Ecosystems and Biogeoeconomy in SPM Biology: How Marks Move Through the System

The ecosystem chapter in SPM Biology pulls together ideas from photosynthesis, respiration, and nutrition into one connected system. Examiners often build Paper 2 questions around a food web diagram and ask you to predict what happens when a species is removed. This guide covers the vocabulary, the cycles, and the reasoning patterns the exam rewards.

Trophic Levels and Feeding Relationships

Every organism in an ecosystem occupies a trophic level. Producers (usually green plants) make their own food via photosynthesis. Primary consumers eat producers; secondary consumers eat primary consumers; tertiary consumers sit at the top. A food chain shows one path of energy flow; a food web shows the interconnected network of all paths in an ecosystem.

Pyramids of Number, Biomass, and Energy

A pyramid of numbers counts individuals at each level; a pyramid of biomass measures total dry mass; a pyramid of energy shows the energy available. Energy pyramids are always upright because energy is lost at each step — roughly 90% is lost to respiration, heat, and undigested waste, leaving only about 10% to pass to the next level. This is why food chains rarely exceed four or five links.

Predicting the Effect of Removing a Species

This is the classic exam question. To answer, trace what eats the removed species and what it eats. If a secondary consumer is removed, its prey (primary consumers) increase, which then overgraze producers, causing producer numbers to crash. Always state the direction of change (increase or decrease) and the reason in the same sentence for full marks.

Biogeochemical Cycles

Matter cycles through ecosystems; energy flows through and is lost. The two cycles you must know are the carbon cycle and the nitrogen cycle. In the carbon cycle, carbon moves from the air (as CO₂) into plants via photosynthesis, through food chains, back to the air via respiration, and into long-term storage as fossil fuels. Combustion of fossil fuels releases stored carbon rapidly, which is the basis of climate change.

The nitrogen cycle is more complex because most organisms cannot use atmospheric nitrogen. Nitrogen-fixing bacteria convert N₂ to ammonia; nitrifying bacteria convert ammonia to nitrites then nitrates, which plants absorb; denitrifying bacteria return nitrates to the air. Each step is performed by specific microorganisms, and naming the correct bacterium earns marks.

Adaptation and Survival

Adaptations are features that help organisms survive in their habitat. Morphological adaptations are physical (thick waxy cuticle to reduce water loss); physiological adaptations are internal (camels concentrating urine); behavioural adaptations are actions (nocturnal activity to avoid heat). When describing an adaptation, always link the feature to a specific environmental challenge.

Biodiversity and Conservation

Biodiversity refers to the variety of species, genes, and ecosystems. High biodiversity increases stability because ecosystems with many species can absorb shocks better than those with few. Threats include deforestation, overharvesting, pollution, and climate change. Conservation strategies include protected areas, legislation, captive breeding, and sustainable resource use.

Exam Technique for Diagram Questions

• Read the labels on the food web carefully before answering.
• Use the exact terms — 'population increases' is better than 'becomes more'.
• When asked about energy loss, mention respiration, heat, and undigested waste.
• For cycles, name the specific process (photosynthesis, nitrification) and the form of the element (CO₂, NO₃⁻).

The ecosystem chapter rewards systems thinking. Instead of memorising isolated facts, see the connections: energy flows one way and is lost, matter cycles through specific organisms, and every change ripples through the web. Master that view and the long Paper 2 questions become some of the most predictable in the paper.