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With growing concerns about the impact of selective harvesting on natural populations, researchers encourage managers to implement harvest regimes that avoid or minimize the potential for demographic and evolutionary side effects. A seemingly intuitive recommendation is to implement harvest regimes that mimic natural mortality patterns. Using stochastic simulations based on a model of risk as a logistic function of a normally distributed biological trait variable, we evaluate the validity of this recommendation when the objective is to minimize the altering effect of harvest on the immediate post-mortality distribution of the trait. We show that, in the absence of compensatory mortality, harvest mimicking natural mortality leads to amplification of the biasing effect expected after natural mortality, whereas an unbiased harvest does not alter the post-mortality trait distribution that would be expected in the absence of harvest. Although our approach focuses only on a subset of many possible objectives for harvest management, it illustrates that a single strategy, such as hunting mimicking natural mortality, may be insufficient to address the complexities of different management objectives with potentially conflicting solutions.
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Biol. Lett. (2008) 4, 307–310
Published online 21 February 2008
Population ecology
Should hunting mortality
mimic the patterns of
natural mortality?
Richard Bischof
*, Atle Mysterud
and Jon E. Swenson
Norwegian University of Life Sciences, PO Box 5003,
1432 A
˚s, Norway
Centre for Ecological and Evolutionary Synthesis (CEES ),
Department of Biology, University of Oslo, PO Box 1066,
Blindern, 0316 Oslo, Norway
Norwegian Institute for Nature Research, 7485 Trondheim, Norway
*Author for correspondence (
With growing concerns about the impact of selec-
tive harvesting on natural populations, research-
ers encourage managers to implement harvest
regimes that avoid or minimize the potential
for demographic and evolutionary side effects.
A seemingly intuitive recommendation is to
implement harvest regimes that mimic natural
mortality patterns. Using stochastic simulations
based on a model of risk as a logistic function of a
normally distributed biological trait variable, we
evaluate the validity of this recommendation
when the objective is to minimize the altering
effect of harvest on the immediate post-mortality
distribution of the trait. We show that, in the
absence of compensatory mortality, harvest
mimicking natural mortality leads to amplifi-
cation of the biasing effect expected after natural
mortality, whereas an unbiased harvest does
not alter the post-mortality trait distribution
that would be expected in the absence of harvest.
Although our approach focuses only on a subset of
many possible objectives for harvest manage-
ment, it illustrates that a single strategy, such
as hunting mimicking natural mortality, may
be insufficient to address the complexities of
different management objectives with potentially
conflicting solutions.
Keywords: demography; life history; simulation;
management; selection; vulnerability
There is growing concern regarding potential demo-
graphic side effects and evolutionary consequences of
selective harvesting on wildlife populations (Harris et al.
2002;Coltman et al. 2003). Perturbations of a popu-
lation’s demographic structure (Mysterud et al. 2002;
Milner et al. 2006) and short- and long-term changes of
morphological traits or life-history strategies due to
artificial selective pressures (Festa-Bianchet & Apollonio
2003) are some of the processes through which selective
hunting may affect populations beyond more obvious,
direct effects on population size and growth rate through
the removal of individuals. In search of management
and are ‘evolutionarily enlightened’ (Gordon et al.
2004), it has been suggested that harvesting regimes
should mimic natural mortality patterns (e.g. Milner
et al. 2006;Loehr et al. 2007;Bergeron et al. 2008).
Surprisingly, the general applicability of this recommen-
dation has received little theoretical or empirical
evaluation. Recently, Proaktor et al. (2007) presented
model-based evidence that selection for lighter weight
at first reproduction in ungulates could be a conse-
quence of harvest and that harvest pressure is more
important in driving this adaptive response than the
degree of harvest selectivity. It seems plausible that this
would apply to other situations in which the benefits of
more and earlier reproduction eventually outweigh its
costs (e.g. lower quality offspring), possibly due to higher
overall mortality and consequently a greater chance of
not reproducing later. To our knowledge, this is the only
strong argument thus far in support of the statement
that harvest selectivity patterns should mimic natural
mortality, because a harvest biased towards younger
(and lighter) individuals could minimize the aforemen-
tioned adaptive response. Even in such a case, simply
targeting small (i.e. young) individuals may lead
to further decreases in the size at, and time to,
maturation as recent literature on fisheries-induced
evolution suggests (Kuparinen & Merila¨ 2007 and
references therein).
