Ecosystems and feeding relationships
An ecosystem is a community of organisms together with their non-living surroundings. Energy enters it as sunlight, captured by producers (plants) in photosynthesis. It then passes along a food chain: producer → primary consumer (herbivore) → secondary consumer → and so on, each stage a trophic level. Many interlinked chains form a food web.
Energy flow
Energy is lost at each trophic level — used in respiration, movement and heat, or in parts not eaten — so only about 10% passes to the next level. This is why food chains are short (rarely more than four or five links) and why there is far more mass of producers than of top predators. Decomposers (bacteria and fungi) break down dead material, returning nutrients to the soil.
Why food chains are short
Roughly 90% of the energy is lost at each step (respiration, heat, waste). After a few levels there is too little left to support another, so food chains rarely exceed four or five links.
The carbon and nitrogen cycles
Nutrients, unlike energy, are recycled. In the carbon cycle, photosynthesis removes CO₂ from the air, respiration and combustion return it, and decomposers release it from dead matter. In the nitrogen cycle, nitrogen-fixing bacteria turn nitrogen gas into compounds plants can use, decomposers release it from dead organisms, and denitrifying bacteria return it to the air.
Human impact and conservation
Human activity disturbs ecosystems: burning fossil fuels raises CO₂ and drives climate change, deforestation destroys habitats, and pollution and overhunting reduce biodiversity. Conservation — protecting habitats, replanting forests, controlling pollution and managing resources sustainably — aims to preserve species and the services ecosystems provide.
A worked example
A food chain is: grass → grasshopper → frog → snake. Explain why the snake population is much smaller than the grass population.
- Grass is the producer, capturing energy from sunlight.Energy enters the chain at the producer.
- Only about 10% of the energy passes to each next level.Most is lost in respiration, heat and waste.
- So less energy reaches grasshoppers, less again the frogs, and least of all the snakes.Energy dwindles up the chain.
- With little energy available, only a small number of snakes can be supported.Population size is limited by the energy available.
Energy is lost (~90%) at each trophic level, so little reaches the top — supporting far fewer snakes than the abundant grass.
What limits a population
A population cannot grow forever — its size is limited by the environment. The availability of food, water and space, the number of predators, the spread of disease, and competition (between and within species) all keep populations in check. The largest population an environment can support long-term is its carrying capacity. When a population grows beyond it, shortages and predation bring it back down, so numbers tend to fluctuate around a balance.
Pyramids of number and biomass
Feeding relationships can be drawn as pyramids. A pyramid of numbers shows how many organisms are at each trophic level; it is usually a pyramid shape but can be odd (one large tree supports many insects). A pyramid of biomass shows the total mass of living material at each level, and — because energy is lost at each step — it is almost always a true pyramid, widest at the producers and narrowing toward the top predators.
Why biomass pyramids taper
About 90% of energy is lost at each trophic level, so there is less living material to support the next level up. That is why a pyramid of biomass narrows steadily from producers to top predators.