Demographic Responses of Myomorph Rodents to Mast
Production in a Beech- and Birch-dominated Northern
Hardwood Forest
Christopher A. Conrod and Leonard Reitsma
Northeastern Naturalist, Volume 22, Issue 4 (2015): 746–761
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C.A. Conrod and L. Reitsma
22001155 NORTHEASTERN NATURALIST 2V2(o4l). :2724,6 N–7o6. 14
Demographic Responses of Myomorph Rodents to Mast
Production in a Beech- and Birch-dominated Northern
Hardwood Forest
Christopher A. Conrod1,2,3 and Leonard Reitsma1,2,*
Abstract - Myomorph rodents play important roles in trophic systems and can have rapid
population-level responses to food pulses, such as mast. The purpose of our study was to
measure such responses and record potential interactions among the rodent species in a
northern hardwood forest. We used mark–recapture methods to estimate abundances of 3
myomorphs commonly found in northern hardwood forests—Peromyscus spp. (deer mice,
hereafter, Peromyscus), Napaeozapus insignis (Woodland Jumping Mouse), and Myodes
gapperi (Red-backed Vole)—over 2 years (2006 and 2007). Seedfall was measured concurrently.
The abundance of Peromyscus and Red-backed Voles substantially increased in
response to the 2006 mast, which was the highest in 7 years of continuously recorded data at
Hubbard Brook Experimental Forest. Adult-mean weights of all 3 species were higher during
the spring following mast production than during the preceding spring. Following these
responses to the mast and during the subsequent lean summer of 2007, Woodland Jumping
Mice completely disappeared from the study area, mean Peromyscus weight dropped to
its lowest level during the study, and reproductive activity of Peromyscus and Red-backed
Voles substantially declined. Woodland Jumping Mice likely dispersed from the area in response
to interference competition from a substantially increased Peromyscus population.
These pulses in food, particularly the larger seeds of Fagus grandifolia (American Beech),
and the consequent population responses corroborate the patterns found by others and may
influence the food webs of these northern-hardwood forest communities beyond the trophic
level of granivores.
Introduction
Rodents comprise a significant portion of the small-mammal community in
northern hardwood forests, and most, if not all, forest rodents include nuts and
seeds in their diet. Moreover, rodents benefit tree populations through ecological
associations such as seed dispersal (Vander Wall 2002) and symbiotic-fungi
dispersal (Terwilliger and Pastor 1999). In turn, rodent-abundance responses to
masting can regulate predator populations (Fryxell et al. 1999, King 1983, Ostfeld
and Keesing 2000) and invertebrate prey populations (Elkinton et al. 1996). Consequently,
granivorous rodents form an important guild that responds to a bottom-up
control and broadcasts trophic reverberations throughout the forest community.
Numerous studies have investigated the response of myomorphs to pulsed
mast–production. Gashwiler (1979) and Falls et al. (2007) performed long-term
1New England Institute for Landscape Ecology, 266 Prospect Hill Road, Canaan, NH 03741.
2Plymouth State University, 17 High Street MSC #64, Plymouth, NH 03264. 369 Perkins Lane,
PO Box 17, South Tamworth, NH 03883. *Corresponding author - leonr@plymouth.edu.
Manuscript Editor: Michael J. Cramer
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C.A. Conrod and L. Reitsma
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demographic studies of Peromyscus spp. (deer mice) in coniferous and Acer sp.
(maple) forests, respectively. Myodes gapperi (Vigors) (Red-backed Vole) and
Peromyscus populations have been studied in relation to Pinus sp. (pine), Quercus
(oak), and maple mast production (Elias et al. 2006, McCracken et al. 1999, Schnurr
et al. 2002). Other multi-consumer studies have been conducted in oak-dominated
forests (McShea 2000, Wolff 1996). Fewer studies, however, have focused upon
Fagus sp. (beech)-dominated forests. Jakubas et al. (2005) studied the demographic
responses of Martes americana (Turton) (American Marten) and Ursus americanus
(Pallas) (Black Bear) to Fagus grandifolia Ehrh. (American Beech) mast production.
Studies in New Zealand (Choquenot and Ruscoe 2000, King 1983) examined
Nothofagus (southern beech) forests, but the effect of American Beech mast production
on granivorous consumer populations has received less attention, and has been
mainly limited to qualitative observations in early literature (e.g., Merriam 1884,
Williams 1936).