In this article, we are specifically concerned about the
lack of scrutiny of the statement with regard to the
immediate disruption caused due to demographic or
other changes as a result of biased harvest. To avoid
ambiguity, we identify a clear objective for harvest
management with respect to selectivity, namely that
harvesting and natural mortality acting on a population
should result in a post-mortality population structure (or
biological trait distribution) that is identical or at least
very similar to the structure that would be expected in
the absence of harvest (see also Harris et al. 2002). With
this objective in mind, we ask the question: should
hunting mortality mimic natural mortality in order to
limit the potential for disruptions caused by demo-
graphic or trait-distribution changes?
The effects of selection on trait distributions are now
relatively well understood (e.g. Lynch & Walsh 1998).
Particularly relevant to our work is a paper by Vaupel
et al. (1979), which explores the effects of viability
selection on the distribution of a trait over time and age
cohorts. The authors termed this trait ‘frailty’ to high-
light the fact that it is related to an individual’s risk of
mortality, and assumed that the probability density
function of frailty follows a gamma distribution. Vau p e l
et al. (1979) further assumed that the force of mortality
(a measure of an individual’s risk) is a function of time,
age and frailty. Although our basic approach is similar,
we develop a slightly different model and explore the
outcome through the simulations. We also extend Vau p e l
et al. (1979) by discriminating between two mortality
causes and by investigating how altering the shape of the
viability selection function affects the post-mortality trait
For model construction, we assume a normally distributed random
variable x(with mean mand variance s) that represents a certain
trait of individuals in the population, with associated probability
density function
pexp KðxKmÞ2
Received 18 January 2008
Accepted 4 February 2008
307 This journal is q2008 The Royal Society
We then assume that risk pis a logistic function of trait x
(figure 1), where the relationship between pand xcan be expressed as
Here, aand bare the intercept and slope of the linear regression
(with the logit link), respectively. Although the assumptions behind
equations (2.1) and (2.2) oversimplify a world where risk probably is
a complex function of multiple variables (morphology, age, experi-
ence, behaviour, space use, etc.), the approximation is sufficient for
our purposes. The above approach centres on a logistic relationship
between risk and a normally distributed continuous feature of the
population, but this representation also allows the incorporation of
discrete or factor variables, as well as other distributions. Following
the precautionary principle and because strong compensation can be
expected to occur only rarely (Lebreton 2005), we assume that
mortality is additive. Furthermore, we ignore potential density-
dependent effects that in real populations may, for example, positively
affect the growth rate of individuals exhibiting trait values that are
selectively targeted.
An interesting finding of Vaupel et al. (1979) was that, as
individuals age, their force of mortality increases more rapidly
than the average force of mortality of the age cohort they belong
to, because the removal of frail individuals decreases the average
frailty of the surviving cohort. The mechanism underlying this
phenomenon applies also to our model, although for simplicity we
did not include an age term. While surviving individuals retain
their original trait value as they move from pre- to post-mortality,
the average trait value shifts towards the less vulnerable end of the
trait spectrum (assuming no recruitment within that time step).
We investigated changes in the probability density distributions of
trait x(e.g. size) in a heterogeneous hypothetical population with
two groups with different mean trait values (e.g. females and males)
resulting from exposure to harvest followed by natural mortality.
We evaluated the effect of different shapes of the logistic function
linking harvest risk and trait value on both the post-mortality trait
distribution and the ratio of the two groups in the population. We
used three main expressions of the logistic function based on its
shape (‘mimic’, ‘inverse’ and ‘unbiased’; figure 2) relative to the
risk associated with natural mortality, by altering the intercept and
slope in the logistic function (equation (2.2)).
We conducted stochastic simulations using R v. 2.5.0
(R Development Core Team 2007). We repeated simulations with
the same settings 100 times and calculated bias and 95% CI limits
from 1000 bootstrapped replicas of the mean parameter values. We
note that, although we chose to illustrate the effect of viability
selection using simulations, the effects of multiplying a distribution
with a function can also be evaluated analytically, e.g. through the
use of conjugate priors within a Bayesian framework ( Fink 1997).
