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Demographic Side Effects of Selective Hunting in Ungulates and Carnivores

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Selective harvesting regimes are often implemented because age and sex classes contribute differently to population dynamics and hunters show preferences associated with body size and trophy value. We reviewed the literature on how such cropping regimes affect the demography of the remaining population (here termed demographic side effects). First, we examined the implications of removing a large proportion of a specific age or sex class. Such harvesting strategies often bias the population sex ratio toward females and reduce the mean age of males, which may consequently delay birth dates, reduce birth synchrony, delay body mass development, and alter offspring sex ratios. Second, we reviewed the side effects associated with the selective removal of relatively few specific individuals, often large trophy males. Such selective harvesting can destabilize social structures and the dominance hierarchy and may cause loss of social knowledge, sexually selected infanticide, habitat changes among reproductive females, and changes in offspring sex ratio. A common feature of many of the reported mechanisms is that they ultimately depress recruitment and in some extreme cases even cause total reproductive collapse. These effects could act additively and destabilize the dynamics of populations, thus having a stronger effect on population growth rate than first anticipated. Although more experimental than observational studies reported demographic side effects, we argue that this may reflect the quite subtle mechanisms involved, which are unlikely to be detected in observational studies without rigorous monitoring regimes. We call for more detailed studies of hunted populations with marked individuals that address how the expression of these effects varies across mating systems, habitats, and with population density. Theoretical models investigating how strongly these effects influence population growth rates are also required.
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Review
Demographic Side Effects of Selective Hunting in
Ungulates and Carnivores
JOS M. MILNER,†‡ ERLEND B. NILSEN,† AND HARRY P. ANDREASSEN
Hedmark University College, Department of Forestry and Wildlife Management, N-2480 Koppang, Norway
†Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of Oslo, P.O. Box 1066, Blindern,
N-0316 Oslo, Norway.
Abstract: Selective harvesting regimes are often implemented because age and sex classes contribute differ-
ently to population dynamics and hunters show preferences associated with body size and trophy value. We
reviewed the literature on how such cropping regimes affect the demography of the remaining population
(here termed demographic side effects ). First, we examined the implications of removing a large proportion
of a specific age or sex class. Such harvesting strategies often bias the population sex ratio toward females
and reduce the mean age of males, which may consequently delay birth dates, reduce birth synchrony, delay
body mass development, and alter offspring sex ratios. Second, we reviewed the side effects associated with the
selective removal of relatively few specific individuals, often large trophy males. Such selective harvesting can
destabilize social structures and the dominance hierarchy and may cause loss of social knowledge, sexually se-
lected infanticide, habitat changes among reproductive females, and changes in offspring sex ratio. A common
feature of many of the reported mechanisms is that they ultimately depress recruitment and in some extreme
cases even cause total reproductive collapse. These effects could act additively and destabilize the dynamics
of populations, thus having a stronger effect on population growth rate than first anticipated. Although more
experimental than observational studies reported demographic side effects, we argue that this may reflect the
quite subtle mechanisms involved, which are unlikely to be detected in observational studies without rigorous
monitoring regimes. We call for more detailed studies of hunted populations with marked individuals that
address how the expression of these effects varies across mating systems, habitats, and with population density.
Theoretical models investigating how strongly these effects influence population growth rates are also required.
Keywords: big game, population dynamics, selective harvesting, trophy hunting, wildlife exploitation, wildlife
management
Efectos Demogr´aficos Secundarios de la Cacer´ıa Selectiva en Ungulados y Carn´ıvoros
Resumen: Los regimenes de cosecha selectiva a menudo son implementados porque las clases de edad y sexo
contribuyen distintamente a la din´
amica de la poblaci´
on y los cazadores muestran preferencias asociadas con
el tama˜
no corporal y el valor como trofeo. Revisamos la literatura sobre los efectos de esos regimenes de cosecha
sobre la demograf´
ıa del resto de la poblaci´
on (denominados aqu´
ıefectos demogr´aficos secundarios). Primero,
examinamos las implicaciones de la remoci´
on de la mayor parte de una clase espec´
ıfica de edad o sexo. Tales
estrategias de cosecha a menudo sesgan la proporci´
on de sexos de la poblaci´
on hacia hembras y reducen la edad
promedio de los machos, lo que consecuentemente puede retardar fechas de nacimiento, reducir la sincron´
ıa de
nacimientos, retardar el desarrollo de la masa corporal y alterar la proporci´
on de sexos de las cr´
ıas. Segundo,
revisamos los efectos secundarios asociados con la remoci´
on selectiva de relativamente pocos individuos
espec´
ıficos, a menudo machos grandes. Tal cosecha selectiva puede desestabilizar las estructuras sociales y
la jerarqu´
ıa de dominancia y puede provocar la p´
erdida de conocimiento social, infanticidio seleccionado
sexualmente, cambios de h´
abitat entre hembras reproductivas y cambios en la proporci´
on de sexos de las
cr´
ıas. Una caracter´
ıstica com´
un de muchos de los mecanismos reportados es que, a fin de cuentas, deprimen el
email jos.milner@hihm.no
Paper submitted March 8, 2006; revised manuscript accepted June 14, 2006.
36
Conservation Biology Volume 21, No. 1, 36–47
C
2007 Society for Conservation Biology
DOI: 10.1111/j.1523-1739.2006.00591.x
Milner et al. Demographic Effects of Selective Hunting 37
reclutamiento y en algunos casos extremos causan un colapso reproductivo total. Estos efectos pueden actuar
aditivamente y desestabilizar la din´
amica de las poblaciones, por lo que tienen un mayor efecto que el esperado
sobre la tasa de crecimiento poblacional. Aunque estudios m´
as experimentales que de observaci´
on reportaron
efectos demogr´
aficos secundarios, argumentamos que esto puede reflejar los sutiles mecanismos implicados,
que pueden no ser detectados en estudios de observaci´
on sin regimenes de monitoreo rigurosos. Hacemos
un llamado para la realizaci´
on de estudios m´
as detallados de poblaciones cazadas utilizando individuos
marcados para abordar la variaci´
on de esos efectos en sistemas de apareamiento, h´
abitats y densidades
poblacionales diferentes. Tambi´
en se requieren modelos te´
oricos que investiguen el impacto de estos efectos
sobre las tasas de crecimiento poblacional.
Palabras Clave: caza deportiva, caza mayor, cosecha selectiva, din´amica poblacional, explotaci´on de vida sil-
vestre, gesti´on de vida silvestre
Introduction
One of the central aspects of conservation biology is the
relationship between human exploitation and the con-
servation of exploited resources. Throughout the world
terrestrial mammals are hunted for sport, subsistence, and
to control population size (Festa-Bianchet 2003). Hunting
thus provides a significant source of meat and income in
rural communities and beyond. Nevertheless, there are
numerous examples of populations being overharvested,
and subsistence hunting may be one of the most urgent
current threats to the persistence of species in tropical
ecosystems (Robinson & Bennett 2000; Milner-Gulland &
Bennett 2003). Over 30% (250 species) of mammals cur-
rently listed as endangered on the World Conservation
Union (IUCN ) Red List are threatened by overexploita-
tion (Baillie et al. 2004). Of these, larger mammal species,
especially ungulates and carnivores, are particularly tar-
geted (Baillie et al. 2004; Fig. 1).
