and Morten Muus Falck gave me advice about wildlife and forest management and
helped in raising funding for field work. I will thank Ivar Mysterud for
lending me radio-telemetry and other field equipment, without it, this project
would never have started. I also wish to thank all the nice folks at the
Division of Zoology for making this period so enjoyable, especially Arild O.
Gautestad, Ivar Mysterud, Iver Mysterud and Jerry Thomas Warren for lunch
discussions, Ottar N. Bjørnstad, Rolf A. Ims and Nigel G. Yoccoz for
bearing my (to them) silly statistical questions. I further thank Rolf A. Ims,
Jerry Thomas Warren, Nigel G. Yoccoz and Eivind Østbye for valuable
comments to the introductory chapter.
Financial support was given by the Norwegian Research Council
through a three year scholarship (1996-1998), to which I am very appreciative.
I am also appreciative to Viltfondet centrally (Directorate for Nature
Management), the County Officers in Buskerud, Hedmark, Oppland, Telemark and
Vestfold Counties, Utviklingsfondet for skogbruket, NORSKOG, and Norges
Skogeierforbund who supported the field work.
I will thank my
parents, Åse and Ivar Mysterud, for making me "love" the deep forests and
showing me my first roe deer, and my brother, Iver, his wife, Marianne, and
their four-legged "son", the dog Rask Mysterud, for friendship and many nice
hikes over the years. Last but not least, I sincerely thank my wife,
Rønnaug S. Mysterud, for her patience during long periods of field work,
for her help during capturing animals and for always being supportive in every
Blindern, February 1998
LIST OF INDIVIDUAL PAPERS
The characteristics of the species (Ch.
The Lier valley, Buskerud
FRAMEWORK, SAMPLING AND METHODS
Sampling and statistical analysis
Estimating resource levels
RESULTS AND DISCUSSION
Resource use strategies
Bed-site selection during winter (paper I)
Bed-site selection during summer (paper II)
Forage site selection and food preference during
winter (paper III)
Effect of snow on forage site selection
Habitat use/availability, trade-off
and temporal scale
Functional responses in habitat use (paper V)
Resource availability and habitat rankings
Space use and social organization
Home range size and the mating system
Seasonal migration pattern and the clan
system (paper VIII)
The ecological processes and mechanisms that determine an
animal's use of resources are scale dependent. For example, patch/habitat
selection within the home range may depend on the shifting phases in the daily
activity cycle of an animal, while the more infrequent home range shifts at the
landscape scale may result from seasonal migrations. Patterns of habitat use
may also reflect various trade-offs, e.g., between foraging and predation risk.
However, these insights have only to a limited extent been incorporated into
the analysis of habitat selection by cervids, such as the European roe deer (
capreolus ), an
important game species.
Habitat selection by roe deer was studied relative to the spatial
distribution of food and cover using tracking, radio-telemetry or a combination
thereof transcending different spatial and temporal scales mainly in Lier,
Buskerud county, in Norway. In the first section, I tested several hypotheses
regarding use of cover as a thermal strategy, an antipredator strategy and/or
as a strategy to find areas with shallow snow. In the second section, a new
method for analysing habitat selection in trade-off situations is presented,
and I tested if habitat rankings based on roe deer habitat selection reflect
resource availability. In the third section, patterns of space use are tested
During winter, roe deer foraged mainly in open habitats, but only
bed-sites far from the forage area had denser canopy cover than that measured
at random locations. Roe deer more often sought cover when bedding during cold
rather than warm periods and when duration of the bedding period was long
rather than short. During summer, roe deer left feeding areas more often before
bedding during warm weather. However, roe deer did not select a more covered
bed-site during periods of heat stress, rather they selected a more humid
bedding substrate, probably to increase heat loss through conduction. Selection
of forage site during winter was to a lesser extent influenced by temperature
than was selection of bed-sites. Roe deer selected more open habitats at night,
and avoided feeding close to human settlement during day. As snow depth
increased, they foraged closer to human settlement, suggesting a trade-off
between energetic demands and antipredator behaviour. Roe deer increased use of
mature forest habitats as winter progressed, and use of mature forest was
greater at higher compared to lower altitudes, corresponding to different snow
The observations that roe deer balance foraging in open habitat
with resting in cover habitat suggest that the relative use (selection) of
these habitats may change with their relative availabilities. However, all
current methods for evaluating habitat selection from use/availability data
assume that use is directly proportional to availability. A new method was
therefore developed that regresses use against availability on a logit scale.
Different hypotheses may be framed in terms of the regression parameters. A
slope of one (b=1) indicates that use is directly proportional to availability,
whereas b=0 indicates that use is consistent with availability, e.g. if the
animal forages in open habitat and rests in cover habitat. The prediction that
food and cover alone may be poor predictors of roe deer habitat selection was
tested with data on habitat use of 27 radiocollared deer and compared with
measures of food and cover availability. As predicted, food availability did
not predict roe deer habitat selection during summer. Though there was a
correlation between roe deer habitat selection and canopy cover availability,
this result may be dependent on the relative availability of food and cover.
Use of forage-rich habitat was higher when deer were active as compared to when
inactive, and tended to be higher at night than during the day.
Finally, data on seasonal migration pattern and home range of
male and female roe deer are presented. Males were found to defend territories
which were almost twice as large as female home range size during the mating
season (summer), whereas the common pattern is equal range size among the
sexes. It is hypothesized that this result may be due to the combination of low
density (low costs) and high resource abundance in Lier (giving small female
home ranges; high benefits). Roe deer in Lier had on average summer ranges at a
higher elevation than their winter ranges. However, summer ranges at high
elevations were large, suggesting these areas were of low quality. Snow depth
is regarded as a major factor influencing migration patterns of temperate
cervids, but it can be assumed that snow depth induces an equal migration
pattern of males and females. However, there was a higher proportion of
stationary males than females, and more females than males were long distance
migrators (>10 km). This supports an earlier hypothesis that roe deer
migration patterns are also influenced by social factors, even though grouping
tendencies are less pronounced at low population density. The influence these
sex-dependent patterns of space use may have on habitat selection is discussed.
This dr. scient. thesis is based on the 8 papers listed below.
They are later referred to by their Roman numerals.
(*Note: References are updated relative to
I. Mysterud, A., and Østbye, E. 1995. Bed-site selection
by European roe deer ( Capreolus capreolus ) in southern Norway during winter. Canadian Journal of Zoology
II. Mysterud, A. 1996. Bed-site selection by adult roe deer
southern Norway during summer. Wildlife Biology 2: 101-106.
III. Mysterud, A., Lian, L.-B., and Hjermann, D.Ø. 1999.
Scale-dependent trade-offs in foraging by European roe deer (Capreolus
capreolus) during winter.Canadian Journal of Zoology 77: 1486-1493.
