1. How Populations Grow
Section 15.1

2. 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

3. Three Key Features of Populations
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

4. Three Key Features of Populations…
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

5. Three Key Features of Populations…
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

6. 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

7. Exponential population growth
Forms a ‘J’ curve
This model only shows the early stages of population growth

8. Logistic population growth
Forms an ‘S’ curve
Where the line levels out – that is the carrying capacity of the environment

9. 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

10. 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