2008_goodrich_survival rates and causes of mortality of amur tigers on and near the sikhote-alin...

Survival rates and causes of mortality of Amur tigers on andnear the Sikhote-Alin Biosphere Zapovednik

J. M. Goodrich1, L. L. Kerley2, E. N. Smirnov3, D. G. Miquelle1, L. McDonald4, H. B. Quigley2,M. G. Hornocker2 & T. McDonald4

1 Wildlife Conservation Society, WCS Russia Program, Bronx, NY, USA

2 Hornocker Wildlife Institute/Wildlife Conservation Society, Bozeman, MT, USA

3 Sikhote-Alin Zapovednik, Terney, Primorski Krai, Russia

4 Western EcoSystems Technology, Cheyenne, WY, USA

Keywords

Amur (Siberian) tiger; mortality; large

carnivores; Panthera tigris altaica; Russia;

survival.

Correspondence

J. M. Goodrich, 50 Years October Street,

5-1, Terney, Primorsky Krai, Russia 692150.

Email: [email protected]

Editor: Nigel Bennett

Received 11 November 2007; revised 26

March 2008; accepted 10 April 2008

doi:10.1111/j.1469-7998.2008.00458.x

Abstract

We examined causes of mortality and survival rates for Amur tigers on and near

the Sikhote-Alin Biosphere Zapovednik. Our objectives were to estimate and

compare survival rates among sex and age classes, estimate cause-specific mortal-

ity, identify conservation issues related to tiger mortality and provide recommen-

dations for reducing human-caused mortality. We used two separate datasets; one

based on radio-tracking tigers from 1992 to 2005 and one based on reports of dead

tigers from 1976 to 2000. We examined causes of mortality for both datasets and

used a Cox proportional hazards models to estimate survival rates using data from

42 radio-collared tigers. Mortality was predominantly human-caused for both

datasets (83% for the telemetry dataset and 78% for the other, n=24 and 53

mortalities, respectively), and 75% of collared animals were poached. All collared

subadult tigers that dispersed were poached (n=6). Annual survival of adult

females (0.81� 0.10) was greater than that of adult males (0.63� 0.20) (z=1.52,

P=0.13) and subadult males (0.41� 0.46) (z=2.07, P=0.04). Survival rates were

precariously low on our study area, which included the largest protected area

within Amur tiger range. Efforts to reduce human-caused mortality should focus

on poaching and reducing deaths from tiger–human conflicts.

Introduction

Knowledge of rates and causes of mortality is important to

understanding of carnivore population dynamics. Mortality

and survivorship data provide real input for population

models, an opportunity to assess whether observed mortal-

ity rates are sustainable, and insight into ways to reduce

mortality. Humans cause most mortality in most large

carnivore populations and are one of the greatest threats to

their survival worldwide (Noss et al., 1996; Woodroffe &

Ginsberg, 1998; Woodroffe, 2001). Most large carnivores

evolved under conditions of high survival of breeding adults

(Weaver, Paquet & Ruggiero, 1996), but human-caused

mortality often takes a heavy toll on this cohort (e.g. Fuller,

1989; Mclellan et al., 1999). Small changes in adult survivor-

ship may have serious consequences for persistence, espe-

cially of isolated carnivore populations and long-term

survival of adults, especially females, is critical to popula-

tion well-being (Knight & Eberhardt, 1985; Weaver et al.,

1996). For example, Kenney et al. (1995) estimated that

extinction risk would increase from 5 to 95% with an

increase in poaching mortality from 4 to 8% for small,

isolated tiger populations.

As a species at the northern edge of its range, Amur tigers

may be particularly extinction prone. They occur at very low

densities – a result of low prey biomass – and have large area

requirements (Smirnov & Miquelle, 1999). Viable popula-

tions cannot be contained within the boundaries of pro-

tected areas and rely on multiple use lands for persistence

(Miquelle et al., 1999a). On these unprotected lands, con-

flicts between tigers and people and associated mortality, are

common; hence, Amur tigers may be more extinction prone

than populations that exist within the boundaries of pro-

tected areas (Woodroffe & Ginsberg, 1998; Woodroffe,

2001; Miquelle et al., 2005).