Although subsistence hunting may take a random
sample of a population, in many other instances
particularly associated with sport hunting of ungulates
and carnivoreseconomic demands, ecological knowl-
edge, and hunter preferences have led to the implemen-
tation of selective harvesting regimes (e.g., Ginsberg &
Milner-Gulland 1994; Solberg et al. 1999). Here the off-
take is focused around predetermined sex and/or age
classes or specific individuals. Such selective hunting
will, in addition to the obvious direct effects of reduc-
ing the population size, also affect the demography of
populations by altering age and sex structures (Ginsberg
& Milner-Gulland 1994) and potentially disrupting social
systems (Swenson et al. 1997). Although such effects have
received far less attention than direct overharvesting, they
are potentially equally undesirable (Festa-Bianchet 2003)
and occur even when the overall offtake is not regarded
as excessively high.
We sought to synthesize the current knowledge on how
selective harvesting regimes affect the performance of
populations. We considered the effects of hunting a large
proportion of a selected sex and/or age class of the pop-
ulation, so affecting the age and sex structure of the re-
maining population and hunting specific individuals for
trophies, so disturbing social structures and dominance
hierarchies. We included recreational or sport hunting
for meat and trophies, and poaching and population con-
trol where specific individuals or sex/age classes are tar-
geted. We focused on ungulates and carnivores because,
with the exception of a vast literature on size-selective
exploitation of fish stocks and its consequences (see e.g.,
Law 2001), these are the groups for which most informa-
tion regarding selective harvesting is available.
Consequences of Perturbing the Population Age
and Sex Structure
Many mammalian populations are strongly structured by
age and sex. Because survival rates typically differ among
age and sex classes (Gaillard et al. 1998), populations of
equal size but differing structures will have different tem-
poral dynamics (Coulson et al. 2001) and will respond dif-
ferently to stochastic environmental variation (Cameron
& Benton 2004). Consequently, by perturbing population
sex and age structure, selective harvesting affects popu-
lation dynamics (Festa-Bianchet 2003).
Theoretically, the most productive populations are
those with a female-biased sex ratio (Caughley 1977).
Male-biased harvesting regimes have therefore been
widely applied to ungulates in North America (McCul-
lough 2001; Stalling et al. 2002), Scandinavia (Langvatn
& Loison 1999; Sæther et al. 2004b), and in wildlife crop-
ping schemes in Africa (Ginsberg & Milner-Gulland 1994).
Even though a more balanced or slightly female-biased
harvest is taken in many European countries (Milner et
al. 2006), harvested ungulate populations invariably have
mortality patterns that deviate significantly from those in
unhunted populations (Ginsberg & Milner-Gulland 1994;
Langvatn & Loison 1999). In particular, mortality rates
of prime-aged adults, especially males, are considerably
higher than in unhunted populations.
Male-biased harvesting regimes have led to severely bi-
ased sex ratios; for example, there are 0.05 adult males
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Volume 21, No. 1, February 2007
38 Demographic Effects of Selective Hunting Milner et al.
Figure 1. The number of ungulate (U ) and carnivore (C) species registered as threatened (all threat categories) at
least partly due to harvesting (IUCN 2004) relative to the total number (in parentheses) of ungulate and carnivore
species evaluated in each region. Shading represents the approximate proportion of ungulate and carnivore species
threatened. There are substantial differences among the regions with the highest proportion of threatened species
(0.37) occurring in south and southeast Asia and west and central Asia.
per female in populations of both North American elk (if
not provided, scientific names are in Table 1 or 2) (Noyes
et al. 1996) and the central Asian saiga antelope (Milner-
Gulland et al. 2003). In addition, the often high harvesting
pressure on mature males for trophies results in harvested
populations with lower average ages of males and fewer
old males than unhunted populations (Langvatn & Loison
1999; Laurian et al. 2000; Apollonio et al. 2003). For exam-
ple, 70% of all males in a Norwegian moose population
are harvested by 3 years of age (Solberg et al. 1999).
In the following we discuss how sex- and age-specific
hunting affects various demographic processes. We do
not discuss genetic and evolutionary effects in detail be-
cause they have been reviewed recently (Harris et al.
2002; Festa-Bianchet 2003).
Effects on Reproduction
Although selective harvesting of males leads to female-
biased adult sex ratios, this does not necessarily lead to a
reduction in fecundity rate because most harvested game
species have polygynous mating systems in which a sin-
gle mature male is capable of inseminating many females
(Ginsberg & Milner-Gulland 1994; Mysterud et al. 2002;
but see Greene et al. 1998 for monogamous species). Con-
sequently in many cases, recruitment rates are resilient
to skewed sex ratios (Table 1) and may even increase
because of higher proportion of females in the adult
population (Solberg et al. 2000). But there may nonethe-
less be a sex-ratio threshold below which fecundity col-
lapses. Indeed, if the offtake is strongly male-biased, pop-
ulation crashes due to reduced fecundity can occur at
lower overall offtake rates than if a random harvest is
taken (Ginsberg & Milner-Gulland 1994). This has been
observed in saiga antelope at a ratio of between 0.025 and
0.009 males per female (Milner-Gulland et al. 2003), cari-
bou (Rangifer tarandus) at a sex ratio of 0.08 (Bergerud
1974), elk populations with a sex ratio of 0.04 (Freddy
1987), and elephants with a sex ratio of 0.013 (Dobson
& Poole 1998). In moose, even moderately, female-biased
sex ratios (0.250.70) can affect the fecundity of primi-
parous females, although the fecundity of older females
seems to be unaffected (Solberg et al. 2002).
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Volume 21, No. 1, February 2007
Milner et al. Demographic Effects of Selective Hunting 39
Table 1. Demographic consequences of a selective-harvesting regime that creates a female-biased adult sex ratio and/or a young average age of males.
Effect of harvesting Demographic consequencea
offspring offspring 1) young 1)
-biased reduced fecundity breeding/ birth sex survival 2) survival 2) adult
Species sex ratio age rate birth date synchrony ratio weight weight condition Reference
Moose (Alces alces)X+Solberg et al. 1999
XbSolberg et al. 2002
X<Sæther et al. 2004
X0c2) 2) 0 Sæther et al. 2003
X0 02)2) 0 Sæther et al. 2003
X2) Garel et al. 2006
X X 0 0 0 Laurian et al. 2000
X0 2)dTaquet et al. 1999
X0 Courtois & Lamontagne
1999
Elk (Cervus elaphus)X Freddy 1987
Xe−− 0 Noyes et al. 1996
X Squibb 1985
XWhite et al. 2001
XX 0 Bender & Miller 1999
Red deer (Cervus elaphus)XX0 Langvatn & Loison 1999
Fallow deer (Dama dama)X−+ Komers et al. 1999
Mule deer (Odocoileus hemionus)X White et al. 2001
White-tailed deer X 0 +b<eOzoga & Verme 1985
(Odocoileus virginianus)
Bighorn sheep (Ovis canadensis) X 0 0 1) 0 1) 0 0 Singer & Zeigenfuss 2002
X 0 0 0 Shackleton 1991
X1)Jorgenson et al. 1997
Dall sheep (Ovis dalli) X 0 1) 0 Murphy et al.1990
X0 1) Singer & Zeigenfuss 2002
X1)Heimer et al. 1984
Caribou (Rangifer tarandus)X Bergerud 1974
Reindeer (Rangifer tarandus)X 0 0 2) 0 Holand et al. 2003
X0 0 2) 0 Holand et al. 2003
(X)f(X)f<g2) Holand et al. 2006
X 2) hMysterud et al. 2003
Saiga antelope (Saiga tatarica)X Milner-Gulland et al. 2003
aKey: 0, no effect; +, positive effect; , negative effect.
bPrimiparous females only, no effect in adult females.
cSkewed adult sex ratio had significantly stronger effect on calving date than young-male age structure.
dOffspring size measured by length of hind foot not body weight.
eNonsignificant trend.
fA group of the females were inhibited from mating during their first cycle, thus conceiving in the second cycle. Simulates skewed sex ratio and/or male age structure.
gSex ratio of calves conceived in second estrus.
hIncreased weight loss during the rut.
