The Effect of Habitat Disturbance on Populations of White-Footed Mice (Peromyscus leucopus) in Northern Michigan

Alexandra Klimovitz

EEB 453 August, 2014

ABSTRACT We contrasted population size and demography of white-footed mice (Peromyscus leucopus) in a recently disturbed and undisturbed habitat in northern Michigan. Disturbed forest had a more open canopy than control forest (t = 14.8461, df = 142, p<0.0001), but there was no relationship between canopy coverage and the abundance of P. leucopus (R2

Disturbed = 0.00011, R2control = 0.00274). We caught a higher proportion of white-footed

mice in disturbed forest (68.83%, n = 53) in comparison to control forest (31.17%, n = 24; X2 = 10.922, df = 1, p = 0.0001). We found an unexpectedly high ratio of male P. leucopus in disturbed areas (observed = 34, expected = 30.1), and a high ratio of females in the control (observed = 14, expected = 10.1). There was a marginally significant difference in this ratio of sexes found in the disturbed versus control forest (X2 = 3.79, df = 1, 0.1>p>0.05). Similarly, we found a marginally significant difference between proportion of ages in individual P. leucopus captured at each site, however we observed approximately the same proportion of individuals that we expected in each age class (X2 = 0.278, df = 1, 0.1>p>0.05). We did not find a difference in the reproductive status or weight of P. leucopus captured at each site (reproductive status: X2 = 0.3887, df = 1, p = 0.3887; weight: t = 0.5468, df = 18, p<0.5912). Our data suggest that disturbance level plays an important role in the population density, sex, and age of P. leucopus.

INTRODUCTION

A major focus in conservation biology examines the response of flora and fauna

to habitat loss and disturbance (Harris, 1984). A significant portion of forest biomass in

the United States has been lost in recent years and it has become urgent to study the

implications of disturbance on the distribution of native species. (Hansen et al, 2013).

This study serves to examine how local small mammal populations are affected by

disturbances in the northern lower peninsula of Michigan.

We selected the white-footed mouse (Peromyscus leucopus) as our study subject

because of its high abundance in northern Michigan. This ecosystem engineer serves as a

significant seed distributer and important prey for many predator populations in the area

(Marshall et al. 2012). It thrives in many habitats including deciduous woodland, grassy

areas, shrubby borders, and even fencerows (Kurta, 1952). Small mammals such as the

white-footed mouse are important indicators of sustainable forest practices due to their

characteristic response to disturbance (Pearce, 2005).

In 2008 researchers working at The University of Michigan Biological Station

(UMBS) began a project to study the role of disturbance and succession in carbon

exchange processes (Curtis et al.). This project, called the Forest Accelerated

ExperimenT (FASET), aims to accelerate the successional transition of an even-aged

aspen-dominated forest to an uneven-aged mixed-deciduous forest (Curtis et al.).

Researchers girdled all aspen (Populus grandidentata/tremuloides) and birch (Betula

papyrifera) trees within a 33 ha treatment stand in order to expedite the death of these

early successional species (Curtis et al.). We chose this study site as a representation of

recently disturbed forest and compared it to a nearby undisturbed forest with a similar

species composition. All forest in this study was subject to intense logging and fire in the

early 1900's thereby allowing us to control for age (Nadelhoffer et al. 2010).

Our goal here is to examine the differences in P. leucopus populations between a

disturbed forest area (FASET) and an undisturbed forest area (control). We intended to

document an early consequence of the girdling and subsequent death of the aspen and

birch overstory. We measured the canopy coverage and recorded the characteristics of

live-trapped P. leucopus in each study site. We hypothesized that 1) we would observe a

more open canopy in FASET owing to disturbance, 2) this difference in canopy coverage

would be associated with differences in capture frequency of P. leucopus, with a greater

abundance of P. leucopus caught in the FASET plot, and 3) we would see differences in

sex, age, reproductive status, and weight of P. leucopus at each study site.

METHODS

Study Site Description

We designated four transects in the FASET forest. We set twenty traps in each

transect approximately 10 meters apart. Transects began at the FASET tower (45.562965,

-84.695129) and extended outward in a semicircular fashion for 200 meters (Fig. 1). We

designated eight transects in the control forest for comparison (45.562228, -84.689829).

Transects were approximately 15 meters apart and extended 80 meters outward from the

road (Fig. 1). We set 8 traps in each control transect approximately 10 meters apart. A

total of 80 traps was set in FASET and 64 traps in the control plot.

Notable tree species present in the FASET forest included dead Paper Birch

(Betula papyrifera), Bigtooth Aspen (Populus grandidentata) and Trembling Aspen

(Populus tremuloides), in addition living Hemlock (Tsuga canadensis) and Red Oak

(Quercus rubra; Fig. 2). The control forest represented the FASET forest prior to

disturbance and included the same tree species, except that Paper Birch and Bigtooth

Aspen were still living.