In this study, we monitored the populations and demographic parameters of 3
species in the suborder Myomorpha before, during, and after a pulsed-mast production
in a northern hardwood forest with co-dominating American Beech, Betula
alleghaniensis Britton (Yellow Birch) and Acer saccharum Marsh. (Sugar Maple).
We determined rodent abundances prior to seedfall, identified changes in abundance
and demographic parameters, and determined the timing of population responses
relative to mast production. Based upon conclusions of previous studies performed
in other forest community types (e.g., Falls et al. 2007, Ostfeld et al. 1996), we expected
significant increases in populations of myomorphs during the year following
a pulsed seedfall of Sugar Maple and American Beech. We also predicted that these
increases would be accompanied by changes in density-dependent demographic
variables including mortality and fecundity.
Field-Site Description
We conducted our study at Hubbard Brook Experimental Forest in central New
Hampshire. The site is a 3160-ha research forest entirely within the White Mountain
National Forest defined by the limits of the Hubbard Brook watershed, approximately
centered at 43º56'N 71º45'W. Elevations vary between 200 m and 1000 m.
Annual precipitation averages ~1400 mm and is evenly distributed throughout the
year (Bailey et al. 2003). About 25–33% of precipitation falls as snow.
With the exception of a few isolated, small watersheds beyond the range of this
study, the entire area is forested. Forest composition is 80–90% northern hardwoods
with 3 co-dominant species of roughly equal abundance: Sugar Maple, American
Beech, and Yellow Birch. Stands of Tsuga canadensis (L.) Carr. (Eastern Hemlock)
occur in the low areas and are occasionally mixed in with the hardwoods. Picea
rubens (Sarg.) (Red Spruce) and Abies balsamea (L.) Mill. (Balsam Fir) dominate
at higher elevations. Our study area was extensively logged in the late 19th and early
20th centuries. Following cessation of logging around 1917, forest disturbance has
been limited to natural causes, primarily ice storms and wind events. Overall, these
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2015 Vol. 22, No. 4
natural disturbances have had minimal effect on forest composition and structure
(Leak and Smith 1996, Peart et al. 1992).
Methods
We established four 72-station trapping grids east of Falls Brook at elevations
of 440–515 m. We labeled grids based on relative compass direction (NW, NE, SE,
SW), with each grid arranged in a 7 x 7 configuration with 25-m spacing between
stations, resulting in a 150-m square grid. We placed one 8 cm x 9 cm x 30 cmlong-
bridge galvanized Sherman trap at each of the 49 stations. We established 23
intermediate stations at random locations within each grid and equidistant between
primary stations, creating a 5 x 7 interior grid with 12.5-m spacing. We placed one
5.5 cm x 6.5 cm x 17 cm galvanized Sherman trap at each intermediate station to
increase the probability of capturing the broadest range of size classes possible. We
provided all traps with synthetic nesting material and covered them with a cedar
shingle for rain protection. Each of the 4 grid systems including both size classes
of traps constituted a multi-grid.
We placed 25 seed collectors within each of the 4 grids to collect American
Beech, Sugar Maple, and Fraxinus americana L. (White Ash) seeds. We based our
collector design on the non-winter seed trap described in Graber and Leak (1992)
and used a systematic random placement protocol. On 25 of the 36 square plots
defined by the primary stations in each grid, we placed a collector 3 m NE of the
closest American Beech tree with dbh ≥ 15 cm and closest to the SW corner of the
plot. We assigned numbers to plots and collectors corresponding to the station on
the SW corner of the plot. On plots containing no American Beech trees, we placed
the collector 3 m NE of the SW corner of the plot. This method maximized the probability
of collecting an adequate sample of American Beech seeds, while allowing
wind-blown Sugar Maple and White Ash seeds to also be collected.
We installed the collector array on the NW grid on 5 October 2005 and checked
it on 3 November 2005. We set out the collectors on the remaining 3 grids in early
June 2006. We gathered seeds in all collectors each year (2006 and 2007) in early
October and early November to coincide with the sample period for the NW grid
in 2005. We counted all American Beech, Sugar Maple, and White Ash seeds, and
thus generated a relative yearly seed-fall index, which we added to long-term finelitterfall
data collected by Fahey (2006) as a measure of changes in available food.
