Carrying capacity is the rate at which a particular ecosystem can support a particular species of life.

In this context, an ecosystem is an example of an intrinsically renewed resource. Each species of life within an ecosystem takes what it needs from that ecosystem for its use. Its use will be sustainable if it uses the resources of the ecosystem at a rate that doesn't exceed the rate at which they can be regenerated.

As an example, consider a very simple ecosystem such as pasture with cattle on it. Pasture is agricultural land that is used to feed grass-eating animals. The rate of use of pasture (the stocking rate) is usually described as a number of animals for each unit of land (usually hectares or acres); for example: three head (of cattle) per hectare.

An area of pasture has a carrying capacity: the number of animals can sustainably live on it. The bigger the area of land the more animals it can carry, so the carrying capacity is usually described as a maximum stocking rate. The actual value will depend on several characteristics of the land, but especially on the quality of the soil and the climate.

If too many animals are placed on the land they will eat the grass down too low; they may even pull up and eat its roots. If the grass is eaten down too low, the amount of its leaves will be reduced so that it won't be able to photosynthesise sufficiently, restricting its source of energy to regrow. The top soil will be exposed to the elements risking damage to the soil biota, leaching, drying, and erosion; if top soil is lost the land may be permanently damaged. These effects will reduce the carrying capacity of the land and slow its recovery.

Conversely, if the pasture is underused it may also be degraded as pasture. Grass has evolved in a symbiotic (cooperative) relationship with the herbivores that eat it. Grass grow a lot of leaf matter very quickly, more than it needs for its own processes; its leaves grow continually from the base, so being cut or bitten off doesn't restrict its ability to continue growing.  This excess material feeds the herbivores, which encourages then to stay.  If this extra material isn't eaten it eventually becomes rank (coarse, slow growing, and unpalatable to herbivores), or dies and shades out new grass shoots.

Grass can't directly compete well with broad leaved plants, especially trees that grow over it and shade it out. Many grass-eating animals have evolved an instinct to destroy these plants: hares and wallabies will ring-bark sapling trees or bite them entirely off; cattle will eat the leaves off small trees and push them over; elephants will push very large trees over. With insufficient herbivores pasture will eventually be overwhelmed by broad leaf plants and trees, and will turn to woodland or forest.

So, too many grass eating animals on pasture is not sustainable because the grass will be over eaten and may die out altogether; but neither is too few, because the pasture will become rank and overgrown by other plants. Between these two extremes lie a range of stocking rates. Somewhere in this range is the optimum stocking rate which gives maximum productivity: a rate that is not so great that the grass never gets a chance to regrow properly, but that is not so low that the grass becomes rank, slow growing and overwhelmed.

If the pasture is not actually destroyed by having its carrying capacity exceeded, it's likely that its exploitation will increase until its productivity falls, eventually becoming no longer worthwhile continuing with exploitation. However, if its productivity recovers, even slightly, it is likely that exploitation will return, so it's likely that its stocking rate will hover around the minimum level, at the edge of collapse.

If the pasture is to be returned to its maximum productivity the stocking rate must be reduced further to allow the grass to regrow to the point where it can be restocked at the carrying capacity to give the optimum level of extraction.

This page is linked from:

An example of Tragedy of the Commons

The condition of the cattle

The condition of the pasture

Indeterminacy of the stocking rate

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