For the case of harvest preceding natural mortality,
simulation results (figure 2,table 1) indicate that (i)
inverse harvest risk prior to natural mortality
diminishes and in extreme cases reverses the biasing
effect of natural mortality on the density distribution
of the biological trait, (ii) unbiased harvest risk keeps
the biasing effect of natural mortality unchanged,
and (iii) mimicking harvest risk amplifies the biasing
effect of natural mortality on the density distribution
of the biological trait. Biased natural mortality alters
the ratio of the two groups in the population, with
additional changes in the ratio due to mimic and
inverse harvest mortalities, but no further alterations
if harvest is unbiased. The altering effect of biased
harvest on the trait distribution and the ratio of the
two groups in the population increases with increas-
ing harvest rate (table 1). Because we assume no
density-dependent effects and, if harvest mortality
is limited by a quota, the above patterns, at least
qualitatively, also hold true for har vest following
natural mortality.
The general statement that harvest mortality should
mimic natural mortality in order to avoid demographic
disturbance or evolutionary consequences is not yet
sufficiently supported, and needs to be qualified. We
found that, for the specific objective of maintaining
(a) (i) (i) (i)
(ii) (ii) (ii)
(iii) (iii) (iii)
0 50 100 150 200 250 300
trait value
0 0.2 0.4 0.6 0.8 1.0
0 50 100 150 200 250 300
trait value
0 0.2 0.4 0.6 0.8 1.0
0 50 100 150 200 250 300
trait value
0 0.2 0.4 0.6 0.8 1.0
Figure 1. Illustration of the link between the density distribution of risk and a normally distributed biological trait x(s.d.Z20;
hashed lines: 2!s.d. boundaries, arbitrary unit), with risk being a logistic function of x. Shifts in the mean trait value ((a(i)–(iii))
100, (b(i)–(iii)) 150 and (c(i)–(iii)) 200) of a hypothetical population (nZ5000) change the density distribution of risk in the
308 R. Bischof et al. Hunting mortality and natural mortality
Biol. Lett. (2008)
unchanged post-mortality distributions of a trait (or
demographic feature), hunting mortality should be
unbiased. This holds true regardless of whether hunting
occurs prior to or after natural mortality. Therefore, in
the absence of strong compensation in mortalities and
until further supporting evidence emerges, we would
limit recommending that hunting mortality should
mimic natural mortality patterns to the following cases.
(i) Natural mortality regimes have been altered, e.g.
as a result of extermination of natural predators.
(ii) The objective is to minimize selective pressure for
earlier reproduction driven by increased overall
mortality as a result of adding harvest.
(iii) An amplification of the biased outcome of natural
mortality is desired.
(iv) The main objective is to minimize the negative
direct impact of harvest on population growth by
targeting those demographic groups whose survival
has the lowest elasticity/sensitivity.
(v) Natural mortality is unbiased.
In our example, increasing overall mortality (whether
the latter is biased or not) by a constant (e.g. adding
unbiased harvest) does not alter the selective pressure
on traits directly linked to risk. We emphasize that
different objectives, such as (i) minimizing the effects of
harvest on the distribution of traits or demographic
features or (ii) limiting the selective pressure for lower
age and size at first reproduction, may have conflicting
solutions, as well as different temporal scopes (see also
Law 2001).
1.0(a) (i) (i) (i)
(ii) (ii) (ii)
50 100 150 200
trait value
50 100 150 200
trait value 50 100 150 200
trait value
Figure 2. Changes in trait distributions as a result of various patterns of hunting mortality relative to biased natural mortality for
a simulated heterogeneous population (two cohorts with nZ1000 each, s.d.Z30, 100 and 150). (ac(i)) show natural and
hunting risks as a logistic function of the normally distributed biological trait x(arbitrary unit; lines: red, hunting; black, natural).
(ac(ii)) show distributions of the biological trait before and after mortality (lines: grey dashed, before mortality (groups
separate); grey solid, before mortality (joint); black, after natural mortality without hunting; red dashed, after hunting and
natural mortality). Risk associated with hunting mortality either (a) mimics natural mortality, is (b) unbiased (slope and intercept
of the logistic function set to 0), or is (c) inverse to natural mortality. Harvest rate was set at 30% of the initial population size.