We estimated the canopy coverage at each trap location with a GRS Densitometer

using the provided vertical sampling methods (Geographic Resource Solutions, 2011).

Trapping Procedure

We trapped P. leucopus in the FASET and control plots for 72 consecutive hours

from July 28th, 2014 to July 31st, 2014. We used 144 (22cm x 7cm x 8cm) Sherman live-

capture traps (Tallahassee, FL) baited with approximately 1 teaspoon of oats. Students

checked traps three times per day (9am, 1pm, 7pm) and brought all trapped individuals

back to the classroom. We replaced and rebaited traps as needed.

Processing and Releasing Procedure

We held all trapped individuals captive with food and an apple slice in the

classroom. We determined the species, sex, age, weight (g), and reproductive status (e.g.

scrotal or abdominal testes; whether nipples were visible, signifying lactation) of each

individual. Additionally, we clipped a small patch of fur on the rear of each individual in

order to allow us to distinguish between previously and newly captured animals. We

released all captured individuals at the location they were trapped when the next group

checked the traps.

Analysis of Data

Using IBM SPSS Statistics we performed a two-tailed t-test to examine the

differences in percent canopy coverage between the FASET and control forests. All

statistical tests relating to trapping used adjusted values to account for the different

number of traps. FASET contained 80 traps whereas control only contained 64, thus we

multiplied all control values by 1.25. We rounded fractional adjusted numbers up. We

executed all statistical tests under the assumption that P. leucopus were equally as easy to

catch in both study areas.

We performed a chi-square test without Yates’ correction using GraphPad

Software ® Quick Calcs to test for differences in proportions of captures, sex, age, and

reproductive status of P. leucopus between the FASET and control forests. We used

linear regression models to examine the relationship between percent canopy coverage

and the number of white-footed mice caught at each site. We performed a two-tailed t-

test using IBM SPSS Statistics to contrast the weight of individuals caught at each site.

RESULTS

We trapped consecutively for 3 days for a 432 trap total in both sites. We caught

90 total P. leucopus individuals of which 18 were recaptures. We captured 53 total P.

leucopus in FASET, 12 of which were recaptures. We captured 24 total P. leucopus in

control forest, 8 of which were recaptures (adjusted).

The FASET plot had a more open canopy (coverage at FASET = 41.75%) than the

control forest (coverage at control = 89.22%; t = 14.8461, df = 142, p<0.0001; Table 1).

There was, however, no significant relationship between the percent canopy coverage and

the number of P. leucopus caught in the FASET plot (R2 = 0.00011; Fig. 3) or the control

plot (R2 = 0.00274; Fig. 4).

We caught a higher proportion of white-footed mice in FASET (68.83%, n = 53)

than in the control, even after adjusting for the difference in the number of traps set

(31.17%, n = 24; X2 = 10.922, df = 1, p = 0.0001; Table 2). We found an unexpectedly

high ratio of male P. leucopus in FASET (observed = 34, expected = 30.1) and an

unexpectedly high ratio of females in the control (observed = 14, expected = 10.1; Table

3). However, there was only a marginally significant difference in the ratio of sexes

found in FASET versus control (X2 = 3.79, df = 1, 0.1>p>0.05; Table 3). We found a

marginally significant difference between the proportion of each age class of individual

P. leucopus captured at each site, however we observed approximately the same number

of individuals that we expected in each age class (X2 = 0.278, df = 1, 0.1>p>0.05; Table

4).

The proportion of reproductively active mice on FASET did not differ

significantly from the proportion on the control (X2 = 0.743, df = 1, p = 0.3887; Table 5).

Additionally, The weights of P. leucopus individuals captured at each site did not differ (t

= 0.5468, df = 18, p<0.5912; Table 6).

DISCUSSION

Canopy coverage was significantly different in FASET versus the control forest,

consistent with hypothesis (1) that disturbance would result in a more open canopy.

While abundance of P. leucopus was higher in the more disturbed FASET forest, this was

not associated with canopy coverage. Our results were not consistent with hypothesis (2).

Relative disturbance level had only a marginally significant effect on sex and age, and no

effect on the reproductive status or weight of P. leucopus. These results were not

consistent with hypothesis (3), that differences between the two sites would be observed.

Many prior studies have shown that the extent of vegetation cover is negatively

correlated with the level of predation (Bechard, 1982; Monamy & Fox, 2000).

Nevertheless, in both FASET and control plots we found no relationship between

abundance and coverage. This observation suggests that a different relationship may exist

in FASET that is influencing the abundance of white-footed mice.