We ran trapping sessions in late May–early June (spring) and late August–early
September (summer) in 2006 and 2007. As bait, we distributed sunflower seeds
near the trap opening at each station 1–2 d prior to each trapping session. Each
session began at 18:00 and continued through 6 nights, ending on the 6th morning,
except the summer 2007 sessions for the SW and NW grids, which ended on the 5th
morning. Traps were continuously open, and we checked them every 12 h, morning
and evening. We recorded species, sex, reproductive status (by appearance of mammae
for females and location of testes for males), weight, and station number for
all myomorphs captured—Peromyscus, Woodland Jumping Mice, and Red-backed
Voles. We marked all animals with a size-1 steel small-ear tag (National Band and
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Tag Company, Newport, KY) at initial capture. Field distinction between P. maniculatus
(Wagner) (Deer Mouse) and P. leucopus (Rafinesque) (White-footed Mouse)
was not possible because we did not record ear lengths (Ridenhour and Cramer
2015, Stephens et al. 2014); therefore, we grouped all Peromyscus spp. captures
together and identified them only by genus.
We determined rodent abundances using the methods of Otis et al. (1978). We
used the program CAPTURE (Rexstad and Burnham 1992) to analyze raw data.
We then employed the best-fit models suggested by the program to calculate abundance
estimations. In the one instance in this study where the best-fit model had no
associated estimator (Mth), we used the jackknife estimator because it is the most
robust of the 5 estimators (White et al. 1982). We made abundance estimations only
for adult Peromyscus, Red-backed Voles, and Woodland Jumping Mice because
capture probabilities for juveniles varied considerably. For this study, we defined
adult mice as all individuals ≥14 g, the lowest weight at which sexual maturity was
common during most trapping sessions. We did not use pelage color to differentiate
between age classes because the gradation of change in the Peromyscus subadult
pelage provided no distinct demarcation point and we observed no definitive color
changes in Red-backed Vole or Woodland Jumping Mouse.
We inventoried trees ≥15 cm dbh in the 4 grids. We chose this threshold because
significant seed production occurs in individuals at this diameter and larger for most
tree species (Garrett and Graber 1995, Houle 1999, Leak and Graber 1993). We did
a full inventory on the SW grid. We recorded species, dbh, and plot number (using
the nomenclature system described above) for all individuals. On the NW, NE, and
SE grids, we subsampled using point-quarter methods. We chose a random point for
each plot (36 per grid), sampled the nearest tree in each compass quadrant around
each sample point, and recorded species, dbh, distance from point, and plot number.
We compared tree composition between trapping grids. We determined importance
values for trees of mast-bearing age (≥15 cm dbh) using the point-quarter data on
relative values of density, sample frequency, and basal area.
We compared rodent demographic parameters between trapping sessions and
between trapping grids. Adult recruitment during seasonal reproduction resulted in
differing age-class structures between seasons; thus, we limited mean-weight comparisons
to spring–spring and summer–summer except where recaptured individuals
provided an adequate sample size. We conducted all demographic statistical tests in R
version 2.11.0 (R Development Core Team 2010). We used Student’s t (t.test) to compare
means. Exact binomial tests (binom.test) determined 95% confidence intervals
for proportions of reproductively active rodents, and we compared these proportions
using a 2-proportion test (prop.test). We used Pearson’s correlation coefficient (cor.
test) for comparisons between vegetative and demographic parameters.
Results
Vegetation and mast production
We determined that American Beech and Yellow Birch were canopy co-dominants
in 3 of the multi-grids (Table 1). Sugar maple was dominant in the NE grid,
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and American Beech and White Ash composed the majority of sub-dominant trees.
Coniferous trees, primarily Eastern Hemlock, were significant canopy components
in the NW and SW grids, Eastern Hemlock was a co-dominant in a small portion of
the SE grid, and coniferous trees were rare in the NE grid.
Deciduous mast production was extremely low from 2003 through 2005 (Fig. 1).
A significant seedfall—the largest from 7 y of available and reliable data—occurred
in 2006 and represented a 14-fold increase over the previous 3 y. American Beech
seeds were the major component of mast production in 2006 in the SW, NW, and
SE grids (Table 2). Sugar Maple seeds were the predominant mast in the NE grid.
White Ash seed production on all grids was insignificant. There were strong correlations
among trapping grids between seedfall and tree species importance values
and basal area. However, importance values were statistically significant only for
American Beech, and basal area was significant only for Sugar Maple (Table 2).
