Populations and Carrying Capacity
- Biology
- Ages 14–18 · Grades 9–12
What you’ll find out
- Populations grow exponentially (J-shaped) when resources are plentiful and logistically (S-shaped) as they approach carrying capacity.
- Density-dependent factors such as competition, predation and disease strengthen with crowding; density-independent factors such as floods and fires do not.
- Predator and prey numbers can rise and fall in linked cycles, as snowshoe hares and lynx do.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
How populations grow
A population is a group of individuals of one species living in the same area. When resources are plentiful, a population can grow exponentially. Each generation adds more individuals than the last, so the numbers climb faster and faster. On a graph, this makes a J-shaped curve. No environment can support such growth for long, however. As food, water, space or shelter become scarce, the birth rate falls and the death rate rises. Growth slows and the population levels off, making an S-shaped curve. The largest population an environment can sustain over time is its carrying capacity. A population that overshoots it usually crashes. Reindeer on St Matthew Island rose from 29 to about 6,000, ate nearly all their food, and then almost all died.
What limits a population
Factors that limit population growth fall into two groups. Density-dependent factors grow stronger as a population becomes more crowded. They include competition for food and space, predation, and disease, which spreads faster when individuals live close together. Density-independent factors affect a population regardless of its size. Examples include floods, wildfires, droughts and severe winters. Predators and prey can also regulate each other. Records from the Hudson’s Bay Company show that snowshoe hare and lynx numbers in Canada rose and fell in cycles of about ten years. When hares were plentiful, lynx thrived. As hares grew scarce, lynx numbers fell. With fewer predators, the hares then recovered, and the cycle began again. Most populations are limited by several factors at once.
Curriculum links: NGSS HS-LS2-1 · NGSS HS-LS2-2 · England KS4: Biology · Cambridge IGCSE: Biology · NCERT Biology, Class 12