Variation in population size

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Population growth curves

The usual pattern of growth for a natural population:

 1. Period of slow growth - intitially small number of individuals reproduce to slowly build up their numbers

 2. Period of rapid growth - the ever - increasing number of individuals continue to reproduce. The population size doubles during each interval of time (steep gradient)

 3. Population growth declines until its size remains stable. Decline may be due to the food supply being limited or increasing predation. Graph levels out with only cyclic fluctuations.

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Population size

Limiting factors in a pond environment:

 > Mineral ions are used up as population becomes larger

 > Population becomes so large that the algae at the surface prevent light reaching those at deeper levels

 > Other species are introduced into the pond, carried by animals / wind, and some may eat or compete with the algae.

 > Winter months in which temperatures are lower and shorter duration of light intensity.

As a result growth of population slows / ceases - but likely to reach a relatively constant size.

Each population has a maximum size that can be sustained over a relatively long period - this is determined by abiotic & biotic factors.

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