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 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… 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… 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