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Undesirable Evolutionary Consequences of Trophy Hunting

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Abstract and Figures

Phenotype-based selective harvests, including trophy hunting, can have important implications for sustainable wildlife management if they target heritable traits. Here we show that in an evolutionary response to sport hunting of bighorn trophy rams (Ovis canadensis) body weight and horn size have declined significantly over time. We used quantitative genetic analyses, based on a partly genetically reconstructed pedigree from a 30-year study of a wild population in which trophy hunting targeted rams with rapidly growing horns, to explore the evolutionary response to hunter selection on ram weight and horn size. Both traits were highly heritable, and trophy-harvested rams were of significantly higher genetic 'breeding value' for weight and horn size than rams that were not harvested. Rams of high breeding value were also shot at an early age, and thus did not achieve high reproductive success. Declines in mean breeding values for weight and horn size therefore occurred in response to unrestricted trophy hunting, resulting in the production of smaller-horned, lighter rams, and fewer trophies.
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Methods
Meteorology
A network of 14 meteorological stations was installed across the Annapurna range before
the 1999 monsoon season, and expanded to 19 stations encompassing 28 rain gauges in
2000. Rainfall is totalled every 30 min. ‘Look-down’ distance rangers and
g
-ray loggers
measure snow depth and total water content, respectively, once a day at high elevations
(.2,500 m in the Greater Himalaya). Only liquid precipitation is measured in the Tibetan
zone, such that the annual (but not the monsoon) total is underestimated here. The data
presented here (Fig. 2b) represent monsoon averages based on the longest record available
from each station.
Apatite fission-track dating
Following mineral separation, apatites were polished, etched and irradiated. Standard and
induced track densities were determined on Brazil ruby muscovite external detectors
(geometry factor 0.5), and fossil track densities were determined on internal mineral
surfaces. Ages were calculated using z ¼ 359 ^ 20 for dosimeter glass CN-5. All ages are
central ages and are reported with 1
j
errors. Long-term erosion rates are conservatively
estimated on the basis of the fission-track age, and assuming a geothermal gradient of
100 8Ckm
21
and an annealing temperature of 140 8C.
Topographic analysis
A 3-arcsec (,90 m) digital elevation model (DEM) is the basis of all topographic analyses.
Hillslope angles are calculated at every pixel in the DEM based on a 3 £ 3 pixel
(,180 £ 180 m) grid. Mean hillslope angles were extracted from a moving, 5-km-radius
window centred on the Marsyandi River. Maximum, minimum and mean elevation (Fig.
2) were calculated along a 50-km-wide swath oriented perpendicular to the strike of the
range and centred on the Marsyandi River (or the Nar-Phu River above its confluence with
the Marsyandi).
Equilibrium-line altitude
Glacial areas were calculated from present and reconstructed ice margins mapped on aerial
photographs, and transferred first to 1:50,000 scale topographic maps and then to the
digital topography. Based on glacial hypsometry, equilibrium-line altitudes were
estimated with an assumed accumulation-area ratio of 0.65. To avoid uncertainty
introduced by avalanches on to glaciers from adjacent high peaks, 29 small glaciers (95%
are ,2.5 km
2
), lacking high headwalls, were analysed. The regional equilibrium-line
altitude gradient shows little sensitivity to accumulation-area ratios ranging from 0.4 to
0.8.
Specific stream power
Analysis was focused on catchments ranging from 3 to 7 km
2
within the non-glaciated part
(,4,200 m elevation) of the study area. These basins drain approximately half of the
landscape and are sufficiently large to be fluvial, as opposed to colluvial/debris flow,
channels. Monsoon rainfall was smoothed across the meteorological network to define an
average precipitation gradient perpendicular to the strike of the topography. This gradient
was then extrapolated parallel to strike across the study area. For each river segment
$500 m long, channel gradients (S) were extracted from the DEM, and discharge (Q)was
calculated as the product of upstream area and rainfall. Discharge is overestimated because
all rainfall is assumed to enter channels. Channel width (W) is calculated as 10
22
Q
0.4
.
Specific stream power (in GJ m
22
yr
21
) is calculated as r
w
QS/W, where r
w
is the density of
water and g is gravitational acceleration. Channel gradients and specific stream power are
binned every 5 km.
Received 20 June; accepted 4 November 2003; doi:10.1038/nature02187.
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Acknowledgements This work benefited from discussions with K. Hodges, J. Lave
´
, A. Heimsath,
K. Whipple, P. Koons, M. Brandon and T. Ehlers. We thank P. Molnar and C. Beaumont for
comments and suggestions. Logistical support from Himalayan Experience and the Nepalese
Department of Hydrology and Meteorology is gratefully acknowledged. This work was funded by
the NSF Continental Dynamics program and by NASA.
Competing interests statement The authors declare that they have no competing financial
interests.
Correspondence and requests for materials should be addressed to D.W.B.
(burbank@crustal.ucsb.edu).
..............................................................
Undesirable evolutionary
consequences of trophy hunting
David W. Coltman
1
, Paul O’Donoghue
1
, Jon T. Jorgenson
2
, John T. Hogg
3
,
Curtis Strobeck
4
& Marco Festa-Bianchet
5
1
Department of Animal and Plant Sciences, University of Sheffield,
Sheffield S10 2TN, UK
2
Alberta Department of Sustainable Development, Fish and Wildlife Division,
Box 1059, Canmore, Alberta T0L 0M0, Canada
3
Montana Conservation Science Institute, Missoula, Montana 59803, USA
4
Department of Biological Sciences, University of Alberta, Edmonton,
Alberta T6G 2E9, Canada
5
De
´
partement de biologie, Universite
´
de Sherbrooke, Sherbrooke, Que
´
bec J1K 2R1,
Canada
.............................................................................................................................................................................
