CB9 — Ecosystems and Material Cycles

Ecosystem terminology

Food chains and food webs

Food chain: shows feeding relationships and energy flow. Always starts with a producer (plant or other photosynthetic organism). Example: grass → rabbit → fox → decomposers.

Trophic levels: Producer (T1) → Primary consumer (T2) → Secondary consumer (T3) → Tertiary consumer (T4).

Energy transfer: only 10–20% of energy at one trophic level is passed to the next (Edexcel: assume 10% for calculations). Losses at each step: heat (respiration), excretion/egestion, material in uneaten parts (bones, roots).

Biomass pyramids: always pyramid-shaped (less biomass at higher levels). Energy pyramids are always pyramid-shaped. Numbers pyramids can be inverted (e.g., one tree → many insects).

Decomposers: bacteria and fungi that break down dead organic material (detritus). Secrete enzymes → absorb soluble products. Essential for nutrient recycling.

The carbon cycle

Carbon moves between the atmosphere (as CO₂), living organisms, and the lithosphere:

1. Photosynthesis: CO₂ absorbed by plants → fixed into organic molecules (glucose).
2. Respiration: CO₂ released by all organisms.
3. Feeding: carbon passed along food chains.
4. Decomposition: bacteria and fungi break down dead matter → CO₂ released by their respiration.
5. Combustion: burning fossil fuels and biomass releases CO₂ stored for millions of years.
6. Fossilisation: small amounts of carbon locked in coal, oil, gas over geological time.

Carbon sinks: oceans (dissolved CO₂ + marine organisms' shells as calcium carbonate) and forests.

Human impact: burning fossil fuels + deforestation → rising atmospheric CO₂ → enhanced greenhouse effect → global warming/climate change.

The water cycle

1. Evaporation: water evaporates from oceans, lakes, rivers.
2. Transpiration: water vapour released by plants through stomata.
3. Condensation: water vapour cools → forms clouds.
4. Precipitation: rain, snow, sleet falls.
5. Surface run-off and groundwater: water returns to oceans/lakes.

The nitrogen cycle

Plants need nitrogen to make proteins and DNA (amino acids contain N). Nitrogen gas (N₂) makes up 78% of air but is inert — most organisms cannot use it directly.

Nitrogen fixation: conversion of N₂ into ammonia (NH₃) or nitrates (NO₃⁻):

• By nitrogen-fixing bacteria in soil (free-living, e.g., Azotobacter) or in root nodules of legumes (e.g., Rhizobium in peas/beans — mutualistic).
• Lightning (small amount).

Nitrification: soil bacteria convert ammonium (NH₄⁺) → nitrites → nitrates. Nitrates absorbed by plant roots.

Denitrification: denitrifying bacteria convert nitrates → N₂ (returns to atmosphere). Occurs in waterlogged/anaerobic soils.

Decomposition: decomposers break down proteins in dead organisms → ammonium ions released (ammonification).

Biodiversity

Biodiversity = variety of different species in an ecosystem AND variety of alleles within a species (genetic diversity).

High biodiversity → stable ecosystems (more resilience to change; more redundancy in food webs).

Threats to biodiversity: habitat destruction, invasive species, overexploitation, pollution, climate change.

Conservation: protected areas (nature reserves, national parks), captive breeding programmes, seed banks, international agreements (CITES, Convention on Biological Diversity).

Food security: sufficient food of adequate nutrition for all people. Threatened by population growth, climate change, water scarcity, food waste, use of land for biofuels. Solutions: GM crops, vertical farming, sustainable fishing (MSC certification), reduced meat consumption.

Human impact on ecosystems

• Deforestation: reduces biodiversity, increases CO₂ (less photosynthesis, more combustion/decomposition), increases surface run-off, reduces water recycling.
• Monoculture: reduces biodiversity, increases vulnerability to disease/pests.
• Eutrophication: excess fertiliser leaches into waterways → nitrates/phosphates → algal bloom → algae block light → aquatic plants die → bacteria decompose dead plants using O₂ (BOD increases) → fish and invertebrates die through lack of dissolved O₂.
• Pesticides: can bioaccumulate up food chains (biomagnification), affecting top predators.