RESPONSE OF BLACKBEARS AT COLDLAKE,ALBERTA,TO
DEMOGRAPHIC
THE REMOVAL
OF ADULTMALES
GLEN A. SARGEANT, U.S. Geological Survey, Northern Prairie Wildlife Research Center, 8711 37th Street Southeast, Jamestown,
ND 58401, USA, email: [email protected]
ROBERT L. RUFF, Department of Wildlife Ecology, 226 Russell Labs, 1630 Linden Drive, University of Wisconsin, Madison, WI
53706, USA
Abstract: Previous reportsdescribedan increase in populationdensity following the removal of 23 adult and 3 subadultmale black bears (Ursus
americanus)from a 218-km2studyareanearCold Lake, Alberta(the CLSA). This findingplays a centralrole in continuingdebatesover population
regulationin bears,buthas been criticizedbecausedensityestimateswere based on assumptionsthatwere not met. Moreover,subsequentdiscussion
has been predicatedon conjecturethat humanexploitationhad minimalinfluence on populationdynamics. We used improvedmethodsof markrecaptureand survival analysis with a combinationof new and previously published data to resolve these issues. Jolly-Seberestimates suggest
numbersof females using the CLSA were relatively stable from 1969 to 1971, then more than doubled when males were capturedand euthanized
during 1971-72. Numbersremainedhigh until 1975, when they declined abruptly. Similarly,numbersof subadultmales capturedannuallyincreaseddisproportionatelyin 1972 andremainedhigh until 1976, when they also declined. Changesin numbersof subadultmales capturedindicated
changes in local density because they were not causedby changes in captureor settlingrates. These findings supportpreviousdescriptionsof trends
in bear density at Cold Lake. However, survivalrecords of 56 bears radiomarkedduring 1974-77 revealed heavier exploitationthan previously
suspected. Annualmortalityratesof radiomarkedbearsrangedfrom 14 to 51%,andhumanscaused at least 25 of 30 observeddeaths. Consequently,
an underlying assumptionof previous interpretations-that the Cold Lake bear populationwas naturallyregulatednear carryingcapacity-no
longer seems reasonable. Ourresults suggest that adultmales deterredbearsin other sex-age groupsfrom using the CLSA; however, we found no
evidence that birth or death rates were affected. Thus, the observed increase in local density should not be construedas a density-dependent
response. Abruptchanges in local density might not have occurredif males had been removedfrom a largerareaencompassingthe CLSA.
Ursus 12:59-68
Key words: Alberta,black bear,compensatorymortality,density dependence,limiting factors,mark-recapture,populationdynamics,population
regulation,Ursus americanus
Naturalregulationof bear populationsultimately involves density-dependentprocesses(Taylor1994). These
processes, however, are poorly understoodand a subject
of continuingcontroversy.Some have suggestedthatadult
males regulate bear populations by killing or evicting
younger males and, to a lesser extent, females (Bunnell
andTait 1981, McCullough 1981, LeCount1993). If so,
removingadultmales by huntingmight increasesurvival
rates of subadultand female bears. Otherscontend that
evidence of densitydependenceis lackingfor blackbears
andwarnagainstharveststrategiesthatanticipateincreases
in survivalor reproductionwhen populationsarereduced
(Miller 1990a,b; Garshelis 1994; Taylor 1994).
of an experimentbegunby Kemp(1972,
Interpretations
and
continued
1976)
by Young and Ruff (1982) are central to this ongoing debate (Garshelis 1994). To determine whether adult males mediated density-dependent
changesin demographicparametersof a blackbearpopulation at Cold Lake, Alberta, Kemp (1976) and Young
andRuff (1982) used mark-recaptureto estimatepopulation size before (1968-70), during (1971-72), and after
(1973-75) an experimentalremovalof adultmales. The
populationwas thoughtto be unexploitedand naturally
regulatedpriorto theremoval(Kemp 1976). After4 years
of relative stability before the removal, populationestimatesmorethandoubledfrom 1971 to 1973 andremained
relatively high through 1975. Moreover,disproportionately large numbersof subadultmales were capturedan-
nuallyduring1972-75. Kemp(1976) andYoungandRuff
(1982) concluded that adult males regulatedpopulation
density by controllingrecruitmentof subadults. These
results are often cited as evidence of density dependence
in black bears (Garshelis1994).