Phenotype-based selective harvests, including trophy hunting,
can have important implications for sustainable wildlife manage-
ment if they target heritable traits
1–3
. Here we show that in an
evolutionary response to sport hunting of bighorn trophy rams
(Ovis canadensis) body weight and horn size have declined
significantly over time. We used quantitative genetic analyses,
based on a partly genetically reconstructed pedigree from a
30-year study of a wild population in which trophy hunting
targeted rams with rapidly growing horns
4
,toexplorethe
evolutionary response to hunter selection on ram weight and
horn size. Both traits were highly heritable, and trophy-harvested
rams were of significantly higher genetic ‘breeding value’ for
weight and horn size than rams that were not harvested. Rams of
letters to nature
NATURE |VOL 426 | 11 DECEMBER 2003 | www.nature.com/nature 655
high breeding value were also shot at an early age, and thus did
not achieve high reproductive success
5
. Declines in mean breed-
ing values for weight and horn size therefore occurred in
response to unrestricted trophy hunting, resulting in the pro-
duction of smaller-horned, lighter rams, and fewer trophies.
Sport harvesting is one of the most pervasive and potentially
intrusive human activities that affect game mammal populations
globally
6
. Hunters are willing to pay large sums to hunt trophy
mountain ungulates in various parts of the world, and many
mountain sheep (Ovis canadensis and O. dalli) populations in
North America are managed primarily to produce large-horned
trophy rams for sport hunters. A world-class trophy ram is an
extremely valuable commodity, and hunting permits have been
auctioned for hundreds of thousands of dollars
7
. One sport hunter
paid over Can$1 million in 1998 and 1999 for special permits to
hunt trophy rams in Alberta, Canada
7
. In many parts of North
America, sport harvest of mountain sheep is often restricted only
by the availability of rams whose horns reach a minimum size
prescribed by regulations. Although the use of income generated
from sport hunting towards enhancing and conserving mountain
ungulate habitat can be seen in a positive light
7
, so far little attention
has been paid to the potential evolutionary consequences, and
hence the sustainability, of harvest regimes
2,3
.
Wildlife management has traditionally focused on demographic
and ecological factors that affect numbers and growth rates in
harvested populations
8–11
. However, the life-history changes experi-
enced by species subject to commercial fisheries strongly suggest
that intensive harvesting practices can elicit an evolutionary
response in wild stocks
12–15
. Experimental size-selective harvesting
treatments on an exploited fish demonstrated evolutionary effects
on somatic growth and population productivity in the opposite
direction of the size bias of the harvest
13
. Recent reviews have called
attention to the potential selective effects of sport hunting on wild
ungulates, in which large-horned or large-antlered males are selec-
tively targeted
2,3
. The increased frequency of tuskless elephants in
many African populations has also been suggested to have occurred
in response to selective ivory poaching
16
. Here we use data from the
long-term study of a harvested bighorn sheep population at Ram
Mountain, Alberta, Canada, to investigate the evolutionary con-
sequences of more than 30 years of selective hunting of trophy rams.
Fifty-seven rams have been shot at Ram Mountain since 1975, or
about 40% of the rams legally available for harvest in each year (see
Methods), for a yearly harvest of between zero and six rams
17
. Most
trophy-harvested rams were shot before reaching 8 years of age (45
of 57 rams), and nine were shot as early as the age of 4 years. In
bighorn sheep, much of the total horn length is added from the ages
of 2 to 4 years, and at Ram Mountain the probability of a ram being
shot before the age of 6 years is positively correlated with cumulative
horn growth over this interval
4
. Animal model’
18
quantitative
genetic analysis of 395 horn-length and 447 weight measurements
taken from 192 rams at ages 2, 3 and 4 years from 1971 to 2002
revealed narrow-sense heritabilities of 0.69 ^ 0.10 and 0.41 ^ 0.11
Figure 1 Selection against high-breeding-value rams imposed by trophy hunting.
a, Breeding values (means ^ s.e.m.) for horn length and weight of trophy-harvested
rams (filled bars) and non-trophy-harvested rams (open bars). b, Relationship between
the age at harvest for trophy-harvested rams and their breeding value. c, Relationship
between the number of paternities assigned to trophy-harvested rams in their lifetime and
their breeding value.
Figure 2 Observed changes in mean weight and horn length and in the population size
from 1972 to 2002. a, Relationship between weight (mean ^ s.e.m.) of 4-year-old rams
and year (N ¼ 133 rams). b, Relationship between horn length (mean ^ s.e.m.) of
4-year-old rams and year (N ¼ 119 rams). c, Changes in population size (taken as the
number of ewes aged at least 2 years plus yearlings
17
) over time.
letters to nature
NATURE | VOL 426 | 11 DECEMBER 2003 | www.nature.com/nature656
(means ^ s.e.m.), respectively (see Methods), and a strong positive
additive genetic correlation between the two (þ0.84 ^ 0.10). Com-
parison of expected genetic ‘breeding values’ (twice the expected
deviation of an individual’s offspring phenotype from the popu-
lation mean owing to the additive effect of the offspring’s inherited
genes
18
) extracted from this model (Fig. 1a) indicates that hunters
selectively harvest rams with high breeding values for horn length
(trophy-harvested mean, þ 0.61 ^ 0.28; non-harvested mean,
21.24 ^ 0.48; t-test: t
148
¼ 24.16, P , 0.001) and weight (tro-
phy-harvested mean, þ0.70 ^ 0.28; non-harvested mean,
20.89 ^ 0.48; t-test: t
148
¼ 23.26, P ¼ 0.0014).
