Population size and competition

Carrying capacity is the maximum population size that can be maintained over a period of time in a particular habitat.

Population size depends on the difference between birth and death rate. It can suddenly increase or decrease, or may oscillate between the two with a regular pattern.

A habitat cannot accomodate large population sizes because of limiting factors on the population:

• water, food, light, oxygen, nesting sites, shelter
• parasites and predators
• competition for resources

Carrying capacity is the upper limit that these factors place on a population size

• When the population size gets bigger, more prey are eaten
• The prey population gets smaller, leaving less food for predators
• Fewer predators can survive and their population size decreases
• With fewer predators, fewer prey are eaten, and their population size increases
• With more prey, the predator population gets bigger
• The cycle starts again    

This happens between members of the same species.

• As food availability decreases, individuals have to compete for it.  
• Those best adapted to obtaining food will survive and reproduce.

Intraspecific competition keeps the population relatively stable:

• If the population size drops, competition reduces, and the population size then increases.
• If the population size increases, competition increases, and the population size then decreases.  

Takes place between organisms of different species. It can affect both the population size and distribution.

Interspecific competition can result in one population being much smaller than the other.