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Volume 21, No. 1, February 2007
40 Demographic Effects of Selective Hunting Milner et al.
Many populations with low male-to-female ratios also
tend to have a low mean male age, which may be a con-
tributing factor to lower fecundity (Solberg et al. 2002).
Nevertheless, even though it has been suggested that
subadults show immature courtship behavior, are socially
disruptive, and prolong the mating season (Squibb 1985;
Shackleton 1991; Singer & Zeigenfuss 2002; Stalling et al.
2002), young males are nonetheless capable of achiev-
ing paternities successfully (Stevenson & Bancroft 1995;
Hogg & Forbes 1997). It is less clear whether they are
able to inseminate as many females as old males (Gins-
berg & Milner-Gulland 1994). Overall, there is little clear
evidence that a reduction in male age affects fecundity
rate per se (Table 1). Rather, the literature points toward
changes in parturition dates, birth synchrony, and off-
spring sex ratio with a reduction in male age.
Selective harvesting may also have indirect effects on
recruitment through its influence on the mean age of
adult females. For example, in an Norwegian moose pop-
ulation in which selective harvesting protects adult fe-
males, the resulting increase in average female age led to
an increase in both calving rate and twinning rate (Solberg
et al. 1999). In other situations, such as game ranching,
cropping results in a general reduction in average female
age and thus in an increased reproductive rate due to
the absence of senescent individuals (Ginsberg & Milner-
Gulland 1994).
Effects on Timing and Synchrony of Birth
Timing and synchrony of birth have important implica-
tions for demography because of their effects on offspring
body weights and survival. Greater birth synchrony leads
to higher survival in species with heavy predation of
neonates (Sinclair et al. 2000), whereas late-born individ-
uals often have lower survival (Clutton-Brock et al. 1987;
Festa-Bianchet 1988) or delayed body mass development
(Sæther et al. 2003; Nilsen et al. 2004; Holand et al. 2006).
In female ungulates this may lead to a delay in onset of
reproduction (Langvatn et al. 1996).
In both reindeer and moose calving is earlier when the
adult sex ratio is even rather than female-biased (Holand
et al. 2003, Sæther et al. 2003). In addition, timing of calv-
ing in moose can be delayed when the male population
is restricted to yearlings (Sæther et al. 2003). Similarly,
birth dates in fallow deer (Komers et al. 1999), timing of
the rut in elk (Noyes et al. 1996), and median date of ac-
cepted mounts in Dall sheep (Singer & Zeigenfuss 2002)
are all significantly earlier in groups or populations with
mature males than when only young males are present, al-
though other studies have shown no such effects (Table
1). Birth synchrony was greater in a moose population
with an even sex ratio compared with a population in
which the sex ratio was experimentally manipulated to-
ward females (Sæther et al. 2003), whereas birth dates are
more synchronous with increasing male age in elk (Noyes
et al. 1996) but less synchronous in fallow deer (Komers
et al. 1999). By contrast, no effects of male age on rutting
behavior or the timing of the birth season were found in
bighorn sheep (Shackleton 1991) or in a hunted moose
population (Laurian et al. 2000).
Effects on Offspring Sex Ratio
In dimorphic and polygynous species birth size is more
strongly correlated with fitness in males than in females
(Kruuk et al. 1999). The Trivers-Willard model (Trivers
& Willard 1973) predicts that mothers in good condi-
tion should therefore produce male offspring because this
yields the highest fitness return (Sheldon & West 2004).
Nevertheless, other factors such as male quality and tim-
ing of breeding may also influence natal sex ratio. For ex-
ample, if females hesitate to mate with young males and
thus conceive late, the model predicts that fitness would
be maximized by producing females because late-born
offspring generally have lower birth and autumn weights
(Holand et al. 2006).
In an experimental study of a Norwegian moose pop-
ulation, a change in male age structure toward younger
males led to a reduction in the proportion of male calves
born (Sæther et al. 2004b), whereas manipulation of the
adult sex ratio had no effect. Similarly, Holand et al. (2006)
showed that reindeer conceived in the first estrus are
more likely to be male, whereas second-estrus offspring
are more likely to be female. They argue that a skewed
sex ratio and young male age structure could result in
fewer adult females conceiving during the first cycle due
to their hesitation to mate with young males. A trend to-
ward more male offspring being sired by older males than
by yearling males has also been observed in white-tailed
deer (Ozoga & Verme 1985).
Effects on Survival
Participation in rutting activities is energetically costly,
and, consequently, winter survival rates of participating
males are typically lower than for other individuals (Geist
1971; Stevenson & Bancroft 1995; Jorgenson et al. 1997).
Subordinate males may engage in high-risk alternative
mating tactics (Hogg & Forbes 1997) and may invest more
heavily in reproductive activities when there is either an
abundance of females relative to males or a paucity of
prime-age males (Squibb 1985; Singer & Zeigenfuss 2002;
Mysterud et al. 2003). One might therefore predict that
young males will be more involved in the rut and suffer
higher winter mortality rates in areas where heavy hunt-
ing of mature males occurs (Geist 1971; Murphy et al.
1990). Evidence for the so-called depressed survival hypo-
thesis, however, is equivocal (Singer & Zeigenfuss 2002;
Table 1). No effect is seen in Dall sheep populations in
which young rams show adult mating behavior in the ab-
sence of mature males (Murphy et al. 1990) or in lightly
hunted populations of desert bighorn sheep and bighorn
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Volume 21, No. 1, February 2007
Milner et al. Demographic Effects of Selective Hunting 41
sheep (Singer & Zeigenfuss 2002). Higher mortality rates
have been detected only among young rams in a heavily
hunted Dall sheep population (Singer & Zeigenfuss 2002).
Where selective hunting leads to high adult mortal-
ity, populations tend to have a high proportion of juve-
niles and yearlings. Because overwinter survival of these
classes is variable from year to year (Gaillard et al. 1998),
such populations are more sensitive to winter mortality in
harsh years than unhunted populations, leading to greater
population variability (Cameron & Benton 2004; Gordon
et al. 2004).
Effects on Body Weights
Another cost to young males participating in the rut is
reduced body growth as resources are diverted to repro-
duction (Stearns 1992). In populations with few mature
males, one might expect increased energy expenditure
of young males participating in mating behavior to lead
to greater weight loss during the rut. This is observed in
male reindeer (Mysterud et al. 2003) and moose (Solberg
&Sæther 1994; Garel et al. 2006). However, Sæther et
al. (2003) found no such effect when mature male moose
were removed from a population, although they found an
indirect negative effect on calf body weight the following
winter due to delayed parturition dates. Similarly, lower
birth and autumn body weights occur in second-estrus
offspring in moose (Schwartz & Becker 1994) and rein-
deer (Holand et al. 2003, 2006). Low mass at birth has
implications for other life-history traits such as survival,
age and body size at maturity, and lifetime reproductive
success (Kruuk et al. 1999).
