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Optimal Width of Movement Corridors for Root Voles: Not Too Narrow and Not Too WideAuthor(s): Harry P. Andreassen, Stefan Halle and Rolf Anker ImsSource: Journal of Applied Ecology, Vol. 33, No. 1 (Feb., 1996), pp. 63-70Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/2405016.
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Journal f
Applied cology
1996, 3,
63-70
Optimalwidth fmovementorridors or ootvoles:
nottoo narrow ndnot oo wide
HARRY P. ANDREASSEN, STEFAN HALLE and
ROLF
ANKER
IMS
Division fZoology,Department fBiology,Universityf Oslo, PO Box 1050
Blindern, -0316 Oslo,
Norway
Summary
1. The
characteristics
f male root vole movements
s a
function
f
corridorwidth
were ested
n
a
310
m
ong
habitat orridor
onnecting
wo habitat atches.Detailed
observations
f movements eremade by means of radiotelemetrynd recording f
footprints.
2. The highest onnectivity,
n
terms
f
transferenceate
f ndividuals
n
the
orridor
system,
as observed
n the
ntermediate
f three orridorwidths ested
3 m,
1
m and
0
4
m).
3. The behaviouralmechanism ehind
he
owerconnectivity
f the narrowest or-
ridorwas a reluctance fvoles to enter t,while inearprogressnthewidest orridor
was
hampered y
a
high
frequency
f
cross-directional
ovements.
4. The relationship etween
orridorwidth nd movement ehaviourwas unaffected
by
the imulated
resence
f competitors
nd
predators.
5. Our results
hallenge
he the-wider-the-better'
rinciple
f
movement
orridor
design,
nd
provide
lements
or n
understanding
f the behaviouralmechanisms
underlying
hemovement
cology
f ndividuals
n
inearhabitats.
Key-words:onnectivity,
orridor
esign,Microtus,
movement
cology.
Journal
fApplied cology
1996) 33,
63-70
Introduction
Habitat ragmentationasbeen ecognizeds a major
threato wildlifeopulationsDiamond 976;Gilpin
& Diamond 980;Higgs& Usher 980; oule 1986;
Lande 1988; ms & Stenseth 989).As continuous
habitats ecome ragmented,n immediateonse-
quence
s that he
mobility
f
organisms
ecomes
restrictedFahrig& Merriam 985; Stamps t al.
1987a,b; ahrig& Paleheimo 988;Burkey 989).
This n turn ecreaseshe ffectiveize, nd, onse-
quently,
he
viability
f
populationsSoule 1986;
Boyce1992). t has been laimed hat henegative
effectsf habitat ragmentationan be reduced y
connecting
solated
ragmentsy
narrow
trips
f
habitat,ermed ovementorridorsWilson Willis
1975;Harris 984;Bennett 990; aunders Hobbs
1991). n the resentaperwe
will
dhere
o the
trict
definitionf movementorridorss linear abitats
allowingormovements,ut ot or ermanentettle-
ment.
The establishmentf movementorridorss cur-
rentlymplementeds an expensive
onservation
strategy
orld-wide
Simberloff
t
al. 1992;
Mann&
Plummer 993). As very
few tudieshave been able
to obtain data about animalmovements
n corridors
(Hobbs 1992), heempirical asis for orridor esign
is poor Simberloff& ox 1987; imberlofft al. 1992;
Mann & Plummer 993).
For instance, he effects f
structural spects
of habitat
corridors,
uch as cor-
ridorwidth nd continuity, n movement ates are
poorly known. Still, specificrecommendations n
optimal orridor esignmay
be found n the iterature
(Noss 1987; Harrison
1992; Merriam & Saunders
1993). The urgent eed
for mpirical tudies ddress-
ingbasic questionsregardingmovement cology
of
individuals in linear habitats, has recentlybeen
stressed
see
Hobbs 1992; nglis
& Underwood
1992;
Simberlofft al.