The procedure of FASET may indicate that the open canopy coverage is a result

of many fallen trees. Zollner and Crane (2003) explored the relationship between canopy

coverage and travel along coarse woody debris (CWD) and found a negative association

between the two. While we did not measure the amount of CWD present on the plots, the

number of recently dead aspen and birch on the FASET plot probably also indicates a

large amount of CWD. This may favor larger populations of mice because they prefer to

travel along logs while foraging (Graves et al, 1988). Future studies in FASET should

incorporate a measure of the abundance of CWD and test for a relationship with mouse

population size.

It is possible that a combination of open canopy coverage and coarse woody

debris affect the populations of P. leucopus; however, the results of our study were

limited by a small sample size over a short period of time. We recommend that longer

time scales be used in further studies of this disturbed forest.

Populations of P. leucopus thrive in disturbed areas (Linzey, 1988). Our results

are consistent with these findings and suggest that disturbance in forest habitats will

positively impact these populations (Linzey, 1988).

While it is possible that no relationship exists between disturbance level and P.

leucopus sex, our values were approaching significance in showing a preference for male

individuals in FASET. Disturbed habitats tend to have higher representations of male P.

leucopus (Linzey, 1988). This suggests that by expanding the time scale and sample size

of a future study we may see a difference in the ratios of male to female and mature to

immature P. leucopus among study sites.

Contrasting to our results, P. leucopus from disturbed habitat typically weighs

more than mice from undisturbed forest (Kaminski et al, 2007). This is due to the greater

structure in habitat of disturbed areas which provide a safe area for foraging (Kaminski et

al. 2007; Monamy & Fox, 2000). One may also argue that these factors should also lead

to increases in the ages of small mammals in disturbed areas. These individuals should be

able to avoid predation and live longer lives, thus increasing the frequency of adults.

However, our results showed no association between age and disturbance.

On several occasions we found traps set off and overturned without an animal

inside. We attributed these disturbances to raccoons, which were likely attracted to the

bait. This may have altered our capture frequency, however we were unable to quantify

this influence.

Our conflicting results with prior research illustrate the importance of studying

forest ecosystems. Disturbance as a result of climate change and humans will continue to

effect forest dynamics and white-footed mice populations, and it is crucial to distinguish

whether the responses will be negative or positive.

FIGURES

Figure 1. Satellite map of study site including locations of all traps in FASET and Control sites.

Figure 2. Cover types in FASET and surrounding forest prior to girdling (Curtis et al.).

UMBS

Table 1. Two-tailed t-test comparing the percent canopy coverage between FASET and control forests. Figure shows a significant difference in the canopy coverage between the two sites, with the FASET forest having a more open canopy. Densiometer FASET Control Mean 0.4175 0.8922 SD 0.2385 0.1028 S.E. of mean 0.0267 0.0129 N 80 64 t value 14.8461** **p<0.0001

Figure 3. Linear regression model comparing the number of P. leucopus caught in FASET versus the percent canopy coverage.

Figure 4. Linear regression model comparing the number of P. leucopus caught in the control plot versus the percent canopy coverage. Table 2. Cross tabulation of proportion of P. leucopus individuals captured between FASET and control forests. Percentages calculated and noted in parentheses.

Total Captures FASET Control Total Observed 53 (68.83) 24 (31.17) 77 (100) Expected 38.5 (50) 50 (38.5) 77 (100) Chi Square 10.922** **p=0.001 Table 3. Cross tabulation of male and female P. leucopus individuals captured between FASET and control forests. Observed/expected values reported. Percentages calculated using observed values and noted in parentheses. Sex FASET Control Total Male 34/30.1 (77.27) 10/13.9 (22.73) 44 (100) Female 18/21.9 (56.25) 14/10.1 (43.75) 32 (100) Chi Square 3.79*** ***p=0.0516 Table 4. Cross tabulation of proportion of immature and mature individual P. leucopus captured between FASET and control forests. Observed/expected values reported. Percentages calculated and noted in parentheses. Age FASET Control Total Immature 32/33 (66.67) 16/15 (33.33) 48 (100) Adult 21/20 (72.41) 8/9 (27.59) 29 (100) Chi Square 0.278*** ***p=0.0516 Table 5. Cross tabulation of proportion of reproductively active and inactive individual P. leucopus captured between FASET and control forests. Percentages calculated and noted in parentheses. Reproductive Status FASET Control Total Active 25 (73.53) 9 (26.47) 34 (100) Adult 27 (64.29) 15 (35.71) 42 (100) Chi Square 0.743*** ***p=0.3887 Table 6. Two-tailed t-test comparing the weight (g) of individual P. leucopus captured between the FASET and control forests. Weight (g) FASET Control Mean 18.653 18.132 SD 4.065 5.456 S.E. of mean 0.587 1.247 N 49 19 t value 0.5468*** ***p=0.5912

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