Carrying Capacity

     Carrying 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!!!

zebras drinkingIf the carrying capacity of the environment is exceeded, organisms die and the environment may be permanently destroyed.

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.

In the page on Reproductive Strategies we read about some biological ways that animals and plants keep their populations within sustainable limits.

Your Planet

     You will 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:

Territoriality

     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.

Reproductive Strategies

R-strategy or K-strategy. See the page on this topic.

Dominance Heirarcharies (Pecking Orders)

     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 carrying capacities.


overgrazingOver-grazing

     One example 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 environment.

     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.

Irrigation

     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.

     The soil 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.

     You notice 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.


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© 1999, 2004.. Elizabeth Anne Viau.  All rights reserved. This material may be used freely for instructional purposes but not sold for a price beyond the cost of reproduction. Please inform the author if you use it at eviau@earthlink.net