1992;
Lindenmayer Nix 1993).
We here
resent
ata from
study
wheremovement
behaviour of
the
root vole Microtus oeconomus
(Pallas)
in
corridors f varying idthwas subject o a
detailed
nalysis.
We usedcorridorwidths themain
treatment
ariable,
ecause
t
s themostbasic struc-
tural haracteristic
f
inear abitats pon which rac-
tical corridor
design nevitably equires
decision.
Root voles were selected s the
studyorganisms
s
they
have
proved
to be veryusefulmodels forfield
63
?
1996
British
Ecological
Society
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64
Optimalwidth f
movement
corridors
experimentsnvolving abitatmanipulationsIms &
Stenseth1989; Ims et al.
1993;
Wiens
et
al.
1993).
Moreover, lthoughweprimarily esigned his tudy
to serve s an empirical
model system sensu
ms
&
Stenseth 989) for
ddressing
asic
biological
mech-
anisms that mpinge n
corridor esign e.g.
move-
mentbehaviour), ur results
may
have some direct
implications or he onservation fremnant ootvole
populations nEurope van Apeldoorn
t
al. 1992).
We show hat ptimalwidth fmovementorridors
in terms f habitat
connectivity
or root voles is
a
compromise
betweennot too
narrow and
not too
wide. Hence, our results hallenge he commonpre-
sumption hatwider orridors ecessarilyre better
corridors Noss 1987; Harrison
1992;
Merriam &
Saunders
1993).
Methods
STUDY PLOT AND MANIPULATIONS
The study ookplaceatEvenstad ieldStation, outh-
east
Norway uring ugust-October
992.The move-
ment behaviourof
individualmale root voles
was
studied n a fenced
ystem
onsisting
f two habitat
patches
onnected
y
a 310
m
long
corridor
Fig. 1).
The
length
f the
corridorwas about one order
of
magnitude onger than the diameter f an
average
male rootvole homerange
n
non-linear abitatsIms
et
al.
1993,
nd
unpublished).
The twohabitat
atches
sed as release
oints size:
5
x
5 m) and the
corridor onsisted f dense,homo-
geneous
meadow
vegetation,
nownto be
preferred
habitat
by
root voles
(Ims et
al.
1993).
An
artificial
runway ystemFig. 1)
was established
s
0
1
m
wide
vegetation-freeaths
long
and acrossthe orridor o
facilitatemovements
n
theotherwise
ery
ense
grass
carpet:
one 310m
long mid-runway
nd cross-run-
ways
t
1
-m ntervals. hese
runways
ended o direct
movements cross footprint lates see below).
The
area between he fences nd thecorridorwas cleared
of vegetative over by means of herbicides Fig.
1).
The fenceswere established o hinder he intrusion
of othervoles and mammalian redators uring he
experiment.We expected hat the absence of female
voles should motivate xploratorymovement Ims
1988) or breedingdispersal Kawata 1989) in the
experimentalmales. To ensure that the olfactorial
environmentn the orridor t the tart fthe
xperi-
mentwas not very ifferento the situation ateron,
some adult males were temporarily eleased
n
the
corridor few
days
before he
first
xperimental
rial
was executed.
We tested hreedifferentorridorwidths
n
com-
binationwith
bsence/presence
f
predators
nd
com-
petitorsTable 1),
which lso
may
affectmovements
(Saunders & Hobbs 1991). The study tarted
with
trials
n
the widest orridor 3 m).
The two narrower
corridorsI
m
and
0 4
m)
weremade
subsequentlyy
mowing nd herbicide se (see time chedulefor he
various reatmentsn Table 1).
Presence f
competitors
nd
predators
were imu-
lated by
six
barrier ages' which
were
permanently
situated
cross the orridor t fixed ntervals
Fig. 1).
The barrier
ages
were made of wire mesh with a
central
assage
tunnel
ncompassing
he
mid-runway
of the corridor.
he
length
f
the barrier
ages
was
adjusted ccording
o the
decreasing
width f the or-
ridor.
For
the treatmentcompetitor' Table 1),
one
live adult male was
kept
n
each barrier
age.
Earlier
studies n whichwire mesh
cages
of a similar ype
have been used to simulate
presence
f
conspecific
individuals
n
microtines
ave
produced significant
responses Ims 1988, 1990;
Nelson
1994).
For
the
treatment
predator',
he barrier
ages
were
upplied
with resh ox
cats ealed
n
small,perforated lastic
boxes. Simulated
presence
f
predatorsby
artificial
additionof
predator
cents
have earlierbeen shown
15m
30m 60m
lOOm
60m 30m
15m
I
i
Iiu
Fence
0
Barrier
age
-
Vegetation
- - -
Runway ystem
* Release point A Tracking late
Fig. . Design f he tudy
lot. he wo abitatatchesnd
he orridoronsistedf ense,
omogeneouseadow egetation
(shaded
rea).The reaoutside he orridor
white rea nside he ence)onsistedfbare
ground. heenlargedection
f
the orridorinset) hows he esign f he
rtificialunwayystem,elease
oint nd he ositioningf rackinglatesn
the
3 mwide
orridor.
unways ere stablished
s 0
1
mvegetation-freeaths
long mid-runway)nd crosshe orridorcross-
runwayst
1
-m ntervals).
rackinglateswere laced n
both ypes frunwaysexcept
n the ross-runwaysor he -4
m
corridor)s well s along he dges f he orridor.
?
1996 British
Ecological
Society,
Journal
fApplied
Ecology,
3, 63-70
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65
H. P.
Andreassen,
S. Halle
&
R.
A.
ms
Table 1.
Experimental rotocol:
Competitor,
ther
dult males
n
thebarrier ages Fig. 1);
Predator, ox cats n thebarrier
cages;
Control, mpty arrier ages. Ten different
ale rootvoles wereused neachofthe9
treatmentypes
Number
f samplerecords
Treatment
Period*
Radiotrackingt
Footprintst
Width,
m
Control
1-9 Aug 10
5
Predator
10-16 Aug
10
2
Competitor 18-26 Aug 9 5
Width,1m
Competitor
26
Aug-i Sep
10
5
Control
2-7
Sep
10
5
Predator
8-15 Sep 10 4
Width,
-4
m
Predator
19-23 Sep
10 5
Competitor
24-8 Sep
9
4
Control
29 Sept-3Oct 10 4
*
The
variable
engths
f the
experimental eriods
e.g. periods
> 5
days) were caused by instances
n
which
ndividual
males n a
trialwerenot
caught
before
8.00
h
the
next
ay.
t
Reduction n the
ample
ize
from
0to
9)
of
radiotracking
ecords or wo
of
the
reatments ere aused
by
dysfunction
of radiotransmitters.
t
Thereductionsn thenumber ffootprintecords rom maximum ffive= number f xperimentalaysper reatment)
for
four
reatmentypes
were
aused
by heavy
ainfalls
making ootprint
lates
unreadable.
to affectpace
use
in
microtinesJedrzejewski
t
al.
1993). For the
control' treatment,he barrier
ages
were eft
mpty.Before he onset of each
treatment,
the barrier
ages were
carefullywashed
to
remove
remaining cent from he previoustreatment. ur-
thermore,
he
cages
were
always placed
on
plastic
sheets
o
prevent
he
oil
beneath
he
cages becoming
contaminated
y
scent.
TREATMENT AND
MONITORING OF THE
STUDY ANIMALS
On
each
day,
wo
radiocollared
Biotrack,
S-2 trans-
mitters),dult male voles were
released
t
08.00
h
in
the two
habitat
patches
n
the
opposite
ends of
the
corridor
one
male
per patch,
cf.
Fig 1).
Each
trial
was terminatedt
18.00
h.
Thereafter,
he
males were
caught y
means f
ive
raps
ctivated
n
the
orridor.
Trials were
generally
onducted n
consecutive
ays,
provided that males from
the
previous
trial
were
caughtbefore 8.00h thenextmorning.
Longitudinal
movements
n
the orridor
ere
moni-
tored
by
locating
he males
to
the
nearest
metre
n
the
ongitudinal
irection f
the
corridor) ymeans
of a
probe
antenna
Andreassen t al. 1993) at
every
fullhour
from
9.00
h
to
17.00
h. The
resulting
ine
radio-trackingositions btainedper
male
wereused
to estimate he
distance
moved,
the
speed
of
move-
ment nd the
ime
pent
n
the
orridor
or
ach
male
(for urther
escriptionf ongitudinal ovement
ar-
ameters,
ee footnotes o
Table
2).
Cross-directional
se of
the
orridor
was
registered
by
the
footprint
ecordingechnique
f
King
&
Edgar
(1977). Devices for
recording ootprints
tracking
plates) were situated
long the corridor
dges and
in themid-runway
nd thecross-runwaysnsidethe
corridorFig. 1). The use of corridor
dges and
the
two runway ypesby
voles were cored
ccording
o
the
number fplateswith ootprints
t the nd of ach
trial. ootprint ounts
were ecorded er
xperimental
day rather han per
individual
ince
footprints f
a
pair
of
ndividuals eleased
imultaneouslyould
not
be distinguished henever
hey adpassed each
other
in
the
orridor. adiotrackinghowed hat uch
ross-
ing
occurred
n
at least 29 of
the trials.Due to
the general ow sample
size offootprint ecords nd
missing ata, eading o
unbalanced esigns
Table 1),
statistical
ests
f
competitor/predatorreatment
ere
not
applied
to the
footprint
ata.
The malesusedinthis tudywere
aboratory orn
F2-F4
generation escendants f
animalscaught n
Pasvik,
north-eastNorway. We chose to use
lab-
oratory-raisednimals o
standardize rior xposure
to factorswhichmighthave influencedmovement
behaviour.
Males were he hosen ex s
they remore
suitablemodels'
than
females or his
ype f
study;
they ppear to be more
motivated
o
disperse
Ims
1989;Kawata 1989),
nd they re ess ubject o
short-
term
behavioural hanges due to
reproductive or-
mone
cycles
hat would
have
introduced
xtraneous
variance
n
thedata.
Before he
rials
n
the
orridor,males
werehoused
separately
n
laboratory ages exposed
to
constant
light,
o
mimic
he
natural ight
regimeduring
he
summern
north-east
orway,
s well s to
avoid any
effects f
changing
hotoperiod n the course
of the
?
1996
British
Ecological
Society,
Journal
fApplied
Ecology, 3,63-70
This content downloaded from 139.133.69.143 on Wed, 9 Jul 2014 08:43:02 AMAll use subject to JSTOR Terms and Conditions
http://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jsp -
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66
Optimal
width f
movement
corridors
Table
2.
The effect fcorridorwidth n movement
nd corridor se parameters. ata are lumpedfor he treatmentactor
absence/presencef
competitor/predator.alues are presented
s
mean
+-standard
error f themean
Corridorwidth
Parameters 3 m
I
m 0 4m
Longitudinalmovements*
Maximumdistance eached
m) 75 +
16 205
+
22
35 + 15
Cumulative istance raversed
m) 163 + 33
440
+ 57
51
+
21
Movement
peed/efficiencyt
Timespent ncorridorh) 7 9 + 0't 5 9 + 0 4 3-9+ 1 1
Maximum peed m/h) 61 + 11 160
+ 19
93
+
24
Average peed m/h) 21
+
4 76
+
11 51
+
13
Progress ate m/h)
10+ 2 40 + 10 35
+
10
Cross-directional ovementstfootprintounts)
Edge
counts 1 42 +
0 51
0 50
+
0 20 0 21
+
0 11
Edge/mid-runway
atio
0
23 + 0-09
0
03
+ 0-01 0 09 + 0-07
Cross-runway/mid-runway
atio
1
56 + 0 31 0-36
+
0 05
*
Longitudinalmovement
arameters
re based on
the
complete
ata set of
radiotracking
ecords
Table 1).
Maximum
distance
eached
=
maximum
istance
eached rom
elease
atch.
Cumulative istance raversed cumulativemoved
during
the rial s revealed
yhourly adiotracking.
t
Movement
peed/efficiencyarameters
nclude
nly
rials n whichmales
entered hecorridor
N3
.
=
26,
NJ...
30 and
No
,
=
8).
Time
spent
n corridor total ime
eriod
male was found n the orridor.Maximum
peed
=
longest
istance
movedduring ne hour.Average peed= totaldistance/timepentncorridor. rogress ate= maximum istance/timepent
in
corridor.
t
Cross-directionalse
parameters
re based on
footprint
ount records
Table 1)
from he
footprint lates
ituated
long
thecorridor
dges edgecounts),
n
the
mid-runway
nd the
cross-runwaysFig. 1).
No estimate
f
the ratiovariable
cross-
runway/mid-runway),
s
given
or he0
4
m
corridor,
ince
footprints
erenot
recorded
n the
ross-runways
or
his orridor
width.
experiment.
ll males
used
were
sexually mature,
adult animals
>
50
g
and scrotal
estes),
nd thusof
adequate
sizefor he
2
5
g
radiocollars.
Twenty-six ifferent
ndividuals
were vailable for
this
tudy.
Most ofthesehad
thus
o
be used
n
several
trials X
= 2
6 + 0
1
[SE] trialsper male), but never
more than once per
treatmentombination.Gener-
ally,10
different aleswere
ested er
reatment
om-
bination Table 1). Males used
n
more han ne treat-
ment were released
n
alternating atches with the
longestpossible nter-trial
nterval o avoid habitu-
ation to the
tudy egime.
Number f trials
er male,
when included as
a
covariate
in
ANCOVA-models,
never xplainedmore han of
variance P > 0 50)
of
any of the movementndices stimatedTable 2).
This result ndicates hatneither abituation orother
possible onfoundingime
ffects,.g. scent ccumu-
lation
see
also
Discussion)
were
important
n
this
study.Males partneredn trials nfrequentlyrossed
each other
n the
corridor
29
of
all
trials)
so it
is
unlikely hat direct nterferenceetween he free-
rangingmales nfluencedhe
results. ence,we find t
justifiable o treat ach trial
er
male as the tatistical
unit
cf.
Table
1)
in
the
nalyses.
Results
Corridorwidthhad a substantial ffectn all move-
ment arametersTable 2)
(two-way NOVAS, all par-
tial
r2
0
25,
P
< 0
001). Presence/absencef pred-
ators nd
competitors
ere
elativelynimportant
or
all
parametersested
with
espect
o
this reatmentall
partial
2
< 0 04,
P
>
0 05).
Voles
generally
moved furthestnd fastestn the
intermediate width (1 m) corridor (Table 2).
Maximum distancesreached by voles in the inter-
mediate width
corridor
were
on
average three nd
sixtimes onger han n the wider nd the narrower
corridors, espectively.
n
the intermediateorridor
more han
one-third
f the
voles reached heopposite
end,
while
this rarelyhappened n the wideror the
narrower orridor Fig. 2).
