How Populations Grow

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