IV. Mysterud, A., Bjørnsen, B.H., and Østbye, E.
1997. Effects of snow depth on food and habitat selection by roe deer
an altitudinal gradient in south-central Norway. Wildlife Biology
V. Mysterud, A., and Ims, R.A. 1998. Functional responses in
habitat use: availability influences relative use in trade-off situations.
VI. Mysterud, A., Larsen, P.K., Ims, R.A., and Østbye, E.
1999. Habitat selection by roe deer and sheep: does habitat ranking reflect
resource availability? Canadian Journal of Zoology 77: 776-783.
VII. Mysterud, A. 1998. Large male territories in a low density
population of roe deer Capreolus capreolus with small female home ranges. Wildlife Biology
VIII. Mysterud, A. 1999. Seasonal migration pattern and home
range of roe deer ( Capreolus capreolus ) in an altitudinal gradient in southern Norway. Journal
of Zoology 247: 479-486.
One of the essential components of Darwin's theory of natural
selection (Darwin 1859) is that only a fraction of each generation survives
until maturity as a result of competition for scarce resources (Milinski and
Parker 1991). A central issue for behaviour and population dynamics is
therefore habitat selection, the study of where and how animals distribute
themselves relative to resources (Fretwell and Lucas 1970, Rosenzweig 1981). A
thorough understanding of habitat selection therefore also forms part of the
basic platform necessary for a skillful management of game species, such as the
European roe deer ( Capreolus capreolus ) (see below), and this has motivated this study.
It is now widely accepted that ecological mechanisms are
scale-dependent (Wiens 1989, Turner 1989, Kotliar and Wiens 1990, Bissonette
1997a), and different mechanisms determining an animal's use of resources seem
to come into play at different spatial and temporal scales (Johnson 1980,
Morris 1987, 1992, Senft et al. 1987, Orians and Wittenberger 1991). For
example, at a fine spatial scale, habitat selection is suggested to be
determined by the differential use of foraging locations within a home range,
whereas at a coarser scale, habitat selection is determined by dispersal and
the ability to relocate the home range (Morris 1987, 1992). Some authors have
suggested up to 4-6 different spatial scales of large ruminant foraging (Senft
et al. 1987, Bailey et al. 1996). Movement decisions at any level may be
simultaneously affected by spatial structures at levels below and above in the
hierarchy (Senft et al. 1987, Wiens 1989, Kotliar and Wiens 1990, Gautestad and
Mysterud 1993, Ims 1995). With respect to temporal scale, there may be
different habitat requirements between rest and activity periods, between night
and day and, at a longer time scale, between seasons. The space and time
dimensions in habitat selection are naturally linked (Fig.
1). Patch/habitat selection within the home range may depend on the
shifting phases in the daily activity cycle of an animal, while the more
infrequent home range shifts at the landscape scale may result from seasonal
migrations. The mechanisms underlying roe deer habitat selection at a coarse
scale, i.e. (density-dependent) dispersal and migration, have recently been
thoroughly studied (Wahlström 1995, Wahlström and Kjellander 1995,
Wahlström and Liberg 1995a, b), and the main focus of this thesis will
therefore be at a within-home range scale.
Figure 1. The approximate relationship between temporal and spatial scales
of roe deer habitat selection considered in this study (adapted after Wiens
1989, Bissonette 1997b). Typically, long time scales are linked to broad
spatial scales. However, a short time scale may give high apparent
predictability regarding coarse spatial scales (Wiens 1989). The time
dimensions are relatively easily defined, whereas the patch-habitat-landscape
entities are difficult to define and often indicates a gradient from fine to
coarse scale in this study.
Optimal patch use within a home range is under the simplest
conditions predicted by the marginal value theorem (Charnov 1976). According to
this, a forager should continue to exploit a patch until its harvest rate in
the patch drops to its average over all patches (including foraging time in
patches and travel time between patches). Most methods used to evaluate habitat
selection from animal space-use observations infer habitat preference as the
disproportional use of some habitats over others (Neu et al. 1974, Johnson
1980, Byers et al. 1984, Aebischer et al. 1993, Clark et al. 1993, Manly et al.
1993, Arthur et al. 1996, Otis 1997; see also Alldredge and Ratti 1986, 1992,
Thomas and Taylor 1990, Cherry 1996), which is valid if the animal uses most of
its time in the food-richest habitats as predicted by the marginal value
theorem. However, the model used to derive the marginal value theorem makes a
number of simplifying assumptions which limit its applicability (Kotler 1997).
These include that foragers have no alternative activities; they only forage or
look for new resources in the environment (Kotler 1997), and that there are no
trade-offs between selection of different resources.
A problem commonly faced by animals is that many habitats do not
have good mixes of patches that allow for all essential activities required for
successful reproduction (Orians and Wittenberger 1991). Both food and cover
have been described as important resources for habitat selection by roe deer
(Dzieciolowski 1976, Papageorgiou 1978, Helle 1980, Henry 1981, Büttner
1983, Cederlund 1983, Stüwe and Hendrichs 1984, Cibien and Sempere 1989,
Aulak and Babinska-Werka 1990a,b, Welch et al. 1990, Selås et al. 1991,
Guillet et al. 1995, Latham et al. 1996, 1997, Tufto et al. 1996, Telleria and
Virgos 1997) as well as by white-tailed deer ( Odocoileus virginianus
) (e.g. Gill
1966, Huot 1974, Armstrong et al. 1983a, b, Lang and Gates 1985). However, even
though in particular Huot (1974), adequately classified cervids habitat
situations as whether the distribution of food and cover were combined or
distinct, and described that selection may vary with activity (Gill 1966, Huot
1974), these early studies were usually without much quantification or
statistical testing (but see Armstrong et al. 1983a, b). On the other hand, the
weaknesses of many recent studies is a consequence of statistical methods which
do not take into account temporal scale or resource distribution (e.g. Tufto et
The main aim of this thesis is therefore to quantify roe deer
habitat selection relative to the spatial distribution of food and cover
transcending different spatiotemporal scales and mechanisms, and to suggest
ways to solve some of the methodological challenges this approach presents. In
the first section, I describe roe deer habitat selection relative to the
spatial distribution of food and cover when selecting bed-sites (winter: paper
I; summer: paper II) and forage sites (winter: paper III, IV). I further test
some predictions regarding use of cover as a thermal strategy (paper I, II,
III), an antipredator strategy (paper III) and as a strategy to find areas with
shallow snow (paper III, IV). I also present a method to correct the bias that
habitat selection may have on estimates of food preference (paper III). In the
second section, the effect of temporal scale on habitat selection in trade-off
situations is discussed in more detail. A new approach of evaluating habitat
selection from animal space use observations pertinent under trade-off
situations is presented (paper V). Based on these arguments, the prediction
that food or cover availability alone may not be a good predictor of roe deer
habitat selection is tested with data from 27 radio-collared roe deer and
habitat rankings established with compositional analysis (Aebischer et al.