Garshelis (1994) subsequently identified potential
sources of bias in populationestimates of Kemp (1976)
and Young and Ruff (1982): Perceivedchanges in population might have resultedfrom (1) age- and sex-related
differencesin recaptureprobabilities,combinedwith the
removal of adult males from the population,and (2) increased numbersof transientbears passing throughthe
areawhen male density was low. Though Kemp (1976)
inferredan increasein settlingratesof subadultsfrom increasedrecapturerates following the removal, Garshelis
(1994) pointedout thatthese high rateswere observedin
only 1 year;thusthe datadid not supportinferencesabout
changes in emigration, immigration, or settling rates.
Garshelis(1994) concludedthatKemp(1976) andYoung
andRuff (1982) reachedunjustifiedandpossibly erroneous conclusions about changes in the Cold Lake black
bearpopulationand called for a re-analysisof theirdata.
We analyzeddatafirstpresentedby Kemp(1972, 1976)
andYoungandRuff (1982), unpublishedmark-recapture
records for 1976 and 1977, and unpublished survival
records for bears radiomarkedduring 1974-77. Objectives of this paper are to (1) use improved methods of
mark-recaptureanalysis to reassess evidence for an in-
60
Ursus 12:2001
crease in bear densities following the removal of adult
males from the CLSA; (2) estimate mortalityrates, identify causes of death, and reconsider the conjecture that
bear densities were naturallyregulated prior to the removal; (3) determinewhethernumbersof subadultmales
capturedon the CLSA were indicative of trendsin numbersusing the area;and(4) reinterpretthe Cold Lakestudy
with respect to density-dependentpopulationregulation
in black bears.
STUDY AREA
The 218-km2Cold Lake bear study area (CLSA) was
located along the Alberta-Saskatchewanborder,240 km
northeastof Edmonton,Alberta (Fig. 1). Boundariesof
the CLSA were definedby Cold Lake andthe provinceof
Saskatchewanto the east, the Medley River and Marie
Lake to the west, the PrimroseLake Air WeaponsRange
to the north,and by agriculturallands to the south. The
CLSA was closed to sport hunting, but bears could be
takenby native people. Some bears that used the CLSA
also used areas where they could be killed by huntersor
to controldepredations.
Topographyof the CLSA was generallyflat except near
the Medley and Martineaurivers. Elevationrangedfrom
535 to 702 m. Vegetation was dominated by aspen
(Populus tremuloides) and mixed stands of aspen and
spruce(Picea glauca), which occupied 66% of the landscape. Dominantunderstoryplantsin these areasincluded
alder (Alnus spp.), rose (Rosa spp.), and lowbush cranberry(Viburnumedule). The remainingthirdof the area
consisted primarilyof muskeg (24%) and spruce (6%),
habitatsthatalso dominatedthe landscapeto the east and
west of the CLSA. The climate was northerncontinental,
with average monthly temperaturesranging from -19?C
in Januaryto 17?C in July. Annual snowfall averaged
138 cm, and snow cover usuallypersistedfrom late October to earlyApril. More detaileddescriptionsof the study
area have been presentedby Kemp (1972, 1976), Tietje
and Ruff (1980, 1983), Young and Ruff (1982), and
Pelchat and Ruff (1986).
METHODS
Bears were captured from April through December,
1968-77, with Aldrichfoot snares(AldrichAnimal Trap
Fig. 1. The Cold Lake bear study area (CLSA). Reproduced with permission from The Journal of WildlifeManagement(Tietje
and Ruff 1980).
POPULATION
DYNAMICS
OF BLACKBEARS* Sargeantand Ruff
Co., ClallamBay, Washington,USA), in culverttraps,by
hand (cubs), in dens, and with tranquilizerdart guns.
Capturedbearswere immobilizedwith phencyclidinehydrochlorideand weighed. In 1971 and 1972, males >90
kg were euthanized. Bears that were released were
marked,when first captured,with colored ear tags and a
permanenttattoo,and a premolartooth was extractedfor
age determination(Willey 1974). Hereafter,2- and3-yearold bears are referredto as subadultsand bears >4 years
of age are called adults.
During 1974-77, 56 capturedbearswere equippedwith
radiocollarsandlocated daily by triangulationwhen they
were withinradiorangeof roads(-5 km). When collared
bearsmoved fartherfromroads,they were relocatedfrom
aircraftwithin 1 week.