While considerable data exist on Amur tiger mortality,

samples are biased because they rely on reports, and tigers

killed in tiger–human conflicts are often reported whereas

most poached animals are not (e.g. Nikolaev, 1985; Niko-

laev & Yudin, 1993; Pikunov, 1994; Matyushkin et al., 1996;

Miquelle et al., 2005). Further, natural deaths of this elusive

animal are rarely detected by hunters and others who

provided information for the above sources. Radio-tracking

provides a less biased sample because mortalities are usually

detected regardless of cause of death. We examined patterns

and causes of mortality, and survival rates for radio-collared

Journal of Zoology

Journal of Zoology 276 (2008) 323–329 c� 2008 The Authors. Journal compilation c� 2008 The Zoological Society of London 323

Journal of Zoology. Print ISSN 0952-8369

tigers on and near the Sikhote-Alin Biosphere Zapovednik

(SABZ), 1992–2004. Our objectives were to estimate and

compare survival rates among sex and age classes, estimate

cause-specific mortality, identify conservation issues related

to tiger mortality and provide recommendations for redu-

cing human-caused mortality.

Materials and methods

Study area

We studied tigers on and near the 390 184 ha SABZ,

Primorski Krai (Province), Russia (441460N, 1351480E).SABZ had minimal human disturbance, with access re-

stricted to scientists and forest guards, but the eastern edge

was bisected by a paved public road which provided access

for poachers (Kerley et al., 2002). The Sea of Japan borders

SABZ to the east and its central feature is the Sikhote-Alin

Mountains, which parallels the coastline and has elevations

reaching 1600m. The land surrounding SABZ is sparsely

populated (about 13 000 people in five villages) and contains

a 70 350 ha buffer zone (1–8 km wide) where human activ-

ities include fishing, hunting and some agricultural practices

(e.g. livestock grazing). Potential predators of tigers and

their cubs include brown bears Ursus arctos, Asiatic black

bears Ursus thibetanus, wolves Canis lupus, lynx Lynx lynx

and a variety of smaller carnivores. Descriptions of the

region and environmental variables influencing tiger mor-

tality can be found elsewhere (Knystautas & Flint, 1987;

Newell & Wilson, 1996; Miquelle et al., 1999b; Kerley et al.,

2002).

Causes of mortality

We examined causes of mortality for two datasets, one

based on radio-tracking and one based on reports. We used

seven categories to define cause of death: (1) tiger–human

conflict (legally killed because it was a threat to human life

or livelihood); (2) poached (illegally killed); (3) suspected

poached (for radio-collared animals only, see below); (4)

road-kill (animal hit by a vehicle); (5) trapped (animals

incidentally captured in legal traps set for furbearers);

(6) natural death (cause of death not directly related to

human actions); (7) unknown.

The first dataset (hereafter the ‘telemetry dataset’) in-

cluded mortalities of radio-collared tigers and their off-

spring detected through our radio-tracking activities,

February 1992 to January 2005 (Goodrich et al., 2001;

Kerley et al., 2002, 2003). Although radio-collared animals

are not a true random sample because capture probabilities

vary, their deaths are the best available approximation

thereof. We primarily detected mortality when radio signals

indicated inactivity for several hours to days and we inves-

tigated the site to determine cause of death. We necropsied

dead tigers when possible, but when animals were poached,

we usually found only a collar cut from the tiger. However,

often when tigers were poached the signal disappeared

because the poachers destroyed the collar. In some cases,

informants later provided data (e.g. a tattoo number) con-

firming a poaching.

We categorized tigers as ‘poached’ when we found a dead

tiger or cut radio-collar or if the animal disappeared and we

received verifiable evidence of poaching. Resident adults

were categorized as ‘suspected poached’ if the signal dis-

appeared despite extensive ground and aerial searches,

ground searches in snow failed to locate tracks, and a new

tiger took over the missing animal’s territory. Because tigers

are territorial and habits and track patterns of study animals

were well known, it was possible to detect the presence of an

individual by its tracks, even when collars were non-func-

tional (Yudakov & Nikolaev, 1987; Kerley et al., 2002). It is

possible that in some cases the collar was destroyed, for

example, in a fight with another tiger that resulted in the

collared animal’s death. However, we never detected the

death of a subadult or adult tiger from predation or fighting.

