Biology: Cells to AP Biology · Module 8 of 15
Ecology
How energy and matter move through ecosystems, and how populations and human activity interact with them.
Levels of Ecological Organization & Biomes
- Levels build up from organism → population (same species, same area) → community (all species in an area) → ecosystem (community + physical environment) → biome → biosphere.
- Biomes are large regions characterized by climate and dominant vegetation/animal life, such as desert, tundra, or tropical rainforest.
All the deer, oak trees, and mushrooms living in one forest — but not the soil or climate — would be considered what level of organization?
Solution. A community: all the populations of different species living in the same area. Adding the nonliving environment would make it an ecosystem.
Energy Flow & Trophic Levels
- Energy flows one direction through an ecosystem: producers → primary consumers (herbivores) → secondary/tertiary consumers (carnivores), with decomposers recycling nutrients from all levels.
- Only about 10% of energy transfers from one trophic level to the next (the rest is lost as heat or used for the organism's own growth, movement, and reproduction, or is never consumed/digested by the next level), which is why food chains rarely exceed 4–5 levels.
Why are there usually far fewer top predators (like lions) than there are producers (like grass) in an ecosystem?
Solution. Because only about 10% of energy transfers between trophic levels, there isn't enough energy available at the top of the food chain to support a large population there.
Population Ecology & Carrying Capacity
- Carrying capacity is the maximum population size an environment can sustainably support given its resources.
- Populations grow exponentially when resources are unlimited, but growth slows and levels off (logistic growth) as they approach carrying capacity due to limiting factors like food, space, predators, or disease.
A deer population in a forest grows quickly at first, then levels off near a stable number for years. What does this pattern describe?
Solution. Logistic growth — the population's growth slowed as it approached the environment's carrying capacity due to limited resources.
Nutrient Cycles & Human Impact
- Matter, unlike energy, is recycled through ecosystems via biogeochemical cycles, such as the carbon, nitrogen, and water cycles.
- Human activities — burning fossil fuels, deforestation, fertilizer runoff — can disrupt these natural cycles, such as adding excess CO2 to the carbon cycle or excess nutrients causing algal blooms.
How does burning fossil fuels affect the carbon cycle?
Solution. It releases carbon that was stored underground for millions of years back into the atmosphere as CO2 much faster than natural processes can remove it, increasing atmospheric CO2 levels.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. Put these levels of organization in order from smallest to largest: community, ecosystem, organism, population.
2. About what percentage of energy transfers from one trophic level to the next?