Texas STAAR Biology Reporting Category 5 (Interdependence within Environmental Systems): a complete overview of ecology, energy flow, matter cycling, populations, and human impact

What Reporting Category 5 actually demands

Reporting Category 5, Interdependence within Environmental Systems, is the ecology core of STAAR Biology and about a fifth of the test points. It covers how the living and non-living parts of the environment are organized, how energy and matter move through ecosystems, how populations are controlled, and how communities change (including under human pressure). The recurring themes are energy and matter and stability and change, and many items are graph-based (energy pyramids, population curves, predator-prey graphs).

This guide ties together the matching dot-point pages, each with its own practice questions: levels of ecological organization, energy flow and food webs, the cycling of matter, population dynamics and carrying capacity, and ecological succession and human impact.

Levels and factors

Ecology organizes life into levels from smallest to largest: organism, population, community, ecosystem, biome, biosphere. An ecosystem is made of biotic factors (living parts) and abiotic factors (non-living parts: water, light, temperature, soil). Knowing which level a question is about tells you which interactions to consider.

Energy flow

Energy enters from the Sun, is captured by producers, and flows one way along a food chain to consumers, with arrows pointing the way energy flows. At each trophic level, only about 10 percent of the energy passes on; the rest is used in life processes and lost as heat. This is why an energy pyramid narrows upward and why food chains are short. You should be able to estimate energy down a pyramid using the 10 percent rule.

Matter cycling

Unlike energy, matter cycles. In the carbon cycle, photosynthesis removes carbon dioxide and respiration (and decomposition and combustion) returns it. In the nitrogen cycle, bacteria make nitrogen usable, it is built into proteins, and decomposers release it. The water cycle moves water through evaporation, condensation, and precipitation. Decomposers are essential across all cycles, returning nutrients so producers can reuse them.

Populations and change

Populations are limited by limiting factors (food, water, space, predators, disease) and level off at the carrying capacity, the maximum the environment supports. Predator and prey populations cycle: more prey lets predators rise, more predators reduce the prey, and so on. Communities change over time through succession (primary, from bare rock; secondary, where soil remains). Human activities (pollution, habitat destruction, overuse, introduced species) reduce biodiversity, but conservation can reduce the impact.

Check your knowledge

A mix of recall, calculation, and reasoning questions covering Reporting Category 5. Attempt them under timed conditions, then check against the solutions.

1. Define a population and a community. (2 marks)
2. Classify each as biotic or abiotic: a frog, sunlight, a tree, soil temperature. (2 marks)
3. Explain why only about 10 percent of energy passes to the next trophic level. (2 marks)
4. A food chain's producers store $8\,000$ kJ. Estimate the energy available to the primary consumers. (1 mark)
5. Explain why matter can be recycled in an ecosystem but energy cannot. (2 marks)
6. State the role of decomposers in the cycling of matter. (2 marks)
7. Define carrying capacity. (1 mark)
8. Explain why a predator population falls after the prey population falls. (2 marks)
9. State the difference between primary and secondary succession. (2 marks)