capacity refers to the size of a population that can
live indefinitely in an environment without doing that environment
any harm. This applies to plants, animals and people.
Populations must not exceed
the carrying capacity of their environments!!!
If the carrying capacity of the environment
is exceeded, organisms die and the environment may be permanently
Our use of
farms, grazing land, and rain forests must include awareness
the carrying capacity of these lands if we want to preserve
the productivity and fertility of these lands.
page on Reproductive Strategies
we read about some biological ways that animals and plants
keep their populations within sustainable limits.
need to look at your planet to see if you have mechanisms to
keep your organisms from exceeding the carrying capacity of your
environment. Mechanisms that work on earth include:
Individuals or groups select and defend
a territory large enough to provide food for the reproducing
adults and their young. Males often defend the territories, as
this takes a lot of energy. Often the females then choose their
mates, no doubt keeping an eye on the size of the territory.
Being female takes a lot of energy, too: producing and laying
eggs, or being pregnant, require females to be able to capture
energy beyond their own physical requirements. Feeding the young
then requires foraging for food (both parents may do this, as
with many birds), or the production of milk. The larger and more
productive the territory, the more food will be available for
the adults and their young.
R-strategy or K-strategy. See the page on this topic.
Many kinds of animals have contests
to determine their status in relationship to the other animals
in their groups. Chickens do this, horses do it, monkeys do it,
wolves do it. Do people do it? The answer is up to you on that!
Having a dominance order is an efficient
way to reduce conflict in a social group. No wolf is going to
argue with the alpha wolf (the one at the top of the hierarchy).
Once the social order has been settled, all the animals react
to the other animals in terms of it. All of them get the benefits
of group membership, and conflict is minimized.
Dominance hierarchies also govern the
distribution of resources. The animals with the highest status
get to eat first, and sometimes they are the only ones in the
group to reproduce. This ensures that the strongest animals stay
strong, and that the new members of the group are descended from
parents who are fit and vigorous. In times when resources are
plentiful all the animals share in using them, but when resources
are scarce, the strongest animals will get what they need to
stay alive. The others animals may go hungry, and may even die.
This seems harsh to us, but life is challenging and carrying
capacity is not elastic.
In nature, the numbers of animals
present in an environment is usually somewhat below the carrying
capacity. This allows for fluctuations in the environment,
such as dry years and wet years. If
the carrying capacity is exceeded the ecosystem is often damaged
severely, and sometimes does not recover. It may become useless
for decades or even indefinitely.
Often we humans do not seem
to understand about carrying capacity. As our population climbs,
we need to think about limits. Population numbers are not the
only problem. Misuse of the land due to thoughtlessness or greed
or poor management practices should also come under scrutiny.
Here are some examples of human activities which interfere with
that we hear about is over-grazing.
In a well-balanced grassy biome the animals eat the plants
and fertilize the ground with their droppings. When much of the
grass has been eaten, the animals move on to where the grass
is tall: the grass in the area where the animals have been feeding
now has few grazers, and is able to renew itself and grow tall
again. After a while, the animals come back, so they always have
enough to eat. But the numbers of animals do not increase indefinitely.
Predators eat some of them. Territorial battles space them out.
Sometimes their numbers are thinned by disease. In a balanced
ecosystem, the numbers of organisms stay in balance with their
resources and each other. This kind of interaction can work forever,
or at least until climate change or other stressors alter the
Humans, however, limit the movement
of grazing animals by putting up fences or by herding them. Animals
are seen as wealth, and the humans want their livestock to increase.
When the number of grazers exceed the carrying capacity of the
pastures, bad things happen. With wild life, there may be a population
crash as the animals compete with each other and food runs out
during the winter. On pastureland, the grass may be damaged beyond
repair, and the land may become barren. Recovery from over-exploitation
can take many years, even centuries. The result may be the land
turning into desert. It is interesting to reflect that the Sahara
Desert was once the grain basket of the Roman Empire. Today it
has a balanced ecology -- but it is a desert ecology now.
Plants need water and sunlight to
grow. Sunlight alone is not enough. Farmers like to plant beside
rivers, where water is abundant. Good crops mean that more babies
live: more people need more food. Farmers begin to water
more land, and irrigation systems develop. This has happened
many times in many different cultures. As the irrigation systems
develop, populations grow: as populations grow, more land
is irrigated. But this water, that once would have flowed to
the sea, carries dissolved salts and minerals. As an irrigation
system reaches its maximum potential, enough water is poured
onto the soil to make the plants grow, but there is not enough
extra water to flow through the soil and carry the salts and
minerals away. Slowly the salts build up in the soil, and plants
do not grow so well. Eventually the soil becomes so salty that
nothing will grow, and only desert remains.
in the San Joachim valley of California is irrigated, and is
beginning to show signs of increasing salinity. We are exploiting
this resource beyond its carrying capacity. Here the limiting
factor is not space or light, but water.
that the examples that I have given concern human activities.
In nature, ecosystems that become unbalanced find a new balance
that will work. Nature does not care if we have leopards and jungles
or cockroaches and grass. Those fitted to survive, survive. Part
of survival fitness is having a mechanism for living within the
carrying capacity of the environment.
Photograph from a Corel CD-ROM
: for viewing only, not for downloading. More Information
1999, 2004.. Elizabeth Anne Viau. All rights reserved. This material
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