We detected fighting between tigers only once, and of 53

tigers handled, we only once found scaring suggestive of

fighting. For dispersing subadults, we categorized an animal

as ‘suspected poached’ if extensive aerial searching did not

produce a signal. In six cases when we classified tigers as

suspected poached, we later received information confirm-

ing our suspicions, whereas we never detected that a ‘sus-

pected poached’ animal was alive, suggesting that we were

usually correct in our classification. We did not classify all

missing tigers as ‘suspected poached’; in two cases we

suspected premature collar failure and in two cases signals

were lost after batteries were due to fail.

We detected mortality of unmarked cubs (o1 year old) of

collared tigresses in three ways. First, remains of cubs were

found when we searched den sites of collared tigresses.

Second, we assumed cubs o6months old died when their

mother was poached or would have died in two cases when

we intervened to save the cubs (Kerley et al., 2003). Third,

we estimated litter size of collared tigresses by tracks and/or

visual observation of cubs associated with their mother. On

average, we determined litter size when cubs were

4.1months old (Kerley et al., 2003). We monitored cubs in

this way until 12months old and assumed cubs had died if

repeated observations failed to detect them with their

mothers. After 12months of age, cubs begin moving inde-

pendently of their mothers, so their absence no longer

indicated mortality. We treated data on cubs separately,

but pooled data for adults and subadults because of small

sample sizes.

We tested for seasonal variation in mortality to determine

if mortality and factors influencing it (e.g. poaching or legal

hunting seasons for ungulates) varied seasonally. We tested

(w2) four separate seasonal classifications: (1) year split intowinter (November–April) versus summer (May–October);

(2) year split into four seasons (January–March, etc.); (3)

year split into three seasons (January–April, etc.); (4) hunt-

ing season (November–February, etc.) versus non-hunting

season (March–October). We tested for seasonal variation

in poaching alone in the same way. We also compared

causes of mortality between sexes. Post hoc, we compared

mean poaching rates (animals killed/year/total animals in

Journal of Zoology 276 (2008) 323–329 c� 2008 The Authors. Journal compilation c� 2008 The Zoological Society of London324

Amur tiger mortality J. M. Goodrich et al.

sample at beginning of year) across three intervals of our

study period (1992–1996, 1997–2000 and 2000–2004)

because data indicated that poaching was higher during

1997–2000.

The second dataset included reports of dead tigers from

SABZ and a 5000 km2 surrounding, 1976–2000 (hereafter

the ‘long-term dataset’). Two tigers from the telemetry

dataset were included because their deaths were detected

regardless of our radio-tracking activities. We did not make

the above comparisons for this dataset because data on sex

and age were often missing.

Estimation of survival rates

Cox proportional hazards models (Venables & Ripley, 1999)

were applied to data collected from 11 February 1992 to 12

November 2004 from 42 radio-collared tigers, to estimate

and compare survival of five sex-age classes of tigers: cubs

(o18months old), female subadults (18–36months old),

male subadults, female adults (436months old) and male

adults. For all animals included, age at capture (Goodrich

et al., 2001) and date of death or censoring was recorded.

Animals were censored from a given age class if contact with

the animal was lost (e.g. the radio failed or fell off) or if the

animal was alive on 12 November 2004.

Animals captured before adulthood and that lived long

enough to enter the next age class were included in all

appropriate age classes. Survival rates in different age

classes of one animal were assumed independent. For

example, F01 was captured at 1 year of age in 1992 and was

still alive and collared on 12 November 2004. She was

initially entered in the ‘cub’ category and subsequently in

the subadult and adult categories. The survival rates in the

three categories were assumed to be independent and the

sample size was increased by three based on data from one

animal. Thus, while 42 tigers were included in the analysis,

overall sample size was 55.

Preliminary analyses indicated interactions between age

and sex (e.g. survival rates for a single age class varied

between sexes), but there were insufficient data to fit a

complicated model using sex, age and their interactions.

Therefore, we assumed interactions existed and fit a separate

Cox proportional hazards model to each of the five sex-age

classes, with the assumption that survival rates for male and

female cubs were the same. We then combined classes that

were judged not significantly different (using a liberal

P-value of 0.15 because of small sample sizes), and com-

pared the rest using an approximate normal z-statistic and

two-sided test.