As a result of expending more energy in avoidance
behavior, female fallow deer in an enclosure with only
young males lost significantly more body weight than fe-
males enclosed with only mature males (Komers et al.
1999). Female white-tailed deer in a low-density hunted
population significantly increased their daily movement
and home range size in peak and late rut, apparently in re-
sponse to low availability of adult males (Labisky & Fritzen
1998). By contrast, Singer and Zeigenfuss (2002) found no
compelling evidence for any negative effects on ewe en-
ergetics of increased harassment of ewes by young rams
in hunted mountain sheep populations.
Consequences of Removing a Few Targeted
Individuals
Trophy hunting typically targets the largest males or those
with impressive ornaments but is generally restricted to
relatively few individuals. Nonetheless, a high proportion
of individuals that qualify as trophy individuals may be re-
moved each year (Coltman et al. 2003). Species subject to
trophy hunting include large carnivores and large horn-,
tusk-, or antler-bearing herbivores. Trophy hunting is usu-
ally associated with a considerable fee, making it an im-
portant tool for wildlife management and conservation
programs, particularly in developing countries, where
it offers potential benefits for rural economies (Festa-
Bianchet 2003). Within Europe and North America, there
is also considerable interest in the trophy hunting of some
relatively common ungulate species that, are also hunted
for meat or population control (Festa-Bianchet 2003; Mil-
ner et al. 2006).
In many mammals the largest individuals are also the
oldest and, as such, play an important role in leading so-
cial groups that benefit from their greater experience.
Nevertheless, these are often the same individuals that
are typically targeted by trophy hunters because of their
size. For example, in elephants, tusk size is related to age,
and hunters or poachers focus their efforts on individuals
with the largest tusks, including matriarchs (Dobson &
Poole 1998). Older matriarchs have social discrimination
abilities that are superior to those of young matriarchs,
so enabling them to make more appropriate responses
during encounters with other elephant groups (McComb
et al. 2001). These factors and a greater knowledge of the
distribution of resources may result in higher per capita
reproductive success for female groups led by older indi-
viduals. Consequently, if groups rely on older members for
their store of social knowledge, then whole populations
may be affected by the removal of a few key individuals
(McComb et al. 2001).
Among lions, the absence of males within a pride en-
ables hyenas to drive females and subadults off their
kills under certain circumstances, constituting a constant
energy drain by forcing them to hunt more frequently
(Cooper 1991). In populations where adult males are
scarce, due, for example, to trophy hunting, cleptopar-
asitism by hyenas is likely to increase.
In most species managers assume that sport hunting
for trophy males only reduces the overall population size
when the rate of male removal is so high that not all
females are impregnated. In many cases it is thought that
sport hunting of males may even have a positive effect
on population growth through compensatory density de-
pendence (McLellan 2005; but see also Miller 1990). In
monogamous species and species in which males pro-
vide parental care, however selective removal of even
a modest number of adult males is predicted to have a
stronger impact on population growth than random re-
movals (Greene et al. 1998).
Effects on Juvenile Survival
Removal of trophy individuals, especially dominant males,
can have far-reaching effects where male replacement is
associated with infanticide. Sexually selected infanticide
(SSI) can occur when a male gains increased mating suc-
cess by killing dependent young he has not sired himself
(Swenson 2003). By killing unrelated offspring a mature
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Volume 21, No. 1, February 2007
42 Demographic Effects of Selective Hunting Milner et al.
Table 2. Demographic consequences of selective removal of a few specific individuals from a population.
Effect of harvesting Demographic consequencea
removal removal fecundity offspring offspring adult
Species dominant dominant rate sex ratio survival condition Reference
Plains zebra (Equus burchelli)X −−Hack et al. 2002
Feral horses (Equus caballus)X Berger 1983
Shackleford Banks horses X −−Rubenstein 1986
(Equus caballus)
Elephants (Loxodonta africana)X Dobson & Poole 1998
XMcComb et al. 2001
Lion (Panthera leo)X Pusey & Packer 1994
X>Smuts 1978
X>Creel & Creel 1997
Brown bear (Ursus arctos)X Swenson et al. 1997
X 0 Miller et al. 2003
XWielgus & Bunnell 2000
Xb()bStringham 1983
Xb()bMcCullough 1981
X 0 McLellan 2005
aKey: 0, no effect; +, positive effect; , negative effect.
bReduced cub recruitment when adult males were removed, but effects on fecundity rate and offspring survival not distinguished.
male can reduce the interbirth period and sire the next lit-
ter. Furthermore, because males tend to roam over larger
areas than females (Nilsen et al. 2005), the turnover of
one male can affect several females. For example, in root
voles (Microtus oeconomus), high male turnover rates
severely hamper population growth (Andreassen & Gun-
dersen 2006). Male infanticide occurs primarily in pri-
mates, terrestrial carnivores, and some rodents.
Among bears, older males may limit the immigration of
younger males (Rogers 1987). Therefore, increasing the
mortality rate of old males can result in a higher immigra-
tion rate of younger, potentially infanticidal, males (Table
2). In Scandinavian brown bears survival rates of cubs are
depressed in areas with high adult-male hunting offtake
( juvenile survival 0.98 vs. 0.72 in unhunted and hunted
populations, respectively; Swenson et al. 1997). A con-
siderable body of evidence points toward infanticide as
the cause of this (Swenson et al. 1997; Swenson 2003). In
North American brown bear populations the evidence for
SSI due to male turnover is still controversial (McCullough
1981; Stringham 1983; Wielgus & Bunnell 2000; Miller
et al. 2003; McLellan 2005). Nevertheless, cases of SSI are
extremely difficult to document in the field, and recent
studies strongly support the SSI model and the adaptive
value of SSI for male brown bears (Bellemain et al. 2006).
In hunted black bear (Ursus americanus) populations
with high male turnover rates, SSI is thought to cause
high intraspecific juvenile mortality (LeCount 1987).
Sexually selected infanticide is also well documentedin
lions (Pusey & Packer 1994), and because trophy hunting
is expected to increase the rate of male takeovers, exces-
sive trophy hunting could limit recruitment through the
negative effects of infanticide on cub survival (Whitman
et al. 2004). Although trophy hunting increases the risk
of population extinction, quite extensive trophy hunting
could be sustained as long as only old males are targeted
(Whitman et al. 2004).
Rare cases of SSI have been documented in some
herbivore species (captive red deer: Bartos & Mad-
lafousek 1994; hippopotamus [Hippapotamus amphibi-
ous]: Lewison 1998; captive plains zebra [Equus
burchelli]: Pluhacek & Bartos 2005). Although the evi-
dence is somewhat circumstantial, this suggests that sim-
ilar effects could arise in ungulates under some conditions
where trophy hunting for adult males takes place.
Effects on Reproduction
In situations where SSI is not documented the removal
of a few adult males may nonetheless have an impact
on demography through other mechanisms. For exam-
ple, when comparing two North American grizzly bear
populations, Wielgus and Bunnell (2000) found that re-
productive rates were suppressed in the hunted com-
pared with the unhunted population (Table 2). These
differences were caused by mature females avoiding food-
rich areas inhabited by potentially infanticidal immigrant
males (sexual segregation), forcing them to use subopti-
mal habitats (Wielgus & Bunnell 2000). Subsequent mod-
eling exercises show that this has a strong negative effect
on the population growth rate and thus increases the risk
of population extinction (Wielgus et al. 2001).