1993) together with information about food and cover availability in the
habitats (paper VI). I end the thesis with a section on space use and social
organization, since ecological factors may affect habitat selection of sex and
age groups differently. I present data on space use during the breeding season
(paper VII) and on seasonal migration pattern and the clan system (paper VIII),
and discuss the influence sex-dependent patterns of space use may have on
The European roe deer is a small (20-30 kg) cervid distributed
all over Europe (except Ireland) and is now regarded as a separate species from
the Siberian roe deer ( Capreolus pygargus ) (Danilkin and Hewison 1996). Roe deer is at its
northernmost range in Scandinavia (Cederlund and Liberg 1995). The species
colonized southern Scandinavia 7-9000 years BP (Hufthammer 1992, Hufthammer and
Aaris-Sørensen 1998), but the present high density is historically more
recent (Cederlund and Liberg 1995, Wahlström and Liberg 1995b).
During summer, roe deer feed mainly on herbs (Cederlund et al.
1980, Selås et al. 1991, Tixier and Duncan 1996), a forage which is rich
in soluble sugars, has low levels of cellulose and hemicellulose, but contains
high levels of tannins and lignin (Tixier et al. 1997). During winter, the roe
deer diet consists mainly of varying amounts of bilberry ( Vaccinium myrtillus
) stems and
twigs/buds from deciduous trees (Cederlund et al. 1980, Helle 1980, Mysterud
and Østbye 1995), but they face severe constraints from their limited
ability to digest these cellulose-rich winter forages (Holand 1992, 1994),
probably because of their small body size (Demment and Van Soest 1985, Illius
and Gordon 1987).
Ruminants typically have a simple foraging-resting-foraging type
of time budget (Bunnell and Gillingham 1985, Mysterud 1998), and during winter,
roe deer use about 55-60% of their time on rumination/rest distributed on about
8 daily bouts, whereas only 45-50% on 12 bouts during summer (Cederlund 1981,
1989, Jeppesen 1989). Changes in activity is mainly related to changes in
digestibility of plants (Cederlund 1989), which is much higher during summer
than winter (Cederlund and Nyström 1981, Holand 1993).
The mating system of roe deer is unique among the Cervidae since
the males defend a (mating) territory from about beginning of April to the end
of August, whereas the females come into estrus during late July/early August
(Cederlund and Liberg 1995, Johansson 1996). The roe deer is only slightly
sexually dimorphic (5-10%), suggesting low levels of polygyny (Loison et al.
1998), although female rut excursions are common in high density populations
(Andersen et al. 1995, Johansson 1996). Females live solitary during summer,
but with some spatial overlap with neighbouring females (Bjar et al. 1991,
Andersen et al. 1995). The roe deer is polytocous, giving birth to between one
and three (four) fawns in early June (Cederlund and Liberg 1995). Females
usually breed for the first time as two-year-olds (Gaillard et al. 1992). The
roe deer is classified as a typical "hider" with respect to the neonatal
antipredator strategy the first few weeks after birth (sensu Lent 1974). Fawns
spend 80% of the time inactive at well hidden places (Linnell 1994), probably
as a strategy to reduce the high levels of predation from red fox (
) (Linnell et
al. 1995, Aanes and Andersen 1996). As one-year-olds, a high proportion of both
sexes leave their natal range and may disperse a long distance before settling
(Wahlström and Liberg 1995a, b). Male yearlings are forced to leave the
natal area by older territorial males (Strandgaard 1972, Wahlström 1994),
whereas females leave voluntarily probably to gain access to more resources
(Wahlström and Kjellander 1995). Since dispersal is often male-biased
(Strandgaard 1972, but see Wahlström and Liberg 1995a), females produce
male-biased litters when environmental conditions are limiting, possibly to
lower local resource competition (Hewison and Gaillard 1996).
During winter, roe deer often congregate in small family groups
(4-5), called clans, especially at high population density (Vincent et al.
1995). The mother and her calves form the core of this unit, often with female
offspring from the previous year and an unrelated male (Linnell 1994). Roe deer
in Scandinavia are prone to large die-offs in snowy winters (critical snow
depth is considered to be 50 cm; Cederlund and Liberg 1995), and red fox may
also kill adults under such conditions (Cederlund and Lindström 1983). In
addition to autumn hunting (adult males: Aug. 16-Dec. 23; females and calves:
Sep. 25-Dec. 23), lynx ( Felis lynx ) may kill all age and sex classes of roe deer in some areas of
Norway (Linnell and Andersen 1995).
The main study area (paper I, II, III, VI, VII, VIII) is about
250 km 2 and situated in the Lier valley in southern Norway (between
59°52'-59°58'N and 10°14'-10°20E). Most of Lier is forested and
situated within the boreonemoral region (Abrahamsen et al. 1977). Vegetation is
varied and dominated by Norway spruce ( Picea abies ) mixed with Scots pine ( Pinus sylvestris
) on the drier
and poorer locations. The forest has been commercially managed, and there are
several clearcuts of varying sizes within the study area. Along the bottom of
the valley on richer soil, deciduous forest predominates, fragmented by small
cultivated fields which roe deer may feed on during winter (Kjøstvedt et
al. 1998). In the deciduous forest, species such as hoary alder ( Alnus incana ) and bird cherry ( Prunus padus ) are dominant, mixed with elm (
) and linden (
) on the richest
The terrain is very hilly, rising from Lake Holsfjorden at 63 m
a.s.l. to over 500 m 1.5-2.5 km from the lake (Fig. 1 in paper VIII). Some
parts of the area are largely flat on a large scale. The bottom of the valley
are undulating and hilly with many ravines on a fine scale (10-100 m between
top and bottom) due to erosion in clay sediments.
The density of roe deer in the Lier valley during the winter
1992/93 was estimated to be 3-5 deer/km 2 (Mysterud 1993). There are red
fox in the area, and occassional visits by lynx is also recorded, though the
main cause of mortality is probably human hunting.