Results

Causes of mortality

Mortality detected through radio-tracking (n=24) was

primarily human-caused (83.3%), with most (75%) asso-

ciated with poaching (i.e. sum of ‘poached’ and ‘suspected

poached’). Natural deaths accounted for o17% of mortal-

ities (Fig. 1). Even when ‘suspected poachings’ (the only

unconfirmed deaths) were removed from the data, human-

induced mortality predominated (poaching=62.5% and

vehicle collisions=12.5%), with natural deaths represent-

ing only 25% of mortalities (n=16). We detected no

differences in causes of mortality between males and females

(w2=0.69, d.f.=2, P=0.7) and no seasonal trends in

poaching or overall mortality (P40.50 for all tests). How-

ever, mean proportion of radio-collared tigers poached per

year was greater from 1997 to 2000 than during other years

(Fig. 2) (ANOVA F=4.8, d.f.=2, P=0.04).

Of four natural mortalities, one tiger died of an unknown

disease, one when a tree fell on it, one when it fell through

the ice on a stream and froze to death and one from injuries

from an unknown cause. When the latter (M50) was

captured, the right side of his head and his left rear knee

were swollen and he had a minor puncture wound on his

head. He died a month later and necropsy revealed that his

left rear leg and left front foot had both been broken several

months before. The diseased tiger (M34) was a resident

0

5

10

15

20

25

30

35

40

45

Poached Suspectedpoached

Road Natural

Per

cent

of t

otal

mor

talit

y

10

8

2

4

Total human-caused mortality = 83%

Figure 1 Causes of mortality detected by radio-tracking tigers cap-

tured on and near Sikhote-Alin Biosphere Zapovednik, 1992–2005.

Sample sizes (number of dead tigers) are above each bar.

0

0.1

0.2

0.3

0.4

0.5

1993–1996 1997–2000 2001–2004

Mea

n po

achi

ng r

ate

Figure 2 Comparison of mean poaching rates (no. of radio-collared

tigers poached/no. of tigers collared at the beginning of the year) of

radio-collared tigers on and near Sikhote-Alin Biosphere Zapovednik,

February 1992 to January 2005. Bars indicate standard error.


Amur tiger mortalityJ. M. Goodrich et al.

adult male found dead in an emaciated condition with no

other apparent pathology; a condition consistent with

canine distemper which has been confirmed in the popula-

tion (Goodrich et al., 2005a).

Forty-three per cent of cub mortality was human-caused,

although much indirectly, that is, cubs died or were removed

from the wild when their mothers were poached (Fig. 3).

One cub was shot by a SABZ guard in an aggressive

encounter. This cub was thin and necropsy revealed a large

abdominal hernia that likely caused his poor condition.

All subadult tigers that dispersed from their natal home

ranges were poached or suspected poached (n=6), whereas

philopatric cubs survived (n=4) (Fisher’s exact test,

d.f.=1, P=0.005). All but one dispersing subadults were

males, and all subadults that settled within their natal areas

were females.

Humans caused 78% (n=53 deaths) of deaths in the

long-term dataset (Fig. 4). Of eight natural deaths, six were

cub remains found in scats of male tigers and were likely

killed by these males, one adult male died when it fell

through the ice into a river and drowned, and one sub-

adult male died of disease clinically diagnosed as canine

distemper.

Survival rates

Estimated annual survival rates varied among sex-age

classes (Fig. 5). No collared tigers died while they were cubs,

so cubs were dropped from further analyses. Annual survi-

val rate of adult females (0.81� 0.10) did not differ from

that of subadult females (0.72� 0.25) (z=0.56, P=0.58),

but was greater than those of adult males (0.63� 0.20)

(z=1.52, P=0.13) and subadult males (0.41� 0.46)

(z=2.07, P=0.04). We detected no differences between

survival rates for adult males and subadult males

(z=0.986, P=0.32), adult males and subadult females

(z=0.394, P=0.69) and subadult males and subadult

females (z=1.107, P=0.27).