Equids often show highly developed multilevel social
organization. Harem-forming feral horses and plains ze-
bras are vulnerable to social instability and a high turnover
of harem males (Hack et al. 2002). The selective removal
of harem stallions can lead to increased stress levels, re-
duced grazing time, and loss of body condition in females
Conservation Biology
Volume 21, No. 1, February 2007
Milner et al. Demographic Effects of Selective Hunting 43
subject to harassment from intruding males, resulting in
induced abortion (Berger 1983) and lower female repro-
ductive success (Rubenstein 1986). Male takeovers in
feral horses led to abortion due to forced copulation in
80% of females <6 months pregnant and due to other
stress factors in a further 10% (Berger 1983). Females
were subsequently reinseminated by new males resulting
in a reduced interbirth interval and genetic investment of
rival males.
As with the selective hunting of specific age and sex
classes, the selective removal of individuals could also
affect other birth characteristics. For example, lion pop-
ulations in which males are hunted, rear a higher propor-
tion of male than female cubs (Smuts 1978; Creel & Creel
1997). According to the sex-allocation theory (Charnov
1982), this could compensate for a high turnover of adult
males (Packer & Pusey 1987) but reduces the number of
lions that can be sustainably harvested before the avail-
ability of females becomes limiting (Greene et al. 1998).
Figure 2. Schematic model of some of the processes and indirect pathways by which selective harvesting may affect
population growth rate. Solid lines are mechanisms and effects that are well documented, and dashed lines
indicate effects that are less well documented. The dotted lines indicate the path by which selective harvesting can
increase population growth rate.
Synthesis and Conclusions
Our review shows that when selective harvesting per-
turbs the sex or age structure in such a way that the mating
system is disrupted, the fecundity and survival of certain
sectors of the population and the offspring sex ratio may
all be affected. The removal of even a few targeted individ-
uals could have similar consequences. Nevertheless, the
evidence for the occurrence of such unintended demo-
graphic side effects is somewhat equivocal (Tables 1 &
2), being more common in experimental than observa-
tional studies. We believe this arises because such effects
are often subtle, indirect, and sometimes involve time
lags (Fig. 2). Changes such as shifts in calving date or
offspring sex ratio are difficult to detect without detailed
monitoring programs, and there is currently a lack of long-
term studies of marked individuals in hunted populations
(Festa-Bianchet 2003). This limits our understanding of
how and when these demographic effects are expressed
Conservation Biology
Volume 21, No. 1, February 2007
44 Demographic Effects of Selective Hunting Milner et al.
across different mating systems, habitat types, and popu-
lation densities.
Many of the processes triggered by selective harvest-
ing indirectly reduce the recruitment of new individuals,
thereby potentially reducing the population growth rate
(Fig. 2). Recruitment is depressed because females hesi-
tate to mate with young males (e.g., Holand et al. 2006),
ovulation is delayed in the absence of stimulation from
mature males (e.g., McComb 1987; Komers et al. 1999),
or, more rarely, there are insufficient males for all females
to be mated (e.g., Milner-Gulland et al. 2003). Concep-
tion rates can be limited by spatial (Mysterud et al. 2002)
and social (Greene et al. 1998) factors influencing ac-
cess to mates and by a physical limit to the number of
females each male can inseminate (Ginsberg & Milner-
Gulland 1994). Although there are clearly differences be-
tween monogamous and polygynous mating systems in
the ratio of adult males to females necessary for all females
to be mated, within polygynous species differences in fe-
male group size (solitary individuals, small social groups,
or large harems) and male mating behavior (e.g., tend-
ing, lekking, or harem holding) also influence access to
mates. In addition, mate access may vary within species
because group size differs with habitat type (Hewison et
al. 1998). Extrapolation of adult sex ratios from domes-
tic populations is not advisable. Generally, daily sperm
production, sperm density, and absolute sperm numbers
are directly related to testes size (Møller 1989), and most
domestic animals have large testes for their body weight
(Ginsberg & Milner-Gulland 1994). Under intense compe-
tition between males, sperm depletion can occur before
the end of the rut, even in species with relatively large
testes (Preston et al. 2001).
Although the mechanisms by which selective harvest-
ing could affect population demography are relatively
well documented (Fig. 2), the extent to which they affect
population growth is still poorly understood (e.g., Wiel-
gus et al. 2001; Whitman et al. 2004). Because the sensitiv-
ity of population growth rate to recruitment is generally
lower than its sensitivity to adult female survival (Gaillard
et al. 2000), demographic side effects that depress re-
cruitment may not have as strong an effect on population
growth rate as the direct harvesting of adult females. Nev-
ertheless, because many of these effects are likely to act
additively (Fig. 2), they may nonetheless reduce the pop-
ulation growth rate more than first anticipated. Although
good estimates are lacking for many parameters, concep-
tual models would be helpful in suggesting when demo-
graphic side effects might start to limit population growth
and in guiding empirical data collection.
The occurrence of demographic side effects of selec-
tive harvesting has implications for the performance of
population viability analyses (PVA). In many of the most
commonly used PVA software programs there is an im-
plicit assumption that sex does not matter as long as the
number of adult males is 1 (Brook et al. 2000). Never-
theless, estimated extinction probabilities are affected by
both population sex ratio and mating system (Ginsberg &
Milner-Gulland 1994; Sæther et al. 2004a). In addition, for
small populations, demographic stochasticity in the sex
ratio could have a direct negative effect on mean popula-
tion growth rate (Sæther et al. 2004a). If the abundance
of one sex is particularly low, chance events could result
in that sex being limiting in certain years. This would be
especially important in small, harvested populations and
in more abundant populations when the sex ratio is close
to the threshold where these effects become important.
Selective harvesting regimes can have destabilizing ef-
fects on populations. The young age structure of har-
vested populations results in less-stable dynamics due to
high stochasticity in juvenile survival (e.g., Gordon et al.
2004). Furthermore, if late-born offspring enter the win-
ter with lower body weights (e.g., Holand et al. 2006),
they are more likely to be affected by random climatic
variation (Festa-Bianchet 1988), which, together with re-
duced birth synchrony, could result in large interannual
fluctuations in juvenile survival. In addition, in species
with SSI, the effect of male removal on population growth
rate is hard to predict because it depends on the number
of offspring killed by immigrant males. In a Scandinavian
bear population Swenson et al. (1997) estimated that the
removal of one male was equivalent to the removal of
0.51.0 females, depending on the extent to which the
immigrant male killed the cubs in the area. In such situ-
ations harvesting juveniles and females will have more
predictable effects.
In response to the demographic side effects discussed
here and the evolutionary consequences of selective har-
vesting (Harris et al. 2002; Festa-Bianchet 2003), wildlife
managers are advised to implement harvesting regimes
that mimic natural mortality patterns more closely. Be-
cause natural mortality is typically higher among juveniles
and old individuals (Gaillard et al.1998), these groups
should be targeted (Ginsberg & Milner-Gulland 1994), al-
though this may conflict with economic considerations
in some areas (Festa-Bianchet 2003; Milner et al. 2006).
