When farmers
and ranchers
began settling
the American
West, they
arrived with livestock to graze,
seeds to plant and a mentality to
tame the West. They perceived the
greatest threat to their livestock
and crops was predators. Predator
management, labeled "o
ne of the
most controversial issues involving
North American wildlife" by James
Trefethen a quarter of a century
ago in his book, "An American
Crusade for Wildlife," continues to
be highly controversial today. And
there are few signs this controversy
is going to lessen.
Six animals are identified as
mule deer predators – gray wolf,
mountain lion, bobcat, coyote,
black bear and grizzly bear. The
first three on the list have to kill
prey species to survive. Coyotes
and bears have a varied diet that
includes plants, thus they can and
do kill prey, but do not have to do
so to survive.
Predators are controversial for
three primary reasons:
- Different segments of society place
different values on predators.
- Agencies responsible for management
of predators are caughtbetween a rock and a hard spot
because of the differing values the
public places on predators.
- Depending on a variety of factors,
reducing predators may or may not
help increase numbers of mule
deer in a given area.
Societal Values
Many segments of society place
differing values on predators, with
ranchers and animal rights activists
on opposite sides of the spectrum.
Ranchers and farmers don’t appreciate
a mountain lion, wolf or coyote
in or around livestock pastures
because predators are seen as a
potential loss of personal property
and income.
Animal rights organizations and
others place a value on predators
as charismatic megafauna, large
wildlife species that embody the
symbol of wilderness.
And biologists have individual
views regarding predators because
of personal experiences, and published
scientific information that is
conflicting in its conclusions about
the roles predators play in the
management of prey populations.
To understand how the values of
these three groups come into play
and affect the ability of state agencies
and provinces to manage
predators requires a short course
in the population dynamics of
mule deer.
The Numbers Game
Mule deer populations increase
when more deer are born than die,
and decrease when more deer die
than are born. Most mortality in
deer herds occurs in young deer
immediately after birth, or during
mid- to late winter. Carrying
capacity, or the ability of the habitat
to support the herd, helps
determine the size of the herd.
Carrying capacity is estimated
based on the body condition of
mule deer and the amount of vegetation
that is browsed by deer.
Additive and compensatory are
the two types of mortality that
occur in mule deer populations.
An increase in one cause of mortality
or the introduction of a new
type of mortality may or may not
increase the total number of animals
that die, depending on
whether that mortality is additive
or compensatory. If the increase or
introduction of mortality increases
the number of deer that die, the
mortality is additive. If it is compensated
for by reductions in other
types of mortality, and therefore
doesn’t change the total number
of deer that die, then it is
compensatory.
It is believed that when a mule
deer population is at carrying
capacity (the ability of the capacity
of the habitat to support it), mortality
is compensatory. Mortality
becomes more additive and less
compensatory as the population
falls further below the carrying
capacity of the habitat.
A mule deer herd that is at or
above the carrying capacity of its
habitat may produce fewer fawns
than one that is below carrying
capacity, and mortality will be
high so that the population
remains stable.
A herd that exceeds the ability
of the habitat to support it will be
in poor body condition, and have
poor birth rates and high death
rates. If the population continues
to remain above carrying capacity,
it will damage its food resources,
so that even when the herd does
recover, carrying capacity may be
reduced and the herd may be
unable to return to previous population
numbers.
Predation and carrying capacity
of the habitat are linked. When a
deer herd is at carrying capacity,
the number of deaths equals the
number of offspring that survive to
age one. In this herd, it is not
important if predators cause some
mortality, because if the predators
are removed, another factor will
cause a similar amount of mortality.
In other words, mortality is
compensatory.
The further below capacity the herd becomes, the
more additive mortality plays a role in reducing the number of mule
deer. The problem is that it is
extremely difficult for biologists to
pinpoint which mortality factors are
playing the greatest role in a mule
deer herd on the sliding scale of
additive and compensatory mortality.
Some biologists believe mortality
is density dependent. For example, if
one type of mortality is reduced in a
deer herd that is nearing capacity,
another type of mortality will replace
it. In a herd that has severe winter as
its only major mortality factor, hunting
does before winter would not
hurt the population. Fewer does will
likely die during the severe winter to
compensate for those that were harvested.
On the other hand, if the doe
hunting occurs before a mild winter,
the mortality could be considered
additive.
What does all of this have to do
with predation? That all depends.
Long-term drought can reduce the
ability of a habitat to support mule
deer, causing significant declines in
some populations. Drought reduces
the quality of the habitat and can
affect the body condition of deer,
potentially making them more vulnerable
to predation.
If predators contribute to significant
mortality in a mule deer population,
and that population is near
carrying capacity, removing predators
may not cause the population to
increase because other types of mortality
may kick in and compensate
for predation. On the other hand, if
predation is causing a mule deer
population to exist below the ability
of the habitat to support them,
reducing predators may allow the mule deer herd to increase until
compensatory factors kick in.
The true question is not whether
predation affects mule deer, but
how much.
Research Results
The few predation studies that
have been conducted on mule and
black-tailed deer have been
somewhat limited in their ability to
draw conclusions across mule deer
populations. But biologists have
been able to glean some useful
information:
- Weather affects the impact
predation may have on mule deer
by changing deer forage and cover,
the densities of prey species and
the physical condition of deer.
