Question 1

What is the difference between gross primary productivity and net primary productivity?

Accepted Answer

Gross primary productivity (GPP) is the total rate of photosynthesis by producers, measured as energy or biomass produced per unit area per unit time. Net primary productivity (NPP) is the energy remaining after producers have used some for respiration (R). The formula is NPP = GPP - R. NPP represents the energy available to consumers in the ecosystem.

Question 2

How does succession lead to a climax community?

Accepted Answer

Succession is the gradual change in species composition over time. Pioneer species (e.g., lichens on bare rock) colonise harsh environments and alter conditions (e.g., soil formation, shade). This makes the habitat more suitable for other species, which outcompete the pioneers. Over time, biodiversity increases, and the community becomes more stable. Eventually, a climax community (e.g., oak woodland) develops, which is in equilibrium with the climate and soil. However, disturbances can reset succession.

Question 3

Why is only about 10% of energy transferred between trophic levels?

Accepted Answer

Energy is lost at each trophic level primarily through respiration (heat), egestion (undigested material), and excretion (urine). Only the energy stored in biomass is passed to the next level. For example, a herbivore may eat 1000 kJ of plant energy, but only about 100 kJ becomes new herbivore biomass; the rest is lost as heat or waste. This limits the length of food chains and the number of top predators.

Question 4

What is the role of decomposers in the carbon and nitrogen cycles?

Accepted Answer

Decomposers (bacteria and fungi) break down dead organic matter, releasing carbon dioxide into the atmosphere (via respiration) and returning nutrients to the soil. In the nitrogen cycle, decomposers convert organic nitrogen in dead organisms into ammonium ions (ammonification). These ammonium ions are then converted to nitrates by nitrifying bacteria, which plants can absorb. Without decomposers, nutrients would remain locked in dead matter.

Question 5

How does deforestation affect the carbon cycle?

Accepted Answer

Deforestation reduces the number of trees, which are carbon sinks that absorb CO₂ during photosynthesis. When trees are cut and burned or left to decay, stored carbon is released as CO₂ into the atmosphere, increasing greenhouse gas levels. Additionally, deforestation reduces the rate of carbon uptake, exacerbating climate change. It also disrupts the water cycle and reduces biodiversity.

Question 6

What is eutrophication and how does it occur?

Accepted Answer

Eutrophication is the enrichment of water bodies with nutrients (mainly nitrates and phosphates), often from agricultural fertiliser runoff or sewage. This causes rapid algal growth (algal bloom), which blocks light and kills submerged plants. When algae die, decomposers break them down, using up oxygen in the water. This leads to hypoxia (low oxygen), killing fish and other aquatic organisms. Eutrophication reduces biodiversity and water quality.

Topic 5: On the Wild Side

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