Small rodent winter survival: snow conditions limit access to food resources

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Small rodent winter survival snow conditions
limit access to food resources

• Derek Leaderer

http//www.zoologi.no/patlas/pat_foto.htm
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Background

• Small rodent populations undergo cycles of growth
  and decline.
• Winter conditions limit food resources in a given
  area, lowering that regions carrying capacity.
  The result is that during the summer an increase
  in population may occur, but the small rodent
  population will likely be unable to be sustained
  come winter leading to a decline.
• The limited amount of food resources should make
  the over-winter survival dependent on its
  availability
• During the winter, many small mammals, including
  voles, reside in the subnivean. The
  characteristics of this environment allow for
  movement and foraging.
• Though it allows for this movement the subnivean
  is not an uniform habitat, but rather is broken
  into accessible and inaccessible regions.

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Sharp boundaries can be observed between ungrazed
and grazed areas of vegetation.
http//acreage.unl.edu/Newsletter/NLS/Feb2006.htm
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Hypothesis

• The objective of this experiment was to assess
  whether the snow cover limits the access to
  subnivean food by physically enclosing patches of
  vegetation and if this in turn reduces winter
  survival
• Korslund and Steen theorized that ice formation
  resulted in the existence of ungrazed patches.
• Additionally, they hypothesized that this ice
  formation reduces food availability and the
  amount of subnivean space limits vole space use,
  thereby affect over-winter survival rates.

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• Study Area
• Finse in south Norway, which is in the low- and
  mid-alpine zones (1300-1400 m. a.s.l.).
• The vegetation contains large numbers of sheep
  fescue (Festuca ovina) and matgrass (Nardus
  stricta)
• Both Norwegian lemmings (Lemmus lemmus) and root
  voles (Microtus oeconomus)
• The climate is alpine with cool summers and mild
  winters and heavy snow cover from late September
  to mid-June

http//www.photoseek.com/norway.html
http//www.nordicskiracer.com/Trails/2004/Norway.a
sp
http//www.seedman.com/Orngrass.htm
http//www.zoologi.no/patlas/pat_foto.htm
http//www.freewebs.com/punkbirder/finlandreport.h
tm
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• Experimental Design
• Four 40 x 40 m trapping grids were spaced 100-500
  m apart in September of 2002 on sites chosen for
  similarities in topography and vegetation.
• The grids remained open allowing for migrations
  and each grid contained 16 permanent trap
  stations arranged in a 4 x 4 grid and with 10 m
  spacing.
• Each trapping station consisted of a 20 x 40 cm
  extendable stainless steel trap chimney with a
  lid and an open base. The lid was insulated to
  keep the temperature consistent with the
  subnivean environment. A passive induced
  transponder (PIT)-tag antenna was placed near the
  chimney in order to record vole movements.
• Temperature loggers were placed through out each
  grid before the onset of winter in order to log
  ground surface temperatures throughout the
  winter.
• Two of the four grids contained a network of 1 m
  wide, 0.5 mm thick corrugated aluminum sheeting
  that connected all the trap stations.
• The sheets, which covered 15 of the grid,
  increased the total amount of subnivean space and
  prevented the formation of compact snow and ice
  on the ground.
• In order to achieve comparable population sizes
  among the four grids and winter onset densities
  similar to those observed during peak years.
  Resident voles were removed from the grids in
  mid-January and replaced with groups of
  laboratory-bred M. oeconomus numbering 25-26 and
  with an even sex ratio that were the offspring of
  adults trapped locally the previous summer. In
  April an additional 9-11 voles were introduced
  into each grid.

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• Data Collection
• Vole populations were monitored once each month
  from January to April and bi-weekly from April to
  mid-June during 8 day sessions.
• Each session was comprised of two parts
• One part consisted of using the PIT-tag system to
  obtain data regarding survival rates and vole
  movement.
• The second portion, when the weather conditions
  permitted, entailed live-trapping. Trapped voles
  were identified, sexed, weighed and their
  reproductive status was determined.
• Immediately after snow melt, the grazing damage
  was recorded.

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Results

• Survival
• Treatment populations had overall higher monthly
  survival rates compared to control populations.
• Female survival rapidly increased with body mass.
  A female with an initial weight of 30 g had a
  44 higher probability of survival compared to a
  female weighing 25 g.
• Body mass had almost no effect on male survival.
  The equivalent difference in survival probability
  between a male weighing 30 g and 25 g was only 3

Estimated monthly survival SE (model averages)
for the seven time intervals. Circles represent
treatment populations and squares represent
controls. Solid and dotted lines represents the
first and second cohort, respectively. Open
symbols represent females and closed symbols
represent males.
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• Body Mass
• There was a negative relationship between body
  mass at introduction and body mass change over
  the first 30 days.
• This was explained most effectively by taking
  into account sex.
• Both large males and females lost weight, while
  small individuals gained weight. This weight
  gain was strongest among males.

Estimated monthly survival of males and females
as an effect of introduction body mass. Solid
line represents males and dashed line represents
females. Dotted lines show standard errors.
Change in body mass during the first 30 days
after introduction as an effect of body mass at
introduction. Open circles represent females and
closed circles represent males.
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• Space Use
• There was a significant difference in individual
  vole space use between the treatment and control
  groups.
• The mean number of trap station visits in
  treatment grids (3.570.24) was almost twice that
  of those in the control grids (1.970.28)

Estimated individual space use for root voles in
treatment and control grids.
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• Population Density
• There was no significant differences in
  population density between the four grids.
• The proportion of voles lost to predation was
  higher in treatment grids compared to the control
  grids
• A portion of mortality throughout the winter was
  suspected to be the result of predation by stoat
  (Mustela erminea), whose feces and tracks had
  occasionally been observed.

http//www.helsinki.fi/science/metapop/english/Spe
cies/Karppa.htm
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• Grazing
• A significant difference in frequency of grazing
  between covered (0.450.044) and uncovered
  (0.100.027) areas within the treatment grid.
• No difference was observed in the control grids
  (0.220.037 and 0.280.041).

Proportion grazed ( SE) as an effect of
treatment and position within grid (n  190).
Dark grey columns represent samples from under
the sheets on the treatment grids and on the
equivalent locations, between trap stations, on
control grids (online). White columns represent
samples taken at other random locations within
the grids (offline).
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Conclusions

• The results supported the hypotheses regarding
  survival and space use. Voles living within the
  treatment grids had a higher rate of survival,
  space use and grazed larger regions. This
  indicates that the physical properties of the
  snow cover limits access within the subnivean.
• The hypothesis pertaining to body mass change
  could not be verified because of the lack of data
  points.
• The corrugated profile of the aluminum sheets
  created a space mimicking the naturally occurring
  subnivean space. This was shown to provide the
  voles access to more foraging space.
• This study implicates the physical properties of
  the snow cover and the consequent fragmentation
  of the subnivean as the cause of the lands
  reduced carrying capacity in winter