Applying a minimum age threshold is a possibility for tro-
phy males if a reliable assessment of age can be made in-
dependently from trophy phenotype, which may be well
developed at a young age in high-quality males (Whitman
et al. 2004). An additional approach would be to consider
the timing of the harvest. Currently many temperate un-
gulates are hunted during the breeding season. If the har-
vest is delayed until after the rut, older males have the
opportunity to breed and could be harvested at the time
of year when their reproductive value is lowest (Kokko
et al. 2001). In lions the optimal time for hunting a pride
male would be as his cubs become independent (Whit-
man et al. 2004). In this way, and by following natural
pride take over intervals, infanticide can be minimized.
We are now starting to understand the mechanisms by
which undesirable side effects of selective hunting occur,
Conservation Biology
Volume 21, No. 1, February 2007
Milner et al. Demographic Effects of Selective Hunting 45
but much less is known about when they occur and the
extent to which they affect population growth. To be able
to make firmer predictions about the effects on popula-
tion growth and viability, both large-scale empirical ma-
nipulations of harvesting regimes and theoretical studies,
including simulation modeling, are urgently needed. Be-
cause most of the effects discussed here operate through
recruitment, monitoring recruitment and juvenile sex ra-
tios should be standard routines for managers, in addition
to assessment of total population size. In addition, stron-
ger emphasis should be put on the timing of the har-
vest. Until the importance of the mechanisms triggered
by selective harvesting discussed here are more clearly
understood, we urge managers to be cautious in their use
of nonrandom harvesting strategies.
Acknowledgments
We thank the Norwegian Research Council for funding
J.M.M. and E.B.N. ( project 156367/530). Furthermore, we
thank colleagues at Hedmark University College for valu-
able discussions and A.J. Loveridge, M. Festa-Bianchet, and
an anonymous referee for helpful comments.
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Conservation Biology
Volume 21, No. 1, February 2007
... On the other hand, evolutionary changes have been recorded in many species subject to selective harvest (e.g., [47][48][49]). Such a type of harvesting is not unusual in both marine and terrestrial habitats. ...
... For example, size-selective harvesting of Atlantic silverside (Menidia menidia) decreased larval growth rate and other characteristics of the species lifecycle [50,51]. Similarly, several sex-selectively (and hence sizeselectively) harvested ungulate populations demonstrate a lower offspring weight [47]. Large and dominant males of such species as grizzly bears (Ursus arctos) and African lions (Panthera leo) also suffer from selective harvest as trophies. ...
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We examine population trends in light of male harvest data considering the long-time series of population data on northern fur seals at Tyuleniy Island. To answer the question has the way males were harvested influenced the population trajectory, we analyzed the visual harem size and birth rate dynamics of the population, as well as the strategy and intensity of the harvest. We analyzed the dynamics of the sex ratio in the early (1958–1988) period to estimate parameters in the late period (1989–2013) based on the observed number of bulls and pups, while utilizing the distribution of reproductive rates obtained from pelagic sealing. Using a matrix population model for the observed part of the population (i.e., the male population), we analyzed the population growth rate associated with changes in both birth and survival rates considering the stochastic effects. Observations allow us to reject the hypothesis of nonselective harvest. Among the variety of natural and anthropogenic factors that could contribute to the decrease in the birth rate in the population, the effect of selective harvesting seems to be the most realistic.
... For instance, the poor reporting compliance by some South African hunters and outfitters (i.e., licensed businesses that employ guides to take care of hunters during hunting expeditions) in 2015 resulted in a year-long leopard hunting moratorium in 2016, which was extended to 2017 (DEA, 2017;CMS/CITES, 2018). If quotas are not well regulated, regularly re-evaluated, and strictly enforced, the effective conservation of leopards and the integrity of the trophy hunting industry are at risk (Balme et al., 2010;Milner et al., 2007;Swanepoel et al., 2014). However, responsibly determined datadriven quotas allow for sustainable harvest and may contribute to species and habitat conservation in the longterm, where leopard conservation also conserves other species that share the same ecosystems and are indicators of ecosystemic functionality (Baker, 1997;Di Minin et al., 2016, 2021Lindsey et al., 2007). ...
Article
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Sustainable offtake of any threatened species and objective monitoring thereof relies on data-driven and well-managed harvest quotas and permit compliance. We used web-sourced images of African leopard (Panthera pardus pardus) trophy hunts to determine whether online photographs could assist in monitoring and documenting trophy hunting in Africa. Of 10,000 images examined, 808 (8%) showed leopard trophy hunts and could be contextualized by date and country. From a subset of photos (n = 530), across six countries between 2011 and 2020, we extracted information on the leopards killed and hunter demographics. We found no significant differences in leopard sex, age, or shot wound position between countries, and most trophy leopards were in good physical condition. Most hunters were White (96%) and estimated at over 40 years old (82%), with the proportion of women hunters in younger age classes significantly higher than in older classes. Rifles, bows, and hounds were used in all countries, except Tanzania and Zambia, where rifles were exclusively used. Online images could not be reasonably compared to the CITES trade database, but in South Africa, more than half (57%) of all nationally registered leopard trophy hunts in the last decade (2010-2020) have been posted online. Online images also reveal hunting violations, including non-permitted hunting of female leopards and illegal hounding. Such monitoring methods may become increasingly useful as social media usage grows and provide valuable insight into this multi-million dollar industry.
... Pour cela, nous pouvons considérer le compromis auquel sont soumises les proies, entre l'acquisition des ressources et la réduction du risque de prédation Lima 1985). phénotypiques importantes dans la morphologie et les traits d'histoire de vie des populations proies exploitées (Ginsberg and Milner-Gulland 1994 ;Coltman et al. 2003 ;Milner et al. 2007 ;Allendorf et al. 2008 ;Darimont et al. 2009). En effet, l'impact écologique des hommes est global et nous savons aujourd'hui qu'il est également à l'origine d'une accélération dans les changements évolutifs des espèces, et particulièrement des espèces qui présentent un intérêt particulier en termes d'économie ou de santé publique, telles que les espèces dites nuisibles, parasites ou à fort intérêt commercial ou récréatif (espèces élevées, chassées ou pêchées). ...
Thesis
Les populations sauvages sont de plus en plus soumises à d’importantes pressions de prédation en lien avec les activités humaines, qui sont la source de multiples facteurs de stress pour les populations sauvages. Parce qu’il est quasiment impossible pour la plupart des organismes desatisfaire l’ensemble de leurs activités fondamentales (alimentation, reproduction, repos,…) sans encourir un risque de prédation, ils sont souvent confrontés à des compromis. Notamment dans le processus d’alimentation, les animaux doivent faire des compromis entre l’acquisition de ressources de bonnes qualités et l’évitement du risque de prédation ou de dérangement, car les meilleurs ressources sont généralement associées à un risque de prédation plus fort. Une des manière dont les animaux peuvent résoudre ce compromis est par la modification de leurs patrons d’utilisation des habitats. Dans cette thèse nous nous sommes intéressés au système Chevreuil-Homme pour comprendre comment les activités humaines peuvent impacter les patrons d’utilisation et de sélection des différents habitats. La population de chevreuils étudiée évolue dans un paysage fragmenté et fortement anthropisé, représentatif des paysages agricoles modernes. Le suivi depuis plus de 10 ans de cette population, avec plus de 300 animaux capturés et équipés de colliers GPS, nous offre une opportunité unique de mieux comprendre les mécanismes qui sous-tendent les stratégies adoptées par les individus au sein du compromis « risque – acquisition des ressources ».Nous avons ainsi montré que le compromis « risque-acquisition des ressources » affecte différemment les patrons de sélection des habitats en fonction des variations spatio-temporelles dans l’intensité du risque et la disponibilité des ressources. L’ensemble de nos travaux a égalementpermis de mettre en évidence l’impact de facteurs environnementaux, tels que la période de chasse ou le moment de la journée, mais également l’impact de facteurs internes, tels que le statut reproducteur ou la sensibilité au stress des individus (probablement liée à leur personnalité), sur lesstratégies d’utilisation des habitats. Les stratégies d’utilisation des habitats résultent donc d’interactions complexes entre les facteurs externes et internes et peuvent avoir potentiellement des conséquences importantes sur la valeur adaptative des individus et, à terme, sur la dynamiquedes populations. La prise en compte de l’ensemble de ces facteurs, et notamment de la variabilité inter-individuelle dans les stratégies d’utilisation des habitats, devrait permettre d’améliorer les outils de gestion et de conservation des populations d’ongulés sauvages.