Seed production was very low in 2007.
Myomorph abundance
Red-backed Voles were common but not abundant throughout this study
(Fig. 2). Grid abundances consistently varied between less than 10 in the spring and ~20
in the late summer, except for the NW grid where we caught no voles during the
spring of 2006 and the most voles during the summer 2007 (n = 34). However,
total spring abundances were more than 3 times higher in 2007 (n = 23) compared
to 2006 (n = 7). Total 2007 estimates showed a weak and non-significant
Table 1. Relative dominance and importance values of the 6 most abundant tree species in our study
plots based on density, frequency in sample plots, and basal area.
Species Relative dominance Importance
SW Grid
Yellow Birch 41.4 95.3
American Beech 24.3 85.6
Eastern Hemlock 19.5 58.7
Sugar Maple 10.1 37.5
NW Grid
American Beech 21.3 91.3
Yellow Birch 37.9 85.7
Eastern Hemlock 23.0 60.1
Red Spruce 10.1 34.8
SE Grid
Yellow Birch 33.3 93.8
American Beech 25.4 85.3
Sugar Maple 23.5 66.4
Eastern Hemlock 8.8 28.9
NE Grid
Sugar Maple 40.6 107.2
American Beech 17.7 78.6
White Ash 26.1 60.3
Yellow Birch 12.9 42.2
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correlation to American Beech seedfall (r = 0.894, P = 0.106); this correlation
was absent in 2006.
Woodland Jumping Mice were common in the study area during 2006 and
they were present on all grids from spring 2006 through spring 2007. Spring 2006
abundances ranged from less than 10 on the NW and NE grids to 24 on the SE grid and 47
on the SW grid. There was a more-even distribution during the summer of 2006,
with individual-grid abundance estimates between 44 and 57. Spring 2007 abundance
estimates on the SW and SE grids were significantly lower than spring 2006
Table 2. 2006 mast production of American Beech and Sugar Maple compared to importance values
and basal area by grid; correlations between sample pairs of all grids are shown as Pearson’s coefficient
(r).
Importance:seedfall Basal area:seedfall
Relative basal Seedfall
Importance area (cm2) (seeds/trap) r P r P
American Beech
NW 91.3 44,209 4.20 0.999 0.001 0.903 0.097
SW 85.7 28,445 3.00
SE 85.3 30,649 2.92
NE 78.7 24,539 1.36
Sugar Maple
NW 37.5 11,773 0.28 0.939 0.061 0.967 0.033
SW 23.6 8100 0.28
SE 66.4 28,372 0.80
NE 107.2 56,417 3.40
Figure 1. Annual mast
production in the vicinity
of Bear Brook, Hubbard
Brook Experimental Forest,
for American Beech,
Sugar Maple, and White
Ash. Data are from Fahey
(2006) for 2001–2005 and
from this study 2006–2007
reported in identical units.
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abundances and more in line with NW and NE grid estimates, which showed little
variation between the 2 years (Fig. 2). Woodland Jumping Mice were completely
absent from all grids during the summer 2007 trapping session. Overall, Woodland
Jumping Mouse abundance increased 250% during the summer of 2006, fell to 50%
of the previous spring abundance in the spring of 2007, and disappeared within the
next 3 months.
Peromyscus was present on all 4 grids throughout the study (Fig. 2). The combined
abundance in all grids increased throughout 2006 and 2007, with significant
reproduction occurring between late summer 2006 and the following spring. Mean
monthly growth rates ([Nt - Nt-1]/ Nt-1 divided by number of months since last trapping
session) were higher for the summer of 2006 than the summer of 2007 (0.57,
0.17, respectively). However, Peromyscus populations peaked on the NE grid in
late 2006 (n = 92) and on the SW grid in the spring of 2007 (n = 135). Given the
reduction in numbers between the fall and spring capture sessions, the NE grid had
a winter decline in abundance before rebounding during the spring and summer
of 2007. There was no significant change in abundance on the SW grid during the
summer of 2007.
Demographic parameters
Red-backed Vole spring mean adult weight increased 14% from 22.3 g in 2006
to 25.5 g in 2007, but this increase was not significant (t = -1.69, df = 25, P = 0.10;
Figure 2. Adult-myomorph abundance by grid and trapping session. Error bars are 95%
confidence limits calculated in program CAPTURE. P = Peromyscus spp., M = Mygodes
gapperi (Red-backed Vole), and N = Napaeozapus insignis (Woodland Jumping Mouse).
