Notes
Parts of an ecosystem
- Biotic factors are living; abiotic factors are nonliving (sunlight, temperature, water, soil). Population, community, and ecosystem are increasing levels of organization.
- A habitat is where an organism lives; its niche is its role.
Energy flow
- Producers capture sunlight. Consumers eat producers or other consumers. Decomposers recycle nutrients.
- Energy flows one way and is lost as heat; matter cycles.
- The 10% rule: only about 10% of energy passes to the next trophic level. Example: $10{,}000 \rightarrow 1{,}000 \rightarrow 100 \rightarrow 10$ kcal.
Biogeochemical cycles
- Water: evaporation, transpiration, condensation, precipitation, runoff.
- Carbon: photosynthesis removes $CO_2$; respiration, decomposition, and burning fossil fuels release it. Forests and oceans are sinks.
- Nitrogen: fixation, nitrification, assimilation, ammonification, denitrification. Fertilizer adds extra nitrogen.
- Phosphorus: no gas phase; slow cycle through rock, soil, and water.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Producers are level 1, primary consumers level 2, secondary consumers level 3, tertiary consumers level 4.
Level 1 to 2 gives 40,000 kcal; level 2 to 3 gives 4,000 kcal.
Photosynthesis takes $CO_2$ from the air, a key step in the carbon cycle.
Water is nonliving, so its shortage is an abiotic limiting factor.
Fertilizers add nitrogen and phosphorus compounds in far greater amounts than natural cycles.
Food webs are more realistic than single chains.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A grassland ecosystem has producers (grasses) containing 100,000 kcal of energy per year. Mice eat the grass, snakes eat the mice, and hawks eat the snakes. Assume the 10% rule applies at each step.
(a) Identify the trophic level of each of the four groups.
Show the answer and scoring guide
Grasses: producers, level 1. Mice: primary consumers, level 2. Snakes: secondary consumers, level 3. Hawks: tertiary consumers, level 4.
Check off the points you earned:
(b) Calculate the energy available to the hawks.
Show the answer and scoring guide
Mice: $100{,}000 \times 0.10 = 10{,}000$ kcal. Snakes: 1,000 kcal. Hawks: 100 kcal.
Check off the points you earned:
(c) Explain where the rest of the energy goes and why there are few hawks.
Show the answer and scoring guide
Most energy is used in respiration for life processes and is lost as heat, or is left in waste and uneaten parts. Because so little energy reaches the top level, the grassland can support only a small number of hawks.
Check off the points you earned: