Andreassen et al 1996.pdf

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1/9

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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2/9

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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3/9

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



4/9

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



5/9

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



6/9

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



7/9

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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