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Sheep Population Size
Size: A population
is defined by all of the animals of a single species that live and
raise their young in a specific area. In Alaska, the Dall sheep
population is currently estimated to be about 70,000 animals. Sheep
live in mountain ranges around the state including the Brooks Range,
the Alaska Range, the Wrangell Mountains, the Chugach Mountains,
the Talkeetna Mountains, and on Kenai Peninsula.
Dall sheep populations
in the Baird Mountains of northwestern Alaska declined substantially
during the early 1990ís following two severe winters. Population
estimates of adult sheep declined 51%, from 811 to 383 between 1989
and 1991. Wildlife biologists conducting population counts found
that the sheep population remained low throughout the 1990ís. They
also observed few lambs during aerial surveys conducted from 1991
to 1994. These declines resulted from the poor nutrition, increased
predation, and severe winter conditions (Adams et al., 1999).
Factors Influencing Animal Populations:
Animal populations change over time as births add individuals to
the population and deaths subtract individuals from the population.
Populations also change when individuals emigrate (leave
an area) or immigrate (move into an area).
Animal populations have
the potential to grow in an exponential fashion. This growth can
be affected by a number of factors, called limiting factors.
Disease, habitat destruction, predators, weather, food availability,
pollution, and human interference can all be limiting factors that
affect Dall sheep populations. A limiting factor either slows the
growth of an animal population or causes the population to decline.
Changes in wildlife populations over time are usually a result of
a combination of limiting factors.
Every population is limited
by the amount of available habitat. The amount of quality habitat
is the factor that ultimately sets the upper limit on the size of
a population. If part of the habitat is not of quality and/or limited,
then the growth of that population may also be limited. For example,
it could be possible that one winter the lower elevation grasses
and shrubs that the Dall sheep eat be buried under 5 feet of snow.
It may be difficult for the animals to forage and get enough food
so all the sheep move into a smaller area or only part of their
habitat to feed. After a period of time, they eat all the available
vegetation and may starve.
Predators are often a
major limiting factor on the growth rate of a prey population. Dall
sheep predators include wolves, bears, and eagles. Prey populations
can also limit the size of the predator population if they are the
only source of food available.
Human activities may
also limit populations. Humans can directly limit populations by
harvesting (hunting) or they can indirectly limit populations through
pollution, habitat destruction, or inadvertent disease transmitted
from domestic animals.
Every population has a maximum size that it can reach before it
exceeds the available habitat. This maximum number is the carrying
capacity of an area. If a population exceeds the habitat carrying
capacity, the population will decrease due to limited resources,
especially food. Healthy wildlife populations fluctuate from year
to year as the limiting factors and carrying capacity of the habitat
change. A classic example of such fluctuations in Alaska are the
cyclic populations of lynx and hare. The hare populations grow when
food is plentiful. The lynx populations then grow from eating the
numerous hare. The hare populations then shrink as they eat the
food and are eaten by the lynx. Then the lynx populations shrink
and the plants grow back, and the cycle starts again. The populations
other large mammals, such as moose, wolves, bears, caribou, and
sheep, also go have cycles, where the population size grows and
shrinks over the years.
Population age structure and its importance:
The age structure of the
sheep population refers to the number of sheep in each age class.
An age class or cohort is made up of all the sheep born in any given
year. For example, sheep born in 1998 as lambs are a cohort. In
1999, this cohort will be 1 year old and so forth. The age structure
of the population depends on the number of sheep in each cohort.
The number of sheep remaining in a particular cohort from one year
to the next is dependent upon their survival rate. As a cohort grows
older, and sheep have been lost to hunger, predation and other limiting
factors, the cohort shrinks. A typical age structure for sheep would
have the greatest numbers in the younger age classes and fewer numbers
in the older age classes. For sheep, life spans vary between ewes
and rams. Ewes may live to be 17 years old, whereas rams reach only
10-12 years of age.
Because a ram's horns
grow in a spiral throughout their lives, they can be used to estimate
their age. The horn grows up and back from the skull and then curls
down and around and up again. A full curl is when the tip of the
horn has circled all the way around to pass the spot where the horn
grows out of its head (don't worry the horn spirals away from the
skull, so the tip of the horn doesn't poke the skull). A three-quarter
curl is when the tip of the horn has curled up, back, and down and
is ready to grow up toward the horn base again. If you imagine the
horn starting at the 3 on a clock, and growing backwards to 12 then
to 9, a three-quarter curl would be when the horn tip gets to the
Rams generally reach
3/4 curl at 7 years old. It takes another 2.5 to 3 years to reach
full curl, around 10 years old. The average number of full curl
rams counted was 18 for 1986-1995; however the three year average
for 1996-1998 is 48. The 1999 count was only 27. So during the years
1996 to 1998 there was a large cohort of older rams, and then by
1999, close to half of the 10 year old rams died. This die off may
not be surprising, since rams don't usually live past 12 years.
Dall sheep populations
in the Baird Mountains have many more ewes than rams. This happens
for many reasons. The number of male and females lambs are approximately
the same, so the difference has to do with how many ewes survive
and how many rams survive. Even though they rarely die directly
from fighting with other males, the rut (male competition for females
in the fall) takes a heavy toll on the rams. They are exhausted
and often injured, leaving them more susceptible to starvation,
predation and illnesses such as pneumonia. The effects of this are
shown by the fact that rams only live until about the age of 10-12,
and ewes closer to 17. Another factor is hunting by humans. Hunters
prefer to hunt the rams with their large horns than the females.
In some areas where moose are hunted heavily, the male:female ration
can go from 60-70:100 all the way to 5:100!