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Effect of cheatgrass on abundance of the North American deermouse (Peromyscus maniculatus).

Invasive species can have detrimental effects on ecosystems (Vitousek, 1990), they can negatively impact native biodiversity (Davis, 2003), and they may contribute to eradication of species (Pimentel et al., 2000). Cheatgrass (Bromus tectorum), an exotic annual from Eurasia, has invaded much of the intermountain west during the past century (Mack, 1981) and it has been implicated in loss of habitat and subsequent decline of several species of animals (Rickard, 1970; Gano and Rickard, 1982; Knick and Rotenberry, 2000; Newbold, 2005; Young and Clements, 2009).

cheatgrass alters dynamics of soil (Belnap and Phillips, 2001) and increases frequency of fire (Young and Clements, 2009). Following overgrazing and wildfires, cheatgrass can become integrated into the understory of shrub-steppe landscape by out-competing native perennial grasses in growth of roots, germination of seedlings, and survivorship of adults (Humphrey and Schupp, 2004); thus, allowing it to displace native grasses and occupy spaces under and around shrubs (Billings, 1990). often, it becomes the dominant plant in a community, further perpetuating the cheatgrass-wildfire cycle (D'Antonio and Vitousek, 1992) and leading to increased abundance of cheatgrass at the expense of the diversity of native plants (Knapp, 1996). Reduced diversity of native plants can affect abundance and diversity of animals by reducing food resources (e.g., seeds, herbaceous forage, invertebrates) and by homogenizing overall structure of plant communities (e.g., shrub fragmentation-loss). cheatgrass also indirectly affects foraging, thermoregulation, refuges, movements, and abundances of some species (Gano and Rickard, 1982; Parmenter and MacMahon, 1983; Stapp and Van Horne, 1997; Newbold, 2005).

The North American deermouse (Peromyscus maniculatus) is an important species because of its role in food webs and dispersal of seeds (Handley, 1999). Yet, little research has been conducted to explore the effects that cheatgrass may impose on populations of this deermouse or on other species of animals inhabiting shrub-steppe habitat (Davidson et al., 1996). On Antelope Island, Davis county, utah, cheatgrass is widespread and may be affecting populations of North American deermice; thus, reducing an important prey for mammalian mesopredators, raptors, and snakes. In a preliminary study in which rodents were incidentally trapped using species-indiscriminate funnel traps, I retrieved fewer North American deermice from traps in areas with higher percentages of cover of cheatgrass. These preliminary observations were the basis for the main objective of the study reported herein; i.e., to examine the relationship between abundance of North American deermice and cover of cheatgrass. I also examined other biological aspects of deermice (e.g., breeding, demographics) to provide a more complete assessment of potential effects of cheatgrass on this species.

MATERIALS AND METHODS--Antelope Island (41[degrees]02'N, 112[degrees]14'W, elevation 1,310 m), Davis Co., Utah, is the largest island (113 [km.sup.2]) in the Great Salt Lake, although it is not a true island due to a highway at the north end and a naturally occurring land-bridge at the south end in dry years. Common native vegetation on Antelope Island consists of bunchgrasses (Poa longiligula, P. spicatum, Agropyron spicatum) and shrubs (Artemisia tridentata, chrysothamnus nauseosus). Cheatgrass is one of the most abundant plants on the island (Hall et al., 2009).

I established seven, 100-m traplines with 10 stations each; 1 station/10 m of trapline. At each station, I placed two Sherman live traps (7.6 by 8.9 by 22.9 cm) ca. 1 m on opposite sides of the trapline (Matlack et al., 2001) for a total of 20 traps/trapline. I opened Sherman traps and baited every evening with rolled oats and balls of peanut butter ca. 1 cm in diameter. To avoid heat stressing trapped animals, I checked traps and removed animals each morning prior to, or at, sunrise.

I selected locations of traplines to reflect a spectrum of cover of cheatgrass by visually estimating size of areas covered by cheatgrass. However, this did not allow me to assess abundances of rodents in areas without cheatgrass, because cheatgrass was in all habitats sampled. Traplines with less cheatgrass had higher diversity of vegetation (i.e., shrubs and grasses); whereas traplines with more cheatgrass had lower diversity of vegetation. All traplines were adjacent to areas with sagebrush (Artemisia tridentata), but sagebrush did not intersect traplines.

For each trapline, I measured cover of cheatgrass by placing a 1 by 1-m wooden frame every 5 m along the trapline and I estimated percentage of cheatgrass within the square to the nearest 5% (Hall et al., 2009). I made 20 estimates of cheatgrass/trapline, which was 20% of the total area of each trapline.

I conducted two, 4-night surveys (23-26 May and 19-22 August 2006) during new moons. Surveys were scheduled to avoid full moons when nocturnal predators potentially could discourage activity by rodents (Daly et al., 1992).