We used capturesof bears>2 yearsof age thatoccurred
during annual captureperiods of May 1-September 30
for mark-recaptureanalyses. Forthese analyses,we used
only capturesaccomplishedwith snaresand culverttraps
because othermethods sometimestargetedspecific individuals.
WeusedJolly-Seber(J-S)open-populationmodels(Pollock et al. 1990) to estimatenumbersof female bearsusing the CLSA during 1969-76. By limiting this portion
of our analysis to females, we eliminatedbias resulting
from sex-relateddifferencesin captureprobabilities. To
investigate possible effects of age-relateddifferencesin
captureprobabilities,we comparedresultsfor all females
with a second set based only on adultfemales. Finally,to
guardagainst spuriousincreases in estimates that might
resultfromthe temporarydepartureof markedbearsfrom
the CLSA, we computeda thirdset of estimatesfromcapturehistoriestruncatedin 1974. To facilitatevisual comparisons of trends resulting from these analyses, which
were based on different-sizedgroupsof bears,we plotted
standardizeddeviatesof estimates(SokalandRohlf 1981)
insteadof estimatesthemselves.
We could not estimate numbersof males by mark-recapture methods because the experimentalremoval of
males resultedin the marking,release, and recaptureof
too few. Thus, we used indirect methods to determine
whethernumbersof subadultmales capturedannuallyon
the CLSA indicated numbersresiding in the area. We
inferredtrends in numbersof subadultmales that were
presentat least temporarilyfrom numbersof individuals
capturedand mark-recaptureestimates of captureprobabilities.Wethenestimatedsettlingratesof subadultmales
(probabilitiesof surviving from 1 captureperiod to the
next while continuingto use the CLSA) fromgeneralized
mark-recapturemodels (Lebretonet al. 1992). Finally,
we considered trends in numbersof subadultsand settling ratesconcurrentlyto determinewhethernumbersof
subadultscapturedreflectednumbersthatused the study
61
areaon a continuingbasis.
We depicted capture histories of individual bears as
vectors of 0's and l's, with l's indicatingcapture. Our
models representedprobabilitiesof possible capturehistoriesas combinationsof settlingandrecaptureprobabilities (e.g., Lebretonet al. 1992:71). Settling rates were
representedby i, k andrecaptureparametersby Pi k where
subscripti denotedthe captureperiod and k indexed age
class (sa = subadults,ad = adults). Thus, 4i,k was the
probabilitya bearin age class k would survive from capture period i to i+l and still be using the CLSA during
captureperiodi+l. P, k was the probabilitya bearin age
class k would be capturedduringperiod i if it was using
the CLSA.
Ourfull model was constrainedby 4 sets of equalities:
(
=
-) 1969,k =4) 1970,k =0 1971,k =. 1976,k 4) k;
ci
1968,k
'pre,
4D1972, k =(D1973, k =I) 1974, k D)1975, k -D) post, k
P
1969, k
=P
1970, k
=P
1971,k
=P
1976, k
=P
1977, k
=P
pre,k;
P 1972, k =P 1973, k =P 1974, k =P 1975,k =P post, k'
Constraintsdividedthe studyinto 2 periodsdictatedby
the volume of data available, which precludedseparate
estimationof every I k and P . Periods corresponded
with the pre-removal(1968-71) andpost-removal(197275) periodsdescribedby Young and Ruff (1982), except
thatwe placed previouslyunpublisheddatafor 1976-77,
when relativelyfew subadultmales were captured,in the
pre-removalperiod. We used nested models and likelihood ratiotests to test for significantdifferencesbetween
parametersof our full model. We used SURGE4.2 computersoftwareto computemaximum-likelihoodestimates
of model parametersand likelihood ratio tests (Lebreton
et al. 1992).
We used the staggered-entryKaplan-Meier(K-M) estimator (Pollock et al. 1989) to estimate survivalrates of
radiomarkedbearsandratiosof age- and sex-specific survival rates. For known or suspectedmortalities,we used
the dateof last radiocontactas the dateof death. Records
for bearswith unknownfates were terminated(rightcensored) at the date of last radio contact. A log-ranktest,
stratifiedby yearandage class, was used to comparerates
for males and females. Age classes were comparedafter
stratifyingby year and sex. We used 1-tailed tests because we anticipatedhighermortalityratesfor males and
subadults than for females and adults. We used SAS
PROCPHREG(Allison 1995), for survivalanalyses.
