Understanding Populations

How Populations Change in Size

Population is all the members of a species living in the same place at the same time.◦ i.e. school of fish, palm trees on the same island

Population also refers to the size of the species that make up the population.

What is a Population?

Populations can be described in terms of size, density, or dispersion.◦ Density is the number of individuals per unit area◦ Dispersion is the relative distribution or

arrangement of its individuals within a given amount of space. Can be even, clumped, or random

Properties of Populations

A population gains individuals with each new offspring or birth and loses them with each death.

Growth rate is the change in the size of a population over a given period of time

Growth Rate = births – deaths

The growth rate can be positive, negative, or zero.

How Does a Population Grow?

A species biotic potential is the fastest rate at which its populations can grow.

Reproductive potential is the maximum number of offspring that each member of the population can produce.◦ i.e. it can take elephants hundreds of years to

produce a million descendents where as it only takes bacteria a few days or weeks.

How Fast Can a Population Grow?

Reproductive potential increases when individuals produce more offspring at a time, reproduce more often, and reproduce earlier in life.

Reproducing earlier in life has the greatest effect on reproductive potential.

Reproducing early shortens the generation time, or the average time it takes a member of the population to reach the age when it reproduces.

How Fast Can a Population Grow?

How Fast Can a Population Grow? Populations can also

undergo exponential growth.◦ They grow faster and

faster.◦ A larger number of

individuals is added to the population in each succeeding time period.

Exponential growth only occur in when populations have plenty of food and space, and no competition or predators.

Population growth never stays consistent.

Several events such as the reduction of resources, increase in deaths, and/or decrease in births can change a populations growth drastically.

Every ecosystem has a carrying capacity.◦ The maximum population that an ecosystem can

support indefinitely.

What Limits Population Growth?

Carrying capacity is the largest population that an environment can support at any given time.

A population may increase beyond this number but it cannot stay at this increased size.

Because ecosystems change, carrying capacity is difficult to predict or calculate exactly. However, it may be estimated by looking at average population sizes or by observing a population crash after a certain size has been exceeded.

What Limits Population Growth?

Carrying Capacity

A species reaches its carrying capacity when it consumes a particular natural resource at the same time it is produced.◦ Limiting resource

As the population approaches its carrying capacity members of the population start competing for resources.

What Limits Population Growth?

Populations are regulated through death Two types of cause of death in a population

◦ Density dependent◦ Density independent

Cause of death is density dependent deaths occur more quickly in a crowded population i.e. limited resources, predation, and disease.

Cause of death is density independent a certain proportion of a population may die regardless of the population density.◦ i.e. severe weather and natural disasters.

Two Types of Population Regulation