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Fig. 3). Fecundity parameters were stable throughout the first 3 trapping sessions
but dropped in summer 2007 (Fig. 4). The frequency of scrotal males declined by
50% (χ2= 20.190, df = 1, P < 0.001). A 38% decline in pregnant/lactating females
was not statistically significant (χ2 = 0.856, df = 1, P = 0.178).
Woodland Jumping Mouse adult mean weights were 15% higher in spring 2007
compared to the previous spring, significantly increasing from 19.9 g to 22.9 g
(t = -5.04, df = 87, P < 0.001; Fig. 3). Apparent increases in spring scrotal males
(χ2 = 1.666, df = 1, P = 0.098) and pregnant/lactating females (χ2 = 2.081, df = 1, P =
0.075) were not statistically significant (Fig. 4). The male/female ratio for spring
2007 was 2.4:1, significantly higher than the 0.71:1 ratio of the preceding seasons
(χ2 = 3.139, df = 1, P = 0.038).
Peromyscus adult mean weights significantly increased between spring 2006 and
spring 2007 (t = -2.04, df, = 283, P = 0.042), but declined between spring and summer
2007. Mass of 26 adult males recaptured in summer 2007 declined from a mean
spring weight of 20.1 g to a mean summer weight of 16.9 g (paired t-test; t = -7.38,
df = 25, P < 0.001), a 16% decline in weight over 3 months. Overall, the mean summer
2007 weight for Peromyscus adults was 1.5 g lower than the previous summer
(t = 5.15, df = 464, P < 0.001; Fig. 3). Fecundity remained stable throughout 2006 and
spring 2007, although fecundity substantially decreased in summer 2007 (Fig. 4).
Scrotal males declined by 83% (χ2 = 99.144, df = 1, P < 0.001) and lactating/pregnant
females declined by 93% (χ2 = 58.823, df = 1, P < 0.001). The 4 lactating females
captured in summer 2007 each had 4 functioning mammae, compared to the usual
Figure 3. Mean weights of Peromyscus spp., Red-backed Vole, and Woodland Jumping
Mouse from spring of 2006 through summer of 2007. Error bars are 95% confidence limits
calculated from Student’s t.
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8 observed in all other trapping sessions. The sex ratio remained relatively constant
throughout the study despite changes in Peromyscus abundance. Females outnumbered
males in all sampling periods except fall 2006. We observed perecentages of
males as follows: spring 2006 = 46.9%, n = 81; fall 2006 = 52.2%, n = 184; spring
2007 = 42.3%, n = 265; and fall 2007 = 46.8%, n = 410.
Discussion
Myomorph abundances and species composition significantly changed apparently
as a result of the dramatic seedfall in summer of 2006. We observed a substantial
Figure 4. Reproductive status of Peromyscus spp. and Myodes gapperi (Red-backed Vole)
from spring 2006 through summer 2007. Error bars are 95% confidence limits calculated
by exact binomial test.
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increase in Peromyscus abundance, and the disappearance of Woodland Jumping
Mice. Demographic parameters showed an increase in both abundance and growth
rates after seedfall for Peromyscus and Woodland Jumping Mouse. However,
growth and reproduction declined for Peromyscus and Red-backed Vole during the
summer following seedfall.
Red-backed Voles
McCracken et al. (1999) found no Red-backed Vole abundance response to masting
of Quercus rubra L. (Red Oak), Pinus strobus L. (White Pine), or Acer rubrum
L. (Red Maple) in Maine, but they reported that adult female Red-backed Voles had
higher mean spring weights following large acorn crops, which the authors attributed
to a high percentage of pregnant females in their study population. Using an
extended dataset (22 vs. 10 years) from the same long-term study in Maine, Elias et
al. (2006) found that Red-backed Vole populations were synchronous with those of
Peromyscus leucopus (Raf.) (White-footed Mouse) and normally peaked the year
after large White Pine seed-crops, but they found no significant associations with
Red Maple or Red Oak. Based on 2 years of abundance estimates in northwestern
Connecticut, Schnurr et al. (2002) reported an increase in Red-backed Vole abundance
the year of a high Red Maple seedfall (late spring–early summer seedfall) and
no association with Red Oak mast production.