Within seasons, mating success in bighorn sheep increases with
dominance rank
19
, age and horn length
5
. The positive effect of large
horns on mating success increases from about 6 years of age
5
, when
rams are capable of defending oestrous ewes during the rut. The age
at which a high-breeding-value ram is harvested is therefore likely to
have an important impact on the number of offspring he can sire.
We found a negative relationship between the age at which a trophy-
harvested ram was shot and his breeding value for horn length
(generalized linear model (GLM) with Poisson errors: x
2
(1)
¼ 4.64,
P ¼ 0.031; Fig. 1b) but not for weight (GLM: x
2
(1)
¼ 1.80, P ¼ 0.18;
data not shown). Trophy-harvested rams with high breeding values
for body and horn size were therefore less likely to reach the ages at
which they achieve high rates of paternity in this population
5
.Asa
consequence, there was a negative relationship between breeding
value for horn length and lifetime mating success, measured as the
number of paternities assigned over their lifetime, among trophy-
harvested rams (GLM with negative binomial error: x
2
(1)
¼ 8.56,
P ¼ 0.0034; Fig. 1c). The mean sire breeding value of individuals
fathered by trophy-harvested rams was therefore significantly less
than zero for both weight (one-sample t-test: mean ¼ 22.41,
s.e.m. ¼ 0.37, t
59
¼ 26.50, P , 0.001) and horn length
(mean ¼ 21.84, s.e.m. ¼ 0.19, t
59
¼ 29.68, P , 0.001). The
mean sire breeding value of individuals fathered by rams that
died a natural death was also significantly less than zero for both
weight (one-sample t-test: mean ¼ 21.24, s.e.m. ¼ 0.17,
t
182
¼ 27.14, P , 0.001) and horn length (mean ¼ 22.10,
s.e.m. ¼ 0.16, t
182
¼ 220.43, P , 0.001). The low breeding values
of rams not harvested (Fig. 1a) and the reduced longevity and
potential reproductive output of the higher-quality trophy-har-
vested rams (Fig. 1b, c) combine to suggest that the selection
imposed by trophy hunting had a negative impact on the evol-
utionary trajectory of horn length and body weight in this popu-
lation during our study.
Is there evidence of a response to selective harvesting at the
population level? Significant declines in both ram weight (linear
mixed-effect model including year of birth and individual as a
random effects, and age, time and resource index as fixed effects:
b
time
¼ 20.30, s.e.m. ¼ 0.09, t
25
¼ 23.42, P ¼ 0.0021) and horn
length (linear mixed-effect model including year of birth and
individual as a random effects, and age, time and resource index
as fixed effects: b
time
¼ 20.35, s.e.m. ¼ 0.12, t
23
¼ 22.97,
P ¼ 0.0068) were observed over the course of the study (Fig. 2a,
b) after controlling for environmental effects such as population
density (Fig. 2c) using an index of resource availability (see
Methods; weight: b
resources
¼ 0.81, s.e.m. ¼ 0.17, t
25
¼ 4.72,
P , 0.001; horn length: b
resources
¼ 0.72, s.e.m. ¼ 0.22,
t
23
¼ 3.32, P ¼ 0.0030). These are very rapid rates of phenotypic
change
20
,correspondingto20.30/12.9 ¼ 20.023 and 20.35/
13.6 ¼ 20.026 standard deviations per year, or 20.14 and 20.15
haldanes (ref. 20) assuming a generation time of 6 years. Analyses of
breeding values are consistent with genetically based responses (Fig.
3). Declines in breeding value (see Methods) were observed for both
ram weight (linear mixed-effect model including year of birth as a
random effect, and time and resource index as fixed effects:
b
resources
¼ 0.037, s.e.m. ¼ 0.025, t
33
¼ 1.49, P ¼ 0.15;
b
time
¼ 20.071, s.e.m. ¼ 0.012, t
33
¼ 26.02, P , 0.001) and
horn length (linear mixed-effect model including year of birth as
a random effect, and time and resource index as fixed effects:
b
resources
¼ 0.050, s.e.m. ¼ 0.024, t
33
¼ 2.08, P ¼ 0.045;
b
time
¼ 20.075, s.e.m. ¼ 0.011, t
33¼
2 6.76, P , 0.001). Such
declines in breeding value over time are indicative of a microevolu-
tionary response to selection
21
in the Ram Mountain population.
Unrestricted harvesting of trophy rams has thus contributed to a
decline in the very traits that determine trophy quality. Hunters
have selectively targeted rams of high genetic quality before their
reproductive peak, depleting the genes that confer rapid early body
and horn growth. Wildlife harvesting that is selective and suffi-
ciently severe might elicit an undesired evolutionary response when
the target trait is heritable. There might also be unexpected effects
on genetically correlated traits, such as female body weight or
disease resistance
22
, that could result in further genetic deterioration
of harvested populations as anthropogenic selection pushes traits
away from their naturally selected optima. Because such changes
will be extremely difficult to reverse, wildlife managers must
consider the genetic effects and the evolutionary implications of
alternative harvest strategies
2,3
. The move to adopt a ‘full curl’
restriction in parts of Alberta in 1996, which limits harvest to
rams with horns whose tip extends beyond the tip of the nose, is one
strategy to minimize further deterioration of the genetic quality of
bighorn sheep. A
Methods
Population and study site
The bighorn sheep population on Ram Mountain, Alberta, Canada (528 N, 1158 W,
elevation 1,080–2,170 m) has been monitored closely since 1971 (refs 17, 23). Immigration
to Ram Mountain from the main species range has not been documented, and is probably
rare because of isolation of the population by about 30 km of coniferous forest. Each year,
sheep were captured in a corral trap baited with salt from late May to early October, and
marked with coloured plastic ear tags or canvas collars for individual identification. Adult
rams were captured once or twice in most summers from early June to mid-July. At each
capture, sheep were weighed to the nearest 250 g with a Detecto spring scale. Horn length
along the outside curvature was measured with tape. The longer of the left and right horn
measurements was used, because rams can have a varying amount of horn removed by
wear. For further details on field methods see refs 17, 23 and 24.
