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CARRYING CAPACITY
Article #94, April, 2005
By Bill Cook
The carrying
capacity idea gets tossed around in a variety of ways, most of which are difficult
to define and defend, if not outright incorrect. Carrying capacity related to
white-tailed deer is likely the most common context in which the idea is used,
and misused.
In some ways,
defining carrying capacity might be theoretical. In other ways it has practical
implications. In any case, most people who use the term often find it hard to
explain exactly what they mean. Notions of carrying capacity are largely value
defined, rather than science defined.
Science-based
evaluations of carrying capacity are difficult to find in the literature. Value-based
evaluations are prolific. Both scientific and social definitions have legitimacy.
The area of greatest conflict occurs when social values are mistaken for science.
Carrying capacity
is the maximum number of individuals of a species that the landscape can support
over time. That's not too hard to understand. However, imbedded in this definition
are a number of difficult questions. Do all species behave the same? Which landscape?
Which season? How are other species impacted? How long in time? Does the maximum
fluctuate? How frequently? For what reasons?
Research has
shown that populations of different species can behave in very different ways.
A species relates to its own kind and with other species. Non-living habitat
elements, such as weather, can have huge impacts. Growth curves showing change
from low numbers to high numbers have more than one shape.
Not all species
populations show the classic rate of increase to a sustainable plateau. For
most species, the curves don't go flat or stabilize. Some species, such as whitetails,
can irrupt and crash. Theoretically, these populations will stabilize in balance
with their environment. However, most ungulate research has demonstrated a continuing
cycle of irruptions and crashes.
When a population
is maintained at their carrying capacity; what impacts does that have on other
parts of an ecosystem? White-tails at carrying capacity can have negative impacts
on the regeneration of timber species and understory plants, which serve critical
habitat for many other wildlife species. Simply because a landscape might be
able to carry a certain number of animals . . . should it?
An ecosystem's
ability to support a population varies widely. For most species in the north,
winter is the season when food is scarcest and environmental conditions most
rigorous. The mix of habitats and constant habitat change are key variables.
It is unrealistic to expect things to stay the same in our actively dynamic
natural environment.
Calculating a
carrying capacity for a species requires knowledge of the species and its environment,
a definition of the range size, and a sense of time measured in years or decades.
Once determined, can all species be managed at that level? Are population irruptions
and crashes natural? Or are these patterns caused by human influence?
Much more commonly,
populations are managed using a set of social values. However, values are seldom
universal and are not static. Deer populations probably provide the best examples
and generate the most controversy. How much tree regeneration can be sacrificed
before it is too much? How many car collisions are acceptable? How much risk
to endangered species shall be tolerated? How much vegetation loss in parks
and natural areas will be allowed? At what point is hunter satisfaction or dissatisfaction
significant? Is a population crash a waste or merely nature taking its course?
Is damage to vegetation permanent or does it recover after a period of time
when deer populations are down? Can deer populations managed at artificially
high populations prevent vegetation recovery? What is artificial? The list of
questions continues.
Most wildlife
and forest management systems are based on sets of societal values, using science-based
data to achieve these values. The science that is used to support a value system
should not be mistaken for science defining nature in a pristine sense or as
a replacement for value judgments.
The body of science
that enables foresters to grow tree plantations, maintain aspen stands, or accelerate
old growth conditions reflects a need to satisfy demands of society. The body
of science used to manage white-tail populations reflects current values of
society held towards deer. Values change.
Science employed
to support one side of an inflammatory issue is usually a misuse of science.
The conflict is over values and should be debated as such. Science, when properly
funded, can tell us, for example, about the level of biodiversity across a defined
landscape. Societal values will tell us whether that level is appropriate or
not.
- 30 -
Trailer
Bill Cook is an MSU
Extension forester providing educational programming for the entire Upper Peninsula.
His office is located at the MSU Upper Peninsula Tree Improvement Center near
Escanaba. The Center is the headquarters for three MSU Forestry properties in
the U.P., with a combined area of about 8,000 acres. He can be reached at cookwi@msu.edu
or 906-786-1575.
Prepared
by Bill Cook, Forester/Biologist, Michigan State University Extension, 6005
J Road, Escanaba, MI 49829
906-786-1575 (voice), 906-786-9370 (fax), e-mail: cookwi@msu.edu
Use
/ reprinting
of these articles is encouraged. Please notify Bill Cook.
By-line should read "Bill Cook, MSU Extension" Please use the article
trailer whenever possible.
Michigan State University is an affirmative action equal opportunity institution. The U.S. Department of Agriculture prohibits discrimination on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital status or family status. (Not all prohibited bases apply to all programs.)
This website is maintained by Bill Cook, Michigan State University Extension
Forest in the Upper Peninsula. Comments, questions,
and suggestions are gratefully accepted.
Last
update of this page was
22 September, 2005
This site is hosted by School of Forest Resources and Environmental Science at Michigan Technological University.
