Ecological Balance in Earth Systems, Biogeochemical Cycles and Food Webs

• Ecosystem: a functioning unit of interacting living organisms and their non-living environment.
• Habitat is where an organism lives; niche is its role — what it eats, what eats it, when and how it lives. Two species cannot keep the same niche indefinitely.
• Producers (autotrophs): photosynthetic or chemosynthetic organisms that build organic matter from inorganic compounds.
• Consumers (heterotrophs): herbivores, primary carnivores and secondary carnivores. Decomposers (bacteria and fungi) break down dead matter externally and absorb it; detritivores (earthworms, woodlice) ingest the debris.
• Trophic level: the feeding position in a chain — level 1 producers, level 2 primary consumers, and so on.
• Food web: the network formed when several food chains cross-link; the more complex the web, the more stable the ecosystem.
• Ecological balance (homeostasis): the ability of an ecosystem to keep its structure and function through the feedbacks between its components.
• Biodiversity: variety of genes, species and ecosystems; it is the main guarantee of resilience.
• Biogeochemical cycle: the circular path of an element between the living (bio), the soil, water and air (geo) and back again.

• Lindeman's 10% rule (trophic-dynamic efficiency): only about 10% of the energy of one level appears as biomass in the next; the rest is lost as respiration, heat, movement and unconsumed or undigested matter.
• Consequence: food chains rarely exceed four or five levels, because there is not enough energy left to support another.
• Example: if plants fix 10,000 kJ, herbivores store about 1,000 kJ, primary carnivores 100 kJ and top carnivores 10 kJ.
• Primary productivity: gross (GPP) minus respiration (R) equals net (NPP) — NPP is what the ecosystem actually offers to consumers; tropical rainforests and coral reefs are the highest, deserts and open ocean the lowest per unit area.
• Nitrogen fixation: N₂ + 8H⁺ + 8e⁻ + 16 ATP → 2NH₃ + H₂, catalysed by nitrogenase in Rhizobium in legume root nodules, free-living Azotobacter and Clostridium, and cyanobacteria such as Anabaena.
• Nitrification: NH₃ → NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter); denitrification by Pseudomonas returns N₂ to the air under anaerobic conditions.

A grazing food chain in a pond: phytoplankton → zooplankton → small fish → large fish → human or osprey. A detritus chain: dead leaves → fungi and bacteria → earthworms → moles → owls. In nature the two run together as a web, so the loss of one species rarely starves another completely.

• In: photosynthesis fixes atmospheric CO₂ into carbohydrate — 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ — and the carbon passes along the food chain.
• Out: respiration in all organisms returns CO₂; decomposers release it from dead matter; combustion of wood, coal, oil and gas returns ancient carbon that had been locked out of the cycle for millions of years.
• The ocean is the largest active reservoir: CO₂ dissolves and equilibrates with HCO₃⁻ and CO₃²⁻, shell-forming organisms make CaCO₃, and when they sink the carbon becomes limestone or is returned by upwelling.
• Because the ocean absorbs CO₂, its pH falls — ocean acidification — which dissolves the shells of corals, molluscs and plankton.
• Terrestrial sinks: living biomass and soils hold far more carbon than the atmosphere, so deforestation both releases carbon and removes the sink.
• Human disturbance: burning fossil fuels and clearing forests move carbon into the air faster than photosynthesis and dissolution can take it back.

Nitrogen — the bacterial cycle

• Plants can only absorb nitrogen as NO₃⁻ or NH₄⁺, never as N₂ gas, so fixation is indispensable.
• Sequence: N₂ —fixation→ NH₃/NH₄⁺ —nitrification→ NO₂⁻ → NO₃⁻ —assimilation→ plant and animal protein —death and excretion— NH₄⁺ —ammonification→ —denitrification→ N₂.
• Legumes host Rhizobium in root nodules in a mutualism: the bacterium gets sugars, the plant gets amino acids — the basis of crop rotation with clover, beans and alfalfa.
• Human interference: the Haber–Bosch process fixes more nitrogen industrially than all natural fixation, and excess fertiliser runs off into water (eutrophication) or is denitrified to N₂O, a powerful greenhouse gas.

Water

Evaporation from oceans, transpiration from plants, condensation into clouds, precipitation, then run-off, infiltration into aquifers and return to the sea — driven by solar energy and gravity, so it is a cycle that needs no chemical change of the substance.

Phosphorus — no gaseous phase

Phosphate is released by the weathering of rocks, taken up by plants, passed through consumers, returned by decomposers, and finally precipitates in sediments on the sea floor; it returns to land only by uplift or by guano and fishing. Because there is no air reservoir, phosphorus is the limiting nutrient in many freshwaters, and its addition from detergents triggers algal blooms.

• An ecosystem keeps its balance through negative feedbacks: prey increase → predators increase → prey decrease → predators decrease. The more species and the more interconnections, the more ways there are to absorb a shock (resistance) and to recover (resilience).
• Keystone species hold a community together out of proportion to their numbers; removing them changes the whole structure.
• Succession is the orderly replacement of communities after a disturbance — lichens and mosses, then grasses, shrubs and finally forest on bare rock (primary succession), or re-growth on existing soil after a fire (secondary).
• Threats: habitat loss and fragmentation, over-exploitation and over-fishing, pollution of water air and soil, invasive alien species that find no natural enemies, and climate change that shifts the ranges of species faster than they can adapt.
• Egypt's measures: protected areas such as Ras Mohammed and Wadi El-Gamal, breeding and re-introduction programmes, the national biodiversity strategy, fishing and hunting regulation, afforestation belts (the "Green Wall") against desertification, and wetland restoration in the north.

1. A field of wheat fixes 20,000 kJ per square metre per year. Estimate the energy available to rats and then to owls. — Rats ≈ 2,000 kJ, owls ≈ 200 kJ; the owl therefore needs a large territory for its food.
2. Why is a pyramid of energy never inverted? — Because each transfer loses at least 90% as respiration and heat; a level can never store more energy than entered it.
3. Name two bacteria and the process each performs in the nitrogen cycle. — Rhizobium fixes N₂ in legume nodules; Nitrosomonas oxidises ammonia to nitrite (nitrification); Pseudomonas reduces nitrate to N₂ (denitrification).
4. A lake receives phosphate-rich detergent effluent and later the fish die. Explain the steps. — Phosphate is the limiting nutrient → algal bloom → algae shade and die → bacteria decompose the biomass → BOD rises and DO falls → fish and invertebrates suffocate.
5. An island lake has one species of fish and a new predatory fish is introduced. Predict and justify. — The native fish, with no evolved defence and no refuges, is likely driven to extinction; the web simplifies and the ecosystem loses resilience — this is the standard invasive-species argument.