Click image to open full size
Population & Community Ecology Cheat Sheet
A printable reference covering population size, density, growth models, carrying capacity, species interactions, and community structure for grades 9-12.
Related Tools
Related Labs
Related Worksheets
Related Infographics
Population and community ecology explain how groups of organisms live, grow, interact, and change over time. This cheat sheet helps students connect population structure, population growth, and species interactions in one place. These ideas are important for understanding biodiversity, conservation, disease spread, invasive species, and ecosystem stability. Population ecology focuses on size, density, distribution, age structure, and growth rate. The most important growth models are exponential growth, which occurs with abundant resources, and logistic growth, which slows near carrying capacity. Community ecology focuses on interactions such as competition, predation, mutualism, commensalism, and parasitism, along with concepts like niches, succession, and keystone species.
Key Facts
- Population density is calculated as density = number of individuals / area or volume.
- Population growth rate can be estimated as change in population = births + immigration - deaths - emigration.
- Per capita growth rate is calculated as r = (births - deaths) / population size when immigration and emigration are ignored.
- Exponential growth follows dN/dt = rN, where N is population size and r is per capita growth rate.
- Logistic growth follows dN/dt = rN((K - N) / K), where K is carrying capacity.
- Carrying capacity is the maximum population size an environment can support over time with available resources.
- Density-dependent factors such as competition, disease, and predation become stronger as population density increases.
- A niche includes a species' role, resource use, habitat needs, and interactions with other organisms.
Vocabulary
- Population
- A population is a group of individuals of the same species living in the same area at the same time.
- Community
- A community is all the populations of different species that live and interact in the same area.
- Carrying Capacity
- Carrying capacity is the largest population size that an environment can support sustainably.
- Niche
- A niche is the full role of a species in its ecosystem, including what it eats, where it lives, and how it interacts.
- Competition
- Competition is an interaction in which organisms use the same limited resource, reducing success for one or both.
- Succession
- Succession is the gradual change in a community's species composition after a disturbance or new habitat formation.
Common Mistakes to Avoid
- Confusing population with community is wrong because a population includes one species, while a community includes many interacting species.
- Using exponential growth for all populations is wrong because real populations usually face limited resources and often follow logistic growth near carrying capacity.
- Forgetting immigration and emigration is wrong because movement into or out of a population can change population size even when births and deaths stay constant.
- Assuming carrying capacity is fixed forever is wrong because environmental conditions, resource supply, climate, and human activity can change K over time.
- Mixing up mutualism and commensalism is wrong because mutualism benefits both species, while commensalism benefits one species and has little effect on the other.
Practice Questions
- 1 A forest plot has 240 oak trees in 12 hectares. What is the population density of oak trees per hectare?
- 2 A rabbit population begins with 500 individuals. During one year, 120 are born, 40 die, 25 immigrate, and 15 emigrate. What is the final population size?
- 3 A population has N = 800, r = 0.2, and K = 1000. Using dN/dt = rN((K - N) / K), what is the logistic growth rate?
- 4 A predator is removed from a community, and one prey species increases so much that plant diversity drops. Explain how this shows the predator may have been a keystone species.