In this study, the higher abundance of Red-backed Voles in the spring of 2007
than in 2006 and the correlation between American Beech seedfall and relative grid
abundance in 2007 suggest a minor response to mast production. Although demographic
parameters for the spring of 2007 showed an increase in mean adult weight
and female fecundity, the small numbers of individuals reduced the likelihood of
detecting statistically significant differences.
Our results offer no explanation as to why the abundance of Red-backed
Voles in the summer of 2007 did not surpass that of the previous year. Fecundity
measurements from the summer of 2007 are typical of a population in decline.
Red-backed Vole and Peromyscus populations are known to exhibit synchrony
(Brooks et al. 1998, Elias et al. 2006, Jameson 1949, Merritt et al. 2001, Miller
and Getz 1977), which makes it unlikely that interspecific competition would be a
factor in the decline. The low relative density of Red-backed Voles throughout the
study period may indicate a lack of optimal habitat, high predation pressure (see
Hanski et al. 2001 for a discussion on a stabilizing effect of generalist predators
on vole populations), or a combination of the 2 factors, thus limiting any response
to an increased food supply. Our results and those of McCracken et al. (1999)
suggest that the response of Red-backed Voles to bursts of food resources is less
distinct than the response of Peromyscus and sciurids. Other factors may influence
and mitigate the response, and long-term studies involving multiple mast
events are required to definitively assess the association between Red-backed
Voles and pulsed mast-production.
Woodland Jumping Mouse
The disappearance of Woodland Jumping Mice during our study was not without
precedent; they are known to occasionally disperse en masse (Sheldon 1938,
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Wrigley 1972). Values for fitness parameters were at their highest in the spring
of 2007; thus, it is unlikely that limited availability of food resources caused the
emigration. It is more likely that interference competition was a causative factor.
The timidity and quick flight-response of Woodland Jumping Mice to agonistic
encounters is well documented (Brower and Cade 1966, Sheldon 1938, Wrigley
1972) and corroborated by observations during this study. Brower and Cade (1966)
and Wrigley (1972) noted complementary abundances of Red-backed Vole and
Woodland Jumping Mouse and attributed the occurrence to the aggressive behavior
of the Red-backed Vole, but abundance of the latter species did not change appreciably
during this study and it is unlikely they were the cause of mass dispersal of
Woodland Jumping Mice. The most significant change we observed between 2006
and 2007 on all study grids was a substantial increase in Peromyscus abundance.
Habitat and resource partitioning (Miller and Getz 1977, Orrock et al. 2003, Wrigley
1972), and complimentary abundances (Jameson 1949, Kirkland and Griffin
1974, Thibault 1969) of Peromyscus and Woodland Jumping Mouse offer the best
evidence explaining the mass dispersal. Wrigley (1972) proposed that the 2 mouse
species were able to co-exist through mutual avoidance. High Peromyscus density
such as we recorded in 2007 would have made avoidance extremely difficult. Orrock
et al. (2003) found that Woodland Jumping Mouse individuals adjusted their
dietary habits when Peromyscus was abundant, and Kirkland and Griffin (1974)
reported that Woodland Jumping Mice did not appear on 1 study site until Peromyscus
abundance had decreased by one third. Thibault (1969) observed a decrease in
Woodland Jumping Mouse activity as Peromyscus numbers increased. It is evident
that, at times of high abundance, Peromyscus populations interfere with those of
Woodland Jumping Mouse. This interference could have caused the mass dispersal
by Woodland Jumping Mice that we observed. Alternatively, Woodland Jumping
Mice may have switched to another food source, making the bait less attractive and
thus resulting in the lack of captures. Another possibility is that this species became
trap-shy. If mass dispersal does occur in the Woodland Jumping Mouse, a better
understanding of the causes and mechanics of this phenomenon is required before
any conclusions can be made.
Peromyscus
The population response of the Peromyscus to a pulsed seedfall of American
Beech and Sugar Maple is consistent with observed responses to masting of Red
Oak (McCracken et al. 1999, McShea 2000, Schnurr et al. 2002, Wolff 1996),
Pseudotsuga menziesii (Mirb.) Franco (Douglas Fir) (Gashwiler 1979), and Sugar
Maple (Falls et al. 2007). The early timing of the peak on the NE grid is likely a
function of the low American Beech seedfall relative to the other grids (Fig. 2).
