A population is a group of organisms of the same species living in the same area at the same time. Population ecology studies how and why the number of individuals changes over time.
Factors affecting population size
Population size increases through natality (birth) and immigration, and decreases through mortality (death) and emigration.
Key idea
Change in population = (births + immigration) − (deaths + emigration).
Population growth and limiting factors
When resources are plentiful, a population may grow rapidly. However, growth cannot continue forever. The carrying capacity is the maximum population that the environment can support. Growth is checked by limiting factors such as:
- Food and water supply, and space.
- Predation, disease and competition.
- Accumulation of waste and, for plants, light and minerals.
Example
When a few rabbits are released into a grassy field, their number rises quickly at first. As food runs short and predators increase, the population levels off near the carrying capacity.
Estimating population size
Because organisms cannot all be counted directly, ecologists use sampling methods:
- A quadrat — a square frame — is used to estimate the number or percentage cover of non-moving organisms such as plants.
- The capture–mark–release–recapture method estimates animal populations by marking captured animals, releasing them, then recapturing a sample.
In the mark–recapture method, the population is estimated using the relationship: total population ≈ (number first marked × total caught the second time) ÷ (number of marked individuals recaptured). The method assumes that the marks do not harm the animals, that marked animals mix freely back into the population, and that there are no large births, deaths or migrations during the study. Sampling like this lets ecologists monitor whether a population is stable, growing or declining without disturbing the whole habitat.
Remember
Use quadrats for plants and slow organisms; use mark–recapture for mobile animals.