Lesson
Populations grow until something says stop
Births, deaths, and carrying capacity are the basic accounting.
Learning goals
- Contrast exponential and logistic growth in words.
- Name density-dependent and density-independent limits.
Explanation
A population is individuals of one species in an area. If resources are briefly unlimited, growth can look exponential. Logistic growth slows as the population nears carrying capacity—the level the environment can sustain. Capacity is not a moral target; it can move when climate, food, or space changes.
Disease and competition often bite harder in a crowd (density-dependent). A late freeze can kill many individuals whether the crowd is large or small (density-independent). Human management can be either kind of force.
Key terms
- Carrying capacity. The population size an environment can sustain given resources and conditions.
- Density-dependent factor. A limit whose effect strengthens as the population becomes more crowded.
Common mistakes
- Drawing exponential growth as if it can continue forever on a finite island.
- Calling carrying capacity a fixed number for all time.
Practice
Original Marlow Works items. Check the answer explanation after you try.
A cell’s surface-area-to-volume ratio is important because it affects:
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
A storm kills 30% of a bird population whether nests are packed or sparse. What kind of factor is the storm?
Answer. Density-independent, because the percent loss does not rely on crowding.
The weather event does not need high density to do its damage.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Biology course page
Official eight-unit framework, science practices, and lab resources.
College Board · All rights reserved · accessed 2026-10-01
Open sourceReference only — do not copy
OpenStax Biology 2e
Biology reference with commercial-use restrictions on the current publisher page.
OpenStax · CC BY-NC-SA 4.0 with additional restrictions · accessed 2026-10-01
Link only unless permission is obtained.
Open source