- Wolves can effectively reduce deer
populations, particularly on island
habitats, and especially if they are
the primary predator.
- In some undisturbed arctic environments,
severe weather or
human over-harvest can cause a
mule deer population to decline.
Predation can further reduce that
population or prevent it from
recovering. However, most of the
environments where mule deer
exist today have been altered by
fire suppression, development,
fragmentation of habitat and other
factors. In these habitats (most of
the West), biologists believe predation
does not cause declines in deer
populations. The effect predators
have on prey populations in these
environments is more complex and
related to how humans affect predators,
prey and habitat, and the
types and densities of predators
that exist.
- The effects of predators are complicated
because there is more than
one species of predator – wolves
have to kill and eat prey species to
survive, while coyotes can survive
on plants. If mule deer and large
mammal populations decrease,
coyotes are less susceptible to these
prey reductions because of their
ability to eat a variety of foods.
- To warrant a reduction in predators,
predation should be identified
as an important mortality factor,
and managers must estimate
the population of deer relative to
the carrying capacity of its habitat.
What does all of this mean?
Despite everything we've learned
about predators in the past century,
they are as "good or bad" as
they were 100 years ago.
Jim deVos of the Arizona Game
and Fish Department said, "In
cases where you can't
demonstrate that predation
is, in fact, a population
regulator, predator
control is nonsensical.
When you can demonstrate
predators are having
an effect, predator
control can be effective."
The effects predators
have on prey populations
are dependent upon
habitat conditions, the
numbers of predators and
prey, and the sex and age
ratios of predator and prey populations.
Sorting through these factors
makes it very difficult to determine
the effects of predation on mule
deer and elk populations because
every mule deer population is
different, and other factors that
affect a mule deer population will
determine the extent of the effect
of predators.
Widespread predator management
may or may not increase a
mule deer population. Smaller
mule deer populations may be
more susceptible to predators than
larger ones. Larger populations can
afford more losses to predation
than smaller ones. If a mule deer
population experiences one or
more severe winters or droughts
and their numbers are low, they
may be more susceptible to
predators until their population
numbers increase.
In cases where you can't demonstrate that predation is, in fact, a population regulator, predator control is nonsensical. When you can demonstrate predators are having an effect, predator control can be effective.
In years when mule deer populations
are lean, some predators
such as mountain lions and wolves
may consume several wildlife
species including elk and small
mammals, causing the predators to
maintain artificially high numbers.
While this has the potential to
slow the growth of mule deer populations,
scientific studies show
that reducing predators does not
increase the number of fawns that
survive to adulthood. And it's the
number of fawns that survive to
adulthood that determines the
growth rate of a mule deer
population.
If there are big changes to habitat
that result in different movement
patterns for mule deer, they
could become more susceptible to
predation. Changes in habitat may
also change predator communities.
In the last century, there has been a shift in the predator community
from wolves to coyotes. Humaninduced
factors have contributed
to loss and change of wolf prey
and wolf habitat, causing the
elimination of wolves in many
parts of the United States.
Recommendations for
Predator Management
Many of the human influences
that have caused changes to how
predators and prey interact make
managing healthy populations of
both difficult and challenging.
Some segments of the public want
effective predator management
programs so that their livestock
and wildlife are protected, while
others place a value on the presence
of predators in wildlands. As
this debate continues, mule deer
populations have been declining.
Increasing concern with
declines in mule deer and blacktailed
deer populations in large
parts of the western United States
prompted several wildlife professionals
to review wildlife research
and make recommendations on
future research and management
of predators.
Wildlife professionals determined
that reducing the number of
predators in an area may help
deer populations if:
- Predator management occurs when
the deer population is lower than the
ability of the habitat to support it.
- Predation is identified as a factor
that is limiting the ability of the deer
population to grow.
- The predator population is reduced
enough to yield results.
- Reduction in predators occurs
just before reproduction of
predators or prey.
- Reduction in predators occurs on a
scale of less than 250 square miles.
They also determined that predator
management did not successfully
improve mule deer populations
when:
- Mule deer populations were at or
near habitat carrying capacity.
- Predation was not a key factor limiting
the ability of the deer population
to grow.
- Reduction of predators did not
reduce predator populations to a
significant degree.
- Reduction of predators occurs on
large-scale areas.
Wildlife professionals recommend
a wildlife management plan be
completed before reducing predator
numbers. That plan should include
the status of mule deer populations
and the population objective desired
from a reduction in predators,
desired removal goals for the predator
species, timing, method and
scale of removal efforts, and a
description of other factors that may
be depressing mule deer populations.
The plan should also include
monitoring and evaluation of predator
and prey populations, and the
thresholds when reduction of predators
will cease or be modified.
Professionals also recommend
long-term studies on coyote, mountain
lion and black bear, and human
dimensions work to better understand
public acceptance of predator
management, and a cost-benefit
analysis of predator control.
The debate about the good and
bad of predators will likely not be
resolved in the near future as habitats
continue to be fragmented and
susceptible to human influences,
and the public continues to align
itself with one or more “stances” on predators.