The other study area (paper IV), Flatdal, is about 6 km
2 and situated in Seljord municipality in Telemark county in
south-central Norway (between 59°32'-59°34'N and 8°32'-8°36'E)
and extends from 170 to 1000 m a.s.l. Slope varies between 15° and 30°
(Bjørnsen 1985). The terrain is hilly also on a fine scale, with many
large boulders and small cliffs. The area is forested and situated within the
northern boreal zone, though it is close to the boreonemoral border-zone
(Abrahamsen et al. 1977). Plant species representative for both vegetation
zones are found within the study area. Norway spruce is the dominant tree
species, but with scattered stands of Scots pine on poorer and drier sites. At
the highest altitudes, subalpine birch ( Betula spp.) is dominant. The forest is commercially managed.
There are no farms or houses within the study area. Roe deer density was about
2-4 deer/km 2 during the time of study. The area is rather undisturbed, but
there is hunting during autumn (paper IV).
Animal movement patterns may be studied using several different
bodies of theory (Ims 1995). Although not often explicitly stated, they usually
address different spatial and temporal scales. Habitat selection theory was
chosen a priori for this study, as is usually the case for this kind of field
studies. Assuming that the theoretical framework is sound, the challenge
becomes to find a sampling design and statistical methods that reflect this
framework. Unfortunately, all statistical methods of habitat selection
(Alldredge and Ratti 1992), as well as foraging models (Laca and Demment 1996),
make explicit or implicit assumptions that limit their applicability (see also
paper III, V). The chosen study design obviously also reflects limitations set
by equipment, time and financial costs. Along with a few definitions and a
brief presentation of methods, I therefore find it necessary to briefly comment
upon some obvious pitfalls in this regard, whereas details of the sampling
design are given in the individual papers.
The term "habitat quality" is often used rather loosely and with
variable meaning. Scientists working with (large) cervids often think of
habitat quality only in terms of food quantity and quality (e.g. Hanley 1984,
Langvatn and Hanley 1993, Hjeljord 1997), whereas others suggest that habitat
quality integrates habitat features such as resource levels, habitat structure,
predation risk, social interactions and demographic parameters (Wiens et al.
1993). I, however, regard food (quantity and quality) and cover as the main
habitat resources for roe deer in this study, based on earlier studies of roe
deer habitat selection (e.g. Henry 1981, Stüwe and Hendrichs 1984). The
entities patch, habitat and landscape were used as the basic, hierarchically
ordered, spatial scales. However, the term habitat selection is seldom used in
a scale-specific manner; e.g. I often include patch selection in habitat
selection. When analysing selection in a hierarchy of spatial scales, it is
common that what is availability at one scale, becomes use at the next (e.g.
Schaefer and Messier 1995a). This is the case when I use compositional analysis
(Aebischer et al. 1993), whereas elsewhere, the scales patch-habitat-landscape
are seen as a continuum from local to coarse scale, and not as strictly defined
The term selection is really intended for situations in which use
differs from availability, whereas I do not always use this strict definition
(see Johnson 1980, Thomas and Taylor 1990 for a discussion on terminology on
preference/selection). Habitat selection is here evaluated in two distinct
ways; either as what I term (1) resource use or (2) based on habitat
use/availability data (details below). I also regard studies of space use such
as scaling of home range size (paper VII) and migration patterns (paper VIII)
as an indirect way of evaluating habitat selection at a landscape scale.
Resource use is evaluated based on measurements of resource
levels at bed-sites or feeding sites (paper I, II, III), and neither patches
nor habitats are demarcated. Habitat selection evaluated from more traditional
data on habitat use relative to availability presumes demarcation of habitat
types. In my case, habitat boundaries were determined according to the standard
habitat classification scheme used in forestry in Norway (paper IV, VI). There
are at least two problems with such an approach. The patch/habitat boundaries
which cervids experience in most field situations can be both diffuse and
distinct, and patch boundaries may have considerable effect on animals ability
to assess resource levels (Stamps et al. 1987, Arditi and Dacorogna 1988,
Kremsater and Bunnell 1992, Schmidt and Brown 1996). Further, animals may not
view the world as we do. Wiens (1989) has urged for the development of
operational ways of defining scale. This is fraught with difficulty due to the
link between variability and scale (R.A. Ims, pers. comm.), but attempts to
identify the "correct" number of habitats based on the animals habitat use have
been made (Knight and Morris 1996). Once habitats are classified, there are
further problems with identifying what is available to the animal, which is a
considerable challenge. The specification of habitat availability is equivalent
to specification of a spatial scale at which to study the selection process
(Otis 1997). The study area scale is set arbitrarily, whereas the home range
scale is defined from the animals use of space. Even though home range borders
are not always easily defined (Spencer et al. 1990, Gautestad and Mysterud
1995), this is not likely to be a great problem except possibly at low habitat
availabilities (paper V).
I have distinguished between rest and activity, night and day and
between summer and winter in this study. Problems of definition are somewhat
less difficult for temporal scales than for spatial scales. However, although
time used for activities other than foraging and resting/ruminating is
negligible during winter (less than 1% for moose: Renecker et al. 1978), males
at least may spend a considerable amount of time defending and patrolling
territories during summer (Johansson 1996). A further problem is that bout
length may affect bed-site selection (paper I), and that the state of the
animal may vary from hunger to near satiation within a foraging bout (Edwards
et al. 1994, Gillingham et al. 1997). Possibly, future studies can combine
radio-telemetry with physiological measures of the state of the animal (see
MacArthur et al. 1979). Within the frame of this study, splitting the analysis
on active/foraging and rest/ruminating is considered to be the best option.
Collection of data on roe deer space use was by either tracking
(paper I, IV), by aid of radio-telemetry (paper VI) or by a combination thereof
(paper II, III). Only radio-telemetry provides the opportunity to collect
unbiased data in space and time in forested areas, and a total of 41 roe deer
were therefore caught and fitted with radio-collars (see below). Fixes from
radio-collared animals were obtained by standard triangulation from at least
two directions, preferably within 300 m of the deer. Although a short distance
between observer and deer will reduce the measurement error-rates, they do not
eliminate them (Springer 1979, see also Saltz 1994). This reduces the power of
the analysis, but Guillet et al. (1995) also showed that radio-tracking based
methods were unable to determine preference of habitat-types which were smaller
than the error polygon. I combine therefore radio-telemetry with tracking in
two studies (paper II, III). Combining the two techniques also allows
measurement of resource levels directly at feeding or bed-sites (see below).
Animals used for analysis based on radio-telemetry (paper II,
III, VI, VII, VIII) were caught during winter (February-March) in boxtraps of
the "Öster-Malma"-type and with "drop-nets". Apples, carrots,
grain-pellets etc. were used as bait. Animals were trapped at traditional
artifical feeding sites (rare in the study area), but I mostly established new
feeding sites. This was important in order to make unbiased samples of deer
with respect to habitat use. However, all trap sites were at low elevations,
and deer in some local areas seemed to be more difficult to catch. All deer
were fitted with motion-sensitive radiocollars (Televilt TXE-3), i.e. they
transmitted with a different pulse rate depending on whether or not the deer
was active. Unfortunately, this function was not reliable on all collars.