Bighorn males on Ram Mountain can be legally harvested by Alberta resident hunters
from late August to the end of October. Until 1996, rams with horns describing at least
four-fifths of a curl (‘trophy’ rams) could be harvested by any hunter holding a trophy
sheep licence
17
. As any resident could purchase a licence, the harvest was limited only by
the availability of trophy rams. A change in regulations in 1996 limited harvest to ‘full-curl’
rams. Consequently, only three rams have been shot since 1996. Individual weight and
horn length measurements from rams captured between 1971 and 2002 were adjusted to
5 June (ref. 24). Because the youngest age at which rams were shot by hunters was 4 years,
we used weight and horn length data from ages 2, 3 and 4 years to avoid bias due to hunter
selection.
Pedigree reconstruction
Maternity was known from field observations for 709 of the 894 (79.3%) marked sheep
Figure 3 Changes in the mean breeding value of cohorts born between 1967 and 2002.
a, Relationship between breeding value (mean ^ s.e.m.) for weight and year of birth
(N ¼ 783 individuals). b, Relationship between breeding value (mean ^ s.e.m.) for horn
length and year of birth (N ¼ 783 individuals).
letters to nature
NATURE |VOL 426 | 11 DECEMBER 2003 | www.nature.com/nature 657
whose fates have been followed since 1971. Tissue sampling for DNA analyses started in
1988. Blood samples were taken from all captured sheep until 1993 and stored in
preservative at 220 8C. Sampling resumed in 1997, when hair samples were taken from all
captured sheep by plucking 50–100 hairs including roots from the back or flank. Hairs
were kept either in paper envelopes or plastic bags containing about 5 g of silica at room
temperature. From 1998 to 2002, a tissue sample from each captured sheep was taken from
the ear with an 8-mm punch. Ear tissue was kept at 220 8C in a solution of 20%
dimethylsulphoxide saturated with NaCl. We sampled 433 marked individuals over the
course of the study.
DNAwas extracted from blood with a standard phenol–chloroform method, and from
either 20–30 hairs including follicles or about 5 mg of ear tissue, using the QIAamp tissue
extraction kit (Qiagen Inc., Mississauga, Ontario). Polymerase chain reaction
amplification at 20 ungulate-derived microsatellite loci, 15 as described previously
5
plus
MCM527, BM4025, MAF64, OarFCB193 and MAF92 (refs 25, 26), and fragment analysis
were performed as described elsewhere
5
. After correction for multiple comparisons, we
found no evidence for allelic or genotypic disequilibria at or among these 20 loci.
Paternity of 241 individuals was assigned by using the likelihood-based approach
described in CERVUS
27
at a confidence level of more than 95% with input parameters
given in ref. 5. After paternity analysis, we used KINSHIP
28
to identify 31 clusters of 104
paternal half-sibs among the unassigned offspring. A paternal half-sibship consisted of all
pairs of individuals of unassigned paternity that were identified in the KINSHIP analysis as
having a likelihood ratio of the probability of a paternal half-sib relationship versus
unrelated with an associated P , 0.05 (ref. 28). Members of reconstructed paternal half-
sibships were assigned a common unknown paternal identity for the animal model
analyses. Paternal identity links in the pedigree were therefore defined for 345 individuals.
Animal model analyses
Breeding values, genetic variance components and heritabilities were estimated by using a
multiple trait restricted-estimate maximum-likelihood (REML) model implemented by
the programs PEST
29
and VCE
30
. An animal model was fitted in which the phenotype of
each animal was broken down into components of additive genetic value and other
random and fixed effects: y ¼ Xb þ Za þ Pc þ e, where y was a vector of phenotypic
values, b was a vector of fixed effects, a and c were vectors of additive genetic and
permanent environmental, e was a vector of residual values, and X, Z and P were the
corresponding design matrices relating records to the appropriate fixed or random
effects
18
. Fixed effects included age (factor) and the average weight of yearling ewes in the
year of measurement (covariate), which is a better index of resource availability than
population size because it accounts for time-lagged effects
4
. The permanent
environmental effect grouped repeated observations on the same individual to quantify
any remaining between-individual variance over and above that due to additive genetic
effects, which would be due to maternal or other long-term environmental and non-
additive genetic effects.
The total phenotypic variance (V
p
) was therefore partitioned into three components:
the additive genetic variance (V
a
), the permanent environmental variance (V
e
) and the
residual variance (V
r
), thus: V
p
¼ V
a
þ V
e
þ V
r
. Heritability was calculated as h
2
¼ V
a
/
V
p
. The VCE
30
program returns standard errors on all variance components and ratios.
Best linear unbiased predictors of individual breeding values were quantified by using
REML estimates of the variance components obtained with PEST
29
. All statistical tests
were conducted in SPLUS 6.1.
Received 11 August; accepted 17 October 2003; doi:10.1038/nature02177.
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Acknowledgements We thank the many students, colleagues, volunteers and assistants that
contributed to this research over the past 30 years.B. Wishart initiated the Ram Mountain project.
Our research was funded by the Alberta Conservation Association, Alberta Fish and Wildlife
Division, Alberta Recreation, Sports, Parks and Wildlife Foundation, Eppley Foundation for
Research, Foundation for North American Wild Sheep, National Geographic Society, Natural
Environment Research Council (UK), Natural Sciences and Engineering Research Council of
Canada, Rocky Mountain Elk Foundation (Canada), and the Universite
´
de Sherbrooke. We are
grateful for the logistical support of the Alberta Forest Service.
Competing interests statement The authors declare that they have no competing financial
interests.
Correspondence and requests for materials should be addressed to D.W.C.
(d.coltman@sheffield.ac.uk).
..............................................................
Theroleofevolutioninthe
emergence of infectious diseases
Rustom Antia
1
, Roland R. Regoes
1
, Jacob C. Koella
2
& Carl T. Bergstrom
3
1
Department of Biology, Emory University, Atlanta, Georgia 30322, USA
2
Laboratoire de Parasitologie Evolutive, Universite
´
Pierre et Marie Curie,
75252 Paris, France
3
Department of Biology, University of Washington, Seattle, Washington 98195,
USA
.............................................................................................................................................................................
It is unclear when, where and how novel pathogens such as
human immunodeficiency virus (HIV), monkeypox and severe
acute respiratory syndrome (SARS) will cross the barriers that
separate their natural reservoirs from human populations and
ignite the epidemic spread of novel infectious diseases. New
pathogens are believed to emerge from animal reservoirs when
ecological changes increase the pathogens opportunities to enter
the human population
1
and to generate subsequent human-to-
human transmission
2
. Effective human-to-human transmission
requires that the pathogen’s basic reproductive number, R
0
,
should exceed one, where R
0
is the average number of secondary
infections arising from one infected individual in a completely
susceptible population
3
. However, an increase in R
0
, even when
insufficient to generate an epidemic, nonetheless increases the
number of subsequently infected individuals. Here we show that,
letters to nature
NATURE | VOL 426 | 11 DECEMBER 2003 | www.nature.com/nature658
... Accompanying changes in abundance, there may sometimes be changes in the mean size of individuals. For example, major reductions in the size at maturity have occurred for some exploited fish species [2,3] as well as with trophy-hunting such as for the horns or tusks of some mammals [4,5]. Set against that backdrop, there is cause for measured optimism with ongoing conservation efforts for some taxa and habitats proving very successful [6]. ...
... [11]). These observations are enigmatic, since declining mean size in other taxa is often linked to overharvesting and population declines [2,4], rather than population increases. One hypothesis to explain the reduction in mean turtle nester size over time is that an increased number of first-time nesters, which are often smaller than experienced nesters, are driving both the increase in nesting numbers as well as the reduction in mean size [11]. ...
... R. Soc. B 289: 20220696 individuals often reflects overharvesting [2,4], the decrease in the mean size of nesting turtles we recorded was not caused by the removal of larger size classes of turtles, since the number of nests increased over time for all size classes. ...
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With some taxa, a reduction in the mean size of individuals may reflect over-harvesting and/or trophy hunting. However, we show that in sea turtles, a reduction in the mean size of breeding individuals may be part of the good news story of an expanding population. We describe a 70-fold increase in annual nest numbers on the island of Sal (Cape Verde, North Atlantic) between 2008 and 2020 (from 506 to 35 507 nests), making this now one of the largest loggerhead ( Caretta caretta ) nesting aggregations in the world. We use 20 128 measurements of the size of nesting turtles to show that their mean annual size has decreased by about 2.4 cm, from 83.2 to 80.8 cm. This decrease in the mean size of nesting turtles was not caused by the removal of larger turtles, for example by selective harvesting. Rather we develop a theoretical model to show than this decrease in mean size can be explained by an influx of first-time nesters, combined with a decrease in the size of those first-time nesters over time. A reduction in mean size of nesting turtles has been reported across the Atlantic, Pacific and Indian Oceans, and may be a common feature of population recoveries in sea turtles.
... Consequently, shifts in mortality from natural to human-induced causes could produce selection of behavioral, demographic, or physical attributes with subsequent effects to population dynamics (Ciuti et al., 2012;Coltman et al., 2001Coltman et al., , 2003. ...
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Understanding the types and magnitude of human-caused mortality is essential for maintaining viable large carnivore populations. We used a database of cause-specific mortality to examine how hunting regulations and landscape configurations influenced human-caused mortality of North American gray wolves (Canis lupus). Our dataset included 21 studies that monitored the fates of 3564 wolves and reported 1442 mortalities. Human-caused mortality accounted for 61% of mortality overall, with 23% due to illegal harvest, 16% due to legal harvest, and 12% the result of management removal. The overall proportion of anthropogenic wolf mortality was lowest in areas with an open hunting season compared to areas with a closed hunting season or mixed hunting regulations, suggesting that harvest mortality was neither fully additive nor compensatory. Proportion of mortality from management removal was reduced in areas with an open hunting season, suggesting that legal harvest may reduce human-wolf conflicts or alternatively that areas with legal harvest have less potential for management removals (e.g., less livestock depredation). Proportion of natural habitat was negatively correlated with the proportion of anthropogenic and illegal harvest mortality. Additionally, the proportion of mortality due to illegal harvest increased with greater natural habitat fragmentation. The observed association between large patches of natural habitat and reductions in several sources of anthro-pogenic wolf mortality reiterate the importance of habitat preservation to maintain wolf populations. Furthermore, effective management of wolf populations via implementation of harvest may reduce conflict with humans. Effective wolf conservation will depend on holistic strategies that integrate ecological and socioeconomic factors to facilitate their long-term coexistence with humans. K E Y W O R D S Canis lupus, carnivore, cause-specific mortality, meta-analysis, telemetry
... Polar bear populations have low intrinsic growth rates, annually at ca. 4.5% at the maximum net productivity level (Regehr et al., 2017), and are thus easy to deplete and slow to recover. How harvest will affect a polar bear population will depend on several factors including abundance, age and sex structure, possible Allee effects (Molnár et al., 2008), distribution (Stirling and Andriashek, 1992), and genetic structure (Coltman et al., 2003). Our study, focusing on harvest numbers, cannot assess such changes. ...