Table 1. Bootstrapped estimates of the mean trait value (m, arbitrary unit) and ratio (r) of groups (group 1 : group 2) of
surviving individuals in a hypothetical population after hunting followed by natural mortality and after natural mortality in
the absence of hunting mortality (m
) from 100 simulation runs for each of the three shapes of the risk function (see text
and figure 2) and two different harvest rates. (The initial population consisted of two groups (nZ1000 each) with mean trait
value mZ100 and 150, respectively, and s.d.Z30. Natural mortality was modelled as a logistic function of x(see text), with
intercept aZK5 and slope bZ0.04.)
risk shape
rate mCIL
mimic 0.25 103.95 103.82, 104.09 109.12 108.99, 109.25 2.10 2.08, 2.12 1.74 1.72, 1.75
0.5 96.65 96.52, 96.78 109.05 108.90, 109.20 2.82 2.78, 2.86 1.77 1.75, 1.78
unbiased 0.25 108.92 108.72, 109.12 109.10 108.96, 109.24 1.77 1.75, 1.79 1.75 1.73, 1.77
0.5 109.14 108.88, 109.38 109.14 109.01, 109.27 1.76 1.73, 1.78 1.74 1.73, 1.76
inverse 0.25 114.47 114.25, 114.70 109.01 108.87, 109.17 1.45 1.43, 1.47 1.77 1.75, 1.78
0.5 123.97 123.67, 124.25 109.01 108.86, 109.17 1.05 1.04, 1.07 1.75 1.73, 1.77
Ninety-five per cent CI limits from 1000 bootstrapped estimates.
Hunting mortality and natural mortality R. Bischof et al. 309
Biol. Lett. (2008)
We focused on the potential of selective harvest to
alter the post-mortality distribution of a single trait
from the distribution that would be expected if natural
scope is required to evaluate all important ecological
and evolutionary consequences of harvesting and to
answer the questions about optimal harvesting strategies
comprehensively. Such models may include age effects
on trait values and risk, density-dependent effects, and
environmental and demographic stochasticity. Further-
more, empirical exploration into how various harvesting
strategies in concert with biased natural mortality affect
trait distributions of natural populations are required to
validate what theory suggests.
We thank S. J. Hegland, A. Ordiz, O. G. Støen, A. Zedrosser
and two anonymous reviewers for their comments. This
manuscript benefited substantially from T. Coulson’s advice,
for which we are grateful. Funding for this project came
from the Norwegian University of Life Sciences (R.B.) and
the Research Council of Norway (A.M.).
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... Some authors argue that wildlife managers should mimic natural predation and harvest younger individuals in order to achieve a most natural population structure 9,34,53 . Others reason that this could be best achieved by a totally unselective hunting except for some special scenarios 52 . Some of these scenarios, however, actually apply to the wild boar. ...
... Given increasing human-wildlife conflicts and ecological threats caused by exponentially growing wild boar populations, a limitation of population growth or even a population reduction is urgently required in many areas 9,13,[27][28][29] . Therefore, an important objective of a selective harvest in wild boar could be to maximise hunting efficiency in order to bring down population growth 52 . ...
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Technical Report
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Svalbardreinen er en unik underart av reinsdyr (Rangifer tarandus platyrhynchus) som bare finnes på Svalbard. Dette innebærer at forvaltningen av arten skal være i overensstemmelse med Svalbardlovens høye miljøkrav. Jakt på svalbardrein er forbeholdt lokalbefolkningen på Svalbard og er lokalisert til Nordenskiöld Land. Siden jakt ble tillatt i 1983 har jaktuttaket økt fra 117 dyr til det dobbelte. I ”Plan for forvaltning av svalbardrein” utarbeidet av Sysselmannen på Svalbard i 2009 ble det påpekt at det ikke har vært gjort noen vurdering av hvor stor effekt dagens forvaltningspraksis med hensyn på jakt, har på bestanden av svalbardrein. I denne rapporten benytter vi matematiske modeller for bestandsdynamikken til svalbardrein til å vurdere dette. De viktigste funnene Resultatene tyder på at jaktuttaket av svalbardrein under dagens forvaltningspraksis har små effekter på bestandene av svalbardrein. Den viktigste gruppen dyr i bestandsdynamikken til svalbardrein er de voksne simlene. Vi anslår at man kan ta ut opptil 13 % av bestanden av