2.6. 3: Population Regulation
The number of organisms in any population continually change. They never remain constant for ever. Many factors can affect population size, but their combined effect is going to be seen in an alteration in one of four conditions: birth rate, death rate, gains from immigration and losses from emigration.
Changes in these four conditions determine later population numbers. These conditions in-turn are affected by the available resources in any ecosystems. As populations grow then competition tends to come into play and resources start to become limited, this can continue until a theoretical point is reached where the amount resources available can not support the current population. The resource available define the maximum number of species (or individuals within a species) that habitat can support throughout their complete life cycle. This is the Carrying Capacity of that ecosystem.
It is the environmental carrying capacity that limits how large a population can grow to. I reality, often the number of individuals in a population is greater than the carrying capacity. When this occurs competition between individuals for the limiting resources takes place. These resources could be; food, mates, breeding sites, water, soil nutrients or anything else. This is an example of Density dependent competition (see 2.1.7: Population Interactions)
With Density dependent competition the larger the population the greater the degree of competition for the limited resources. As the population grows so fewer individuals will get the resources they require to survive.
This need not in itself result increased mortality but may also cause emigration of individuals to areas where the resource is in greater supply or a lowering of reproductive success. This can be illustrated by examining the declining Swallow population in Europe. Since the 1970’s concern has been raised about an apparent decline in the breeding populations of these birds across Europe. Two possible explanations are linked to changes in farming practices. Extensive use of pesticides on crop plants has reduced the number of insects that swallows can feed on. However a another factor is the loss of old brick and stone farm buildings that Swallows require for nest sites: this is an effect on their reproductive success, fewer breeding sites means fewer young and so eventually fewer adults.
Explore population growth in an ecosystem through changing the parameters in a simple model
