UNDERSTANDING POPULATIONSHow Populations Change in Size

Charles Darwin once calculated that Charles Darwin once calculated that a single pair of elephants could a single pair of elephants could theoretically produce 19 million theoretically produce 19 million descendants within 750 years.descendants within 750 years.

Darwin made the point that the Darwin made the point that the actual number of elephants is actual number of elephants is limited by their environment.limited by their environment.

WHAT IS A POPULATION?

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.

PROPERTIES OF POPULATIONS

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

HOW DOES A POPULATION GROW?

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.

HOW FAST CAN A POPULATION GROW?

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.

POPULATION GROWTH 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)

EXPONENTIAL GROWTH

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.

WHAT LIMITS POPULATION GROWTH?

Carrying capacity

Resources

Competition within a population

TYPES OF POPULATION REGULATION

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.