voksne simler årlig uten at bestanden vil bli betydelig redusert. For de siste 10 år beregner vi at uttaket av voksne simler ligget på 4-10 % av bestanden. Sannsynligvis kan man ta ut 400-450 dyr årlig på hele Nordenskiöld Land, noe som innebærer mulighet for fortsatt vekst i antall fellingsløyver. I tillegg til å ha lite effekt på bestandsstørrelsene har jakten slik den praktiseres og forvaltes også lite effekt på kjønnsfordelingen i bestanden. Dagens uttak av like mange bukker som simler synes å stabilisere kjønnsfordelingen nært den fordelingen man ville hatt uten jakt. Det er mulig å øke uttaket av bukker, men dette vil kunne føre til en betydelig skjevere kjønnsfordeling i bestandene. Miljøgevinst Arbeidet gir en avklaring på spørsmålet om hvor stor effekt rekreasjonsjakten på svalbardrein har på bestandene. Den gir støtte til dagens forvaltningspraksis og dermed faglig støtte til at dagens rekreasjonsjakt på svalbardrein kan fortsette. Forslag til tiltak Rapporten foreslår ingen umiddelbare tiltak, men rapporten gir innspill til de vurderinger som bør gjøres hvis det blir aktuelt å fortsette å øke jaktuttaket. Hva er viktig for miljøforvaltningen? Analysene tyder på at det totalt kan felles oppimot 450 rein på Nordenskiöld Land årlig. I dag gis det 300-350 fellingstillatelser årlig hvorav 60 % blir benyttet. Hvis andelen som benytter fellingstillatelsen øker betydelig vil dette innebære en betydelig vekst i antall dyr felt innenfor jaktområdene. Det er mulig at dette vil kunne gi lokal overbeskatning i enkelte jaktområder. Dette vil være avhengig av hvor isolerte bestandene i jaktområdene er fra bestandene i omliggende områder der det ikke jaktes. De varslede klimaforandringene i Arktis kan forandre levevilkårene til svalbardreinen dramatisk. Forvaltningen må derfor være forberedt på at jaktuttaket kan måtte justeres. Oppfølging Det anbefales at man undersøker i hvilken grad bestandene i de forskjellige jaktområdene er isolerte fra bestandene i omliggende områder. Særlig gjelder det jaktområdene ved Grønnfjorden og Sassendalen. For å utvikle bedre bestandsmodeller for svalbardrein trengs det en innsats for å bedre estimatene på kalvers overlevelse gjennom vinteren, og en bedre forståelse for hvordan klimatisk variasjon påvirker bestandene.
... As the most productive lands were cultivated and settled by humans (Overland and Hjelle 2009;Hjelle et al. 2010), red deer were forced into poorer habitats. Long-term size-or sex-selective harvest (Coltman et al. 2003;Saether et al. 2003;Fenberg and Roy 2008), or even just persistent high harvesting intensity (Proaktor et al. 2007;Bischof et al. 2008;Mysterud 2011), also may have caused a decrease in body size and earlier age of maturation. Our estimate of the sex ratio of the prehistoric harvest indicates a relatively pronounced sexually biased harvest with . ...
Large-scale variation in mammalian body size has often been found to be related to environmental conditions. A general finding among large herbivores is that body size increases with decreasing temperature (Bergmann's rule), because animals with larger body size have better heat conservation or fasting tolerance, or because higher quality forage occurs in colder environments. Using a data set on the skeletal morphology of Norwegian red deer (Artiodactyla, Cervidae: Cervus elaphus) spanning the last approximately 7,100 years, we document an inverse relationship between climatic conditions and body size. The size of Norwegian red deer, as estimated from both teeth and weight-bearing bones, was significantly larger during the warmer and wetter middle Holocene than it is today. However, the reduction in body size does not seem to be related to changing climatic conditions. Rather, this decrease happened during a period of large-scale human-mediated habitat fragmentation, increased populations of domestic herbivores, and heavy hunting pressure that reduced population density. The size of teeth was reduced as much as, or even more than, the size of weight-bearing bones, which indicates an evolutionary response rather than phenotypic plasticity to changing forage and environmental conditions. Decreased body size may be a general response in wild ungulates to a more human-dominated landscape, resulting from reduced access to optimal habitats and high adult hunting mortality.
