Ecology -- Diagnostic Tests

Ecology — Diagnostic Tests

Unit Tests

UT-1: Food Chains and Webs

Question: A food chain in a woodland ecosystem is: oak tree $\to$ caterpillar $\to$ blue tit $\to$ sparrowhawk. (a) Identify the producer, primary consumer, secondary consumer, and tertiary consumer. (b) If the oak trees store 50,000 kJ of energy, calculate the energy available to the sparrowhawk (assuming 10% transfer efficiency at each level). (c) Explain what would happen to the blue tit population if the caterpillar population decreased by 50%. (d) Draw a pyramid of biomass for this food chain.

Solution:

(a) Producer: Oak tree (converts light energy to chemical energy via photosynthesis). Primary consumer: Caterpillar (herbivore). Secondary consumer: Blue tit (carnivore that eats caterpillars). Tertiary consumer: Sparrowhawk (top predator).

(b) Oak tree: 50,000 kJ. Caterpillar: $50,000 \times 0.1 = 5000$ kJ. Blue tit: $5000 \times 0.1 = 500$ kJ. Sparrowhawk: $500 \times 0.1 = 50$ kJ.

(c) The blue tit population would decrease because caterpillars are their primary food source. With 50% fewer caterpillars, there is less food available, leading to increased competition for the remaining caterpillars. Some blue tits may starve or fail to breed, reducing the population. In the longer term, the blue tit population would stabilise at a lower level. The sparrowhawk population would also eventually decrease due to less prey (blue tits).

(d) The pyramid of biomass has the largest bar at the base (oak tree biomass) and progressively smaller bars: oak tree (largest) $\to$ caterpillar $\to$ blue tit $\to$ sparrowhawk (smallest). Each bar is approximately 10% of the one below. The pyramid is always pyramid-shaped for biomass (unlike pyramids of numbers, which can be inverted).

UT-2: The Carbon Cycle and Water Cycle

Question: (a) Describe the main processes in the carbon cycle. (b) Explain how deforestation contributes to increased atmospheric CO $_2$ . (c) Describe the main stages of the water cycle. (d) Explain why peat bogs are important carbon stores and what happens when they are drained.

Solution:

(a) Carbon cycle processes:

Photosynthesis: Plants absorb CO $_2$ from the atmosphere and convert it to organic compounds (glucose).
Respiration: Plants and animals release CO $_2$ back into the atmosphere by breaking down glucose.
Combustion: Burning fossil fuels (coal, oil, gas) and biomass releases stored carbon as CO $_2$ .
Decomposition: Decomposers break down dead organic matter, releasing CO $_2$ .
Feeding: Carbon is transferred through food chains.
Fossilisation: Dead organisms that are not fully decomposed are compressed over millions of years to form fossil fuels.

(b) Deforestation increases atmospheric CO $_2$ because: (1) Trees are cut down, reducing photosynthesis (less CO $_2$ is absorbed from the atmosphere). (2) Cut trees are often burned (releasing stored carbon as CO $_2$ immediately). (3) Dead plant material decomposes, releasing CO $_2$ . (4) The soil carbon store is disturbed and oxidises to CO $_2$ .

Evaporation: Water from oceans, lakes, and rivers evaporates due to solar energy.
Transpiration: Water evaporates from plant leaves.
Condensation: Water vapour rises, cools, and condenses into tiny water droplets, forming clouds.
Precipitation: Water droplets in clouds merge and fall as rain, snow, sleet, or hail.
Run-off/infiltration: Water flows over land (run-off) into rivers and oceans, or soaks into the ground (infiltration) to become groundwater.

(d) Peat bogs accumulate partially decomposed plant material over thousands of years in waterlogged, acidic conditions that prevent full decomposition. This stores vast amounts of carbon. When drained: (1) The waterlogged conditions are removed, allowing aerobic decomposition to resume, releasing stored carbon as CO $_2$ . (2) The peat may be harvested as fuel, releasing carbon through combustion. Drained peatlands are a major source of greenhouse gas emissions.

UT-3: Biodiversity and Human Impact

Question: (a) Define biodiversity. (b) Describe three ways in which human activities reduce biodiversity. (c) Explain the importance of biodiversity for: medicine, agriculture, and ecosystem stability. (d) Describe three methods of conservation.