RESULTS
Ourcriteriafor mark-recapturemodeling(age >2, capturedduringMay-Sep) were met by 692 capturesof 276
individualbears. However, only the first capturewithin
each annualcaptureperiod was relevantto our analysis.
Afterremovingduplicatecaptureswithinperiods,we were
62
Ursus 12:2001
left with 65 females captured123 times as adults,40 females captured46 times as subadults,93 males captured
136 times as adults,and 125 males captured146 times as
subadults.Twenty-threeadultmales and3 large subadult
males capturedduring1971-72 were euthanized.
Weused Jolly-Sebermodels withtime-specificsurvival
and captureprobabilitiesto estimatenumbersof females
(Model A of Pollock et al. 1990) because they fit better
than models with constantsurvivalprobabilities(Model
B: all females X25= 12.3, P = 0.03; adultsonly X23= 10.9,
P = 0.01; truncatedcapturehistoriesX23= 8.6, P = 0.03)
and models with constant survival and recaptureprobabilities(ModelD: all femalesx213= 27.3, P = 0.01; adults
= 25.84, P = 0.02; truncatedcapturehistoriesX28
only %213
= 14.4, P = 0.07). Further,goodness-of-fit was satisfactory only for Model A (all females X29= 4.07, P = 0.91;
adultsonly X25= 3.52, P = 0.62; truncatedcapturehistories 24= 3.28, P = 0.51). Estimatednumbersof females
(Table 1) were relatively stable from 1968 to 1970, increasedabruptlyduring1971-72, remainedhigh through
1974, then declined abruptly(Fig. 2).
The abruptnatureof the increasein estimatednumbers
of female bears suggests a change in spatialdistribution,
ratherthana changein reproductionor survival. Survival
recordsof radiomarkedbears suggest thatmortalitymay
have been partiallyresponsiblefor the abruptdecline in
1975 (Table 2). K-M estimates of mortalitycaused by
humans were similar for radiomarkedmale and female
bears (M:F risk ratio = 1.05, X21= 0.014, P = 0.91) and
were not significantlylower for subadultsthanfor adults
(ad:sarisk ratio = 0.80, X21= 0.20, P = 0.66).
Table1. Estimatednumbersof blackbearsusinga 218-km2
area near Cold Lake,Alberta,1968-77. Estimatesof total
numbers(allbears)are fromYoungand Ruff(1982)unless
otherwisenoted. Estimatesforfemalesarefromthis study
and are based on Jolly-Sebermodels with time-specific
survivaland recaptureprobabilities.
All bears
All females
A
A
N
84
71
N
Adult females
A
SE
N
a
15.9
21.4
a
2.88
4.36
_a
16.1
17.1
31.6
45.8
9.49
11.38
18.9
40.4
15.60
46.7
1973
24.61
59.1
1974
5.85
25.5
1975
5.26
34.2
1976
c
99bc
1977
aNotestimableby Jolly-Sebermethod
bR. L. Ruff, unpublisheddata
CLimiteddataprecludedestimation
31.5
41.4
Year
1968
1969
1970
1971
1972
92
75
117
175
136
137
123b
22.9
25.0
S
cn
w
2-
N
N0
a:
Z -1I
1968
1970
1972
1974
1976
YEAR
Fig.2. Trendsinestimatednumbersof bearsusingthe CLSA
during1968-77, standardizedfor comparison. Estimates
fromYoungand Ruff(1982)are based on maleand female
bearsaged >2 years (U). Othersare Jolly-Seberestimates
forfemales(age >4years[o];age >2years[.]; capturesprior
to 1975,age >2 years[o]).
Humanactivitywas the proximatecause of at least 25,
bears
andprobably28, of 30 deathsthatbefell radiomarked
during1974-77. Principalcauses of mortalitywere depredationcontrol (11 deaths) and a combinationof legal
huntingby nativepeople andpoaching(11 deaths),which
could not be distinguished. Legal sporthunting,a drug
overdose, a collision with a train,anotherbear,and other
naturalcauses each caused 1 death. Fatesof 3 bearscould
not be confirmedby recoveryof a carcassor radiocollar,
but each was rumoredto have been shot.
