Acta Chiropterologica, 9(1): 297–303, 2007
PL ISSN 1508-1109 © Museum and Institute of Zoology PAS
Describing roosts used by forest bats: the importance of microclimate
JUSTIN G. BOYLES
Center for North American Bat Research and Conservation, Department of Ecology and Organismal Biology,
Indiana State University, Terre Haute, 47809 USA; E-mail: [email protected]
Adequate descriptions of roosting habitat are vital to the management and conservation of bats. However, most
studies on bat roosting preference report only structural characteristics of roosts and surrounding habitat, and
ignore potentially important factors in roost selection. I argue that the current methods for describing the
roosting habitat of tree-roosting bats can be improved, and that more emphasis should be placed on designing
studies to determine why bats choose particular roosts. Herein, I focus on measuring microclimate in roosts
because it universally influences habitat selection. Specifically, roost temperature is easily measured and is
likely an important microclimate variable used by bats in roost selection. Variation in structural characteristics
of roosts is often assumed to correlate with variation in microclimate of the roost; however, empirical data are
too scarce to verify this assumption. I suggest improvements to the current methods of describing roost
characteristics and suggest the inclusion of new methods to describe microclimate. In summation, I argue that
there are methods of measuring roost characteristics that may be beneficial to use in conjunction with the
methods currently being used, and that microclimate should be considered when designing future studies.
Key words: Chiroptera, habitat selection, study design
INTRODUCTION
Many species of bats roost in trees for
at least part of the annual cycle. Conservation of forest habitat is important to bat
management; however, management of
appropriate habitat for bats is logistically problematic, partly because of current
paradigms used for describing bat roosting
habitat. Generally, studies are designed to
report the characteristics of known roosts
that are unique compared to those available.
Such studies are important to bat ecology,
but they do not answer the ultimate question of why bats choose a particular roost
at a given time. Some authors speculate
about the causal factors in roost selection,
but the experimental design of most studies
is not appropriate to determine why bats
chose a particular roost. My purpose here
is to suggest that in addition to studies
aimed at answering the proximate question of what characteristics describe roosts,
studies should also be designed to answer
the question of what ultimate factors are
important in roost selection. Herein, I focus on the measurement of microclimate
as a causal factor in roost selection because it universally affects roost selection;
however, each species may use several
other factors in determining roost selection
(e.g., sociality, predation risk). I use the
term ‘roost’ to mean any location used
by bats at some time during the annual cycle, regardless of occupancy at the time of
measurement.
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J. G. Boyles
Most studies, and therefore management
plans, describe bat habitat in forests based
on the structural properties of roost trees
(e.g., Kurta et al., 1993; Vonhof and Barclay, 1997; Menzel et al., 2001). This method is well established and allows for comparison to previous studies and meaningful
meta-analyses (Lacki and Baker, 2003; Kalcounis-Rüppell et al., 2005); however, it is
lacking in regard to microclimatic properties of roosts or physiological (e.g., production of metabolic heat and propensity to use
torpor) and behavioral characteristics (e.g.,
clustering) of bat species. It is possible that
measuring simple structural characteristics
of a tree and the area surrounding may provide a proxy for variation in microclimate
within the roost (Sedgeley, 2001; Willis and
Brigham, 2005), but this assumption should
not be made without adequate testing.
Microclimatic characteristics within
roosts have often been suggested as important factors influencing habitat selection by
bats (e.g., Kunz, 1982; Gardner et al., 1991;
Cryan et al., 2001), but they have only rarely been measured systematically. For example, since 1996, I know of 41 studies that
have been published describing tree or foliage roosting habitat of North American
bats and only five of those (Vonhof and
Barclay, 1997; Kalcounis and Brigham,
1998; Hutchinson and Lacki, 2001; Menzel
et al., 2001; Willis and Brigham, 2005)
have reported any data for microclimate
variables. These studies report microclimate, but only when a bat is actually in
the roost or shortly thereafter. This may
lead to a bias and make comparison to data
on microclimate difficult because no random controls are available for comparison.
Studies that have reported systematically collected data for roost microclimate
generally focus on temperature (Vonhof
and Barclay, 1997; Kalcounis and Brigham,
1998; Hutchinson and Lacki, 2001; Smith
and Racey, 2005; Willis and Brigham,
2005) and occasionally relative humidity
(Sedgeley, 2001) or wind speed (Willis and
Brigham, 2005). However, roost microclimate has only been compared to the characteristics of spatial controls (i.e., random
non-roost trees) and not temporal controls
(i.e., season-long measurements of known
roosts), so conclusions can only be drawn
about the causes of spatial variation and not
temporal variation (or the interaction of the
two) in roost use.
Many species use more than one type of
structure for roosting. For example, most
species that naturally roost in tree cavities
have also been recorded in buildings, bat
houses or rock crevices, despite the obvious
structural differences (Cryan et al., 2001;
Lausen and Barclay, 2006). In addition,
most tree and foliage dwelling bat species
use a variety of tree species of different
sizes in varying stages of decay (Brigham
et al., 1997; Carter and Feldhamer, 2005;
Boyles and Robbins, 2006). The variety of
roosts used indicates that simply describing
the structural characteristics of roosts is insufficient (Cryan et al., 2001), or perhaps
even misleading, for determining the ecological and physiological underpinnings of
roost selection.