The estimated probability that a subadult female would

reach 36months was 0.65 given that she reached 18months,

and the estimated probability that a subadult male would

reach 36months of age was 0.32 given that he reached

18months. Estimated probabilities of annual survival (Fig. 5)

are consistent with expected lower survival rates for males

versus females in each age class.

Survival rates were highest for adult females, and pro-

gressively lower for subadult females, adult males and

finally subadult males. Whereas some adult females were

expected to survive 7 years, given that they reach their third

0

5

10

15

20

25

30

35

40

45

50

Motherpoached

Tiger-humanconflict

Natural Unknown

Per

cent

of t

otal

mor

talit

y 8

2

1

10

Total human-caused mortality ≥43%

Figure 3 Causes of mortality for cubs of radio-collared tigresses

captured on and near the Sikhote-Alin Biosphere Zapovednik,

1992–2005. Sample sizes (number of dead cubs) are above each bar.

Poaching34%

Vehicle8%

Trap4%

Natural15% Unknown

13%

Tiger-humanconflict26%

Figure 4 Causes of mortality for 53 tigers on and near the Sikhote-Alin

Biosphere Zapovednik, based on confirmed reports from 1976 to

2001.

1.0

0.8

0.6

0.4

0.2

0.0

0 500 1000 1500 2000 2500

Lifespan in age group (days)

Tiger survival curves

Adult femaleSubadult femaleSubadult maleAdult male

Sur

viva

l rat

e

Figure 5 Survival curves for four sex-age categories of radio-collared

tigers on and near the Sikhote-Alin Biosphere Zapovednik,

1992–2004.



birthday, few individual adult males were expected to

survive that long. Standard errors were large indicating that

precisions of the estimates were low. However, differences

between survival rates as great as those observed between

adult females and subadult males (0.81 vs. 0.41), are a strong

indication of real differences that cannot be attributed to

random variation and small sample sizes. Precision of these

statistics is low because of small samples, but new data

would have to indicate no differences in survival rates or

even opposite results to those in the current analysis to

reverse the trends.

Discussion

Deaths from natural causes were rare in either dataset. Of

four radio-collared tigers that died of natural causes, two

may have died from human-related causes. M34 likely died

of canine distemper, a disease usually fatal to tigers that is

common in and probably transmitted by domestic dogs in

Russia (Goodrich et al, 2005a). While impacts of this disease

on tiger populations are unknown, distemper caused 34%

mortality in Serengeti lions (Roelke-Parker et al., 1996), so

the issue warrants further investigation. M50’s pathologies

were consistent with a vehicle collision, but could also have

been obtained during a predation attempt. We detected six

cases of probable infanticide based on cub remains in tiger

scats. Infanticide may also have caused the death of two

litters born to radio-collared mothers, whose father died

shortly before the cubs were born. Both litters disappeared

shortly after they left their natal dens and about the same

time when a new male immigrated to the area. Infanticide

has been reported for Amur and Bengal tigers elsewhere

(Smith & McDougal, 1991; Nikolaev & Yudin, 1993) and

probably increases reproductive success of immigrating

males by cycling resident females into estrus more quickly

(Smith & McDougal, 1991; Smith, 1993).

Poaching was the most common cause of death in both

datasets, but was higher for radio-collared animals. This

dataset was less biased because we could detect deaths

regardless of cause. The long-term dataset depended on

reports which favored conflict situations because they were

widely publicized and reported by people seeking govern-

ment assistance (Miquelle et al., 2005); whereas, poachers

maintained secrecy. Nonetheless, it is clear that conflict is an

important cause of mortality. No collared tigers died in

tiger–human conflicts, but most lived on SABZ where there

was a low potential for conflict because it was closed to the

public and livestock. On non-protected areas, we would

expect higher mortality from both poaching and tiger–

human conflict (Miquelle et al., 2005), an alarming fact,

considering that human-caused mortality made up 83% of

radio-collared tiger deaths. All other studies of Amur tiger

mortality detected similar rates of human-caused mortality

(e.g. Nikolaev, 1985; Nikolaev & Yudin, 1993; Pikunov,

1994; Matyushkin et al., 1996; Miquelle et al., 2005) and it

was the leading cause of tiger death in Nepal and India

(Schaller, 1967; Sunquist, 1981; Smith, 1993).

