Ecosystem: Structure, Food Chains and Energy Flow

What is an ecosystem?

An ecosystem is a functional unit in which living organisms (biotic components) interact with each other and with their physical surroundings (abiotic components such as sunlight, water, soil and temperature). The term was coined by A.G. Tansley (1935).

Components

• Producers (autotrophs): green plants and phytoplankton that fix solar energy through photosynthesis.
• Consumers (heterotrophs): herbivores (primary), carnivores (secondary/tertiary) and omnivores.
• Decomposers: bacteria and fungi that break down dead matter and recycle nutrients.

Food chains and food webs

• Grazing food chain starts with green plants: grass → grasshopper → frog → snake → eagle.
• Detritus food chain starts with dead organic matter: leaf litter → earthworm → bird.
• Interlinked food chains form a food web, which makes an ecosystem more stable.

Energy flow: the 10% law

Energy flow is unidirectional (sun → producers → consumers) and is lost as heat at each step. According to Lindeman's 10% law, only about 10% of energy passes to the next trophic level. That is why food chains rarely have more than 4–5 links.

Ecological pyramids

• Pyramid of energy: always upright.
• Pyramid of numbers: can be inverted (e.g. a single tree supporting many insects).
• Pyramid of biomass: inverted in aquatic ecosystems (phytoplankton biomass is smaller than zooplankton at a given time).

Biomagnification

Non-biodegradable pollutants such as DDT and mercury increase in concentration at each higher trophic level. Top predators (e.g. fish-eating birds) are affected most.

Prelims tips

1. Energy flow is one-way; nutrient cycling is cyclic.
2. Remember which pyramids can be inverted and where.

Analogy

An ecosystem is like a relay race where the baton keeps shrinking. The sun hands energy to plants, plants hand it to herbivores, and so on — but at every hand-over roughly 90% of the baton is dropped as heat. After four or five runners there is almost nothing left to pass, which is why food chains are short.