With the relatively course measurements
taken in most studies (e.g., diameter at
breast height, tree height, estimated canopy
cover) and the general exclusion of measurements directly related to roost microclimate (e.g., water content of tree, variation in
the amount of solar radiation on the roost),
it is unlikely that variation in microclimate
is accounted for. For example, specific heat
capacity, a good indicator of the thermal
properties of wood, can vary by nearly
100% as a function of water content of
the wood and ambient temperature (Ragland et al., 1991). Statements suggesting
that bats prefer trees in specific size and decay classes may be confounded by the fact
that trees of similar size and decay class, but
Microclimate in roosts of forest bats
of different species, may exhibit different
thermal characteristics due to inherent differences in wood properties (Hengst and
Dawson, 1994).
A further complication is that microclimate will be different when a roost is occupied versus not occupied based on structural characteristics of the tree (e.g., cavity size, specific gravity of the wood) as
well as due to fluctuations in group size
and individual use of torpor (Bakken and
Kunz, 1988). Even with technological advances, it is still easier to collect only data
on structural characteristics of roosts; however, I argue that it is important to collect
microclimate data as well.
MEASURING ROOST TEMPERATURE
Bakken and Kunz (1988) reviewed the
techniques and procedures appropriate for
measuring microclimate in roosts. Temperature-sensitive dataloggers have been used
for several decades to describe temperature
in bird nests (Horvath, 1964) and bat roosts
(Humphrey et al., 1977). With the recent
advent of small, affordable dataloggers, systematic studies of temperatures in bat roosts
are becoming more common (Weibe, 2001;
Chruszcz and Barclay, 2002; Dechmann et
al., 2004), but are still rare for tree and foliage roosting bat species (although see
Hutchison and Lacki, 2001; Sedgeley, 2001;
Willis and Brigham, 2005). This is despite
the fact that microclimatic characteristics,
especially temperature, can be measured for
long periods of time allowing for comparisons between sites when bats are or are not
present and comparisons between roosts
and randomly selected non-roost-sites (Hutchison and Lacki, 2001; Willis and Brigham, 2005). No studies have measured temperature in roosts throughout the season,
and short sampling periods are insufficient
to describe temporal heterogeneity in the
roost.
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Some authors have suggested that roost
microclimate should not be measured while
bats are present because of the effect of the
bats on microclimate (Sedgeley, 2001), but
I argue that it is important to measure microclimate while bats are and are not present to fully characterize roost microclimate.
Comparing microclimatic characteristics of
known roosts when occupied and unoccupied will lead to a more realistic ecological
and physiological explanation of roost selection based on a long-term (i.e., seasonal
or annual) understanding of roost microclimate. It will also help explain variation in
use of individual roosts throughout the year
and from year to year. Measuring roost microclimate only when bats are absent will
likely misrepresent true microclimate selection for two reasons. First, if bats are not
present, it may be because the roost does not
meet microclimate requirements given current environmental conditions. If roosts
with a specific microclimate prove to be
common, this may be unimportant; however, it is currently speculative to assume
microclimates are not limiting. Second, by
not measuring microclimate when bats are
present, the behavior and physiology of individual bats as well as the cooperative benefits of colonial roosting are not taken into
account (especially for species that roost in
tree cavities or beneath bark). Microclimate
selection criteria should be determined by
the interaction between thermal properties
of the roost and the effects of thermoregulation by bats.
I suggest that when measuring roost microclimate, it is important to have both spatial and temporal controls. Most species that
roost under bark or in tree cavities show
enough year-to-year fidelity to roosts, especially for trees used as maternity sites
(Barclay and Brigham, 2001; Willis et al.,
2003), that season-long measurements are
possible and important. By placing dataloggers in roosts before bats return, it is
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J. G. Boyles
possible to assess both the microclimate
variation and effects of roosting bats on microclimate throughout the season. Bats generally rest against cavity walls in tree cavities, but variation in temperature caused
by uneven wall thickness or differential
heating caused by varying amounts of solar
radiation will be difficult to account for. If
possible, several dataloggers placed around
the interior of the cavity will give an estimation of temperature variation, but it may
be more practical to suspend dataloggers in
the middle of the cavity to measure the
average roost temperature. In large cavities, microclimate should be measured at
several locations to encompass variation in
the roost. Temperature data from roosts
under sloughing bark will be more difficult
to acquire, but in many roosts it should be
possible to place a datalogger where the
bats will not rest directly on it. In some
large cavities and under sloughing bark,
thermal imaging equipment will allow
measurements of variation in temperature
throughout the roost, but season-long measurements of this type will be impractical.
Measuring the microclimate of foliage
roosting species with dataloggers (Hutchinson and Lacki, 2001; Willis and Brigham,
2005) may be insufficient to fully characterize these roosts. Foliage roosting species
are more likely to show low roost fidelity than other tree roosting species (Lewis,
1995) so season-long measurements may be
impractical.
