Ecosystems and human activity

Ecosystems — key terms

• Ecosystem: all the living organisms in an area (community) plus their non-living (abiotic) environment.
• Population: all individuals of one species in an area.
• Community: all populations of all species in an area.
• Habitat: the place where an organism lives.
• Niche: the role of a species within its ecosystem (what it eats, what eats it, when it is active, etc.).

Food chains and energy flow

A food chain shows the direction of energy flow: Producer → Primary consumer → Secondary consumer → Tertiary consumer

Producers (plants, algae): convert light energy into chemical energy via photosynthesis. Consumers: feed on other organisms (herbivores, carnivores, omnivores). Decomposers (bacteria, fungi): break down dead organic matter → return minerals to soil.

Energy transfer efficiency

Only ~10% of energy is transferred between each trophic level. The rest is lost as:

• Heat from respiration
• Undigested waste (faeces, urine)
• Energy used in life processes (movement, growth, reproduction)

This is why food chains rarely exceed 4–5 trophic levels, and why there are fewer organisms at higher levels (pyramid of numbers/biomass).

The carbon cycle

Carbon moves through ecosystems via:

1. Photosynthesis: CO₂ from atmosphere fixed into organic molecules (glucose) by plants.
2. Respiration: all organisms release CO₂ back to atmosphere.
3. Feeding: carbon passed along the food chain.
4. Decomposition: decomposers break down dead matter → CO₂ + minerals released.
5. Combustion: burning fossil fuels/wood releases stored carbon as CO₂.
6. Fossilisation: carbon locked in fossil fuels (coal, oil, gas) over millions of years.

Human impact: burning fossil fuels increases atmospheric CO₂ → enhanced greenhouse effect → global warming.

The nitrogen cycle

Plants need nitrogen to make amino acids and proteins. Nitrogen gas (N₂) is very unreactive — most organisms cannot use it directly.

1. Nitrogen fixation: Rhizobium bacteria (in root nodules of legumes) convert N₂ → ammonium ions (NH₄⁺); lightning also fixes N₂.
2. Nitrification: nitrifying bacteria convert ammonium → nitrite → nitrate (NO₃⁻) in soil — usable by plants.
3. Absorption: plants absorb nitrates through roots; use them to make amino acids and proteins.
4. Feeding: nitrogen passed to consumers through proteins.
5. Decomposition: decomposers (bacteria, fungi) break down dead organisms/urea → ammonium ions.
6. Denitrification: denitrifying bacteria convert nitrates back to N₂ (in waterlogged/anaerobic soils) — removes nitrogen from the cycle.

Human impacts on ecosystems

Eutrophication

1. Fertiliser (nitrates + phosphates) washed into lake/river.
2. Algae bloom (rapid growth → algal bloom).
3. Algae block light to submerged plants → plants die.
4. Decomposers break down dead plants → population explosion of decomposers.
5. Decomposers use up dissolved oxygen (aerobic respiration).
6. Fish and other aquatic organisms die due to lack of oxygen (deoxygenation).

Conservation

Why conserve?

• Moral: species have intrinsic value.
• Ecological: biodiversity keeps ecosystems stable.
• Medical: many drugs derived from wild species.
• Economic: tourism, genetic resources.

Methods:

• Protected areas (nature reserves, national parks, marine reserves)
• Captive breeding programmes → reintroduction
• Seed banks (e.g. Svalbard Global Seed Vault)
• Legislation (CITES — trade in endangered species)
• Habitat restoration