... Caddy and Sharp (1986) argued what they labelled a utopian case, in which each component of an ecosystem would be exploited in proportion to the rate at which natural mortality removes biomass. Linking mortality rates in hunting to natural mortality rates has also been debated in terrestrial animals (Bischof et al. 2008). Most recently, the idea of making fishing mortality proportional to productivity has been advocated (Garcia et al. 2012). ...
Size-at-entry regulations in fisheries cause major disruption to aquatic ecosystems, including truncation of age- and size-structures, destabilization of fish stocks, directional selection on phenotypic traits and a by-catch of unwanted species and sizes. Here, we use simple dynamic models of size-spectra to examine an alternative, so-called balanced harvesting. Balanced harvesting helps in retaining the approximate power-law size-structure of natural ecosystems, whereas size-at-entry regulations do not. Balanced harvesting is less likely to destabilize steady states than size-at-entry regulations set close to the size at maturation. Surprisingly, our numerical results suggest that steady-state biomass yield can be substantially increased by switching from size-at-entry to balanced harvesting. On the basis of these results, we argue that the goals of conservation and of greater yields seem less difficult to reconcile than have previously been thought. However, to work towards these goals require a change in our approach to fishing.
Conference Paper
Because of large home ranges, low population densities, and long dispersal distances, brown bear and other large carnivore populations often extend over several administrative borders, and are consequently subject to sometimes very different management regimes. Since many populations are transboundary in nature, it is vital that their conservation and management are done in a coordinated and cooperative manner between all administrative units sharing such population. This is also recommended by the Guidelines for population level management plans for large carnivores in Europe. However, very little is known about the effects of harvesting on demography of a population that has different management regimes and harvesting strategies employed in different parts of its range. Brown bears (Ursus arctos) in Slovenia are a perfect study case for this problem: 1.) Slovenia represents only a part of the larger brown bear population, 2.) structure and the number of bears removed from population in Slovenia differs considerably from the neighbouring countries, 3.) bears in Slovenia are intensely managed and human-caused mortality represents the majority of all recorded mortality, and 4.) good long-term monitoring data about the removed bears are available. Bears in Slovenia are the north-western part of the Dinaric-Pindos population. The landscape continues without any physical barriers towards south-east into neighbouring Croatia, and the bears readily cross the national border. We have observed significant changes in demographic structure at the periphery of the bear population in Slovenia, with a steep decline of both bear densities and proportion of females towards the population edge in the north. The behaviour and space use of bears at the periphery also differs considerably from those of the bears in the core area (e.g. males at the periphery have larger home range sizes and appear to perform directional movements during the mating season). We analyzed demographic structure of the recorded brown bears that were removed from population in Slovenia in the 1998-2008 period (n = 927). Most bears were removed through hunting (78 %) or in traffic accidents (18 %). Most of the bears removed from population in Slovenia are young (the average age of all removed bears is 2.3 years), which is in strong contrast with neighbouring Croatia, where mainly adult males with high trophy values are harvested. According to the data from a mark-recapture study using noninvasive genetics performed in 2007, approximately 25 % of the bears living in Slovenia were removed each year, which is much higher than in other brown bear populations. Using virtual population analysis and stochastic age- and sex- structured models we have shown that such high removal rates were only possible because of a steady influx of immigrating bears from neighbouring Croatia, where removal rates during the study period were much lower. Slovenia thus represents a sink for the Dinaric-Pindos brown bear population. However, even with the high removal rate, the bear numbers in Slovenia were generally increasing during the study period. This justified the high removal rates in order to reach stabilization of the population growth, which was set as the national bear management goal. In our case, the adaptive management based on monitoring of population trends through systematic observations at constant feeding places and previous harvesting quotas enabled the managers to achieve this goal, which could be missed if only absolute data on reproductive potential and survival rates were considered with no regard to the immigration from Croatia, which apparently influences the population dynamics in Slovenia. Whether neighbouring countries should strive to equalize the number and structure of the harvested animals is disputable. But as our study shows, it is crucial that situation in neighbouring countries that share the population is monitored, and that management is coordinated. For example, recent increase in bear hunting quotas in Croatia will probably decrease the immigration rate to Slovenia. This will have to be adequately taken into consideration when planning the future harvest in Slovenia.