Solution:

(a) Biodiversity is the variety of living organisms in an ecosystem, encompassing three levels: (1) Species diversity — the number of different species. (2) Genetic diversity — the variety of genes within a species. (3) Ecosystem diversity — the range of different habitats.

(b) Three ways: (1) Deforestation — destroys habitats, displaces species, reduces food sources. (2) Pollution — chemicals, plastics, and oil spills poison organisms and disrupt ecosystems. (3) Overfishing — depletes fish populations, disrupts marine food webs. (4) Urbanisation — converts natural habitats to built environments. (5) Climate change — alters habitats faster than species can adapt.

(c) Medicine: Many medicines are derived from natural sources (e.g., penicillin from fungi, aspirin from willow bark). Losing biodiversity means losing potential future medicines from undiscovered species.

Agriculture: Genetic diversity in crop wild relatives provides resistance to diseases and pests. Pollination by diverse insect species is essential for crop production. Soil biodiversity maintains fertility.

Ecosystem stability: Diverse ecosystems are more resilient to environmental changes and disturbances. Each species plays a role in the ecosystem; losing keystone species can cause collapse.

(d) Three conservation methods: (1) Protected areas (national parks, nature reserves) — legally protect habitats from development. (2) Captive breeding programmes — breed endangered species in zoos for eventual reintroduction to the wild. (3) Seed banks — store seeds of threatened plant species to preserve genetic diversity. (4) Sustainable management — quotas for fishing, sustainable forestry, and organic farming.

Integration Tests

IT-1: Ecosystem Dynamics (with Bioenergetics)

Question: A lake ecosystem has the following biomass values (in kg): phytoplankton $= 4000$ Zooplankton $= 400$ Small fish $= 40$ Large fish $= 4$ . (a) Calculate the percentage energy transfer at each level. (b) A farmer applies fertiliser to a field near the lake. Explain how this could lead to eutrophication. (c) Calculate the total biomass if the farmer introduces 500 kg of an invasive fish species that competes with small fish, reducing the small fish biomass by 60%. (d) Explain how the invasive species affects the stability of the ecosystem.

Solution:

(a) Zooplankton: $400/4000 \times 100\% = 10\%$ . Small fish: $40/400 \times 100\% = 10\%$ . Large fish: $4/40 \times 100\% = 10\%$ .

(b) Eutrophication process: (1) Fertiliser (rich in nitrates and phosphates) is washed by rain into the lake (leaching/run-off). (2) The increased nutrients cause rapid growth of algae (algal bloom). (3) The algal bloom blocks sunlight from reaching deeper water, killing aquatic plants below. (4) Dead algae and plants are decomposed by aerobic bacteria, which consume large amounts of dissolved oxygen. (5) Oxygen levels drop, killing fish and other aquatic organisms. (6) The lake becomes stagnant and lifeless.

(c) Small fish biomass after reduction: $40 \times 0.4 = 16$ kg. Large fish biomass would also decrease (less food): $16 \times 0.1 = 1.6$ kg. Zooplankton may increase (less predation): possibly $400 \times 1.5 = 600$ kg. Invasive fish: 500 kg. Phytoplankton may decrease due to invasive fish competing for resources.

(d) The invasive species disrupts the existing food web by competing with native small fish for food and habitat. This cascading effect reduces the large fish population (less prey), may increase zooplankton (less predation from small fish), and potentially reduce phytoplankton (more consumers). The loss of native species reduces biodiversity and genetic diversity, making the ecosystem less resilient to environmental changes. The invasive species may have no natural predators, allowing it to dominate.

IT-2: Biogeochemical Cycles and Climate Change (with Cell Biology)

Question: (a) Explain how increasing atmospheric CO $_2$ contributes to global warming, including the mechanism of the greenhouse effect. (b) The concentration of CO $_2$ has increased from 280 ppm to 420 ppm since pre-industrial times. Calculate the percentage increase. (c) Explain how ocean acidification occurs as a result of increased CO $_2$ And its effect on marine organisms with calcium carbonate shells. (d) Describe two biological consequences of global warming on ecosystems.