A feasible, but unexplored option for
mapping the temporal and spatial heterogeneity in roosts is to use copper-bodied
taxidermic mounts to measure operative
temperature (Te; with unheated mounts) or
standard operative temperature (Tes; with
heated mounts — Bakken et al., 1985;
Bakken and Kunz, 1988). Measurements of
Te are advantageous over simple measures
of air temperature because they incorporate
all aspects that may affect the thermal envi-
ronment experienced by an individual (e.g.,
air temperature, wind, solar radiation; see
Bakken, 1992 for a review and explanation
of Te and Tes). The purpose of Te is to simplify the thermal environment an animal experiences into one measurement when adequate measurements of the climatic variables that affect microclimate are unrealistic
(which is the case when describing many
roosts over long periods). Taxidermic
mounts have been successfully used to map
the thermal environment of many species
(Bakken et al., 1981; Vispo and Bakken,
1993; Fortin et al., 2000), but they have not
been used to map the thermal environment
of bat roosts (Bakken and Kunz, 1988). An
inherent drawback is that construction and
calibration of taxidermic mounts that simulate endotherms is time consuming, but the
use of simpler designs may alleviate these
problems (Bakken et al., 2001).
In addition to measuring roost microclimate directly, I argue that improvements in
the measurement of structural characteristics that affect microclimate are needed. For
example, several studies have measured
structural characteristics of cavity roosts
(Sedgeley and O’Donnell, 1999; Ruczyñski
and Bogdanowicz, 2005), but none have
measured water content of the tree or bark,
a variable which is important to the thermal
characteristics of the roost and one which is
simple to measure (Ragland et al., 1991).
More commonly, researchers have used visual estimations of canopy cover as a proxy
for solar radiation (e.g., Brigham et al.,
1997; Crampton and Barclay, 1998; Hutchison and Lacki, 2000), but this method is
limited by problems including lack of repeatability and the difficulty in determining
the parts of the canopy which most affect
the roost. Small, portable devices are available to measure solar radiation directly
(Hyer and Goetz, 2004) and these should be
used as measures of solar radiation at the
roost.
Microclimate in roosts of forest bats
Logistically, studies of microclimate
may be difficult to conduct and are likely to
face the same statistical limitations of other
bat studies (Lacki and Baker, 2003). However, microclimate data will increase our
understanding of how and why bats select
particular roost sites. Considering that microclimate has been successfully measured
in bird nests for several decades (Hovath,
1964), it is unclear why microclimate measurements are rarely made for bat roosts.
Climbing roost trees can be difficult and
dangerous, but it has been successfully done
(e.g., Ruczyñski and Bogdanowicz, 2005)
and it is possible to place dataloggers in
some roosts without climbing the tree (Hutchinson and Lacki, 2001). By combining
better measures of roost microclimate with
data on individual behavior gained from
passive integrated transponders (Kerth and
Reckardt, 2003), radiotransmitters (Willis
and Brigham, 2004), or other technology, an
accurate picture of the conditions under
which and for what reasons bats use specific roosts can be established.
CONCLUSIONS
I do not mean to imply that studies of the
structural characteristics of bat roosts are
obsolete, but rather that they may lack resolution in explaining roost selection that is
useful to ecologists and land managers
alike. First, studies that report structural
characteristics of roosts are designed to answer the proximate question of what roost
bats are using, and can only lead to speculation of the fitness costs and benefits of using
a roost. Second, most studies that describe
structural characteristics of roosts are forced
to group roosts into temporal categories (often based on reproductive characteristics of
females) because of small sample sizes.
Hypothesis-driven studies designed to determine the reason(s) for the selection of
a particular roost can avoid this problem
with good experimental design and the use
301
of experimental methods not commonly
used in descriptive studies.
The current methods used to describe the
roosts of tree dwelling bats, while valid,
should to be enhanced to account for all potentially important roost selection factors.
I suggest that researchers use new technologies to describe microclimate within roosts
and more stringent methods of measuring
structural characteristics that affect microclimate. Furthermore, speculation about the
connection between structural characteristics and microclimate should be avoided because the link is largely anecdotal and has
not been systematically tested (but see Sedgeley, 2001). It is possible that variation in
structural characteristics will adequately encompass variation in roost microclimate, but
this cannot be assumed without validation.
In this paper, I have focused on the characteristics used to describe tree roosts because studies on forest species are common.
However, the suggestions I make herein are
equally applicable and important in all habitats and roost types. By adding new techniques and solidifying those methods already used, we can move beyond simple descriptions of bat roosts and begin determining the implications of roost selection on
survival and fitness. Measuring structural
characteristics is very practical and important in the conservation and management of
forest habitat but an understanding of why
bats choose roosts will advance that cause
and be invaluable if replacement of habitat
with artificial structures becomes necessary.
ACKNOWLEDGEMENTS
I thank D. Aubrey, R. M. Brigham, P. Cryan, M.
Dunbar, D. Sparks, J. Storm, and one anonymous reviewer for helpful discussion or comments on early
versions of this manuscript.
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Received 02 June 2006, accepted 11 January 2007