It doesn’t seem possible
that raindrops or
snowflakes would affect
the ability of mule deer
to thrive. But in fact,
precipitation is a key factor that
drives mule deer populations.
The amount and timing of precipitation
affects plant growth and
quality, deer diet and nutrition, the
ability of a habitat to support a
population of deer, distribution
and movement of deer, predation
and management. Also, extreme
weather such as droughts, prolonged
periods of extreme cold or
excessive snow can directly cause
mule deer mortality.
Plant growth and quality
Precipitation affects soil moisture,
ambient temperatures and
annual plant growing seasons, all
of which affect the plants that
mule deer eat on winter and summer
ranges. In desert environments,
precipitation is critical for
new spring growth. In northern climates,
severe winters with large
amounts of snowfall may increase
mortality because deer do not
have access to food.
Habitat and Deer Nutrition
Mule deer eat browse tips, forbs
and grasses, although their diet is
primarily shrubs. How much of
each kind of food a mule deer eats
is directly related to the availability
of it in the environment. During
years of normal rainfall, deer may
feed primarily on nutrient-rich
deciduous shrubs. In years of
drought, they may eat mostly evergreen
and drought-resistant plants.
Extremes in precipitation can
directly affect the ability of mule
deer to eat nutritious foods. For
example, deep snow can reduce
availability of forage in northern
climates, while drought can lessen
availability of forbs and grasses in
desert environments.
Inadequate precipitation
reduces the availability, annual
growth, digestibility and quality of
important plants, and forces deer
to eat more food with less nutritional
value. Each of these affects
energy levels and the overall ability
of a habitat to support a mule
deer population.
If winter and spring precipitation
are low and there is little new
growth of plants, deer are forced
to eat older plants that have less
nutritional value and are more difficult
to digest. Poor forage can
delay the age at which deer
become sexually mature. Poor
nutrition makes it difficult for does
to successfully rear fawns because
of the inability to provide adequate
milk during lactation. Even if
fawns survive, their small size may
make them more susceptible to
predation, or the rigors of winter.
Severe winters with significant
snowfall and cold temperatures
may lead to malnutrition of deer,
resulting in fewer fawns produced,
and a higher than average death of
fawns and adults.
Drought is a long-term, natural,
cyclic event that is linked to
declines in mule deer populations,
particularly in arid regions. Both
seasonal and long-term droughts
can affect the survival of deer
fawns. In general, higher levels of
rainfall correlate to improved production,
whereas lower levels of
rainfall have been associated with
declines in production and numbers
of mule deer.
Distribution and
Movement of Deer
Precipitation influences the distribution
and movements of mule
deer in northern, cooler climates
and drier, desert climates. Home
ranges of mule deer increase with
a decrease in quality habitat
because the animals need to range
farther to meet their nutritional
needs.
During severe winters with deep
snowfall, deer may be crowded on
small winter ranges, unable to
travel through deep snow to meet
their nutritional needs. Providing
good cover to help deer conserve
energy is critical during severe
winters.
In desert environments, plant
density and rainfall seem to be
good predictors of distribution
and productivity.
In mountainous regions,
seasonal migrations are based on
availability of food resources.
Snowstorms cause deer to migrate
in fall, while migrations from winter
to summer ranges occur with
lush, spring growth and receding
snowlines.
In mountainous regions
with less severe climates, migration
may be closely associated with
changes in relative humidity.
Predation
Interactions between predators
and prey are related to the ability
of a habitat to support a population
of deer, weather, human use
patterns, the type of predator and
changes in habitat. For example,
during drought periods in Texas,
mule deer numbers are below
carrying capacity, and predation by
mountain lions may be significant.
Disease
Little scientific research exists to
suggest there is a direct relationship
between spread of disease in deer
and precipitation. Deer may concentrate
around water during dry, hot
summers, however it is more likely
that drought causes poor nutrition in
deer, leading to greater susceptibility
to disease.
Pollution
Acid precipitation can damage
foliage and roots of vegetation and
destroy soil nutrients and organisms,
resulting in greater susceptibility to
disease, drought, and frost, and
reduced germination and seedling
survival. Acid precipitation may also
lower trace minerals in forage, an
important component in the diet of
ungulates.
Management
Deer populations may be
managed with greater accuracy by
making correlations between climate
and survival of deer fawns. Mean
snow depth, ambient temperatures,
wind speed, drought severity index
and several other climatic factors
can help managers predict fawn
survival, particularly in extreme
desert scrub and montane conifer
environments.
