U NDERSTANDING P OPULATIONS How Populations Change in Size

Charles Darwin once calculated that a single pair of elephants could theoretically produce 19 million descendants within 750 years. Darwin made the point that the actual number of elephants is limited by their environment.

W HAT IS A P OPULATION ? Is all the members of a species living in the same place at the same time. The word population refers to the group in general and also to the size of the population – the number of individuals it contains.

P ROPERTIES OF P OPULATIONS Described in terms of size, density or dispersion. Density = number of individuals per unit area Dispersion = distribution or arrangement of its individuals Can be even, clumped, or random

Clumped Uniform Random

H OW D OES A P OPULATION G ROW ? A change in the size of a population over a given period of time is the growth rate. The growth rate is: birth rate – death rate * Growth rates can be positive, negative or zero.

H OW F AST C AN A P OPULATION G ROW ? A species’ biotic potential is the fastest rate at which its population can grow. The biotic potential is limited by the reproductive potential- the maximum number of offspring each member can produce.

P OPULATION G ROWTH CONT. Reproductive potential increases when individuals produce more offspring at a time, reproduce more often, and reproduce earlier in life. Reproducing early shortens the generation time (average time it takes to reach a reproductive age)

E XPONENTIAL G ROWTH Exponential growth means that populations grow faster and faster (example: dogs) Exponential growth occurs in nature only when populations have plenty of food and space, and have little or no competition or predators.

W HAT L IMITS P OPULATION G ROWTH ? Carrying capacity Resources Competition within a population

T YPES OF P OPULATION R EGULATION Cause of death in a population may be density dependent or density independent. Density dependent – deaths occur because of crowding in a population. Density independent – a certain proportion of a population dies regardless of the population’s density.