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Ecology & Population Dynamics Cheat Sheet
A printable reference covering population growth, carrying capacity, survivorship curves, community interactions, energy flow, and nutrient cycles for grades 10-11.
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Ecology studies how organisms interact with each other and with the physical environment. Population dynamics focuses on how population size changes over time due to births, deaths, immigration, emigration, and limiting factors. This cheat sheet helps students connect ecological patterns to formulas, graphs, and real biological examples. It is useful for reviewing population growth models, community relationships, and ecosystem energy flow. The most important ideas include population density, exponential growth, logistic growth, carrying capacity, and limiting factors. Students should recognize that real populations rarely grow forever because resources, predators, disease, and competition affect survival and reproduction. Energy moves through ecosystems by feeding relationships, while matter cycles through systems such as the carbon and nitrogen cycles. Strong ecology answers use both quantitative evidence and biological reasoning.
Key Facts
- Population density = number of individuals / area or volume.
- Population change = births + immigration - deaths - emigration.
- Per capita growth rate r = (births - deaths) / initial population when immigration and emigration are ignored.
- Exponential growth is modeled by ΔN/Δt = rN and produces a J-shaped curve when resources are unlimited.
- Logistic growth is modeled by ΔN/Δt = rN((K - N) / K), where K is the carrying capacity.
- Mark-recapture population estimate = (number marked first sample × total caught second sample) / number marked in second sample.
- Only about 10 percent of energy is transferred from one trophic level to the next in a typical energy pyramid.
- Density-dependent limiting factors, such as disease, competition, and predation, become stronger as population density increases.
Vocabulary
- Population
- A population is a group of individuals of the same species living in the same area at the same time.
- Carrying Capacity
- Carrying capacity is the maximum population size an environment can support over time with available resources.
- Limiting Factor
- A limiting factor is any biotic or abiotic condition that restricts population growth.
- Niche
- A niche is the role of a species in its ecosystem, including how it uses resources and interacts with other organisms.
- Trophic Level
- A trophic level is a feeding position in a food chain or food web, such as producer, primary consumer, or secondary consumer.
- Survivorship Curve
- A survivorship curve is a graph showing the proportion of individuals in a population that survive to each age.
Common Mistakes to Avoid
- Using birth rate alone as population growth is wrong because deaths, immigration, and emigration also change population size.
- Confusing exponential and logistic growth is wrong because exponential growth assumes unlimited resources, while logistic growth slows near carrying capacity.
- Treating carrying capacity as a fixed number is wrong because K can change when climate, food supply, disease, or habitat conditions change.
- Reversing energy flow in a food web is wrong because energy moves from producers to consumers and is lost as heat at each transfer.
- Calling all limiting factors density-dependent is wrong because events such as drought, fire, and extreme temperature can affect populations regardless of density.
Practice Questions
- 1 A pond contains 240 frogs in an area of 60 square meters. What is the population density in frogs per square meter?
- 2 A population starts with 500 rabbits. During one year, 120 are born, 40 die, 25 immigrate, and 15 emigrate. What is the final population size?
- 3 In a mark-recapture study, 80 fish are marked in the first sample. Later, 100 fish are caught, and 20 are marked. Estimate the total fish population.
- 4 A deer population rises quickly after predators are removed, then food becomes scarce and the population levels off. Explain which growth model fits this pattern and why.