The
low successof the narrowest orridor
n
con-
necting
he
habitatpatcheswas caused by reluctance
of voles to enter t
(Fig. 2);
the
narrowest
orridor
(04 m) was entered nly
n
27 of the trials,while
the two
wider orridors
were
ntered
n the
majority
of
the trials 100 and 83 entrance ate
n
the
1
m
and the 3m corridor, respectively) X2= 40.3,
d.f.= 2, P < 0 001). Thisapparent version y voles
to the
narrowest orridors corroborated y the hort
time
spent
in
this corridor
by voles that actually
enteredt;on average,males enteringhewidest or-
ridor pent wice s much ime
n
the orridor s males
entering
henarrowest orridor
Table 2).
The
speed
of linear movementwas
considerably
lowerformales n the 3 m wide corridor ystem han
in
the two
narrower ystems.
or
instance, he pro-
gress
ate
expressed s maximum istance eached
n
thecorridor
ividedby
time
pent
n
corridor)
n
the
1
m wide
corridorwas
on
average four imeshigher
than n the
3
m
wide
corridor,
hereas his
parameter
?
1996 British
Ecological Society,
JournalfApplied
Ecology,
3, 63-70
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67
H. P.
Andreassen,
S. Halle
&
R.A.Ims
25 Corridor idth: m
20
15
10
+
25
Corridor idth:
m
201.
15.,
10
+
5
F
171
251+
Corridor idth: -4m
20O.
15fl
101
of
0
60
120 180
240
300
Fig. . Frequency istribution
f maximum istances
reached20-m ntervals)rom
he elease
oint seeFig.1)
for he hreeestedorridoridths. lack ars t the nd f
the orridor
ive
henumberf timesmales id not eave
their elease atch
r
reached heopposite atch, espec-
tively.
rrowsenote hemediansf he hreeistributions.
did
not differ
ignificantlyetween
he
1
m and 0
4
m
corridor
Table 2).
The
footprint
ecords ndicated
n increased se of
corridor dges edge counts;Table 2) with ncreasing
corridorwidth. he two ratios
edge counts/mid-run-
way
counts'
and
'cross-runway ounts/mid-runway
counts',
whichboth
may
serve s indicesfor cross-
directional se
of
the
corridor,
had
highest
mean
values in the
3
m
wide
corridor
Table 2).
This indi-
cates that therewere
more
cross-directional
ove-
ments n thewidest orridor han
n
the wo narrower
corridors.
Discussion
Many
observational tudies ave ndicated hat inear
structures
n
fragmentedandscapes may
play
an
important
ole as movement orridors nd
hence,
influence he abundance and
spatial structuring
f
fragmented opulations (e.g. Wegner& Merriam
1979;
Middleton & Merriam
1983;
Hansson
1987;
Szacki
1987;
Verboom
&
van
Apeldoorn1990;Zhang
& Usher
1991; Fitzgibbon1993). However,
very
ew
studies
have
assessed movement
ates of
individuals
as functions f
qualitative
nd
quantitativespects
f
such
corridors,
hich n
turn ould serve s empiri-
cally
based
guidelines
or
design
of movement or-
ridors
or
onservation
urposes.Preferentially,
uch
guidelines hould stemfrom xperimentaltudies n
which the causal relationship between corridor
characteristicsnd connectivityould be established.
The great ifficultiesf conducting tudies ddressing
questions about corridordesign,for which
all
the
required lements f proper xperimentalesigns i.e.
randomization f reatmentombinationss to ensure
independent easurements)re satisfied, as recently
been discussed
y nglis
& Underwood
1992).
Our study
acks
some
statistical
igour
f a
proper
experimentn the ense hatwe werenot able to have
more than one corridor ystem.To obtain proper
randomization f treatment ombinationswiththe
same sample
size
would have required
45
corridor
systems. The present study should therefore e
regarded s a thoroughly ontrolled,manipulative,
observational tudy ather
han
proper xperiment.
However,we
are
not
able
to imagineconfounding
variables that could have producedthe very pro-
nounced pattern f our results.With respect o the
most obvious confounding actor
of the
study, .e.
time, ur results
re not
compatible
with consistent
timebias, e.g. brought bout by
an
accumulation
f
scent hrough
ime.
uch
a bias shouldhave
generated
a
general
ecrease r increase
n
movement stimates
bothwithin nd between he reatments.