However, American Beech seedfall volume does not account for the high abundance
of Peromyscus in the summer of 2006 relative to the other grids. Although
there was no evidence of an early Sugar Maple or White Ash seedfall on the NE
grid that could explain the spring–summer increase in abundance, the presence
of these 2 species as canopy dominants (Table 1) is indicative of an enriched soil
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containing a high amount of fine organic matter (Leak 1978). Such a condition
would likely lead to greater productivity in the herbaceous stratum and consequently,
a more diverse food resource during the growing season. We observed a
denser herbaceous stratum on the NE grid; particularly abundant were Uvularia
sessilifolia L. (Wild Oats) and Medeola virginiana L. (Indian Cucumber Root)
(C.A. Conrad, pers. observ.).
The increase in mean weight of Peromyscus after seedfall in the spring is consistent
with the findings of McCracken et al. (1999; Red Oak mast) and Falls et al.
(2007; Sugar Maple mast). Falls et al. (2007) observed decreased weight gain in
juveniles throughout the season following seedfall, which is consistent with our
supposition that Peromyscus individuals that we classified as juveniles by weight
(less than 14 g) in 2007 may have included a higher proportion of older mice than in the
previous year. Our fecundity measurements and abundance estimates following
seedfall indicate that reproduction slowed down during the winter but did not cease.
Falls et al. (2007) found that reproduction stopped and populations declined during
the winter after seedfall, but reproduction resumed earlier in the spring than in nonpeak
years. This difference can be attributed to differences in geography between
the sites of the two studies (latitude and continental position); a milder winter climate
allows reproduction when food resources are readily available. Consequently,
Hubbard Brook mice displayed a faster and uninterrupted population response to
seedfall compared to that measured by Falls et al. (2007) in Ontario. Gashwiler
(1979) found that Peromyscus could sustain reproduction throughout the winter
following a pulsed seedfall in Oregon, as did Wolff (1996) in Virginia.
The precipitous drop in fecundity and significant adult weight loss corroborates
the results of Falls et al. (2007). Although loss of fecundity in Peromyscus has been
associated with high population-density (Canham 1969, Rintamaa et al. 1976), the
Red-backed Voles were not at high density when fecundity declined. This circumstance,
in combination with a drop in Peromyscus weight, suggests that it was not
density alone that caused loss of fecundity but rather a density-dependent response
to low food-availability. Relative density on the 4 grids in summer 2007 varied by
as much as 33%, yet weight and fecundity dropped comparably on all grids.
Implications
The results of our study corroborate those of previous investigations of
myomorph responses to pulsed mast-production, and further demonstrates that
responses vary quantitatively, temporally, and demographically by species. Differences
in foraging strategies and food preferences can dictate the timing and
intensity of responses, both positive (population growth, fitness) and negative (loss
of fitness and population decline through loss of productivity and emigration). Responses
can be direct or indirect; for some species the response can be mediated by
interference competition.
The demographic responses of granivores to pulsed mast-production can reverberate
through other trophic levels in an ecosystem. Rodent responses to mast
production have been documented to influence numerical responses in predators
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2015 Vol. 22, No. 4
such as American Marten (Fryxell et al. 1999, Jakubas et al. 2005) and Mustela
erminea L. (Ermine; King 1983). Other studies have investigated egg predation by
granivores (McShea 2000, Reitsma et al. 1990). Mast consumption can also have
rebound effects on seed recruitment and forest composition (Schnurr et al. 2004).
Direct and indirect effects of an ecosystem perturbation such as pulsed mastproduction
can be either attenuated or amplified as they pass through successive
trophic levels (Yang et al. 2010). Within the granivore guild in this study, greater
understanding will come through more intensive focus upon the importance of
habitat associations, dietary and foraging behaviors, and interspecific and intraspecific
agonistic behavior.
Acknowledgments
L. Brown, M. Gagne, and M. Evans assisted with fieldwork. T. Boucher provided statistical
guidance. S. Bolton was adept at acquiring research supplies on short notice. The
folks at Hubbard Brook Experimental Forest and the Hubbard Brook Research Foundation
provided a study area in a supportive community. Funding for the study was provided in
part by the Biological Science Department of Plymouth State University and from a seed
grant through the PSU Center for the Environment.
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