The numberof subadultmales capturedon the CLSA
was largerduringeach year of 1972-75 (range= 21-25)
thanduringany year of 1968-71 or 1976-77 (range= 217; Table3). We believe this resultindicatesa difference
in the numberof subadultsthatwere at least temporarily
presenton the studyareabecauseestimatedcaptureprob=
abilities of subadultswere not significantlyhigher (X%2
1.13, P = 0.29) during the post-removal period
(PostAa
=
0.55, SE = 0.11) thanduringthe pre-removalperiod(P pre
= 0.72, SE = 0.13).
SE
a
3.11
3.51
5.80
15.63
19.42
27.02
7.24
Table 2. Kaplan-Meierestimates of mortality rates from
deaths due to all agents and natural agents only for
radiocollared black bears at Cold Lake, Alberta, 1974-77.
Year
1974
1975
1976
1977
Naturalmortality
SE
Rate
0
0
0.079
0.08
0.077
0.03
All mortality
SE
Rate
0.122
0.14
0.086
0.51
0.087
0.28
0.080
0.24
Bears
11
36
30
26
Deaths
1
16
7
6
POPULATION
DYNAMICS
OFBLACKBEARS* Sargeant and Ruff
Moreover,estimatedsettling rates did not differ enough
between periods ( pre,sa = 0.49, SE = 0.10; post, sa=
0.49,
sa
SE = 0.07) to offset the increasein captures. Thus, numbers of subadultmales using the study area were probably largerduring 1972-75 thanduringotheryears.
DISCUSSION
Changesin Density
Studies of black bears embody most limitations of
mark-recaptureanalysis:sample sizes are usually small,
capturerates are often low, and necessary assumptions
are seldom met. Thus, estimatesof abundanceare often
biased andimprecise. In the case thatinspiredthis work,
Kemp (1976) and Youngand Ruff (1982) estimatedbear
numbersfor each year by averaging6 Lincoln-Petersen
estimates, 3 capture efficiency estimates (Meslow and
Keith 1968), and 1 estimate obtainedfrom the distributionof capturefrequencies(Hartley1958). Estimateswere
stable from 1968 to 1971, increasedabruptlyfrom 1971
to 1973, then declined from 1973 to 1977.
Garshelis (1994) subsequently showed how the
Lincoln-Petersenestimatescould have been influencedif
(1) age class and sex-relateddifferencesexisted in capture rates and (2) the composition of the population
changed over time. According to Garshelis(1994), the
Lincoln-Petersenestimateswere severely biased and had
so much influence on results thattrendsin bearnumbers
could have been misinterpreted.
Unlike previousanalyses, our J-S estimatesfor female
bearswere not affectedby sex-relateddifferencesin capturerates. We believe perceptionsof trendswere robust
to age-relateddifferencesbecausethey were affectedonly
slightly when we restrictedour analysisto adultfemales.
Estimateswere similarfrom 1969 through1970,increased
abruptlyduring 1971-72, remainedhigh through 1974,
anddeclinedabruptlyin 1975. Thus,ourresults,like those
of Kemp(1976) andYoungandRuff (1982), suggestbear
Table 3. Numbers of individual black bears captured
annually at Cold Lake, Alberta, May-Sep, 1968-77.
Males
Year
1968
1969
1970
Subadults Adults
2
14
Females
Subadults Adults
3
14
9
15
14
0
4
1971
5
14
7
1972
21
17
1973
25
8
8
1974
24
22
17
7
12
31
13
6
7
3
6
7
12
25
18
1975
1976
1977
13
Total
33
14
12
28
45
5
6
32
6
14
58
5
46
44
49
79
51
63
density increasedwhen adult males were removed (Fig.
2). However,neithertheirresultsnorours shouldbe used
to calculateactualdensities (i.e., bears/km2).The CLSA
was small relative to home range sizes of bears;consequently,the effective area sampled was not known with
sufficient accuracy for density estimation (White et al.
1982).