For cubs, the second most common cause of death was

associated with poaching of their mothers. Four of eight

tigresses poached had young cubs, and in three cases, the

cubs were too young to survive on their own. Tigresses with

cubs may be more susceptible to poaching. They move less

and more slowly, making them easier for poachers to track

down in snow. Also, tigresses often aggressively protect

their cubs, making them easier targets for poachers.

Poaching and related cub mortality was clearly associated

with roads (Kerley et al., 2002). High human-induced

mortality along roads has been demonstrated for other large

carnivores, for example grizzly bears, cougars and wolves;

often the result of increased hunting and poaching (reviews

in Noss et al., 1996; Kerley et al., 2002). If the current trend

of increasing and improved roads, and increased vehicles per

capita continues within Amur tiger habitat, we expect

human-induced mortality to increase.

Following high levels of poaching from 1997 to 2000,

SABZ improved anti-poaching activities on the public road

through SABZ, including increased patrols and construc-

tion of a guard station and two observation cabins (Astafiev,

2005); poaching subsequently decreased. However, while

such intensive anti-poaching activities may be sustainable

within protected areas, the remainder of tiger habitat is too

vast and the number of roads too numerous to patrol

adequately. From 1998 to 2000, over $800 000 were invested

in tiger-related law enforcement activities in Russia, yet

poaching remains high (Christie, 2006). Thus, solutions

must include attacks on other fronts, such as changes in

Russian legislature to increase fines for poaching and

eliminate loopholes allowing possession of endangered spe-

cies, road restrictions, educational programs and economic

incentives not to poach.

Dispersal is essential to maintaining genetic connectivity

within the Amur tiger population (Miquelle et al., 1999b).

However, all dispersing collared tigers were poached before

they settled and reproduced. That successful dispersal does

occur is clear because some tigers immigrated to our study

area, suggesting they dispersed from somewhere, settled,

and survived to reproduce (Goodrich et al., 2005b). How-

ever, our data indicate that successful dispersal is rare and if

current levels of poaching continue, maintaining suitable

habitat for dispersal corridors alone may not be enough to

retain genetic exchange across the range of Amur tigers.

One model of tiger population dynamics suggests that

tiger populations will decline human-induced mortality

exceeds 10% and if survival of breeding adults is o80%

(Chapron et al., in press). Estimated survival for our

population was 0.81 and 0.63 for adult females and adult

males, respectively; that is, precariously low for a protected

area that should act as a source population. However, our

survival estimates reflect means from a 13-year period and

survival was much lower from 1997 to 2000, when 34% of

our radio-collared tigers were poached per year. Kenney

et al. (1995) suggested that for small (o120 individuals)

isolated populations of tigers, annual poaching levels of 4%

were acceptable, but twice that level would result in a 95%

probability of extinction. However, Karanth et al. (2006)



estimated survival of 77% in a stable population in India.

While it is unclear what level of poaching may be sustainable

by the Amur tiger population, which is relatively large

(about 450 individuals) and unfragmented (Miquelle et al.,

1999a), there is little doubt that poaching rates from 1997 to

2000 (34%) were unsustainable. Efforts to reduce human-

caused mortality should focus on poaching and reducing

deaths from tiger–human conflicts.

Acknowledgments

Funding was provided by theWildlife Conservation Society,

21st Century Tiger, National Geographic Society, National

Fish and Wildlife Foundation Save the Tiger Fund, Na-

tional Wildlife Federation, Exxon Corporation, the Charles

Engelhard Foundation, Disney Wildlife Fund, Turner

Foundation, US Fish and Wildlife Service Rhino and Tiger

Conservation Fund, Richard King Mellon, Avocet Charita-

ble Lead Unitrust, Robertson Foundation, Starr Founda-

tion and Goldman Environmental Foundation. I. Nikolaev,

B. Schleyer, N. Rybin, A. Rybin, A. Kostirya, I. Seryodkin,

V. Melnikov, A. Saphonov, V. Schukin and E. Gishko

assisted with data collection. Director A.A. Astafiev and

Assistant Directors M. Gromyko and Y. Potikha of SABZ

provided logistical and administrative support, and the

Russian Ministry of Natural Resources provided permits

for capture and political support.

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