Solution:

(a) Greenhouse effect mechanism: CO $_2$ (and other greenhouse gases like methane and water vapour) in the atmosphere absorb infrared radiation (heat) emitted by the Earth’s surface. This radiation would otherwise escape to space. The absorbed heat is re-radiated in all directions, including back towards the Earth’s surface, warming the planet. Increasing CO $_2$ concentration increases the greenhouse effect, trapping more heat and raising global temperatures.

(b) Percentage increase $= (420 - 280)/280 \times 100\% = 140/280 \times 100\% = 50\%$ .

(c) Ocean acidification: The oceans absorb about 30% of atmospheric CO $_2$ . Dissolved CO $_2$ reacts with water to form carbonic acid ( $\text{H_2\text{CO_3$ ), which dissociates into hydrogen ions ( $\text{H^+$ ) and bicarbonate ( $\text{HCO_3^-$ ). The increased $\text{H^+$ concentration lowers the ocean pH (makes it more acidic).

Effect on marine organisms: Organisms with calcium carbonate shells (e.g., corals, molluscs, some plankton) are affected because the lower pH reduces the availability of carbonate ions ( $\text{CO_3^{2-}$ ), making it harder for them to build and maintain their shells. This weakens the organisms and can lead to coral bleaching and death, threatening marine food webs.

(d) Two consequences: (1) Range shifts: Species migrate towards the poles or to higher altitudes as temperatures rise, disrupting existing ecosystems and food webs. Species that cannot migrate quickly enough face extinction. (2) Phenological mismatch: The timing of seasonal events (flowering, migration, breeding) changes at different rates for different species. For example, insects may emerge earlier than the plants they pollinate, or birds may arrive after the peak food supply.

IT-3: Fieldwork Techniques (with Organisation)

Question: A student investigates the distribution of daisies across a field using a quadrat. The field is 100 m $\times$ 50 m. (a) Describe how to use a quadrat and explain why random sampling is important. (b) The student places a $0.5 \text{ m \times 0.5 \text{ m$ quadrat at 10 random positions and counts: 3, 5, 7, 2, 4, 6, 8, 3, 5, 4 daisies. Calculate the mean number of daisies per quadrat and estimate the total population in the field. (c) Explain two ways to ensure the results are reliable. (d) The student also measures soil pH at each quadrat position and finds that daisies are more abundant where pH $\gt$ 6.5. Explain what type of investigation this is and suggest a hypothesis.

Solution:

(a) Quadrat method: A quadrat (a square frame of known area) is placed on the ground, and the number of organisms of the target species within it is counted. Multiple quadrats are placed at random positions across the field to get a representative sample.

Random sampling is important because it avoids bias. If the student chose where to place the quadrat (e.g., only in areas with many daisies), the results would not represent the whole field. Random placement (using random number coordinates) ensures every part of the field has an equal chance of being sampled.

(b) Mean per quadrat $= (3 + 5 + 7 + 2 + 4 + 6 + 8 + 3 + 5 + 4) / 10 = 47/10 = 4.7$ daisies per quadrat.

Quadrat area $= 0.5 \times 0.5 = 0.25$ m $^2$ . Density $= 4.7 / 0.25 = 18.8$ daisies per m $^2$ .

Field area $= 100 \times 50 = 5000$ m $^2$ . Estimated population $= 18.8 \times 5000 = 94,000$ daisies.

(c) Two ways to ensure reliability: (1) Large sample size — use more quadrats (e.g., 20 or 30) to reduce the effect of anomalies and get a more representative mean. (2) Repeatability — repeat the investigation at different times or have another student independently carry out the same method to check consistency.

(d) This is a correlational investigation studying the relationship between an environmental variable (soil pH) and species distribution (daisy abundance).

Hypothesis: “Daisies are more abundant in soil with a pH greater than 6.5” or “Soil pH affects the distribution of daisies, with higher pH favouring daisy growth.” The investigation does not prove causation — other factors (light, moisture, competition) may also be correlated with pH.

Summary

The key principles covered in this topic are linked in the sub-pages above. Focus on understanding the definitions, applying the formulas or frameworks, and evaluating strengths and limitations of each approach.

Worked Examples

Worked examples demonstrating the application of key concepts are covered in the detailed sub-pages linked above.

Common Pitfalls

Confusing terminology or concepts that appear similar but have distinct meanings.
Overlooking key assumptions or boundary conditions that limit applicability.

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