I estimated food quantity as coverage of main groups of forage
plants (paper II, VI), whereas I used a food index that took into account both
food quantity (availability) and quality (the selection index) in paper III.
Plant quality may vary with site quality and shade conditions (reviewed in
Jefferies et al. 1994), and the delayed phenological plant growth under cover
may be an important factor for cervid habitat selection (Hanley 1982, Hjeljord
et al. 1990). However, excluding plant quality will not likely bias the
results, since I focus little on use per se, but rather on short term changes
in use, e.g. between night and day, which could not be predicted from
variations in plant quality.
I measured cover characteristics both as canopy cover closure and
hiding cover (sighting distance along ground) with a spherical densiometer
(Lemmon 1956, 1957, Nuttle 1997) and a cover board (Nudds 1977, Griffith and
Youtie 1988), respectively.
Trade-offs can be identified in several different ways. If
availability of one resource declines (or distance to resource increases) and
availability of another resource simultanously increases e.g. when deer chooses
a bed-site, then a trade-off between selection of these resources is suggested.
In some cases, I correlate the two resources directly based on measurements of
availability at bed-sites (paper II). A (perfect) trade-off situation gives a
negative correlation (r=-1), whereas r=1 indicates no trade-off (Fig. 2). Given the same range of food and cover values, a
correlation coefficient between this, e.g. r=0, indicates that there are
patches with combinations of high/high, high/low, low/high and low/low
availability of the two resources, i.e. local trade-offs may arise. Note that
this measures trade-offs mainly at a patch (local) scale, though one usually
discuss trade-offs related to habitat scale (see Introduction, paper V, VI). I
also regard it as a sufficient indication of a trade-off if selection of only
one resource varies with temporal scale (paper VI), since it is then apparently
traded with some other (unknown) resource.
Figure 2. Selection of two resources may either be maximized at the same
time (no trade-off, r>0) or traded-off (r<0). However, given the same
variance in both resources over all r (e.g. both food and cover availability
vary from 0-100%), the level of r indicates the extent of the trade-off. For
example, if there is an equal number of patches with high/high, high/low,
low/high and low/low availability of food and cover, respectively, the
correlation will be r=0, which indicates that local trade-offs may arise. This
assumes that the animal scales home range size to one resource only, so that it
is unable to attain a sufficient amount of high food/high cover patches only
(if they do, then the trade-off is at a coarser spatial scale).
Resource use was evaluated in two distinct ways. Resource
availability at bed-sites or feeding sites was compared with availability at a
point 50 m away in a random direction (with Wilcoxon pair-tests). This distance
was set arbitrarily, but comparing with a random location 10 m away showed a
similar pattern (Mysterud 1993). It is assumed that this measures patch
selection in the sense of a local choice. When evaluating how resource use
correlates with environmental characteristics such as temperature, snow depth
and period, resource availabilities at the feeding or bed-sites were correlated
with the different predictor variables (mainly general linear models). This
does not account for differences among individuals in availability or changing
resource availability with time, but this is regarded as a small problem given
a random sample of observations. This approach in itself does not say anything
about the scale at which the selection process occurs. Ideally, differences
between home ranges are removed before analysis by adjusting estimates with
each individual means (paper III), so that any landscape level selection can be
accounted for. Further, any contradictory result between the two analyses may
indicate the scale at which selection is made. In paper III, this approach was
brought a step further by using contrasted values (feeding site-random site) as
response variable in a similar analysis.
Most methods designed to evaluate habitat selection compare some
measure of use relative to some measure of availability (see Introduction),
except for experimental situations when availability can be kept equal (Elston
et al. 1996). A crucial step toward a sound analysis are to have availability
well defined or demarcated. When data is from unknown individuals and hence
pooled (paper IV), availability is calculated for the whole study area (design
1, sensu Thomas and Taylor 1990), and I used the Bonferroni Z-statistics (Neu
et al. 1974, Byers et al. 1984). In such a case, it is not possible to separate
selection of a home range (landscape scale) from selection within home range
(habitat scale), although attempts to partly account for this were made (paper
IV). When data are from radiocollared deer (paper VI), one can calculate
availability both for the study area and for the individual home ranges (design
3), and hence selection of home range and habitats within home range can be
separated. I then used the compositional analysis of log-ratios (Aebischer et
al. 1993). This method is currently regarded as the best option for evaluating
habitat selection at two spatial scales since it also incorporates the unit-sum
constraint, has a proper level of replication and allows testing for effects of
biological relevant factors such as sex and age (Aebischer et al. 1993).
Compositional analysis takes into account much of the criticism earlier methods
treating use/availability data for habitat evaluation have received (i.e. that
such data only apply for ideal-free foragers after equilibria is set, Van Horne
1983, Hobbs and Hanley 1990), at least at the home range scale. Note, however,
that these habitat types alone do not say anything about resource levels. I
therefore either combine this approach with separate measurements of food and
cover in transects (paper VI), or make qualitative assessments of resource
levels (paper IV).
Both food and cover have been termed important for habitat
selection by smaller temperate cervids such as roe deer, white-tailed deer and
mule deer ( Odocoileus hemionus ) (e.g. Armstrong et al. 1983a, b, Lang and Gates 1985,
Schmitz 1991, Armleder et al. 1994). Obviously animals must eat to acquire
energy, but the effect of cover on habitat use is less understood (reviewed in
Peek et al. 1982, Mysterud and Østbye 1998). Several different
hypotheses (not mutually exclusive) have been proposed about the mechanisms
that can explain variation in the use of cover. It may be (1) a strategy to
find areas with a favorable thermal regime (e.g. Moen 1968, 1973), and hence,
use of cover is predicted to increase during severe cold (paper I) or heat
(paper II). It may also be (2) a strategy to lower the risk of predation (e.g.
Lima and Dill 1990). It is more difficult to assess whether, or to what degree,
the use of cover is influenced by risk of predation, since predation pressure
is not easily quantified. However, use of cover can be predicted to be higher
during day than night when chances of visual detection are higher, if the
thermal regime can simultanously be statistically controlled for (paper III).
Use of cover may also be (3) a strategy to find areas with shallow snow (e.g.
Armleder et al. 1994). The use of cover areas can be predicted to increase with
increasing snow depth for two reasons (paper IV): (a) Snow depth increases
proportionally between habitats (e.g. a snow depth of 20 cm in dense and 40 cm
in open habitat increases to respectively 40 and 80 cm) and (b) energetic costs
of movement increases at a much higher rate when snow depth exceed breast
height (Parker et al. 1984). Note that all of these predictions assume some
food-cover trade-off, which is discussed in more detail in the next section.