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Change in the size of populations over space and time is, arguably, the motivation for much of pure and applied ecological research. The fundamental model for the dynamics of any population is straightforward: the net change in the abundance is the simple difference between the number of individuals entering the population and the number leaving the population, either or both of which may change in response to factors intrinsic and extrinsic to the population. While harvest of individuals from a population constitutes a clear extrinsic source of removal of individuals, the response of populations to harvest is frequently complex, reflecting an interaction of harvest with one or more population processes. Here we consider the role of these interactions, and factors influencing them, on the effective harvest management of waterfowl populations. We review historical ideas concerning harvest and discuss the relationship(s) between waterfowl life histories and the development and application of population models to inform harvest management. The influence of population structure (age, spatial) on derivation of optimal harvest strategies (with and without explicit consideration of various sources of uncertainty) is considered. In addition to population structure, we discuss how the optimal harvest strategy may be influenced by: 1) patterns of density-dependence in one or more vital rates, and 2) heterogeneity in vital rates among individuals within an age-sex-size class. Although derivation of the optimal harvest strategy for simple population models (with or without structure) is generally straightforward, there are several potential difficulties in application. In particular, uncertainty concerning the population structure at the time of harvest, and the ability to regulate the structure of the harvest itself, are significant complications. We therefore review the evidence of effects of harvest on waterfowl populations. Some
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This report reviews conjugate priors and priors closed under sampling for a variety of data generating processes where the prior distributions are univariate, bivariate, and multivariate. The eects of transformations on conjugate prior relationships are considered and cases where conjugate prior relationships can be applied under transformations are identied. Univariate and bivariate prior relationships are veried using Monte Carlo methods.
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Possible evolutionary consequences of sport hunting have received relatively little con -sideration by wildlife managers. We reviewed the literature on genetic implications of sport hunting of terrestrial vertebrates and recommend research directions to address cur -rent uncertainties. Four potential effects can be ascribed to sport hunting: 1) it may alter the rate of gene flow among neighboring demes, 2) it may alter the rate of genetic drift through its effect on genetically effective population size, 3) it may decrease fitness by deliberately culling individuals with traits deemed undesirable by hunters or managers, and 4) it may inadvertently decrease fitness by selectively removing individuals with traits desired by hunters. Which, if any, of these effects are serious concerns depends on the nature and intensity of harvest as well as the demographic characteristics and breeding system of the species at issue. Undesirable genetic consequences from hunting have been documented in only a few cases, and we see no urgency. However, studies specif -ically investigating these issues have been rare, and such consequences require careful analysis and long time periods to detect. Existing information is sufficient to suggest that hunting regimes producing sex-and age-specific mortality patterns similar to those occur -ring naturally, or which maintain demographic structures conducive to natural breeding patterns, will have fewer long-term evolutionary consequences than those producing highly uncharacteristic mortality patterns.
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Summary • Wild large herbivores provide goods and income to rural communities, have major impacts on land use and habitats of conservation importance and, in some cases, face local or global extinction. As a result, substantial effort is applied to their management across the globe. To be effective, however, management has to be science-based. We reviewed recent fundamental and applied studies of large herbivores with particular emphasis on the relationship between the spatial and temporal scales of ecosystem response, management decision and implementation. • Long-term population dynamics research has revealed fundamental differences in how sex/age classes are affected by changes in density and weather. Consequently, management must be tailored to the age and sex structure of the population, rather than to simple population counts. • Herbivory by large ungulates shapes the structure, diversity and functioning of most terrestrial ecosystems. Recent research has shown that fundamental herbivore/vegetation interactions driving landscape change are localized, often at scales of a few metres. For example, sheep and deer will selectively browse heather Calluna vulgaris at the edge of preferred grass patches in heather moorland. As heather is vulnerable to heavy defoliation, in the long term this can lead to loss of heather cover despite the average utilization rate of heather in a management area being low. Therefore, while herbivore population management requires a large-scale approach, management of herbivore impacts on vegetation may require a much more flexible and site-specific approach. • Localized impacts on vegetation have cascading effects on biodiversity, because changes in vegetation structure and composition, induced by large herbivores affect habitat suitability for many other species. As such, grazing should be considered as a tool for broader biodiversity management requiring a more sophisticated approach than just, for example, eliminating grazing from conservation areas through the use of exclosures. • Synthesis and applications. The management of wild large herbivores must consider different spatial scales, from small patches of vegetation to boundaries of an animal population. It also requires long-term planning based on a deep understanding of how population processes, such a birth rate, death rate and age structure, are affected by changes in land use and climate and how these affect localized herbivore impacts. Because wild herbivores do not observe administrative or political boundaries, adjusting their management to socio-political realities can present a challenge. Many developing countries have established co-operative management groups that allow all interested parties to be involved in the development of management plans; developed countries have a lot to learn from the developing world's example. Journal of Applied Ecology (2004) 41, 1021 –1031
