Ecosystems and Food Webs
FoundationAn ecosystem = a community of organisms + their environment. Energy flows from producers (plants) to consumers (herbivores → carnivores); decomposers break down remains. A food web is interlinked food chains — arrows show energy flow. Only ~10% of energy passes to the next level.
- Producers → consumers → decomposers; arrows = energy flow
- Only ~10% of energy passes each level
- More links = more stable ecosystem
§Text Passage
1An ecosystem is the living community plus its environment. Energy starts from the Sun: photosynthesis stores it in plants (producers), which pass it to herbivores (primary consumers) and then to carnivores (secondary, tertiary). Bacteria and fungi decompose remains, returning nutrients to the soil.
2A food web is many food chains interwoven. Arrows point from the eaten to the eater — the direction of energy flow. On a grassland: grass → rabbit → eagle and grass → mouse → snake → eagle — the eagle can eat both rabbit and snake, and the more links, the more stable the web.
3Energy transfer follows the 'one-tenth rule': only about 10% is stored at each step; the rest is spent on respiration, excretion or never eaten. That is why top predators are fewest. If one species crashes, ripples spread across the whole web.
1Summary 重點摘要
An ecosystem = a community of organisms + their environment. Energy flows from producers (plants) to consumers (herbivores → carnivores); decomposers break down remains. A food web is interlinked food chains — arrows show energy flow. Only ~10% of energy passes to the next level.
2Vocabulary cards Vocabulary
Makes its own food by photosynthesis — grass, trees.
Eats other organisms: primary (herbivore), secondary/tertiary (carnivore).
Breaks down dead matter and waste, returning nutrients to the soil — bacteria, fungi.
Many interlinked food chains forming a network.
A community of organisms plus their environment, living and non-living.
A sequence of who eats whom, showing the flow of energy.
A plant-eating animal — the primary consumer, e.g. a rabbit.
An animal-eating consumer — e.g. eagles and snakes.
Energy per trophic level — widest at producers, with ~10% passing up each step.
3Step by step Steps
- Know the rolesIn an ecosystem, producers (plants) make their own food by photosynthesis and start every food chain; primary consumers are the herbivores, such as rabbits; secondary consumers prey on the primary ones, such as snakes; tertiary consumers stand at the top, such as eagles. Bacteria and fungi act as decomposers, breaking down the nutrients in remains and returning them to the soil for producers to absorb again, keeping the nutrients circulating within the system.
- Arrow directionFood-chain arrows point from the eaten to the eater, showing the direction of energy flow: in the chain of grass, rabbit and eagle, the arrow runs from grass to rabbit and then to eagle. Common error: drawing the arrow from the eagle to the rabbit. Remember that the arrow points at whoever benefits. Energy climbs the chain level by level like this, and the arrows never run backwards — the first rule of drawing any food chain.
- Energy pyramidOnly about 10% of the energy passes up each level of a food chain — the rest is lost in respiration, leaves in excretions, or is never eaten at all. Producers therefore hold the greatest numbers and energy while top predators hold the fewest, forming an energy pyramid with a broad base and a narrow top; chains rarely exceed five levels, apex predators are destined to be scarce, and each needs a particularly large range.
- StabilityThe more interwoven the food web, the more stable the ecosystem: an eagle can eat rabbits or snakes, so when one species declines its predators still have alternative foods and no chain snaps at once. A single food chain, once broken, affects every organism on it; systems with few species are the most fragile — one failed link can ripple through the whole web, and recovery takes the longest.
- Living and non-living factorsAn ecosystem consists of living factors — animals, plants, bacteria and fungi — and non-living factors such as sunlight, water, temperature, soil and air. The non-living conditions decide where species live: mangroves grow on mudflats where fresh and salt water meet, home to mudskippers and fiddler crabs. Once the environment changes — pollution in the river or rising water temperature — the whole food web feels the impact.
- Matter cycles, energy flows one wayIn an ecosystem matter cycles endlessly: decomposers return carbon, nitrogen and other nutrients from remains to the soil, where producers absorb them again, so matter is used over and over. Energy, however, flows one way only: it enters producers from the Sun, passes up the food chain level by level, and is finally lost as heat, never to be recycled. This is the fundamental difference between matter and energy flow, and the reason ecosystems need a constant input of solar energy.
4Diagrams Diagrams
5Worked example Example
Grassland: grass → rabbit → eagle. What happens if eagles are heavily hunted?
- Fewer eagles → rabbits lose their predator
- Rabbit numbers soar → grass is over-grazed
- Less grass → rabbits starve, numbers crash
- The whole system oscillates before rebalancing
- Answer: With eagles heavily hunted, rabbits lose their predator and boom at first, over-grazing the grass; once grass declines, rabbits starve and their numbers crash, so the system oscillates before settling again. Do not stop at 'more rabbits': give the chain 'predators down → rabbits up → grass down → rabbits crash' (a domino effect) and note that the web finally reaches a new balance. Point out that 'the more complex the food web, the more stable the ecosystem' for extra credit.
6Common mistakes Mistakes
7Quick check Quick check
Q1In 'grass → locust → frog → snake', which consumer level is the frog?
Q2What is the role of decomposers?
Q3Why is a food web more stable than a single food chain?
Q4What is the ultimate source of energy in an ecosystem?
Q5Why does a food chain rarely have more than five levels?
8Recap Recap
- Producers → consumers → decomposers; arrows = energy flow
- Only ~10% of energy passes each level
- More links = more stable ecosystem
- Arrows point from the eaten to the eater — the direction of energy flow
- One-tenth rule: only ~10% of energy passes up each level