Louisiana LEAP 2025 Biology LS2 (Ecology and Interdependence): a complete overview of energy flow, the cycling of matter, carrying capacity, ecosystem stability, and human impact

What the LS2 ecosystems core idea demands

Ecosystems: Interactions, Energy, and Dynamics (LS2) is the ecology core idea of the Louisiana LEAP 2025 Biology course. This guide runs from how energy flows through organisms, through how matter cycles, to how populations are controlled, how ecosystems stay stable and recover, and how humans affect them and can reduce the harm. The recurring crosscutting concepts are energy and matter (flow and cycling) and systems and system models (the ecosystem as interacting parts). Several LS2 items ask you to use mathematics (the ten percent rule, growth curves) or design solutions.

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 when producers capture sunlight by photosynthesis. Consumers get energy by eating; decomposers break down dead matter. A food web links many food chains. Only about ten percent of the energy passes to the next trophic level (the rest is lost as heat through respiration and in life processes), so energy pyramids narrow upward and chains are short. Energy flows one way and must be resupplied by the sun.

The cycling of matter

Unlike energy, matter cycles. In the carbon cycle, photosynthesis removes carbon dioxide and fixes carbon into glucose, while respiration (and decomposition and combustion) returns it. Decomposers release nutrients from dead matter. The nitrogen cycle uses bacteria to make atmospheric nitrogen usable. The key contrast: matter cycles, energy flows one way.

Population dynamics and carrying capacity

Carrying capacity is the maximum population an environment can sustain. Limiting factors (food, water, space, predators, disease, competition) set it. Populations grow exponentially (J-curve) when resources are plentiful, then level off logistically (S-curve) at the carrying capacity. Density-dependent factors intensify with crowding; density-independent factors (weather) do not.

Ecosystem stability and resilience

A stable ecosystem stays steady; a resilient one recovers from disturbance. Higher biodiversity increases both, because some species are more likely to survive a disturbance and there are more pathways in the food web. A keystone species has an outsized effect, so its loss causes large changes. Species interactions (predation, competition, symbiosis) help regulate populations.

Human impact

Humans reduce biodiversity through habitat loss, pollution, climate change, invasive species, and overexploitation, which lowers ecosystem stability. HS-LS2-7 asks you to design a solution (protect or restore habitat, reduce pollution at its source, control invasives), then evaluate it by measuring an outcome and refine it.

Check your knowledge

A mix of recall and reasoning questions covering the LS2 ecosystems core idea. Attempt them under timed conditions, then check against the solutions.

1. State what producers do and why they are essential to a food web. (2 marks)
2. About what percentage of energy passes from one trophic level to the next? (1 mark)
3. A producer level holds 20,000 units of energy. Estimate the energy available to a primary consumer. (1 mark)
4. Name the two processes that move carbon between living things and the atmosphere. (2 marks)
5. Explain why matter cycles but energy flows one way. (2 marks)
6. Define carrying capacity. (1 mark)
7. Name two limiting factors and state whether each is density-dependent or density-independent. (2 marks)
8. Explain why high biodiversity makes an ecosystem more resilient. (2 marks)
9. Name two human activities that reduce biodiversity. (2 marks)