For each rodent, I recorded trapping station, species, age, sex, breeding condition (e.g., males with scrotal testes; pregnancy or size of mammae in females), and weight to the nearest 0.1 g using a Pesola spring scale (Pesola AG, Baar, Switzerland). Each individual was marked by hair-clipping as described by Matlack et al. (2001). Recaptured individuals were not included in statistical analyses because markings of individuals were not unique. Rodents captured during spring were marked above the right shoulder and those captured in summer were marked above the right side of the rump. Once marked, I released rodents at site of capture.

I calculated mean percentage cover of cheatgrass for each trapline. For each trapline in spring and summer, I calculated relative abundance (number captured per trapline during 4 nights), percentage of breeding adults, percentage of adults, mean weights of males and females, and percentage of adult males in the total sample of adult males and females. I calculated overall percentages of breeding individuals, percentage of adults, percentage of adult males in the total sample of adult males and females, overall mean weights for males and females. using linear regression, I determined if there was a relationship between relative abundance of rodents and percentage cover of cheatgrass. I used additional linear regressions to assess whether percentage of adult males in the total sample of adult males and females, percentages of breeding individuals, percentage of adults, and mean weights were associated with percentage cover of cheatgrass.

Using chi-square analysis, I assessed whether percentage of adult males in the total sample of adult males and females between survey periods significantly deviated from parity. Data for juvenile deermice were removed from percentage of adult males in the total sample of adult males and females, percentage of breeding individuals, and mean weights so that these factors could be compared using the same demographic group. Statistical analyses were performed using the SPSS software package (SPSS version 15.0, SPSS, Inc., Chicago, Illinois). Statistical tests were considered significant at [alpha] [less than or equal to] 0.05.

RESULTS--Amount of cheatgrass varied among traplines (Fig. 1). During May, 183 North American deermice (62 males, 121 females) were captured and during August, 150 (78 males, 72 females) were captured; thus, a total of 333 deermice was captured in 1,120 trap-nights. Eight montane voles (Microtus montanus; 2 males, 6 females) were captured in May and three Ord's kangaroo rats (Dipodomys ordii; 1 male, 2 females) were captured in August. Because of small samples for these two species, they were excluded from analyses.

Linear-regression equations revealed that relative abundance of the North American deermouse was negatively associated with percentage cover of cheatgrass in both spring ([F.sub.1,5] = 97.20, P < 0.01, [R.sup.2] = 0.95, n = 183) and summer ([F.sub.1,5] = 11.80, P = 0.02, [R.sup.2] = 0.70, n = 150; Fig. 1). In summer, percentage of adult males in the total sample of adult males and females from each trapline was related negatively to percentage cover of cheatgrass. That is, fewer males were likely to be in areas with a high percentage of cheatgrass ([F.sub.1,5] = 17.60, P = 0.01, [R.sup.2] = 0.78). In spring, percentage of adult males in the total sample of adults was not statistically related to percentage cover of cheatgrass ([F.sub.1,5] = 0.14, P = 0.72, [R.sup.2] = 0.03). During spring, percentage of adult males in the total sample of adults significantly deviated from parity due to the large number of females that were captured ([chi square] = 23.50, df = 1, P < 0.01). However, in summer, percentage of adult males in the total sample of adults did not deviate significantly from parity (%2 = 0.17, df = 1, P = 0.68).


Percentage of adult deermice captured on each trapline was not significantly related to percentage cover of cheatgrass during spring or summer ([F.sub.1,5] = 1.97, P = 0.22, [R.sup.2] = 0.28; [F.sub.1,5] = 1.36, P = 0.30, [R.sup.2] = 0.21, respectively). In addition, there was no statistically significant relationship between percentage of breeding deermice and percentage cover of cheatgrass during spring ([F.sub.1,5] = 0.51, P = 0.51, [R.sup.2] = 0.09) or summer ([F.sub.1,5] = 0.16, P = 0.70, [R.sup.2] = 0.03). Mean weights of deermice for each trapline were not significantly associated with percentage cover of cheatgrass during spring (males: [F.sub.1,5] = 0.01, P = 0.97, [R.sup.2] < 0.01; females: [F.sub.1,5] = 0.02, P = 0.89, [R.sup.2] = 0.01) or summer (males: [F.sub.1,5] = 0.19, P = 0.68, [R.sup.2] = 0.04; females: [F.sub.1,5] = 2.80, P = 0.15, [R.sup.2] = 0.36).

