No population keeps a J-curve for long. In How Populations Grow food and space were plentiful. Sooner or later something runs short.
Key idea
A limiting factor is anything that keeps a population from growing: food, water, space, shelter, light, disease, predators. The largest population an environment can support over time is its carrying capacity (K). A population growing toward K follows an S-shaped curve and then rises and falls around K.
Why it works
When a population is small, each individual finds plenty of food, so births beat deaths. As the population nears K, each individual gets less: more go hungry, disease spreads more easily in crowds, and fewer young survive. Births and deaths come into balance. If a good year pushes the population above K, food runs short and it drops back.
Worked example: Which factor limits?
A pond has plenty of insects for its frogs and lots of water, but only a few logs and rocks where frogs can hide from herons. What limits the frog population?
- Food: plenty. Water: plenty.
- Shelter: scarce. Frogs without cover are caught by herons.
Answer: Shelter (and the herons that catch unsheltered frogs).
Worked example: Read the curve
A deer population grows from 50 to 400 in eight years, then stays between 380 and 420 for the next ten years. About what is the carrying capacity?
- The population stops growing and moves up and down around one level.
- The middle of 380 to 420 is 400.
Answer: About 400 deer.
Common mistake
Students think a population sits exactly at carrying capacity, or that one plentiful resource lets it grow forever. Plenty of food does not help if there is no water or shelter: whatever runs short first sets the limit. And real populations rise and fall around K from year to year.
Carrying capacity can change. A wet year grows more grass, so a meadow can feed more rabbits; a drought lowers K.