Our results orroborate hree arlier tudies how-
ing that structural spects
of
corridors ffectmove-
ment
ates
n smallmammals
Lorenz &
Barrett
990;
Merriam& Lanoue
1990;
La
Polla & Barrett
993).
However, ur data
do not end
support
o the
notion
thatwider orridors ecessarily onnecthabitatpat-
ches more
fficiently
hannarrow orridors
e.g.
Har-
rison
1992).
Of three orridor idths
ested,
he
nter-
mediate width stimulated
he
highest requency
f
long-distance movements. Similar results have
recently
een obtained
for
meadow
voles
Microtus
pennsylvanicusLa
Polla & Barrett
993)
undermore
natural, ut esscontrolled onditions.
n
themeadow
vole
study,
herewere ess movements
n
5
m
than
n
1
m
wide corridors
onnecting
abitat
patches
with
free-rangingubpopulations
f
voles. Whereas the
meadowvole corridors ere nly10
m
long La Polla
&
Barrett
1993)
and
probably supported mainly
within-home
ange movements,
he
long-distance
movementsn our 310 m
long
corridor
re
probably
more
representative
or
xploratory
movements ur-
ing dispersal sensu
Stenseth Lidicker
1992). Typi-
cally exploratorymovements, .g. during dispersal
events,
ake
place
on
ground
which
s
novel
to
the
moving nimal this tudy),whilewithin-home-range
movements
the study
of
La Polla
&
Barrett
1993)
are
normally
ased
on
spatial memory Ims 1994).
However,
he
elationshipetween orridor idth
nd
movement ate was the same for the
two
Microtus
species, despite
the fact
that
they
were studied
n
different
ettings, resumably erforming
ifferent
movement
ypes
at different
patial
scales
(Wiens
1989;
Johnson
t al. 1992).
This
bears promisefor
generalizations.The potential robustness of our
results s furthermphasized y thefact hat hegen-
eral relationship etween orridorwidth nd move-
ment ehaviour n M. ceconomus as not changedby
?
1996 British
Ecological Society,
Journal fApplied
Ecology, 3, 63-70
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68
Optimalwidth f
movement
corridors
circumstancesuch s simulated resence f
predators
or competitors
n the
corridor.
he
apparent
unim-
portanceof thesetwo
potentiallymodifying actors
mighthave been due to a general ailure o
simulate
predators and competitors dequately.
However,
similarmethods sed in earlier tudies although
not
conducted
n
corridors) roduced
lear
responses
Ims
1988, 1990;Jedrzejewskit al. 1993;Nelson
1994).
The
detailed bservations
made during hepresent
study uggest everal
behaviouralmechanisms hat
may accountfor the variation n connectivity
ith
corridorwidth.
The
low efficiency
f
the
narrowest
corridor
n
transferring
oles
between
abitat atches
seemed o
be
caused by a behavioural version y
the
voles
to
enter
he
corridor cf. the ow entrance ate
in the 04
m
corridor; ig. 2). Moreover,
voles that
entered he corridor pent
ess time n the narrow
and the ntermediateorridor ompared
o the
widest
corridor
Table 2).
For the ntermediate idth orri-
dor, however,
the short residency
ime
probably
reflectshehigh
inearmovement aterather han ny
aversion ehaviourcf. 100 entrance ate n the1m
wide
corridor; ig. 2).
The voles
frequently
eached
the
opposite
nd
quickly
when
ested
n
the
1
m
wide
corridor.
he
ow
inearmovement
peed
n thewidest
corridor, espite relatively igh
ntrance
ate
83 ),
may
havebeencaused
by
a
highfrequency
f
cross-
directional
zig-zag)
movements.
his
was indicated
by
the
highproportionate
se of
cross-runways
nd
edges
n the 3 m
wide
corridor
cf.
the
ratio-variables
based
on
footprint
ounts;
Table
2). Zig-zaggingmay
be a movement
mode
which
s
commonly
sed
by
animals
n
the absence of
physical
orientation
ues
(Bell 1991;
Ims
1994).