... Sex differences, including reproductive strategies and habitat segregation, are important ecological and evolutionary issues (Clutton-Brock et al. 1987, Mysterud et al. 2002, Rankin and Kokko 2007 and key factors in sexually dimorphic large mammals. For example, males with larger antlers, such as deer, are targeted for trophy hunting, and such selective hunting causes body size declines (Coltman et al. 2003) and sexratio biases in populations, which may destabilize social structures and reproductive collapse (Milner et al. 2007). However, deer populations are increasing worldwide (Matsuda et al. 1999, Clutton-Brock et al. 2002, Milner et al. 2006, Loe et al. 2009), and a male-biased dispersal is often observed in the distribution periphery of local deer populations (Carden et al. 2011). ...
Article
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Sex differences in large mammals with sexual dimorphism are important ecological and evolutionary issues and key factors for wildlife management. To examine the potential use of drone (unmanned aerial vehicle; UAV) observation using thermal infrared images for sex ratio monitoring of deer, we conducted UAV surveys at night in a sparse forest located on the distribution periphery of sika deer Cervus nippon and wild boar Sus scrofa local populations during summer and winter. Of the 163 thermal infrared images of large mammals detected, 132 (81.0%) and 16 (9.8%) were identified for deer and wild boar, respectively. In addition, velvet antlers of deer were visually recognized during summer, and 92% of the detected deer were antlered. This biased sex ratio would be a characteristic in the distribution periphery of local deer populations. Therefore, monitoring abundance and sex ratio using thermal infrared sensors on UAVs can improve deer management especially in the distribution periphery of local populations.
... More generally, differences between female and male demography can affect the reproductive output of a population under environmental change [77] and promote selective harvesting of males via hunting, thus altering population structure and evolutionary life-history trajectories [78]. Sex differences in reproductive behaviours within local populations may be coupled with sex-biased dispersal [79,80], and understanding this bias has been important to understanding invasion [81] and modelling population dynamics [82]. ...
Article
Life-history strategies are diverse. While understanding this diversity is a fundamental aim of evolutionary biology and biodemography, life-history data for some traits-in particular, age-dependent reproductive investment-are biased towards females. While other authors have highlighted this sex skew, the general scale of this bias has not been quantified and its impact on our understanding of evolutionary ecology has not been discussed. This review summarizes why the sexes can evolve different life-history strategies. The scale of the sex skew is then discussed and its magnitude compared between taxonomic groups, laboratory and field studies, and through time. We discuss the consequences of this sex skew for evolutionary and ecological research. In particular, this sex bias means that we cannot test some core evolutionary theory. Additionally, this skew could obscure or drive trends in data and hinder our ability to develop effective conservation strategies. We finally highlight some ways through which this skew could be addressed to help us better understand broad patterns in life-history strategies.
... red deer:Frantz et al. (2012) documented the barrier effect of a motorway in Wallonia, and motorways also separated populations in Hesse. In addition, a large artificial canal prevents gene flow among adjacent red deer populations in Northern Germany, and management practices affect the genetic diversity and behavior of red deer(Hartl et al. 1991;Milner et al. 2007;.Negative inbreeding effects have already been identified among two populations; Reiner & Willems 2019) accompanied by alarmingly low numbers of effective population sizes indicating the severe situation of red deer and the need for habitat connectivity. But statements about red deer connectivity in Germany are not consistent, some studies found a sufficiently high genetic diversity and gene flow between populations despite potential barriers (e.g., Hochkirch 2012;Kühn et al. 2003), whereas others found low diversity and significant genetic differentiation, presumably caused by isolation(Hmwe 2005;Nielsen et al. 2008;Meißner et al. 2009;. ...
Thesis
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This thesis investigates the effects of environmental variables on gene flow and migration of two contrasting species across Germany. Using red deer (Cervus elaphus) and European wildcat (Felis silvestris silvestris) as study species, the impeding effects of human infrastructure and current species management are demonstrated. Comparing genetically derived corridor models with already existing corridor models for both species, similarities and differences are presented.
... En la mayor parte de los casos se lo caza por su carne, por lo que no se seleccionan ejemplares. Aunque se trate de una especie exótica e invasora, es importante regular la cacería con base en estudios ecológicos (e.g., demográficos), ya que la eventual selectividad puede provocar efectos contrarios a los deseables, incluyendo aumentos poblacionales(Milner et al. 2007).5.-Prioridades de investigación y gestiónLa información de campo sobre la especie en Uruguay es muy escasa. Para tener un panorama básico que haga posible su manejo efectivo con base científica, es necesario profundizar sobre diversos aspectos de su ecología. ...
Chapter
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El ligustro, Ligustrum lucidum W.T. Aiton (Oleaceae), es una de las principales especies exóticas invasoras de los bosques de Uruguay. Su alta producción de semillas y banco de plántulas, dispersión por aves frugívoras, rápido crecimiento y amplia tolerancia ambiental, le confieren un gran potencial invasor. Este árbol originario de Asia templada (principalmente China) ha invadido actualmente todos los continentes, excepto la Antártida. En Uruguay, fue introducido a mediados del siglo XIX como especie ornamental, para ser usado en parques, plazas y cercos vivos. En la actualidad, se ha registrado su presencia en el 4.3 % de las 1467 parcelas relevadas por el Inventario Forestal Nacional, principalmente en bosques ribereños y parques. Se distribuye principalmente en el sur, litoral oeste y centro del país. Por el momento, los registros en el norte y este son escasos. La invasión del ligustro genera impactos sobre la diversidad de los bosques, alterando su estructura y funcionamiento, pudiendo llegar a extinguir localmente a algunas especies leñosas. Si bien la superficie de bosques invadidos no llegaría al 5% en la actualidad, debido a la gran extensión geográfica (14-15 departamentos) y alto potencial invasor, la erradicación del ligustro es prácticamente imposible a nivel nacional. Una estrategia razonable sería enfocarse en cuatro tipos de acciones: (1) Reducción de fuentes de propágulos, mediante la prohibición de venta de ligustros en viveros y el control en centros poblados (cercos vivos, plazas). (2) Monitoreo y prevención, para erradicar invasiones recientes, principalmente en las regiones norte y este del país. (3) Control para reducir abundancia en áreas de relevancia ecológica que cuenten con recursos para su gestión a largo plazo, como es el caso de áreas protegidas públicas o privadas. (4) Investigación sobre ecología de la invasión y métodos de restauración de bosques invadidos.
