| Introduction Large mammalian herbivores are known to cause major
changes in plant community composition and structure. This is a major concern of
Rocky Mountain National Park (RMNP) where large numbers of elk were found to suppress
heights, leader lengths, productivity, and annual production f willow communities in the
park.
High densities of moose have also been shown to influence the dynamics and
structure of ecosystems. Bark stripping by moose in Denali National Park may
increase the rate of succession in aspen-spruce communities due to tree mortality,
and in the taiga of interior Alaska, browsing on willow/alder communities by
moose reduced fine root longevity and accelerate carbon turnover.
The Colorado Division of Wildlife started the reintroduction of moose to
Colorado in 1978. Two groups of 12 moose were released near the town of Rand in
North Park Colorado, in 1978 and 1979, 8 miles west of the National Park boundary.
The first observation of moose within the park was in June of 1980. Today, between
30-50 moose are believed to summer in the park.
There is growing concern that moose may have the same effect on willow
communities that was found for elk. Willow is an important component of the
summer diet of moose in montain regions of the western United States, and moose summer
diets can be comprised of between 80 and 90% willow.
It is the park's objective to reduce human impact on the ecosystem and
restore biodiversity. Willow provides habit and food for a wide variety of different
animal and bird species, including beaver who's population has been in decline since the
1960's. It is a major concern of the park that grazing by herbivores, both moose and
elk, is causing major changes and declines in willow communities which in turn may cause
reduction of other plant and wildlife species that were historically found in the
park.
Carrying Capacity
Carrying capacity is the maximum number of animals with specific nutritional
needs that can be supported by a given habitat. This simple idea of how many animals
can live in a given ecosystem is fundamentally important to management of wildlife
populations and the habitat they occupy. In the early 1980's researchers suggested that
looking at the nutrient needs of animals through the winter when food is most limited, and
estimating the amount of those nutrients that the plants in the environment can provide
may give an idea of how many animals of a certain species the environment could
support. This idea of nutrient-based carrying capacity is highly dependent on the
nutritional quality and amount of food available to animals, which varies throughout the
season and from year to year. An early attempt to predict carrying capacity based on
nutrient requirements of the animal and the ability of an ecosystem to supply those
nutrients was made for elk wintering in RMNP, and was considered a valid technique.
Summer range as a limiting factor to wildlife
populations is less studied. The summer is when moose feed on willow almost
exclusively in the park, and may be a cause of their decline. Basic information on
how many moose can be supported in the summer in RMNP might give park service biologist an
idea if there are to many moose in the park presently, or how many moose can live in the
park without causing damage to willow communities in the summer. By knowing how many
moose can live in the park, park biologist can devise a management plan to keep moose
populations at that number. Managing moose populations could help willow communities
survive, and provide food and habitat for other animals as well, therefore supporting the
park's goal of continued biodiversity.
Several factors are looked at when trying to
predict the nutrient-based carrying capacity of a landscape. These include; activity
patterns of the animals and the energetic cost of those activities, cost of thermal
regulation in terms of energy expenditure, forage abundance, digestibility and nutrient
content of available forage, and intake rates (how fast an animal eats) of the
animal. All of these tend to vary among and within seasons. Some of these
factors will be discussed as they pertain to moose in RMNP.
Factors
The abundance of forage used by moose varies
between different habitats and seasons. Forages used by moose tend to be more
abundant in spring and summer, and decline in fall and winter with the loss of deciduous
leaves and aquatic vegetation. Because moose selectively feed on leaves of deciduous
browse, forbs, and aquatic plants in summer and primarily woody stems in winter, their
summer diets are generally more digestible. Nutritional quality of vegetation consumed by
moose is often expressed as digestibility. Foods that are easily digested provide
more nutrients to an animal than foods that can't be digested. Non-digestible plants
tend to pass threw the animal without their nutrients being extracte for use by the
animal. Quality therefore varies with seasons, with spring and summer diets being 1.5 -3
times more nutritious than winter diets.
