How Populations Grow
Section 15.1
What is a population?

All the same species
 Similar
organisms that can interbreed and
produce fertile offspring

In the same place

At the same time

Scientists study population dynamics – the size
and how populations change over time
Three Key Features of Populations
1.
Population size

Scientists may use different techniques to estimate
population size, especially if animals are difficult to
locate or move around often

The actual number of individuals

Small animal populations are at risk for extinction

Smaller populations also usually have less genetic
diversity
Three Key Features of Populations…
2.
Population density

The number of individuals that live in a given area

Populations that are spread out over a large area may
reproduce less often
Three Key Features of Populations…
3.

Population dispersion
The pattern of arrangement in the area – three
possibilities:
 Random
distribution – there is no pattern
 Even
distribution – there is even spacing between
individuals
 Clumped
distribution – individuals are in bunches such
as flocks or herds
Modeling Population Growth


Growth rate = the change in the population in a given amount
of time

Can increase: rate = a positive number

Can decrease: rate = a negative number

Or can remain the same: rate = zero
Two types of graphs are used to show population growth rates:

Exponential growth curve – unlimited population growth

Logistic growth curve – shows the impact of limiting factors
on population growth
Exponential population growth

Forms a ‘J’ curve

This model only shows the early stages of population
growth
Logistic population growth

Forms an ‘S’ curve

Where the line levels out – that is the carrying capacity of
the environment
Limiting Factors

Density-dependent
 An
increase in the population size causes a decrease in
the availability of the resource
 Example:

food
Density-independent
 An
increase in the population size has no impact on the
factor
 Example:
bad weather
Different species have different
patterns of growth

Rapid growth: r-strategists
 Reproduce
at a high rate and population increases rapidly
 Usually
small animals who mature quickly and have short
lifespans
 Example:

insects
Slow growth: K-strategists
 Reproduce
at a slow rate and population stays close to the
carrying capacity
 Usually
larger animals who have long lifespans and care for
their young
 Example:
elephants