During the snow-poor winter 1992/93, snow tracking of roe deer in
Lier indicated that they foraged mostly in open habitats (average 1% canopy
cover) such as small agricultural fields, deciduous forests and clear-cuts
(paper I). Distance from feeding sites to bed-sites varied from 0 to over 200
m, with fewer bed-sites as distance from feeding sites increased. Only
bed-sites far from feeding sites were more covered than random sites. Seeking
cover for bedding was hence the same as moving further away from the feeding
site, and there was apparently a trade-off between selection of food when
foraging and of cover when resting.
As predicted, there was more canopy cover above roe deer
bed-sites during cold than during mild periods, whereas temperature had no
effect on cover characteristics at feeding sites (paper I). Though the more
detailed study also showed an effect of temperature on selection of feeding
sites (paper III), relatively little energy is used for thermoregulation
compared to that which can be gained by choosing high quality forage (Moen
1968). The effects of temperature and wind-chill were not separated, but roe
deer had more hiding cover in direction of incoming wind, suggesting that
windchill was an important part of the thermal environment. Studies on
white-tailed deer and red deer ( Cervus elaphus elaphus ) also conclude that the combination of low temperature
(correlated with radiation) and high windspeed may be detrimental for the
energy budget during winter (Moen 1973, Staines 1976). It was further shown
that roe deer sought more cover when bedding for long rather than short periods
of time (the estimates of duration of bedding period was based on observations
of degree of snow melt in the bed-site, and hence this result should be
interpreted with some caution). That roe deer use cover more often when resting
for long periods and in colder weather, suggests that use of cover is linked to
the energetic costs of reaching cover habitats. This is further evidenced by
Stüwe and Hendrichs (1984) who reported that roe deer in an open field
habitat sougth cover for bedding only when distance from feeding site to cover
Bed-site selection of 3 male and 2 female roe deer was studied
during summer 1994 (paper II). There was a higher coverage of herbs at
bed-sites than at random sites, suggesting that bed-site selection is
influenced by selection of feeding site. There were no correlation between
cover and herb availability. Hence, there seemed to be no general trade-off
between selection of food and cover during summer (see also Tufto et al. 1996),
although such may exist at a local level for some patches/habitats.
During warm weather, the coverage of herbs at bed-sites was lower
than during cooler periods, indicating movement away from feeding sites (paper
II). However, this movement was not correlated with an increase in canopy cover
above bed-sites (but see Belovsky 1981, Schwab and Pitt 1991, Demarchi and
Bunnell 1995), but rather roe deer selected areas that provided a more humid
substrate. The conductive heat loss, i.e. the direct transfer of energy between
the deer and substrate, may be the most important component of the energy
budget for a bedded ungulate (Moen 1973). This has been measured to approach
30% of sheep's ( Ovis aries ) minimum heat production on cold, poorly insulated ground
(Gatenby 1977). This strategy has also earlier been suggested as part of the
thermal strategy for several other ungulates (sheep: Gatenby 1977; white-tailed
deer: Jacobsen 1980; elk Cervus e. canadensis : Merrill 1991; mule deer: Sargeant et al. 1994), but
without any quantitative measures of humidity.
Feeding site selection and food preference of 5 male and 5 female
radio-collared roe deer was studied by snow-tracking during winter 1996 (paper
III). A forager's diet results from the two-step decision process of selecting
where to seek food (habitat selection) and then, once there, selecting what to
eat or pursue among available foods (e.g. Nudds 1980, Senft et al. 1987, Brown
and Morgan 1995). Despite this theoretical knowledge, empirical studies of
hierarchical foraging by cervids are few (Hanley 1997; but see Edwards et al.
1994, Ward and Saltz 1994, Schaefer and Messier 1995a, b for bovids), and
current methods for evaluating food selection based on use/availability data
from field observations have not incorporated this element (e.g. Neu et al.
1974, Aebischer et al. 1993). We therefore present a new way of using a method
originally presented to control for autocorrelation in habitat selection
(Arthur et al. 1996). The use of this method allowed us to compare the chosen
diet (use) with availability within each foraging site in succession, rather
than to some home range or study area average (Neu et al. 1974, Aebischer et
al. 1993). There was no evidence of variation in food preference between males
and females or between day and night, though statistical power was low.
As predicted, roe deer used more open areas at night than during
the day. They also avoided feeding close to human settlement during day. As
snow depth increased, they foraged closer to human settlement, suggesting a
trade-off between energetic demands and antipredator behaviour. Also,
observations that the deer seemed very shy and selected bed-sites that were
well hidden compared to random plots both during winter (paper I) and summer
(paper II), suggest that the antipredator aspect of cover use may be very
important for how roe deer select habitats. However, the strategy will likely
vary with the predators hunting strategy. For example, it has been observed
that roe deer seek open fields in areas with lynx (V. Holthe and S.R. Gjems,
pers. comm.), a typical ambush predator (Murray et al. 1995).
Roe deer's selection of forage sites was random with regard to
cover when measured against a random location only 50 m away, whereas selection
of cover varied with temperature and period. In contrast, there was more forage
at the chosen patches than at the random locations, but there was no effect of
either temperature or snow depth. This suggests that the selection of cover is
at a coarser level (habitat scale) as compared to food (patch scale). This was
further supported by the use of contrasted values (feeding site-random site).
Snow decreases availability of forage to deer, and increases the
cost of locomotion profoundly, especially when snow depth exceed breast height
(Parker et al. 1984). Shallow snow may hence also ease escape from predators
(Messier and Barrette 1985), and is therefore regarded as a major factor
determining habitat selection of North-American cervids (Huot 1974, Kucera
1976, Armstrong et al. 1983b, Beier and McCullough 1990, Pauley et al. 1993,
Armleder et al. 1994). Although snow depth influenced distance to human
settlement, no effect of increasing snow depth on selection of canopy cover was
found in Lier (paper III). However, this may be due to the shallow snow that
did not reach the critical limit (see prediction 3b). Hence, I reanalysed a
data set from Flatdal in Telemark in which snow depth (Fig. 4 in paper IV) in
some habitats exceeded the critical limit (50 cm, Cederlund and Liberg 1995).
Consistent with the predictions, use of open habitats tended to
decrease as winter progressed and was lower at higher than at lower altitudes
in accordance with differences in snow depth. Snow depth also seemed to affect
habitat selection at a coarser scale, since use of higher altitudes decreased
as winter progressed (paper IV).