Daily and seasonal energy expenditure in
different environments is rarely constant because of changes in basal metabolism,
activity, and thermal regulation. Basal metabolism is the energy an animal uses to
support its basic life functions such as heart rate and digestion. Basil metabolic
rate (BMR) has been defined as the energy expenditure of an animal that is resting, has
fasted, and is in an environment that is neither too hot nor cold for the animal.
This measure gives us a base line to measure the energy expenditure due to other
activities. Because the animal is fasting it does not use energy for digestion,
because the animal is resting it does not expend energy for such things as movement, and
because it is neither hot or cold the animal does not expend energy to maintain
thermoregulation. Seasonal estimates of BMR for moose fluctuated with highs in the
summer, and lows in the winter. These seasonal changes in BMR are believed to be related
to changes in body weight and feed intake.
Energy expenditure varies seasonally and
among individuals. Much of this energy expenditure is due to the daily activity of
animals. Moose are highly active in the summer while food is abundant, and less
active in the winter when food is less available. Male moose also become more active
during mating season, and all moose expend more energy when trying to escape predators.
Moose are highly adapted to cold climates, but
susceptible to heat stress in warmer temperatures. Metabolic rates increase to
maintain normal body temperature when environmental temperature rises of falls below
critical limits therefore increasing energy expenditure. The large body size of moose
helps minimize heat loss during winter, but also affects the ability of moose to dissipate
surplus heat during summer of unusually hot winters. The inability of moose to deal
with these high temperatures could be a limiting factor controlling the southern
distribution of moose in North America.
It is necessary to understand how much energy
an animal is expending so that one can look at how much the animal must eat to maintain
it's health. If an animal expends more energy than its food intake can support, then
it will use up fat reserves, and in extreme conditions starve to death. By knowing
how much nutrients an animal needs to maintain its self, we can look at how much forage is
available in the environment to meet those energy demands, and therefore estimate carrying
capacity.
Conclusion
A few studies have looked at elk summer range
carrying capacity, but no studies have looked at summer range carrying capacity for moose.
Summer is a key period in which moose build up fat reserves that take them threw the
winter. Summer diets are generally 1.5 to 3 times more nutritious than winter
diets. Moose enter winter with an estimated 20-26% body fat, and this excess fat is
used in winter when winter diets are insufficient to meet nutrient requirements.
Summer habitats that don't provide the
available nutrients to build up these fat reserves could represent a limiting factor in
the number of moose that survive the winter regardless of quality and availability of
winter forage. Temperature may also play a large role in the ability of moose to
build up fat reserves. High temperatures increase metabolism, heart rate, and
respiratory rates, representing an increased energy cost to moose during summer, and moose
reduce intake rates and lose body weight during warm summer periods.
Moose in Colorado represent the most southern
range of moose in North America. High temperatures in summer may restrict the amount
of time spent foraging, increase metabolic and activity costs, and reduce the amount of
fat reserves built up by moose during summer, therefore possibly controlling the
population of moose in RMNP.
In conclusion, I believe that an estimate of
moose summer range carrying capacity in RMNP may be beneficial to park biologist in
determining the effects that moose are having on willow communities, and that summer range
may be a limiting factor in the number of moose that RMNP can support.
By estimating the number of moose that can
live in RMNP in the summer, managers can use this information to keep the population level
of moose at levels that don't negatively affect willow communities. By restricting the
amount of moose that feed on these willow communities, it may give theses communities a
chance to recover. With the rebounding of willow communities, other animals such as beaver
may increase in population and therefore increase biodiversity. Maintaining
biodiversity is the main goal of RMNP.
Anyone interested in learning more about nutrient based
carrying capacities, or general moose nutrition might read the following
articles:
Hobbs, N. T., D. L. Baker, J. E. Ellis, D. M. Swift,
and R. A. Green. 1982. Energy and
nitrogen based estimates of elk winter range carrying capacity. Journal of
Wildlife
Schwartz, C. C. 1992. Physiological and nutritional
adaptations of moose to northern
environments. ALCES Supplement 1:139-155.
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