Use of Four Grassland Types by Small Mammal Species in Southern Minnesota

Methods

This study was conducted on the 160-acre Schottler Wildlife Management Area located 6.5 miles south of Austin, Minnesota (43°59′97.71″ N and 92°92′45.58″ W). A suite of 12, 4.5-hectare plots were planted in 2005 with one of four seed mixes (Figure 1). The site had been in a corn-soy rotation for at least 25 years prior to the reconstruction of grasslands. The plantings represent the only grassland habitat, other than road ditches, larger than a few acres within a one-mile radius. Post-planting management of the site was minimal with controlled burns occurring in half the plots during spring 2009 and the remaining half during spring 2010.

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Figure 1.

Planting design at the Schottler Wildlife Management Area (WMA) south of Austin, Minnesota in Mower county, SE Minnesota. CSG = cool season grass dominated fields, WSG = warm season grass dominated fields, F = forb fields lacking cup plant and compass plant, and TF = forb fields characterized by the presence of cup plant and compass plant. Both TF and F has greater than 65% forbs by rooted abundance. Each plot was approximately 11 acres in size. Study utilized approximately half of the total WMA area. Plots inside the yellow boundary were used for small mammal surveys. Photo Credit: Google Earth.

Eight plots were selected for this study and represented a block of side-by-side habitats with duplicate plots of each grassland type (Figure 1). Two plot types were dominated by grasses. One was predominantly tall (> 1m) native warm-season grass (WSG) such as Andropogon gerardii (big bluestem) and Sorghastrum nutans (indian grass). The other grass dominated plot type consisted of short non-native cool-season grasses (CSG) such as Dactylis glomerata (orchard grass), Festuca pratensis (meadow fescue), and Phleum pretense (Timothy grass). The other two plot types were dominated by forbs, one with a greater proportion of tall forb species such as Silphium perfoliatum (cup plant), Silphium laciniatum (compass plant) and Aster novea-angliae (New England aster). Other than the differences in taller forb species, both forb plots (F and TF, Figure 1) were characterized by similar species such as Monarda fistulosa (bergamot), Zizia aurea (golden alexander), Solidago rigida (stiff goldenrod), Penstemon digitalis (white beardtongue), Rudbeckia hirta (black-eyed susan), Ratibida pinnata (yellow coneflower), Liatris pycnostaycha (prairie blazing star), and Gentiana alba (cream gentian). The forb dominated areas had over 45 species, with forbs contributing greater than 70% of all rooted plants. Post-planting floristic surveys were conducted in the summer of 2011, the sixth growing season, to quantify any changes in the reconstruction since seeding (see Port and Schottler 2017 for further details).

Results

During a total of 7680 trap nights, 1807 animals representing at least seven different species groups were captured across the four plot types. Microtus pennsylvanicus were the most common animals captured (n = 1233) with M. ochrogaster (n = 434) and Peromyscus (n = 102) abundant in some of the survey years. Additional species captured included Blarina brevicauda (northern short-tailed shrew), Zapus hudsonius, Cryptotis parva (least shrew), and Mustela erminea (ermine) (Table 1). Total richness over the duration of the study was seven (range 3–7, mean = 4.6).

Discussion

These grasslands were dominated by M. pennsylvanicus, M. ochrogaster, and Peromyscus. Small mammal communities in the Midwest typically have low species diversity, often with three to five species within an area (Getz and Hofmann 1999, Heske 1999). Our prediction that small mammal species richness would differ between plot types was not supported, at least not during the June trapping period. We cannot discount the possibility that patterns do exist later in the growing season as reproduction increases population densities and dispersal. It also remains possible that sampling in the greater field would have produced different results due to greater variability between and within habitats. We were only able to replicate our plots twice as opposed to three times because several plots bordered a road and a large wet meadow and were potentially structurally different from the eight plots studied. This may have affected our results as well.

Individuals from each species were found in all plot types during the study and no plot type had biologically significant differences in the number of unique captures. We did find some evidence of movement between plot types by individuals, but these movements were rare during June, with most recaptures occurring at traps within 20 m of the original location. The ability to rapidly colonize new areas is a life history trait of Microtus species (Ebenhard 1990, Glorvigen et al. 2013) and it is possible dispersal occurs later in the summer as fecundity increases population pressures within plots.

Microtus pennsylvanicus were the dominant species throughout all plot types with unique captures ranging from 11 to 421 during the study. Microtus pennsylvanicus are known to exhibit pronounced population cycles every 4–5 years (Krebs 2013) but it remains unknown if this population demonstrated a similar pattern. The rapid decline in the M. pennsylvanicus population along with trapping success of other species in 2013 coincided with periods of heavy rainfall, localized flooding, and cooler temperatures throughout the trapping period and may reflect the impact of weather on the local population, rather than a periodic population event.