... En la mayor parte de los casos se lo caza por su carne, por lo que no se seleccionan ejemplares. Aunque se trate de una especie exótica e invasora, es importante regular la cacería con base en estudios ecológicos (e.g., demográficos), ya que la eventual selectividad puede provocar efectos contrarios a los deseables, incluyendo aumentos poblacionales(Milner et al. 2007).5.-Prioridades de investigación y gestiónLa información de campo sobre la especie en Uruguay es muy escasa. Para tener un panorama básico que haga posible su manejo efectivo con base científica, es necesario profundizar sobre diversos aspectos de su ecología. ...
Chapter
Full-text available
El caracol rapana, Rapana venosa (Valenciennes, 1846) es un predador activo de más de nueve especies nativas de moluscos que ha llegado a aguas del Río de la Plata en 1998 desde el sudeste asiático, posiblemente en aguas de lastre. Desde su llegada su distribución se ha ido ampliando hacia el sur por la costa de la provincia de Buenos Aires y hacia el norte por la costa uruguaya. En la actualidad parece estar desplazándose, ampliando su distribución, hacia la zona Sur de la costa argentina. El presente capítulo describe resultados de los estudios desarrollados para el estuario del Río de la Plata con énfasis en aspectos de distribución, y en los efectos ecosistémicos que puede ocasionar. Se presentan las acciones de control desarrolladas en Uruguay y Argentina hasta el momento y se sugiere que el uso como recurso alimenticio explotable podría tener viabilidad. Se destaca el elevado valor comercial como ítem exportable y el desarrollo de importantes pesquerías en aquellos lugares donde es consumido. De acuerdo a los resultados presentados, claramente rapana representa un riesgo para la malacofauna en general y para la de importancia económica de la zona, en especial para el mayor recurso malacológico explotado en Uruguay, el mejillón azul. También ocurre competencia, por el espacio, con especies nativas y causa impacto directo sobre la tortuga verde a través del biofouling sobre el carapacho, generando problemas de flotabilidad y nado. Se recomienda, entre otras acciones, la promoción de su captura como recurso pesquero sobre-explotable y su potencial uso como indicador de calidad ambiental en el Río de la Plata y zona oceánica adyacente. Palabras clave: caracol rapana, especie invasora, estuario, mitigación y control, predación
... En la mayor parte de los casos se lo caza por su carne, por lo que no se seleccionan ejemplares. Aunque se trate de una especie exótica e invasora, es importante regular la cacería con base en estudios ecológicos (e.g., demográficos), ya que la eventual selectividad puede provocar efectos contrarios a los deseables, incluyendo aumentos poblacionales(Milner et al. 2007).5.-Prioridades de investigación y gestiónLa información de campo sobre la especie en Uruguay es muy escasa. Para tener un panorama básico que haga posible su manejo efectivo con base científica, es necesario profundizar sobre diversos aspectos de su ecología. ...
Chapter
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CAPÍTULO VIII. Limnoperna fortunei (Mejillón dorado): características bióticas, distribución, impactos y manejo poblacional en Uruguay Ernesto Brugnoli, Jennifer Pereira, Carolina Ferrer, Ivana Silva, Leandro Capurro, Ana Laura Machado, Juan María Clemente (†), Lucía Boccardi, Soledad Marroni, Daniel Fabián, Fabiana Rey, María Jesús Dabezies, Iván González-Bergonzoni, Daniel Naya, Alejandro D’Anatro, Franco Teixeira de Mello, Claudio Martínez, Guillermo Goyenola, Carlos Iglesias & Pablo Muniz. Resumen El mejillón dorado (Limnoperna fortunei) es una especie de molusco originario de Asia que ingresó a inicios de 1990 al Río de la Plata por medio de aguas de lastre; actualmente es considerada como especie invasora en la cuenca del Plata. Presenta hábitos bentónicos, epifaunal bisado, comportamiento gregario, desarrollo indirecto, presencia de estadios larvales y ciclo reproductivo asociado a la variación de la temperatura del agua. Invade los principales cuerpos hídricos de la región (Argentina, Brasil, Paraguay, Bolivia y Uruguay). En nuestro país se reporta en sistemas hídricos de las cuencas de los ríos Uruguay, Negro, Santa Lucía, zonas interna y media del Río de la Plata, Laguna Merín y Laguna del Sauce, presentando diferencias en las tasas de invasión a nivel regional y local. En la región se reporta asociado a sustratos consolidados naturales y artificiales, incrementado sus abundancias poblacionales, ocasionando modificaciones en las comunidades bentónicas y planctónicas, en hábitos alimenticios de peces autóctonos y modificaciones en los parámetros de calidad de agua. Se presentan estudios desarrollados en Uruguay sobre efectos ecosistémicos y usos de la especie como bio-monitor de calidad de agua. El mejillón dorado ocasiona efectos de macrofouling (incrustaciones) adhiriéndose a infraestructuras hidráulicas, generando problemas en servicios ecosistémicos en diferentes usuarios de los recursos hídricos de la cuenca del Plata. Se describen generalidades sobre estrategias de manejo poblacional desarrolladas para la prevención, control/mitigación y erradicación de especies invasoras que ocasionan macrofouling mostrando ejemplos nacionales sobre prevención y control (mecánico, pinturas antiincrustantes, biológico) realizados en estudios de grado, posgrado y proyectos de investigación para el manejo poblacional de la especie. Palabras claves: efectos ecosistémicos, especie acuática invasora, macrofouling, manejo poblacional, servicios ecosistémicos.
Article
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We present data from 4 studies of radiomarked brown bears (Ursus arctos) in Alaska to evaluate the effects of hunting and differential removal of males on cub survival and litter size. In the Susitna area in southcentral Alaska, the proportion of males declined during a period of increasing hunting pressure (1980-96). Cub survivorship was higher in the heavily hunted Susitna population (0.67, n = 167 cubs) than in a nearby unhunted population in Denali National Park (0.34, n = 88 cubs). On the Alaska Peninsula, in coastal areas rich in salmon (Oncorhyncliits spp.) and with higher brown bear densities, cub survivorship was significantly higher in the hunted Black Lake population (0.57, n - 107 cubs) than in an unhunted population in Katmai National Park (0.34, n = 99 cubs). The Black Lake population had alternate-year hunting, and cub survivorship was similar during years with and without hunting during the preceding fall and spring. In both coastal and interior comparisons, litter sizes were either larger or not significantly different in hunted areas than in nearby unhunted national parks. We found no evidence that removal of adult male bears by hunters reduced cub survival or litter size. For populations below carrying capacity, convincing evidence is lacking for density dependent effects on cub survivorship or litter size. In our studies, variations in cub survivorship and litter size were best explained by proximity to carrying capacity; local environmental factors and stochastic events probably also influence these parameters. We believe that cub survivorship in our national park study areas was lower than in nearby hunted areas because of density-dependent responses to proximity to carrying capacity.
Article
Reviews literature on Ursus arctos and U. americanus supporting a direct relationship between increased hunting (or decreased number of males) and increased survivorship of subadults (reproductively immature bears). Such studies usually have not concluded the existence of a relationship between hunting and increased subadult survival. Evidence from 1 area in Alaska suggests the absence of a relationship between heavy grizzly bear hunting and cub survivorship. -from Author