DISCUSSION--Studies have revealed that abundance of rodents may be influenced by cheatgrass (Gano and Rickard, 1982; Gitzen et al., 2001; Ostoja, 2008). In addition, areas invaded by cheatgrass are prone to wildfires (Young and Clements, 2009). Gano and Rickard (1982) reported that rodents were less abundant in an area that had burned 4 years previously, but McGee (1982) observed that rodents were affected only temporarily by fires (i.e., 1-3 years). If vegetation in an area has sufficient time to return to pre-burn conditions following a wildfire, then reestablishment of populations of rodents is conceivable. However, presence of cheatgrass often increases intervals between fires (Knapp, 1996), thus decreasing recovery time following burns. Due to the self-perpetuating cycle of wildfires and succession by cheatgrass, populations of rodents may be reduced over time.

Homogenization of landscapes due to presence of cheatgrass may reduce diversity and abundance of foods that are available to rodents. This probably would affect specialists more than generalists; thus, partially explaining why more North American deermice, which are generalists (Sieg et al., 1986), were captured by me in cheatgrass-dominated habitats compared to other species of rodents. Deermice are selective opportunists that primarily exhibit granivorous and insectivorous habits in the west, with coleopterans and hymenopterans comprising most of their diet (Sieg et al., 1986). Cheatgrass alters communities and abundances of beetles (Rickard, 1970). This is consistent with my detection of lower abundances of deermice in areas with higher percentages cover of cheatgrass.

Although cheatgrass can be a small component in the diet of deermice (Sieg et al., 1986), studies in the laboratory and field demonstrated that they do not prefer leaves or seeds of cheatgrass (Kelrick et al., 1986). Given that cheatgrass can be abundant and that diet of deermice has the potential to vary seasonally and differ in composition (Sieg et al., 1986), cheatgrass could be important in the diet of deermice. Areas with greater productivity of foods support more deermice (wolff, 1996), but results of my study and others indicate that cheatgrass supports fewer deermice. This may be because cheatgrass is an undependable source of food (Young and Clements, 2009) due to its variability in producing forage.

Loss and fragmentation of shrubs due to increased frequency of fire and subsequent colonization by cheatgrass may also be affecting the community of small mammals (Gitzen et al., 2001). Deermice are more abundant in shrub communities (Stapp and Van Horne, 1997) where they use open microhabitats with less herbaceous cover (Belk et al., 1988); perhaps, to facilitate capturing insects (Pearson et al., 2001). Furthermore, shrubs serve as protection during times of lunar illumination (Price et al., 1984). However, Parmenter and MacMahon (1983) discovered that following removal of shrubs, deermice continued to inhabit shrubless areas and other species of rodents migrated elsewhere. They believed that shrubs were important as sources of food and that they increased foraging times.

Mobility of deermice may be affected by cheatgrass (Rieder et al., 2009), which may negatively affect reproductive success in areas with cheatgrass (Groves and Steenhof, 1988). In my study, lack of a statistical relationship between percentages of breeding deermice and cover of cheatgrass did not indicate that proportions of breeding deermice were reduced because of encumbered mobility. I could not assess if reproductively active deermice were successful at mating; however, dense cheatgrass may make it difficult for deermice to encounter one another. This might explain why relative abundance of deermice was lower in areas dominated by cheatgrass. Reduced mobility also could affect success in foraging on arthropods and ability of deermice to escape predation.

My study of deermice concurs with those of others indicating that cheatgrass affects composition of communities and distribution of rodents. This is detrimental for ecosystems invaded by cheatgrass because deermice play an important role in food webs and in dynamics of the dispersal of seeds. Disruption of these interactions could lead to more serious disturbances of processes in the shrub-steppe ecosystem. Much of the landscape that is susceptible to invasion by cheatgrass probably has been affected to some degree. The extent to which cheatgrass has affected plants and animals remains unclear. Species of animals inhabiting areas invaded by cheatgrass are at risk (Billings, 1990) and unless cheatgrass can be effectively managed, it will not surrender its territory in the semi-arid grasslands of the Intermountain West. This leaves the future of many shrub-steppe species uncertain, especially those that may be more sensitive to disturbance than the North American deermouse.

I thank E. Hall, K. Hall, B. Richardson, C. Webb, K. Peterson, J. Mull, J. Cavitt, K. Stone, S. Bates, and J. Hatch who provided time, resources, and expertise. I also thank J. Mull, J. Cavitt, S. Zeveloff, S. Newbold, and D. Kaufman for improving the manuscript. Financial support was provided by Antelope Island State Park and the Phyllis Crosby Gardner Undergraduate Research Grant through the Weber State University Office of Undergraduate Research. This project was approved by the Weber State University Institutional Animal Care and Use Committee.


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Submitted 12 November 2009. Accepted 26 March 2012.

Associate Editor was Richard T. Stevens.


Department of Zoology, Weber State University, Ogden, UT 84408-2505

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Author:Hall, Lucas K.
Publication:Southwestern Naturalist
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Geographic Code:1USA
Date:Jun 1, 2012
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