Changes in estimatednumbersof females could have
resulted from (1) parallel changes in the effective area
sampled by trappingor (2) the temporarydepartureof
bears markedprior to 1975; however, these alternative
explanationsseem comparativelyunlikely.First,distances
betweencapturelocationssuggest movementsof females
may have been more extensive during1968-71 thanduring 1972-75 (YoungandRuff 1982). Resultingchanges
in the effective areatrappedwouldhave dampened,rather
than produced,the trendsin our estimates. Second, the
abruptincreasein estimatesduring1971-72 persistedeven
when we made departureseffectively permanentby truncating capturehistories in 1974. Finally, our results are
consistentwith subjectiveperceptionsof field staff from
the Cold Lake study,who reporteda notable increase in
the frequencyof bearsightingsaftermales were removed.
An abruptincrease during 1971-72 in the numberof
females using the CLSA neverthelessseems surprising.
Typically,adult females monitoredfor >2 years occupy
the same home range in successive years (e.g., Alt et al.
1980, Reynolds andBeecham 1980, Powell 1987). Emigrationby females is thought to be uncommon for reasons related to reproductive fitness (Rogers 1987).
However, shifts in home ranges of adultfemales are not
unprecedented.Forexample,adultfemales monitoredby
Lindzey et al. (1986) on Long Island,Washington,USA,
adjustedhome rangesandincreasedtheiruse of the adjacent mainlandas bear densities and competition for resourcesincreased. Moreover,the removalof adultmales
probablysubjectedbearsto abruptchangesin social pressureor competitionfor resourcesthatwere more extreme
thantypicallyoccurwhile individualbearsarebeingmonitored.
Female bears radiomarkedwithin the CLSA during
1974-77 sometimesmade long-rangeexploratorymovements (Pelchat and Ruff 1986, R.L. Ruff unpublished
data). Presumably,bearsliving nearthe CLSA did so as
well and were awarethe areacomprisedprimarilyaspen
andaspen-sprucehabitats,a bettersourceof food (Pelchat
and Ruff 1986), preferredover muskeg and spruce that
dominatedthe surroundingarea (Young and Ruff 1982,
Pelchat and Ruff 1986). Thus, habitatdifferences may
have motivatedfemale bears from the surroundingarea
to make greateruse of the CLSA after adult males were
removed. Subadultmales may have been attractedto potentialmates as well as by habitat.
64
Ursus 12:2001
Settlingand Survival
Kemp (1972, 1976) and Young and Ruff (1982) noted
an increase in numbersof subadultbears caught on the
CLSA after males were removed. However, Garshelis
(1994) pointedoutthatit was impossibleto tell, fromtheir
results, whethersubadultscapturedon the CLSA settled
there. Recent advances in mark-recapture analysis
(Lebretonet al. 1992) enabledus to estimatecaptureand
settling rates. Neither changes in trappingeffectiveness
nor changes in settlingrates offset the increasednumber
of subadultscaptured.It thus seems likely thatremoving
adultmales did, in fact, encouragecontinuinguse of the
CLSA by an increasednumberof subadults.
Settlingrateswereprobabilitiesthatmarkedbearswould
survive from 1 captureperiod to the next and continue
using the studyarea. They were not annualsurvivalrates
anddid not distinguishmortalitiesfromdepartures.Thus,
we were able to estimate annualmortalityrates only for
bearsmarkedwith radios during1974-77. Results were
surprisingbecause the CLSA was closed to sport hunting. Mortalityrateswere much higherthanthose of previously studied unhuntedpopulations(Bunnell and Tait
1985, Clarkand Smith 1994, Doan-Criderand Hellgren
1996). During 1975-77, mortalityrates equaled or exceeded estimates of the maximum sustainableby black
bears (22-24%; Bunnell and Tait 1981).
Annualvariationin survivalrateestimateswas substantial,butregulationsgoverninghumanexploitationof bears
did not change duringthe study. Thus, immigrationmay
have played an importantrole, even priorto the removal
of adult males, in sustaining bear densities within the
CLSA. This new findinghas importantimplications,because previous interpretationsof this study have been
predicatedon the belief thathumanexploitationwas negligible. Bear densitiesmay not have been naturallyregulated at the outset of the Cold Lake study,and high rates
of human-causedmortalitymay have contributedto the
abruptdecline in density that occurredin 1975. Young
and Ruff (1982) previously reporteda more gradualdecline resultingfrom changes in social structureas males
maturedon the study area.
