Ecosystem components
An ecosystem includes biotic factors (living things -- plants, animals, microorganisms) and abiotic factors (non-living things -- sunlight, temperature, water, soil). These factors interact constantly: abiotic conditions shape which organisms can survive, and organisms in turn affect their physical environment.
Food chains and food webs
A food chain shows a single feeding path, always starting with a producer: producer primary consumer secondary consumer tertiary consumer. A food web links many overlapping food chains together, showing that most organisms eat (and are eaten by) more than one other species.
Example 1: In the food chain grass grasshopper frog snake, identify the producer and the secondary consumer.
Grass is the producer (makes its own food via photosynthesis). The grasshopper is the primary consumer (eats the producer), the frog is the secondary consumer (eats the primary consumer), and the snake is the tertiary consumer.
Energy flow through an ecosystem
Energy enters an ecosystem via producers capturing sunlight, then flows through each feeding level (trophic level) as organisms are eaten. At each transfer, only about 10% of the energy is passed on to the next level -- the rest is lost as heat, used for movement, or lost in waste. This is why food chains rarely have more than 4-5 levels, and why there are always far more producers than top predators.
Example 2: A food chain has 10,000 units of energy available at the producer level. Estimate how much energy reaches the tertiary consumer (three transfers later), using the 10% rule.
Each transfer keeps about 10%: units -- only about 10 units of the original energy reach the tertiary consumer.
Nutrient cycles
The carbon cycle moves carbon between the atmosphere (as ), living organisms (via photosynthesis and respiration), and long-term stores like fossil fuels. The nitrogen cycle moves nitrogen between the atmosphere, soil, and living things via nitrogen fixation (bacteria converting atmospheric nitrogen into usable forms), nitrification, and decomposition -- since plants and animals can't use atmospheric nitrogen gas directly.