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Wildlife harvest remains a conservation concern for many species and assessing patterns of harvest can provide insights on sustainability and inform management. Polar bears (Ursus maritimus) are harvested over a large part of their range by local people. The species has a history of unsustainable harvest that was largely rectified by an international agreement that required science-based management. The objective of our study was to examine the temporal patterns in the number of polar bears harvested, harvest sex ratios, and harvest rates from 1970 to 2018. We analyzed data from 39,049 harvested polar bears (annual mean 797 bears) collected from 1970 to 2018. Harvest varied across populations and times that reflect varying management objectives, episodic events, and changes based on new population estimates. More males than females were harvested with an overall M:F sex ratio of 1.84. Harvest varied by jurisdiction with 68.0% of bears harvested in Canada, 18.0% in Greenland, 11.8% in the USA, and 2.2% in Norway. Harvest rate was often near the 4.5% target rate. Where data allowed harvest rate estimation, the target rate was exceeded in 11 of 13 populations with 1–5 populations per year above the target since 1978. Harvest rates at times were up to 15.9% of the estimated population size suggesting rare episodes of severe over-harvest. Harvest rate was unrelated to a proxy for ecosystem productivity (area of continental shelf within each population) but was correlated with prey diversity. In the last 5–10 years, monitored populations all had harvest rates near sustainable limits, suggesting improvements in management. Polar bear harvest management has reduced the threat it once posed to the species. However, infrequent estimates of abundance, new management objectives, and climate change have raised new concerns about the effects of harvest.
... More precisely, a common policy-related argument is that reasons offered for intensive wolf hunting (i.e., to mitigate threats to human safety, livestock loss, opportunities to hunt ungulates) do not outweigh the reasons to refrain from intensive wolf hunting (Vucetich et al., 2017). Those reasons for refraining include moral considerations (Vucetich et al., 2015), ecological consequences of allowing wolves to naturally regulate prey populations (Ripple et al., 2014), evolutionary consequences of allowing wolves to naturally regulate prey populations (Coltman et al., 2003;Festa-Bianchet, 2013). Our work contributes to better understanding the robustness of that argument by providing evidence for how unharvested wolf populations may influence the incidence ofosteoarthritis, a genetically based non-communicable disease. ...
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It is widely accepted that predators disproportionately prey on individuals that are old, weak, diseased or injured. By selectively removing individuals with diseases, predators may play an important role in regulating the overall health of prey populations. However, that idea is seldom tested empirically. Here we assess the extent that wolves (Canis lupus) select adult moose (Alces alces) in Isle Royale National Park on the basis of age-class and osteoarthritis, a chronic, non-communicable disease. We also assess how temporal variation in kill rates (on moose by wolves) were associated with the subsequent incidence of osteoarthritis in the moose population over a 33-year period (1975–2007). Wolves showed strong selection for senescent moose and tended to avoid prime-aged adults. However, the presence of severe osteoarthritis, but not mild or moderate osteoarthritis, appeared to increase the vulnerability of prime-aged moose to predation. There was weak evidence to suggest that senescent moose with osteoarthritis maybe more vulnerable to wolves, compared to senescent moose without the disease. The incidence of osteoarthritis declined following years with higher kill rates–which is plausibly due to the selective removal of individuals with osteoarthritis. Together those results suggest that selective predation plays an important role in regulating the health of prey populations. Additionally, because osteoarthritis is influenced by genetic factors, these results highlight how wolf predation may act as a selective force against genes associated with developing severe osteoarthritis as a prime-aged adult. Our findings highlight one benefits of allowing predators to naturally regulate prey populations. The evidence we present for predation’s influence on the health of prey populations is also relevant for policy-related arguments about refraining from intensively hunting wolf populations.
... In other words, a decrease in the anthropocentric Equation 2 leads to a decrease in the ecocentric Equation 3, but the causal link (Equation 4) is still supposed to be valid. In addition, trophy hunting can lead to unexpected evolutionary consequences (Coltman et al. 2003), overharvesting of young males (Lindsey et al. 2007b), and disproportionate pressure on threatened species (Palazy et al. 2011(Palazy et al. , 2012(Palazy et al. , 2013 and therefore to population declines and potential detrimental effects on biodiversity. That means that I(C humans ) in Equation 4 should be carefully examined. ...