Microtus ochrogaster populations varied from year to year. Klatt et al. (2015) suggested at least some of this distributional pattern may be the result of interspecific interactions, with M. ochrogaster dominant in sparse cover and M. pennsylvanicus dominant in heavier cover. Batzli et al. (1999) observed that presence of M. pennsylvanicus resulted in lower densities of M. ochrogaster. While both species of voles preferentially seek out vegetative cover (Getz et al. 2005), Microtus pennsylvanicus are thought to favor relatively tall grass vegetation structures due to foraging preferences (DeJaco and Batzli 2013) and nesting requirements, whereas M. ochrogaster nest in burrows below ground and feed preferentially on forbs (Klatt and Getz 1987, Lin and Batzli 2001, Getz et al. 2005). Survival rates of M. pennsylvanicus are positively associated with density of cover compared to M. ochrogaster, perhaps as a consequence of the greater vulnerability of M. pennsylvanicus to predation (Lin and Batzli 2001, Getz et al. 2005). Klimstra et al. (2015) documented the movement of M. pennsylvanicus out of hayed fields, potentially as the result of lost cover and increased vulnerability to predation. As a result, these species are frequently reported to have opposite habitat preferences within grassland ecosystems (Getz et al. 2005). We found little difference in the distribution of either Microtus species within the four different reconstruction types at any point during the study, although M. pennsylvanicus was far more abundant in most years across all plot types.

There was a significant difference in the mean mass of male M. pennsylvanicus in the cool-season grass (CSG) plots compared to forb (F) dominated fields. This may suggest higher habitat quality, at least for M. pennsylvanicus, for these plot types although vegetational characters were not evaluated in this study. Getz et al. (1979) reported differences in M. pennsylvanicus weights by habitat type although found no correlation between survival or reproduction and body weight. CSG plots were dominated by non-native species including orchard grass, meadow fescue, and Timothy grass that may have provided protective dense thatch layers as well as forage. Interestingly, young shoots of orchard and Timothy grass were reported as favored food items by M. ochrogaster (DeJaco and Batzli 2013) and the heavier mean mass of M. pennsylvanicus within these field types may reflect a higher nutritional quality of these types of graminoids for voles in general, and in particular during the June growing season when trapping occurred. Kaufman and Kaufman (2017) suggested cool season fields dominated by brome may provide higher quality forage year-round and be favored by herbivores over native prairie grasses. No differences in mass between plot types were found in M. ochrogaster although Cole and Batzli (1979) and Getz et al. (1979) did report differences with habitat in their studies.

Peromyscus are generalists and favor landscapes with less cover and avoid high cover vegetation such as brome monocultures (Henderson and Sample 1995, Getz and Hofmann 1999, Heske 1999, Stone 2007). Such traits predict that these mice would be most common early in the establishment of the grassland reconstructions. It is possible the disappearance of Peromyscus from the site reflects the changes in the vegetation composition, density and height, although they were not caught in large numbers at any time during the study. A shift in dominance from Peromyscus mice toward a community dominated by voles might be the expected trajectory for most grassland reconstruction projects in the Midwest (Mulligan et al. 2013).

In summary, we did not find significant differences in species composition or abundance between warm season or cool season grass reconstructions or forb versus grass dominated plots. In our location, Microtus voles, particularly M. pennsylvanicus, dominated all reconstruction types. Cool season grasses supported a population with a greater body mass but not a greater number of individuals, at least not during the spring period when trapping was done. It is unknown whether the increased body mass corresponded to increased survivorship or fecundity later in the season. In contrast to other studies, we did not find clear habitat preferences between M. pennsylvanicus and M. ochrogaster, instead finding both species within all planting types. The vegetation types used in this planting may have lacked differences significant to voles, although clear differences have been reported among other taxa using these seed mixes at this location (Port and Schottler 2017) and other locations (DeGolier et al. 2015, Schottler et al. 2008).

Small mammals, especially grassland species such as Microtus voles and Peromyscus mice, remain key components of grassland ecosystems and serve as indicators of ecosystem stability and health (Clark et al. 1989, Stone 2007), and attract various predators to restored grasslands (Schmidt et al. 2008). While it may not always be practical to increase the plant diversity of seed mixes used to create these grassland ecosystems, care must be taken when reconstructing these grassland ecosystems to consider needs for these often-overlooked components of grasslands. Even if an area is small, we encourage managers to consider planting a diversity of habitat types, including some that may not be floristically rich but offer a different habitat structure. Our research, and that of others (Getz et al. 2005, DeGolier et al. 2015), suggests that while habitat requirements for many small mammal species are somewhat flexible, planting a diversity of habitats is superior for increasing animal biodiversity.