North Carolina Biology EOC: Ecology and Ecosystems - a complete overview of energy flow, the cycling of matter, population dynamics, ecosystem stability, and human impact

What the ecology and ecosystems content demands

The Ecosystems strand is about interactions, energy, and dynamics at the level of communities and ecosystems. This guide runs from how energy flows through food webs, through how matter cycles, how populations are regulated, what keeps ecosystems stable, to how humans affect them. The recurring crosscutting concepts are energy and matter, systems and system models, and stability and change.

This guide ties together the matching topic pages, each with its own practice questions: energy flow and food webs, the cycling of matter, population dynamics and carrying capacity, ecosystem stability and resilience, and human impact on ecosystems.

Energy flow and food webs

Energy enters from the Sun; producers capture it by photosynthesis, and it passes to consumers along food chains and food webs. Each feeding level is a trophic level. Only about 10 percent of energy passes to the next level (the ten percent rule), because most is lost as heat from respiration. This is shown by an energy pyramid and limits food chains to a few levels.

The cycling of matter

Matter cycles while energy flows. In the carbon cycle, photosynthesis takes carbon dioxide in and respiration releases it (with combustion adding more). In the nitrogen cycle, nitrogen-fixing bacteria convert nitrogen gas into usable forms, because organisms cannot use nitrogen gas directly. The water cycle moves water by evaporation, condensation, and precipitation. Decomposers recycle matter in every cycle.

Population dynamics

Carrying capacity is the maximum population an environment can support. Limiting factors regulate growth: density-dependent (food, predators, disease, worse with crowding) and density-independent (disasters, weather, regardless of density). Exponential growth is a J-shape; logistic growth is an S-shape that levels off at the carrying capacity.

Ecosystem stability and resilience

Ecosystems stay stable through interactions (competition, predation, and symbiosis: mutualism, commensalism, parasitism) and biodiversity, which increases resilience. A keystone species has an outsized effect, so removing it changes the whole ecosystem. After a disturbance, ecosystems recover through ecological succession, demonstrating resilience.

Human impact

Humans harm ecosystems through pollution, habitat destruction, invasive species (which spread because they lack natural predators), overuse of resources, and climate change (from carbon dioxide released by burning fossil fuels). Conservation reduces the harm: protecting habitats, cutting pollution, conserving resources, using renewable energy, and controlling invasive species, all to protect biodiversity.

Check your knowledge

A mix of recall and reasoning questions covering the ecology and ecosystems content. Attempt them under timed conditions, then check against the solutions.

1. State the original source of energy for almost all ecosystems and how it enters the food web. (2 marks)
2. An energy pyramid has 10,000 units in the producers. Estimate the energy two levels up and explain. (2 marks)
3. Name the two processes that cycle carbon between organisms and the atmosphere. (2 marks)
4. Explain why nitrogen-fixing bacteria are essential in the nitrogen cycle. (2 marks)
5. Define carrying capacity. (1 mark)
6. State one density-dependent and one density-independent limiting factor. (2 marks)
7. Name the three types of symbiosis and who benefits in each. (3 marks)
8. After a fire, an ecosystem gradually rebuilds its community. Name this process. (1 mark)
9. Name three human activities that harm ecosystems and one way to reduce harm. (4 marks)