In several papers (paper I, V, VI), I assume that the trade-off
are between foraging (in open habitat) and resting (in cover habitat), or
between selection of night and day areas (paper III, VI). Even though this
approach demonstrates that strength of trade-offs varies with temporal scale
(paper I, VI, see also Cowlishaw 1997a, b), it does not quantify the trade-off
per se. Based on estimates of forage quantity and quality, one may estimate the
trade-off by contrasting the predicted and observed use of each habitat.
However, the spatiotemporal variation in resource levels and possible
confounding factors, such as differing availability, limit the value of this
approach. A different approach is to measure giving-up-densities (GUDs) on
artificially planted depletable food trays (Brown 1988, see various
applications in Brown 1992, 1996, Brown et al. 1994, Kotler et al. 1994, Morgan
and Brown 1996, Schmidt and Brown 1996, Kotler 1997, Morgan et al. 1997, Morris
1997), or to titrate food and safety by controlling levels of energy content in
forage presented in different habitats (e.g. Kotler and Blaustein 1995). This
allows for much more controlled measures of trade-offs, and I believe further
progress on habitat use by cervids can come from adapting these methods that
until now have been used mainly for rodents (but see Kotler et al. 1994 for
Nubian ibex Capra
ibex ). However,
radio-telemetry and use/availability data will obviously have their central
place also in the future, especially when studying sex or age dependent
patterns of habitat selection (see next section). In this section, a new method
that can be used to test for trade-offs for such data is presented (paper V),
and I explore to what extent use/availability data can be predicted to reflect
resource levels (paper VI).
During winter, roe deer balance foraging in open habitat against
bedding in cover habitat (paper I). This suggests that the relative use (i.e.
selection) of food and cover areas may change with their relative
availabilities (paper V). However, all current methods (see Introduction) for
evaluation of habitat selection from use/availability data implicitly assume
that use is directly proportional to availability. We therefore present a new
approach to evaluate the effects of availability on relative use. We propose to
use logistic regression to regress proportional use against proportional
availability for a sample of individuals. Different statistical hypotheses may
be framed in terms of the regression parameters. A slope of 1 (b=1) will
indicate that use is directly proportional to availability. If, however, an
animal spends a consistent amount of time in each habitat with changing
availability (e.g. bedding in cover habitat and foraging in open habitat), the
slope is expected to be equal to 0 (b=0). There may also be several other types
of what we call a functional response in habitat use, i.e. a change in
selection with availability of one of two main habitat types (paper V).
For this method to be applicable, availability must vary between
individuals in the population. This is probably closely related to how animals
scale their home range. When food is the limiting factor, the animal may adjust
home range size to include a certain minimum amount of food. For example, if
satiation is reached for the feeding habitat at 50 ha and for the cover habitat
at 5 ha, and if the availability of feeding habitat is <=10%, then the
animal will establish the home range so as to include at least 50 ha of feeding
habitat. Within this home range, the amount of cover habitat will vary, but
often will exceed 5 ha if habitats are sufficiently mixed. Assuming all home
ranges include the same amount of feeding habitat, then with increasing home
range size, the ratio of food to cover in the home range decreases (Fig. 2 in
paper V). Thus availabilities of the different habitats will vary even though
the amount of feeding habitat in all home ranges is constant. If the time
budget remains stable with availability, then the strength of selection or the
correlation between habitat and activity must change. Note that this also
suggests that the way compositional analysis (Aebischer et al. 1993) evaluates
habitat selection at broad scales is moot, since preference for a habitat will
be totally dependent on the arbitrary definition of the study area
A limitation with the method presented is that it requires a
composition of only two habitat types. This is, however, a rather common
situation if one considers functional habitat types (sensu Armstrong et al.
1983b). For example, it may not be important which of four habitats that is
used for cover. Any unjustified lumping of habitat categories will be
identified as a lack-of-fit of the data to the model (i.e. an overdispersed
error term). This was the case for the roe deer data set presented in paper VI,
and the logistic regression approach could therefore not be used. For true
multiple habitat situations, an option is to incorporate temporal scales into
the MANOVA-models of compositional analysis. Even though this is a clear
improvement over pooled data and may provide valuble insight, the MANOVA
approach assumes that use is directly proportional to availability for each of
the temporal scales (i.e. b=1 for each factor, or equivalently, b=0 for the
pooled data if two temporal scales). Compositional analysis with several
temporal scales also requires a large number of radio-locations per individual,
especially if there are many habitat types, and a different approach was
therefore chosen in the following (paper VI).
Based on the arguments above and in the Introduction, it is
expected that food or cover alone may be poor predictors of roe deer habitat
selection. We tested these predictions with radio-telemetry data on habitat
selection by 27 roe deer, and contrasted this with similar data of 10
free-ranging domestic sheep on sympatric forest range in a mix of 12 habitat
types with different amounts of food and cover. The rationale for including two
species was to contrast one species (roe deer), where a trade-off was expected
to play a role in determining habitat rankings, with another (sheep) for which
no trade-off was expected. Habitat selection by sheep in Norway has earlier
been studied in several areas (Warren and Mysterud 1991, Warren et al. 1993).
Sheep are originally mountain animals with grouping and use of precipitous
terrain as their main antipredator tactic, and their habitat selection seem to
be little influenced by cover availability (Warren and Mysterud 1991, Warren et
As predicted, habitat rankings of sheep, as estimated with
compositional analysis, were highly correlated with food (grass) availability
at both the home range and study area scales (paper VI). In contrast, there was
no correlation between roe deer habitat rankings and food (herb) availability.
Even though there was significant correlation between habitat rankings based on
roe deer selection and cover availability, this result may be dependent on the
relative availability of food and cover areas (paper V). Roe deer used forest
habitats with more forage when active than when inactive, and tended to use
more forage rich habitats at night. This indicates that the selection of food
is traded with some other resource. However, the predictions (Table 1 in paper
VI) that roe deer use more open habitat when active or at night were not
clearly supported (using hiding cover gave similar results, A. Mysterud,
unpubl. data). Although the trend was in the predicted direction, this may
reflect that also other resources than food and cover, such as bedding
substrate (paper II), influence habitat selection by roe deer.
I have so far presented data on how ecological factors such as
food and cover, and mechanisms such as temperature and snow depth, affect
habitat selection by roe deer. I have to this point assumed that these measures
have direct and equal benefits to all individuals. This is probably not the
case, partly due to the social setting in which these animals live. In the
following, I will present data on space use of male and female roe deer during
summer, which hence is a study of the mating system (see Ch. of sp.), and also
on how the clan system may affect migration patterns of male and female roe
deer differently. The relevance of these sex-dependent patterns of space use
for habitat selection are discussed.
In mammals, only female reproductive success is limited by access
to resources, whereas male reproductive success is limited by access to females
(Trivers 1972, Davies 1991). The spatial distribution of females therefore
generally reflects the spatial and temporal resource distribution, whereas
distribution of males is influenced more by the distribution of receptive
females (Trivers 1972, Ims 1988), at least during the breeding
Male roe deer are territorial during the summer, whereas females live
solitarily with some spatial overlap to neighbouring females (see Ch. of sp.).
Range sizes of males and females are typically of equal size at densities
varying from medium to high (>10 deer/km 2, see Ch. of sp. and references
in paper VII). In Lier (3-5 deer/km 2, Mysterud 1993), however, males defended territories that were
almost twice as large as female home range size during the same period (summer)
(paper VII). Earlier, it has been shown that male territory size, but not
female range size, decreases with increasing density (ratio male/female range
size was 1.5 and 1.1, respectively, at 5-7 and 25 deer/km 2, Vincent et al. 1995). The
magnitude of the difference was nevertheless surprising, especially since the
size of male territories was similar to the size of female home ranges in
another low density area in Sweden (3-4 deer/km 2, Cederlund 1983). In the Swedish
study area, however, female home range size was large, and hence resource
levels probably low when compared to Lier. I therefore hypothesize that the
large territories of males in Lier, seen relative to female home range size,
may be due to the combination of low density (costs) and high resource
levels/small female home ranges (benefits) (the female dispersion hypothesis,
paper VII), although a skew in the sex ratio also has the potential to explain
this pattern (the male density hypothesis, paper VII).
This result thus provides some support to the suggestion that
male space use, and therefore possibly male coarse scale habitat selection
during summer, is more influenced by the distribution of females than the food
and cover distribution. All studies that are not experimental must face the
fact that there may be confounding factors underlying the observed patterns
(Ims and Yoccoz 1995). It is thus possible that males only track female habitat
use during summer also at finer scales, and that the rather similar pattern of
selection (paper VI), hence, arises for very different reasons.
It has also been questioned whether use of cover conveys any
direct benefit to adult females due to low levels of adult mortality due to
predation during summer (Tufto et al. 1996). The set of behaviours associated
with the hider strategy of fawns (see Ch. of sp.) also include changes in the
mothers behaviour (Linnell 1994). It has been suggested that predators may use
the mother as a cue to find the hidden fawn, and that this is why females use
cover during summer (pronghorn Antilocapra americana : Byers and Byers 1984; roe deer: Tufto et al. 1996).
Although data are limited, observations of yearling females (without fawn) do
suggest that their use of cover is rather similar to adult females (with
fawns). During winter, roe deer with following young tended to use more well
hidden feeding sites than those without (paper III, see also references therein
for other ungulates). However, the males also responded to period in the use of
cover, even though predation rates on adults are low in shallow snow (Cederlund
and Lindström 1983). Low predation rates may indicate that the behavioural
antipredator tactic is effective, and not necessarily that predation is
unimportant for behaviour. I therefore conclude that the mother-infant
relationship may not be necessary to explain why adult roe deer use cover
during summer, although it may affect the strength of the food-cover trade-off.
Typically, migrating cervids in the temperate region choose a
high elevation summer range and a low elevation winter range (see paper VIII
and references therein), and roe deer in Lier were no exception (paper VIII).
The downhill migration during autumn is regarded as a strategy to find areas
with less snow (e.g. Nelson 1995), whereas the uphill migration during spring
is less understood. It has been suggested that high altitude (coastal) summer
ranges have a higher food quality for red deer (Albon and Langvatn 1992),
whereas the opposite may be the case for moose (in the inland, Hjeljord 1997).
The increasing home range size of roe deer with increasing altitude in Lier,
and the low proportion of the population migrating to high elevations, suggest
that these areas are of low quality.
If snow depth is the only mechanism affecting migration, then
migration pattern of males and females should be equal, since there is minimal
sexual size dimorphy in roe deer (Loison et al. 1998). However, I found a lower
frequency of stationary animals and a higher frequency of long-distance
migrators among female than male roe deer in Lier (paper VIII); several females
migrated much further than expected from local snow depth gradients (see also
Holand et al. 1998). It has been reported earlier that female white-tailed deer
migrate further than males in some populations (Nelson and Mech 1981, Nixon et
al. 1991), and Wahlström and Liberg (1995a) observed that only female roe
deer migrated in flat terrain in Sweden. A possibility is of course that the
females choose habitats for their following (shortlegged) calves, though, the
propensity to migrate seem to be most common in single females (Wahlström
and Liberg 1995a).
A much more plausible hypothesis is linked to the dispersal
pattern and the winter clan system (see Ch. of sp.) of these cervids (Nelson
and Mech 1981, Wahlström and Liberg 1995a). It has been observed that
yearling female deer that disperse from their natal range in spring, return in
autumn and become part of their mothers winter clan (Nelson and Mech 1981,
Wahlström and Liberg 1995a, paper VIII). Dispersing males often join a
winter clan at their new location, probably due to their higher social rank. It
was therefore interesting that this pattern also occured in Lier, since
grouping tendencies was less evident at low (5-7 deer/km 2) than at high (25 deer/km 2) density in a forested habitat
in France (Vincent et al. 1995). Further, some individuals continue to migrate
after having their own calves (Wahlström and Liberg 1995a, paper VIII).
This suggests that other factors, such as familarity with the range (Clarke et
al. 1993, Stamps 1995), may also play a role (Wahlström 1995).
On a fine scale, Gilbert and Bateman (1983) found that the
presence of other individuals was the most important factor for bed-site
selection of white-tailed deer in an experiment where cover characteristics
were manipulated. Although conspecifics can be used as a cue to ensure the
choice of a suitable habitat (Smith and Peacock 1990, Reed and Dobson 1993,
Lima and Zollner 1996), it is likely that deer receive direct benefits from
group living through a good trail systems in periods of deep snow (Messier and
Barrette 1985) and increased vigilance (Linnell 1994). However, it is yet to be
tested whether roe deer in groups use habitats in a different way than single
This study provides empirical support to the hypothesis that
spatiotemporal scaling is crucial to the study of habitat selection by roe
deer. For example, the temporal scale (active/inactive, night/day) clearly
affected the trade-off between selection of food and cover. Reliable
predictions regarding the temporal and spatial scales of habitat selection can
only be inferred when the two scale dimensions are seen together. For example,
cover affected roe deer selection of bed-site at both a patch and habitat
scale, but cover only affected selection of feeding site at a habitat scale.
The strength of the food and cover trade-offs, and hence the demographic
consequences, remains to be explored. More data on sex (and reproductive
status) and age dependent food-cover trade-offs are also needed to further
advance our understanding of the adaptive basis for roe deer habitat selection.
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