Ecosystems: Structure, Function and Biogeochemical Cycles

An ecosystem is a self-organising community of organisms interacting with one another and with their non-living surroundings, exchanging matter and energy in regulated cycles. The term was coined by British botanist Sir Arthur Tansley in 1935.

Components

1. Abiotic — sunlight, temperature, water, soil, atmospheric gases, nutrients (N, P, K, micronutrients).
2. Biotic — three functional groups:
   • Producers (autotrophs): green plants, algae, cyanobacteria that fix solar energy through photosynthesis.
   • Consumers (heterotrophs): primary (herbivores), secondary (carnivores), tertiary (top carnivores), omnivores.
   • Decomposers (detritivores): bacteria and fungi that mineralise dead organic matter.

Energy flow

Energy enters an ecosystem from the Sun and flows in one direction through trophic levels, dissipating as heat at each step. The 10% Rule (Lindeman, 1942) states that only about 10% of energy at one trophic level is transferred to the next; the remainder is lost in respiration, heat and excretion.

The basic photosynthesis equation:

6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂

This fixes solar energy into chemical bonds, ultimately powering nearly all life.

Productivity

• Gross Primary Productivity (GPP) — total energy fixed by photosynthesis.
• Net Primary Productivity (NPP) = GPP − R (autotrophic respiration). NPP is what is available to consumers.

Biome	NPP (g C/m²/yr, approx.)
Tropical rainforest	2,000
Temperate forest	1,200
Grassland	600
Desert	90
Open ocean	125

Food chains, food webs, ecological pyramids

• Food chain: linear sequence (grass → grasshopper → frog → snake → hawk).
• Food web: interconnected food chains.
• Ecological pyramids of number, biomass and energy. The energy pyramid is always upright; biomass and number pyramids can be inverted (e.g. plankton supporting whales).

Biogeochemical cycles

These are global circulations of essential elements between living organisms and the abiotic environment.

Carbon cycle

• Reservoirs: atmosphere (CO₂), oceans, fossil fuels, biomass, soil.
• Flux: photosynthesis (sink) and respiration/combustion (source).
• Atmospheric CO₂ has risen from ~280 ppm (pre-industrial) to over 420 ppm (2023).

Nitrogen cycle

• 78% of the atmosphere is N₂ — but plants cannot use it directly.
• Nitrogen fixation by Rhizobium, Azotobacter, cyanobacteria and lightning converts N₂ to NH₃/NH₄⁺.
• Nitrification: NH₄⁺ → NO₂⁻ → NO₃⁻ by Nitrosomonas and Nitrobacter.
• Denitrification: NO₃⁻ → N₂ returned to atmosphere by anaerobic bacteria.

Phosphorus cycle

• The only major cycle without a gaseous phase — sedimentary in nature.
• Source: phosphate rocks weathering to soluble phosphate, taken up by plants.
• Over-application of P fertilisers is a leading cause of eutrophication.

Water (hydrological) cycle

Evaporation, transpiration, condensation, precipitation, runoff, infiltration. Powered by solar energy and gravity.

Major biomes

A biome is a large regional ecosystem defined chiefly by climate and dominant vegetation.

Biome	Climate	Example
Tropical rainforest	Hot, wet year-round	Amazon, Congo
Tropical savanna	Hot, seasonal rains	East Africa
Desert	Arid	Thar, Sahara, Cholistan
Temperate grassland	Cold winters	Steppes, prairies
Temperate forest	Four seasons	Eastern USA
Boreal/Taiga	Long cold winters	Siberia, Canada
Tundra	Polar, permafrost	Arctic Circle
Mediterranean	Dry summer	Coastal California
Mangrove	Tidal estuaries	Indus Delta

Pakistan's ecosystems

Pakistan, despite its arid average, has remarkable ecological diversity:

1. Indus delta mangroves — among the world's largest arid-zone mangrove forests, dominated by Avicennia marina.
2. Cholistan and Thal deserts — xerophytic vegetation.
3. Western dry mountains — juniper forests, Juniperus excelsa near Ziarat.
4. Moist temperate Himalayan forests — deodar (Cedrus deodara), spruce, fir; Galyat, Kaghan.
5. Sub-tropical pine forests — Murree hills.
6. Riverine forests (bela) along the Indus.
7. Alpine pastures above 3,500 m.

Threats to ecosystems

• Habitat fragmentation by infrastructure.
• Invasive species — Prosopis juliflora (mesquite) in Sindh; water hyacinth in Pakistani waterways.
• Over-extraction — Indus delta starved of fresh water flows.
• Climate change — alpine and mangrove zones at high risk.
• Pollution — agrochemical runoff, plastic.