DensityDependenceand Population
Regulation
Up to this point, we have used such terms as population, densitydependence,andregulationin the same context as they were used by our predecessors(Kemp 1972,
1976; Young and Ruff 1982). However, these terms are
frequentlyused to mean differentthings (Sinclair 1991,
Wells and Richmond 1995). Semantic misunderstandings result so frequentlythat evaluating the concept of
density-dependentpopulationregulationis largelya problem of determiningwhat ecologists mean by it (Murray
1994). In this section, we hope to define our meaning,
and that of our predecessors,in a way that will clarify
previousdiscussionandpreventfuturemisunderstandings
of events at Cold Lake.
Ourpredecessorsdescribeda populationthatcomprised
bearsusingthe CLSA (Kemp 1972, 1976;YoungandRuff
1982). Thus, populationgrowth rates were determined
by 4 processes:reproduction,mortality,immigration,and
emigration. Density-dependencewas indicatedif any of
these processeswere influencedby abundance.Unfortunately, this common perspective (e.g., Caughley 1977,
Shenk et al. 1998) poses 2 difficulties for interpretation
when small "populations"are arbitrarilyspatiallydelimited from larger groups of individuals (Harrison and
Cappucino1995). First, the occurrenceof immigration,
emigration,and changes in density are scale-dependent
(Fig. 3; Wells and Richmond 1995). Second, dynamics
of populationnumbers over time are not distinguished
from movements of individuals (Allen and Hoekstra
1992), althoughthe distinctionis of critical importance
for bearmanagement.
To resolve semanticdifficulties,we believe bearbiologists shouldreservepopulationstatusfor disjunct(Wells
and Richmond 1995) or, minimally,much largergroupings (e.g., local populations;Goodwin and Fahrig 1998)
of bears than was studied at Cold Lake. When populations are defined this way, the criticalprocesses of population dynamicsare birthand death (Allen and Hoekstra
1992); movements of individualsare no less important,
butaresaidto influencelocal densitiesandmetapopulation
dynamics.
Regardlessof how they are defined, populationscannot increasewithoutlimit and seldom dwindle to extinction. Regulationis perhapsbest defined as the tendency
of populationsto fluctuate,over the long term,aboutstationarymean densities (Turchin1995). Limitingfactors
influence these means (Sinclair 1991). Direct density
dependenceis aninverserelationbetweenpercapitapopulation growthrates and presentor past populationdensities (Murdochand Walde 1989, Turchin 1995). Direct
density-dependenceis a necessary(butnot sufficient)condition for population regulation (Murdoch and Walde
1989).
Like most studiesof blackbears,the Cold Lakeexperimentwas not controlled.Thus,theremovalof adultmales
was confounded with other influences on bears of the
CLSA. However,the timing, magnitude,anddurationof
changes in numbersof bears using the CLSA strongly
suggest thatthe presenceof adultmales influencedlocal
bear densities. In keeping with their use of population,
our predecessors interpretedthis effect as evidence of
density dependence. However, we note that (1) effects
were documentedonly for a localizedarea;(2) adultmales
DYNAICS OF BLACKBEARS* Sargeantand Ruff
POPULATION
Before
a
/
0
o0
o\
0o
\
0
\
0
0
0
\0
0
After
0
/
o0
o0
o
0e
Q-
o
^
Fig. 3. Demographicconcepts are ambiguouswhen small
study populations are arbitrarilydelimited. Removing
individualsfrom the circle causes the actual density to
declinein the square,but immigration
maintainsthe actual
densityin the circle. Relativelocal densitiesare increased
withinand reducedoutsidethe circle.
depresseddensities of bears in othersex and age classes;
and (3) interactionswith conspecifics are 1 of many factorsthatmay operateon local densitiesin a relativesense,
across a wide rangeof actualdensities. Thus, in keeping
with our synthesisof terminology,we concludethatadult
males limited relative local densities of female and subadult male bears. Our results do not permit inferences
aboutdensitydependenceor populationregulation,as we
defined these terms. We could not estimate population
growthrates,test for changesin survivalor reproduction,
or estimate population densities across an area large
enough to encompass effects of the experimentaltreatment.
forResearchand
Implications
Management
Until effects on survivalor reproductioncan be documented, we supportthe views of Miller (1990a,b) and
Taylor (1994): Sustainableyield projectionsshould not
be based on anticipatedincreasesin vital ratesthatmight
result from reductionsin black bear populationdensity.