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Perspectives in conservation are based on a variety of value systems. Such differences in how people value nature and its components lead to different evaluations of the morality of conservation goals and approaches, and often underlie disagreements in the formulation and implementation of environmental management policies. Specifically, whether a conservation action (e.g. killing feral cats to reduce predation on bird species threatened with extinction) is viewed as appropriate or not can vary among people with different value systems. Here, we present a conceptual, mathematical framework intended as a tool to systematically explore and clarify core value statements in conservation approaches. Its purpose is to highlight how fundamental differences between these value systems can lead to different prioritizations of available management options and offer a common ground for discourse. The proposed equations decompose the question underlying many controversies around management decisions in conservation: what or who is valued, how, and to what extent? We compare how management decisions would likely be viewed under three idealised value systems: ecocentric conservation, which aims to preserve biodiversity; new conservation, which considers that biodiversity can only be preserved if it benefits humans; and sentientist conservation, which aims at minimising suffering for sentient beings. We illustrate the utility of the framework by applying it to case studies involving invasive alien species, rewilding, and trophy hunting. By making value systems and their consequences in practice explicit, the framework facilitates debates on contested conservation issues, and complements philosophical discursive approaches about moral reasoning. We believe dissecting the core value statements on which conservation decisions are based will provide an additional tool to understand and address conservation conflicts.
... Likewise, restored bezoar goat populations might create co-benefits for local communities via legal, controlled hunting and/or income from trophy hunting (Di Minin, Leader-Williams, & Bradshaw, 2016;IUCN, 2016;Michel et al., 2015). This can reduce poaching incentives and pressure, but also entails risks as trophy hunting can lead to undesirable evolutionary and fitness outcomes (Coltman et al., 2003;IUCN, 2016). We caution that legal hunting can only become an option once bezoar goats have attained (much) larger populations and any offtake quotas would have to be scientifically grounded. ...
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Mountain ungulates around the world have been decimated to small, fragmented populations. Restoring these species often is limited by inadequate information on where suitable habitat is found, and which restoration measures would help to increase and link existing populations. We developed an approach to spatially target threat-specific restoration actions and demonstrate it for bezoar goats (Capra aegagrus) in the Caucasus. Using a large occurrence dataset, we identified suitable habitat patches and evaluate them in terms of connectivity, protection status, and competition with other mountain ungulates. We found extant bezoar goat populations to be highly isolated, yet with widespread areas of suitable, unoccupied habitat between them. Many unoccupied habitat patches were well-connected to extant populations, were at least partly protected, and have low potential for competition with other Capra species. This signals substantial pressure on bezoar goats, likely due to poaching, which currently prevents natural recolonization. Our study shows how restoration planning is possible in the face of multiple threats and scarce data. For bezoar goats in the Caucasus, we pinpoint priority patches for specific restoration measures, including reintroductions and anti-poaching action. We highlight that many patches would benefit from multiple interventions and that transboundary restoration planning is needed, a situation likely similar for many mountain ungulates around the world.
... Harvesting species based on their headgear size raises the potential of imposing an artificial selection pressure (Coltman et al. 2003, Hengeveld and Festa-Bianchet 2011, Pigeon et al. 2016; however, alternate hypotheses argue variation in environmental conditions, demography, and density dependence are the primary drivers underlying such phenotypic variation (Monteith et al. 2013(Monteith et al. , 2018. Management approaches for harvested populations of ungulates often include a form of headgear size criteria; for example, in some bighorn sheep jurisdictions only males with a minimum degree of curl can be taken. ...
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... There is also evidence to suggest that hunting pressure can cause a size reduction in animals over time [93][94][95]. The survivorship profile of Polderweg 1 indicates a high adult mortality rate with few individuals surviving beyond the age of five years old. ...
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Detecting microevolutionary responses to natural selection by observing temporal changes in individual breeding values is challenging. The collection of suitable datasets can take many years and disentangling the contributions of the environment and genetics to phenotypic change is not trivial. Furthermore, pedigree-based methods of obtaining individual breeding values have known biases. Here, we apply a genomic prediction approach to estimate breeding values of adult weight in a 35-year dataset of Soay sheep ( Ovis aries) . Comparisons are made with a traditional pedigree-based approach. During the study period, adult body weight decreased, but the underlying genetic component of body weight increased, at a rate that is unlikely to be attributable to genetic drift. Thus cryptic microevolution of greater adult body weight has probably occurred. Genomic and pedigree-based approaches gave largely consistent results. Thus, using genomic prediction to study microevolution in wild populations can remove the requirement for pedigree data, potentially opening up new study systems for similar research.
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Abstract Parasite resistance and body size are subject to directional natural selection in a population of feral Soay sheep (Ovis aries) on the island of St. Kilda, Scotland. Classical evolutionary theory predicts that directional selection should erode additive genetic variation and favor the maintenance of alleles that have negative pleiotropic effects on other traits associated with fitness. Contrary to these predictions, in this study we show that there is considerable additive genetic variation for both parasite resistance, measured as fecal egg count (FEC), and body size, measured as weight and hindleg length, and that there are positive genetic correlations between parasite resistance and body size in both sexes. Body size traits had higher heritabilities than parasite resistance. This was not due to low levels of additive genetic variation for parasite resistance, but was a consequence of high levels of residual variance in FEC. Measured as coefficients of variation, levels of additive genetic variation for FEC were actually higher than for weight or hindleg length. High levels of additive genetic variation for parasite resistance may be maintained by a number of mechanisms including high mutational input, balancing selection, antagonistic pleiotropy, and host-parasite coevolution. The positive genetic correlation between parasite resistance and body size, a trait also subject to sexual selection in males, suggests that parasite resistance and growth are not traded off in Soay sheep, but rather that genetically resistant individuals also experience superior growth.
