Ecosystem

An ecosystem is a unit of nature in which living organisms (the biotic community) interact with one another and with their non-living surroundings (the abiotic environment). A pond, a forest, a grassland and a desert are all examples of ecosystems. Ecosystems may be natural (forest, pond, ocean) or artificial (a crop field, an aquarium).

Every ecosystem has two kinds of components:

• Abiotic components — the non-living parts: sunlight, temperature, water, air, soil and minerals.
• Biotic components — the living organisms, grouped by how they get their food:
  • Producers (autotrophs) — green plants (and algae) that make their own food by photosynthesis. They are the foundation of every ecosystem.
  • Consumers (heterotrophs) — organisms that eat others. They include herbivores (eat plants), carnivores (eat animals) and omnivores (eat both).
  • Decomposers — bacteria and fungi that break down dead bodies and wastes, releasing nutrients back into the soil. They are nature's recyclers.

The functioning of an ecosystem depends on the flow of energy (from the Sun, through producers and consumers) and the cycling of nutrients (through decomposers back to producers). This is why every component, even tiny decomposers, is essential.

Energy and food pass from one organism to another in a sequence called a food chain. For example: grass → grasshopper → frog → snake → eagle. Each step in a food chain is a trophic (feeding) level: producers are the first level, herbivores the second, and so on. In reality, organisms eat many kinds of food, so many food chains link together into a food web — a more realistic picture of who eats whom in an ecosystem.

The ultimate source of energy for almost all ecosystems is the Sun. Producers capture sunlight by photosynthesis and store it as chemical energy in food; this energy then flows up the food chain as one organism eats another. A key rule is that energy flow is one-way (it does not cycle back) and that only about 10% of the energy at one level passes to the next — the rest is lost as heat and in life processes. This is the 10% law. Because so much energy is lost at each step, food chains are usually short (4–5 levels).

This pattern can be shown by ecological pyramids — diagrams of the numbers, biomass or energy at each trophic level. The pyramid of energy is always upright (energy decreases as we go up), because of the 10% law. (Pyramids of number or biomass can sometimes be inverted, but energy never can be.)

Unlike energy, which flows one way and is lost, nutrients (like carbon, nitrogen and water) are used again and again — they move in cycles between the living organisms and the environment. These are called biogeochemical cycles. Two important examples:

• Carbon cycle — carbon moves between the air (as CO₂), plants and animals, and back. Plants take in CO₂ for photosynthesis; respiration, decay and burning of fuels release it again.
• Nitrogen cycle — nitrogen from the air is fixed into the soil (by microbes like Rhizobium), taken up by plants, passed to animals, and returned to the soil and air by decomposers and other microbes.

Decomposers play a vital part in all nutrient cycles by breaking down dead matter and releasing the nutrients back to be reused.

Healthy ecosystems give humanity many free benefits called ecosystem services. These include: producing oxygen and food, purifying air and water, pollinating crops, forming and enriching soil, controlling floods, regulating climate, and supporting wildlife and recreation. Forests, for instance, give us oxygen, timber, medicines, clean water and a stable climate. Because these services are so valuable yet often taken for granted, protecting ecosystems is essential for our own survival. Damaging an ecosystem — through pollution, deforestation or over-use — harms the very services we depend on.

An ecosystem can be measured by how much new living material it produces. Productivity is the rate at which this biomass is made. There are two kinds:

• Primary productivity — the rate at which producers (green plants) make biomass by photosynthesis. The total amount made is the gross primary productivity (GPP); what is left after the plants use some in respiration is the net primary productivity (NPP), the food actually available to consumers.
• Secondary productivity — the rate at which consumers store energy as new biomass.

Decomposition is the process by which decomposers (bacteria and fungi) break down dead bodies and wastes (called detritus) into simple substances, releasing nutrients back to the soil. It involves steps such as fragmentation (breaking detritus into pieces), leaching, catabolism (enzyme breakdown), humification (forming dark humus) and mineralisation (releasing minerals). Decomposition is essential for recycling nutrients.

Ecological succession is the gradual and orderly change in the community of an area over time, until a stable, mature community called the climax community is reached. Primary succession begins on a bare area where no life existed before (such as bare rock or a new volcanic island) — pioneer species like lichens arrive first, followed by mosses, herbs, shrubs and finally trees. Secondary succession occurs on a disturbed area that already had soil and life (such as an abandoned field or a burnt forest), and is faster because soil is already present.