Althoughremovingadultmales encouragedbearsin other
65
sex andage classes to use the CLSA, implicationsfor bear
harvestmanagementare not obvious. Otherthanreports
of a few bearskilled by conspecifics (Kemp 1976, Young
and Ruff 1982), we have no evidence that adult males
influenced rates of population growth. Moreover, the
CLSA was unusual because most hunted areas are not
adjacentto large, unhuntedreservoirsof potentialimmigrants(Garshelis1994).
At Cold Lake, adult males were removed from a restrictedareaduringa shortperiod. The removalof bears
frombroadgeographicareasover long time spansis more
typical of huntedblack bear populations. In such cases,
hunterharvestsof adultmales are unlikely to cause suddenchangesin thedispersionof subadultandfemalebears.
Further,the response observed at Cold Lake may have
occurredonly because the CLSA was a populationsink
comprisingaspen and spruce habitats,which bears preferredover surroundingmuskeg. Removing males from
less desirablehabitatsmight not have a similareffect.
Density-dependentpopulationregulationis a topic of
special interestto bear biologists interestedin maximizing sustainable harvest rates (McClellan 1993, 1994;
DerocherandTaylor1994; Garshelis1994;Taylor1994),
buteffortsto conclusivelydemonstratedensitydependence
within black bearpopulationsmay be doomed to failure.
Formidablelogistical barrierslimit the accuracyandprecision of density estimates. Unambiguousdetection of
densitydependenceis difficult(PollardandLakhani1987),
even with errorfree estimates,and some tests designedto
accommodate sampling error are invalid (Shenk et al.
1998). Moreover,most black bear populationsare studied at moderatedensities,where changes in densityprobably have little effect on reproductionor survival(Fowler
1981a,b; Strong 1986). Finally, even at high densities,
regulatoryagentsmay reducesurvivalor suppressreproduction at irregularintervals, so that a "vague"(Strong
1986) relationshipbetweendensityanddemographicperformanceof a populationshouldbe expected.
Similarly,density dependenceis difficult to detect by
comparing the demographic performance of different
populationsthatvary in density. Garshelis(1994) noted
some obstacles,includingthe difficultyof measuringrelevantparameterswith sufficientaccuracy,complications
that arise from methodologicaldifferences among studies, andfactorsotherthandensitythatinfluencevitalrates.
More important,however,is the fact thatcarryingcapacity variesamongpopulations.Thus,densityaloneis likely
to be a poorpredictorof demographicperformance,even
if compensatoryresponsesarestrongandpopulationdensity can be estimatedaccurately.
In largemammals,strongdensity-dependentresponses
usually occur only when populationsnear carryingcapacity arereduced(Fowler 1981a,b). Probablyfew man-
66
Ursus 12:2001
aged bearpopulationsare nearcarryingcapacity (Taylor
1994). Thus,muchcurrentinterestin densitydependence
and populationregulationmight more profitablybe directed towardfactors that limit growth rates of populations at low densities. The managementsignificance of
such factors may be much greater. Relief from factors
that limit growth rates can buffer low populationsfrom
overharvestand increase sustainableyields, whetheror
not density-dependentresponses occur (Gasaway et al.
1992, VanBallenbergheandBallard1994). Althoughwe
could not assess effects of adult males on population
growth rates, their influence on relative local densities
should encouragefurtherstudy of this issue, especially
with respectto mortalityratesand causes of deathduring
juvenile dispersal.
ACKNOWLEDGMENTS
Fundingfor this projectwas providedby the Northern
PrairieWildlife Research Center of the Biological Resources Division, U.S. Geological Survey (BRD). Administrative support was provided by the BRD
CooperativeWildlife ResearchUnit and the Department
of Wildlife Ecology at the University of WisconsinMadison. J.D. Clark,D.L. Garshelis,D.H. Johnson,J.D.
Nichols, A.B. Sargeant,T.L. Shaffer,and an anonymous
refereereviewed and greatlyimproveddraftsof this paper. We especiallythankG.A. Kemp,the late B.F. Young,
the AlbertaDepartmentof EnergyandNaturalResources
(Fish andWildlife Division), the ResearchCommitteeof
the GraduateSchool at the University of WisconsinMadison,andGulf CanadaResources,Inc., for theirroles
in datacollection.
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