is true (Meunier et al. 1999). In some regions Peromyscus maniculatus (deer mice) have been found to use the right-of-way as a preferred habitat over habitats that lie farther away from an interstate (Adams & Geis 1983).
Another factor to consider is the effect that an interstate highway has on gene flow and the possibility that it aids in the formation of two distinct subspecies of deer mice. Morphological and ecological variability has resulted in the recognition of more than 65 subspecies (Hall 1981). According to the usual geographic concept of subspecies employed by zoologists, no two subspecies should occur in the same area (except under extraordinary conditions) because they would be expected to interbreed and lose their distinctness (Dice 1931). Gene flow is much more extensive over a smaller geographic region (Calhoun et al. 1998). However, Blair (1950) discovered that populations of deer mice could exhibit significant differences in pelage and body dimensions, even though those populations were only separated by a few kilometers. This may be related to the findings of McLellan & Finnegan (1990). They discovered that environmental factors such as the orientation of roads, towns, cultivated areas, and break sites that provide shelter may constrain dispersal patterns, such that gene flow among populations may not correspond directly to the geographic proximity of populations.
The purpose of this study was to examine if an interstate highway acts as a barrier to free movement of Peromyscus maniculatus.
Materials and Methods
This study was conducted along U. S. Interstate-80 in central Nebraska at three different locations (Fig. 1) during the middle of the summer (15 July to 31 July 2003) and the early fall (15 September to 30 September 2003). Covering 482 miles (Nebraska Department of Roads 2002), U.S. Interstate 80 serves as Nebraskas main roadway cutting across the state. It is a divided 4-lane interstate running parallel to one of Nebraskas major ecosystems (the Platte River). Test sites were based on two criteria: accessibility and surrounding habitat.  Fig. 1. Map of study area with the three study sites marked. Kearney is located in central Nebraska, along the Big Bend region of the Platte River.
Site 1 was between mile markers 270 and 271. The North side of the test site was a disturbed roadside habitat. It contained a mixture of invader species and tall grass prairie species. Plant cover was somewhat dense. It was bordered by a pasture, which was grazed with livestock. The south side of the test site contained short grass prairie species, with fewer invader species. Plant cover on this side was also less dense than the opposite side. It was bordered by a cornfield (Fig. 2).  Fig. 2. Site 1 was located between mile markers 270 and 271 on U. S. Interstate-80 in Buffalo County, Nebraska. Exact study area is outlined in white. The habitat adjacent to the north side of the study area was grazed pasture. The south side of the study area was adjacent to row crop farming.
Site 2 was at mile marker 278. Floral make-up of both the north and south sides was primarily short grass prairie species with some invaders species. A cornfield bordered the north, while the south side was bordered by riparian habitat (Fig. 3).  Fig. 3. Site 2 was located at mile marker 278 on U. S. Interstate-80 in Buffalo County, Nebraska. Exact study area is outlined in white. The habitat adjacent to the north side of the study area was a cornfield. The south side of the study area was adjacent to riparian habitat.
Site 3 was on the west side of the Nebraska Highway 10 and U. S. Interstate-80 interchange (Fig. 4). Located between the on/off ramps and Interstate-80, this site had very dense habitat. The north side was extremely dense with tall grass prairie species, making it difficult at times to locate traps. The south side contained virtually the same plant species; however this site was disturbed by tire tracks half-way through the summer collection period. Road construction had been done in the area, but was finished before trapping began.  Fig. 4. Site 3 was located at the interchange of U. S. Interstate-80 and Nebraska Highway 10 in Buffalo County, Nebraska. Exact study area is outlined in white. Plant coverage was extremely dense, providing excellent habitat for small mammal.
Sherman-type live traps were set out one week prior to trapping. Traps were set in a way similar to the method that was used by Richardson et al. (1997). Two rows of 10 traps were set out on opposite sides of Interstate-80 at 5-meter intervals. The first row of traps was set 7.5 m from the roadway in the unmowed section. The second row was placed 5 m from the first row. This was repeated at all three sites. The traps were baited using creamy peanut butter (Kaufman et al. 1988) on a 1-in square piece of wax paper. Cotton batting material was placed in the traps during the last half of the fall trapping season (Kaufman et al 1988, Lambin & Mackinnon 1997). To help with camouflage and temperature regulation, the traps were covered with straw, which was held in place using thin plastic tape. Summer trapping began on 15 July 2003 for Sites 1 and 2. Due to road construction, Site 3 was not started until 25 July. Traps were set for seven nights at each site for a total of 418 trap-nights. The total was not 420; 2 trap-nights were subtracted due the destruction of one trap and movement of another. Early fall collection began on 15 September 2003 and concluded on 30 September. Traps were set for 11 nights, for a total of 660 trap night traps. Deer mice caught on the north side of the road were marked with a permanent red marker on the inside of the left ear. Those caught on the south side were marked in a similar manner with a permanent black marker. All other species caught were noted and released.
Results and Discussion
The results of this study show, with very strong evidence that U. S. Interstate-80 acts as a barrier to the movement of the deer mouse, and most likely other small mammals. At no time during the study were any deer mice recorded crossing the roadway. Trapping results are shown in Table 1.
Fig. 5 shows the results from the midsummer trapping period. Trap success was high for several small mammals (45.44 captures/100 trap-nights); however, the dominant species was P. maniculatus (33.25 captures/100 trap-nights). Other species caught during this sampling period included (in decreasing order) Blarina brevicada (northern short-tailed shrew), Spermophilus tridecemlineatus (thirteen-lined ground squirrel), Peromyscus leucopus (white-footed mouse), Microtus pennsylvanicus (meadow vole), Dipodomys ordii (Ord's kangaroo rat) and Zapus hudsonius (meadow jumping mouse.) Recapture rates varied slightly between the sites. Site 1 had the highest rate of recapture. Fig. 5. Summer 2003 trapping success of small mammals along U. S. Interstate-80 in central Nebraska.
Of all individuals captured one time, 34.3% were recaptured. Site 3 had a recapture rate of 27.5%, while Site 2 was the lowest rate, at 20.4%. These figures do not reflect any deer mice found dead in the trap. The dead mice were recorded to see if they represented a recapture on the opposite side. Otherwise they were not recorded as a part of the population. Fig. 6 shows the results from the early fall trapping period. Capture rates were less than half of what the summer results were, however, the recapture rate was much higher for this study period (site one 61.4%, site two 63.2%, and site three 42.2%.) This was to be expected, as many small mammals begin to enter daily torpor. The number of transient species would be decreased, providing for an increase in the recapture rate, which might be assumed the resident species of the area. Species diversity was also much lower because of the changing of the seasons and the decline in temperature. The deer mouse was by far the most prevalent species collected at all three sites, with the short tailed shrew and white-footed mouse also being present. In addition, the thirteen-lined ground squirrel became non-existent as the temperature lowered.  Fig. 6. Fall 2003 trapping success of small mammals along U. S. Interstate-80 in central Nebraska.
The deer mouse has a home range of approximately 534 m2 (Wolf 1985) and is probably the most widespread and abundant mammal native to North America (Lansman et al. 1983). Deer mice nest in all types of environments including rock piles, brush piles, stream banks, and tree bases (Sharpe & Millar 1990). An average home range for the deer mouse contains 20 nest sites on the average. These sites serve many purposes ranging from food storage to nesting (Blair 1951). Breeding females have been found to relocate to different nesting sites three times more often than non-breeding females. Males and females of this species tend to relocate at least one to as many as seven times over distances as large as 320 m (Sharpe and Millar 1990). Lactating females increase food intake by 145%. Despite all of these characteristics, the results from this study show that the deer mouse will not (or cannot successfully) cross an interstate highway. These results ar similar to those of Oxley et al. (1974). They also found that small mammals do not cross a four-lane divided roadway. Their findings support the idea that the most important factor in small mammals not crossing the roadway might be the lack of cover on the roadway. This, however, cannot be the only reason. Oxley et al. (1974) refered to Sadlier's (1965) study in which a marked deer mouse was observed moving completely across a freshly plowed field. While movement this far is rare, it is possible. Traffic density has also shown to have an adverse affect on the movement of small mammals. While this may directly correlate with road width, Richardson et al. (1997) found that the movement of Apodemus sylvaticus (wood mice), Clethrionomys glareolus (bank voles), and Microtus agrestis (field voles) were all negatively affected by the density of traffic. Light also plays a definitive role in the movement and foraging behavior of the deer mouse. Travers et al. (1988) showed that deer mice selectively forage in shaded and covered patches as opposed to open patches. Kaufman & Kaufman (1982) found this was also true for the Ord's kangaroo rat. An interstate highway provides neither cover nor shade. In fact, the night sky is illuminated with the headlights of automobiles that travel on the roadway at night. The last factor to consider is the corridor (a travel way that connects one habitat to another) effect of the roadside as suggested by Meunier et al. (1990). While they were unable to distinguish between transient and resident species in their study, this might help explain why no deer mice were recorded crossing the road (in this study), but were found in significant numbers within the right-of-way. While I was unable to find a specific reason (or reasons) as to why an interstate highway is a barrier to the movement of the deer mouse, I did prove that it does act as a barrier. This is important from the aspect of gene flow. An interstate highway could lead to the formation of two distinct subspecies which lie less than 150 m apart. In effect, we have created a barrier that cannot be crossed, except in the rare events. LITERATURE CITED
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