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Summary 1. In this review, we focus on how males can affect the population dynamics of ungu- lates (i) by being a component of population density (and thereby affecting interpreta- tion of log-linear models), and (ii) by considering the mechanisms by which males can actively affect the demographic rates of females. 2. We argue that the choice of measure of density is important, and that the inclusion or exclusion of males into models can influence results. For example, we demonstrate that if the dynamics of a population can be described with a first-order auto-regressive proc- ess in a log-linear framework, the asymmetry between the effects of females on the male dynamics and vice versa can introduce a second order process, much in the same way that the interaction between disease and host or predator and prey can. It would be use- ful for researchers with sufficient data to explore the affects of using different density measures. 3. In general, even in harvested populations with highly skewed sex ratios, males are usually able to fertilize all females, though detailed studies document a lower propor- tion of younger females breeding when sex ratios are heavily female biased. It is well documented that the presence of males can induce oestrus in females, and that male age may also be a factor. In populations with both a skewed sex ratio and a young male age structure, calving is delayed and less synchronous. We identify several mechanisms that may be responsible for this. 4. Delayed calving may lower summer survival and autumn masses, which may lead to higher winter mortality. If females are born light, they may require another year of growth before they start reproducing. Delayed calving can reduce future fertility of the mother. As the proportion of calves predated during the first few weeks of life is often very high, calving synchrony may also be an important strategy to lower predation rates. 5. We argue that the effects of males on population dynamics of ungulates are likely to be non-trivial, and that their potential effects should not be ignored. The mechanisms we discuss may be important - though much more research is required before we can demonstrate they are.
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Human activities have indirectly modified the dynamics of many populations, accelerating considerably the natural rate of species extinction and raising strong concerns about biodiversity. In many such cases, the underlying ‘natural’ dynamics of the population has been modified by human-induced increases in mortality, even if the populations are not exploited or harvested in the strict sense. Both dynamical and statistical models are needed to investigate the consequences of human-induced mortality on the overall dynamics of a population. This paper reviews existing approaches and the potential of recent developments to help form a conceptual and practical framework for analysing the dynamics of exploited populations. It examines both the simple case of an extra source of mortality instantaneously in time, and the theory involved when both risks compete over a continuous time scale. This basic theory is expanded to structured populations, using matrix population models, with applications to the conservation biology of long-lived vertebrates. The type and degree of compensation expected and approaches to detect it are reviewed, and ways of handling uncertainty are discussed.
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In sexually dimorphic ungulates, sexual selection favoring rapid horn growth in males may be counterbalanced by a decrease in longevity if horns are costly to produce and maintain. Alternatively, if early horn growth varied with individual quality, it may be positively correlated with longevity. We studied Alpine ibex Capra ibex in the Gran Paradiso National Park, Italy, to test these alternatives by comparing early horn growth and longevity of 383 males that died from natural causes. After accounting for age at death, total horn length after age 5 was positively correlated with horn growth from two to four years. Individuals with the fastest horn growth as young adults also had the longest horns later in life. Annual horn growth increments between two and six years of age were independent of longevity for ibex whose age at death ranged from 8 to 16 years. Our results suggest that growing long horns does not constrain longevity. Of the variability in horn length, 22% could be explained by individual heterogeneity, suggesting persistent differences in phenotypic quality among males. Research on unhunted populations of sexually dimorphic ungulates documents how natural mortality varies according to horn or antler size, and can help reduce the impact of sport hunting on natural processes.
Life table methods are developed for populations whose members differ in their endowment for longevity. Unlike standard methods, which ignore such heterogeneity, these methods use different calculations to construct cohort, period, and individual life tables. The results imply that standard methods overestimate current life expectancy and potential gains in life expectancy from health and safety interventions, while underestimating rates of individual aging, past progress in reducing mortality, and mortality differentials between pairs of populations. Calculations based on Swedish mortality data suggest that these errors may be important, especially in old age.