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The topographic development and the relationship of elevation to erosion within major collisional mountain ranges are investigated through analytical and numerical solutions of the diffusion equation for a continuously uplifted terrain. The solutions are conditioned by observations of topographic and uplift patterns from the Southern Alps of New Zealand. A simple one-dimensional steady state solution for elevation as a function of uplift rate (v), ridge-stream spacing (L), and the effective erosional diffusivity (K*) demonstrates that one-dimensional steady state elevations vary linearly with uplift rate, as the square of channel spacing and inversely with K*. The Southern Alps ridge peak pattern is a function of the response of the slope forming processes to the imposed major stream gradients and is established within ~250 ka for K* ~107 m2/Ma. The ridge-peak pattern within the high rainfall region of the western slope is shown to be an intrinsic topographic pattern for many different uplift rate and diffusivity patterns and is similar to that observed in the Himalayan collision zone. -from Author
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The consumptive use of wildlife, in particular trophy hunting and game ranching of ungulates, has been advocated as a tool for conservation in Africa. We show that these methods of harvesting differ significantly from natural predation, with trophy hunting showing extreme selection for adult males and game ranching leading to disproportionate harvests of young males. Little information, either theoretical or empirical, exists concerning the effect of these harvesting regimes on the long-term population dynamics of ungulate populations. Despite that, the potential effects of sex-skewed harvests are numerous. In this paper, we investigate one potentially deleterious effect of sex-skewed harvests. Both theory and experimental data suggest that male ungulates are limited in their absolute ability to inseminate females. Using a Leslie-Matrix model and published data on impala, we show that the interaction between sperm limitation and harvests with highly male-biased sex ratios can lead to greatly reduced female fecundity (defined as the number of young born) and population collapse. These results are robust and suggest that present methods of harvesting may not be optimal, or viable, in the long term.
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We evaluate methods for measuring and specifying rates of microevolution in the wild, with particular regard to studies of contemporary, often deemed "rapid," evolution. A considerable amount of ambiguity and inconsistency persists within the field, and we provide a number of suggestions that should improve study design, inference, and clarity of presentation. (1) Some studies measure change over time within a population (allochronic) and others measure the difference between two populations that had a common ancestor in the past (synchronic). Allochronic studies can be used to estimate rates of "evolution," whereas synchronic studies more appropriately estimate rates of "divergence." Rates of divergence may range from a small fraction to many times the actual evolutionary rates in the component populations. (2) Some studies measure change using individuals captured from the wild, whereas others measure differences after rearing in a common environment. The first type of study can be used to specify "phenotypic" rates and the later "genetic" rates. (3) The most commonly used evolutionary rate metric, the darwin, has a number of theoretical shortcomings. Studies of microevolution would benefit from specifying rates in standard deviations per generation, the haldane. (4) Evolutionary rates are typically specified without an indication of their precision. Readily available methods for specifying confidence intervals and statistical significance (regression, bootstrapping, randomization) should be implemented. (5) Microevolutionists should strive to accumulate time series, which can reveal temporal shifts in the rate of evolution and can be used to identify evolutionary patterns. (6) Evolutionary rates provide a convenient way to compare the tempo of evolution across studies, traits, taxa, and time scales, but such comparisons are subject to varying degrees of confidence. Comparisons across different time scales are particularly tenuous. (7) A number of multivariate rate measures exist, but considerable theoretical development is required before their utility can be determined. We encourage the continued investigation of evolutionary rates because the information they provide is relevant to a wide range of theoretical and practical issues.
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Fission track analysis of apatite and zircon is used to investigate the late Cenozoic vertical kinematics (amount, age, and rate of rock uplift) of the Pacific continental crust in South Island, New Zealand. New data which constrain the spatial variations in the amount, age, and rate of late Cenozoic rock uplift across the leading edge of the Pacific plate presently in collision with the Australia plate are presented. The pattern of late Cenozoic rock uplift is strongly asymmetrical with respect to the Alpine fault and varies from a maximum of about 19 km at the Alpine fault near Fox Glacier to about 3 km at the east. Late Cenozoic uplift rates have averaged no more than 2.8 mm/yr right at the Alpine fault near Fox Glacier but have been sufficient in 5-8 m.y. to expose amphibolite grade schist near the fault. From 6.7 to 1.3 Ma, cooling rates were of the order of 22 C/m.y., and inferred uplift rates of 0.8 mm/yr are too low to have significantly perturbed the geotherms.
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The Thakkhola graben is one of many north-trending rifts that define the Neogene structural pattern of the southern Tibetan Plateau. Lying at the southern margin of the plateau and extending to the crest of the Himalaya, the graben provides an opportunity to evaluate the kinematic relationships between east-west extensional strain in southern Tibet and north-south extensional strain in the Himalaya. Neotectonic and structural mapping of the Dangardzong fault along the western margin of the graben reveals a southward-decreasing component of normal slip coupled with a southward-increasing component of right-lateral slip that affects Pleistocene basin-fill sediments. We present 14C ages for river terraces in the Thakkhola graben that provide a ca. 17.2 ka minimum age on the latest stage of Dangardzong fault movement. Near the southern termination of the graben, the Dangardzong fault apparently offsets the Annapurna detachment, an early (Miocene) strand of the east-striking South Tibetan fault system. However, the Dangardzong fault itself terminates against a young (i.e., younger than ca. 17.2 ka) strand of the South Tibetan fault system, the Dhumpu detachment. Structural relationships among the Dangardzong, Annapurna, and Dhumpu faults suggest that the Dangardzong structure is a tear fault in the South Tibetan allochthon that accommodates differential amounts and rates of displacement along the South Tibetan fault system. Thus, although the South Tibetan fault system first developed as part of the structural architecture of the Himalaya in Miocene time, at least some strands have been active as recently as the Pleistocene. In a regional context, the South Tibetan fault system serves to accommodate the strain gradient between extension in Tibet and shortening in the Himalaya.