Energy Flow (The 10% Rule): Energy enters ecosystems via sunlight and is converted by producers, but only about 10% of the energy at one trophic level is passed to the next. The remaining 90% is lost primarily as metabolic heat or through undigested material, which limits the number of trophic levels an ecosystem can support.
Nutrient Cycling: Unlike energy, matter (such as carbon, nitrogen, and phosphorus) is not lost but cycles continuously through the biotic and abiotic components. Decomposers play a critical role here by breaking down organic waste into inorganic nutrients that producers can reuse.
Photosynthesis and Respiration: These are the two fundamental chemical processes driving ecosystem energy. Photosynthesis () stores energy in glucose, while cellular respiration releases that energy for biological work.
| Feature | Habitat | Niche |
|---|---|---|
| Definition | Physical location/environment | Functional role and resource use |
| Focus | Where the organism lives | How the organism survives and reproduces |
| Scope | Can be shared by many species | Unique to a single species in a stable system |
| Example | A tropical rainforest floor | A decomposer breaking down leaf litter |
The 10% Calculation: When asked to calculate energy transfer, always move the decimal point one place to the left for each step up the pyramid. For example, if producers have , primary consumers have , and secondary consumers have .
Identify the Source: Always look for the primary energy source in a diagram; if sunlight is present, the first biological box must be a producer (autotroph). If the diagram starts with organic matter, you are likely looking at a detrital food web.
Check the Units: Biomass is usually measured in mass per area (e.g., ), while productivity is measured in mass per area per time (e.g., ). Confusing these two is a common way to lose marks.
Verify Logic: If a question asks about the impact of removing a top predator, remember to analyze the 'top-down' effect, where the immediate prey population usually increases, leading to a decrease in the level below that.
Energy Cycling: A very common mistake is stating that energy 'cycles' through an ecosystem. Energy is never recycled; it enters as light and leaves as heat, necessitating a constant external supply from the sun.
Decomposers vs. Detritivores: While often used interchangeably, detritivores (like earthworms) physically ingest decaying matter, whereas decomposers (like fungi) secrete enzymes to break down matter externally and then absorb the nutrients.
Apex Predator Energy: Students often assume apex predators have the most energy because they are at the top. In reality, they have access to the least amount of the original solar energy captured by the ecosystem due to cumulative losses at each level.