In movement
orridors,
he
edges
re
ikely
o
function
s themain ue
facilitating
directionalmovements.
While
nimalsmoving
n nar-
row corridorsmay
be
able
to
perceive heedges
even
from
he
nterior f thecorridor, ig-zagmovements
may
be
necessary
n
wider inear
trips
f habitat
o
obtain
frequent
isual ontactswith he
dges.
n
fact,
animals
will
probably
ot
perceive
linear
atch
s a
movement orridor bove a
givenwidth,
but as an
elongated
habitat
see
also
La Polla
& Barrett 993).
It has
recently
een mphasized
hat
eneral rinciples
in
landscape
ecology will most likely merge
from
detailed studies
unravelling
he behavioural mech-
anisms nvolved n the movementcologyofspecies
(Johnson
t al.
1992;
Wiens et al.
1993;
ms
1994).
n
particular,
we believe that this
argument
pplies
to
the
generation
f
principles
f movement orridor
function
nd
design.
Mortality
ates
of
animals
n
movement orridors
have never een
stimated,lthough
t s an
extremely
important
actor etermining
hether orridors
unc-
tion as
links
or
sinks
sensu
Pulliam
1988)
in the
dynamics f fragmentedopulations Fahrig
& Mer-
riam1985;Henein& Merriam 990). till, ne premise
underlyinghe presumption
hatwider orridors re
better orridors s
an inverserelationship etween
mortalityate nd corridor idth.Movements n nar-
row
orridors
rebelieved
o be more isky hanmove-
ments n wide corridors
ue to a higher nfluence f
various dge effects,uchas predation Simberloff
Cox 1987; Noss 1987; Andren & Angelstam1988;
Henein & Merriam 1990; Saunders & Hobbs 1991;
Harrison 1992; Merriam
& Saunders 1993; Paton
1994). The voles' reluctance o enter he narrowest
corridorn our study reflectedy low entrance ate;
27 ), may reflect
isk-aversion ehaviour McNa-
mara & Houston 1987). However, our results lso
pointto thepossibility f
a
higher ccumulated
mor-
tality
isk
n
wide
corridors ince voles
spent
more
time
n
the widest orridor, nd
since
they
used the
corridor dges
more
ntensivelycf. number
f
edge
counts;Table 2). Studies pecificallyesigned
o mea-
sure how mortalityates
hangewith orridorwidth,
will be an
important
ext
tep toward
a
knowledge
about which
characteristics aydetermine hether
corridors unction
s links or sinks n a
landscape
context.
Conclusion
We
have shownthat theefficiency
f linearhabitats
as corridors onnecting solated habitat patches
s
dependent
n
their
width,
ut
in a different anner
than ften
upposed.Furthermore,
e have dentified
what
may
be
important
behavioural mechanisms
underlying
ovement
atterns
n
inearhabitats.
Acknowledgments
Ottar
Bj0rnstad,
orun
auske,
Barbara
Halle,
Edda
Johannesen,
iotr
Pawlak, Gytis Racius
and
Anna-
Barbara
Utelli
participated uring
hefieldwork.Bar-
bara
Halle
computerized
he
data.
Ottar
Bj0rnstad,
Gary Fry,
Lennart
Hansson,
Karine
Hertzberg,
ar-
ald
Steen
and Nils Chr. Stenseth
rovided
valuable
comments o drafts
f
the
manuscript.
his
study
s
part
of
the
project
The effect f habitat corridors
on
biodiversity
n
agriculturalandscapes'
which s
supported inanciallyy
heResearch ouncilofNor-
way NFR).
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