A Level H2 Biology Ecology Quiz

Answer Key - A-Level Biology H2 Quiz: Ecology

1. Trophic Level & Limitations [3]

• Definition: The position an organism occupies in a food chain. [1]
• Limitation: Energy is lost at each level (approx. 90%) via respiration, excretion, and uneaten parts. [1]
• Result: Insufficient energy remains to support higher trophic levels. [1]

2. Energy Transfer Efficiency [3]

• Efficiency is generally similar (approx. 10%) across levels. [1]
• However, primary producers often have higher biomass, but the transfer to primary consumers is limited by indigestible matter (e.g., cellulose). [1]
• Tertiary consumers often have very small populations due to the cumulative loss of energy. [1]

3. Eutrophication [4]

• Nutrient runoff (nitrates/phosphates) $\rightarrow$ algal bloom. [1]
• Algae block sunlight $\rightarrow$ submerged plants die. [1]
• Aerobic bacteria decompose dead organic matter. [1]
• Bacteria use up dissolved oxygen $\rightarrow$ hypoxia $\rightarrow$ fish suffocate/die. [1]

4. Ammonification [3]

• Decomposers (bacteria/fungi) break down organic nitrogen (proteins/nucleic acids) from dead organisms/waste. [1]
• This converts organic nitrogen into ammonium ions ($\text{NH}_4^+$). [1]
• This makes nitrogen available for nitrifying bacteria to convert into nitrates for plant uptake. [1]

5. Carbon Sequestration [3]

• Peat bogs have anaerobic/waterlogged conditions. [1]
• This inhibits the activity of decomposers (bacteria). [1]
• Carbon remains trapped in dead organic matter instead of being released as $\text{CO}_2$, reducing the greenhouse effect. [1]

6. GPP vs NPP [2]

• GPP: Total rate at which energy is captured by producers via photosynthesis. [1]
• NPP: GPP minus energy lost through respiration ($\text{NPP} = \text{GPP} - \text{R}$). [1]

7. Pyramids of Numbers vs Energy [4]

• Numbers: Can be inverted (e.g., one large producer tree supporting thousands of insects). [2]
• Energy: Always upright because energy is lost as heat at each transfer (Second Law of Thermodynamics). [1]
• Energy cannot be created; thus, the base must always be the largest. [1]

8. r-selected Species [3]

• Traits: Small size, early maturity, many offspring, little to no parental care. [1]
• Advantage: Rapid colonization of new/unstable habitats. [1]
• High growth rate allows them to exploit resources before competition increases. [1]

9. Population Regulation Factors [4]

• Density-dependent: Impact increases as population density rises (e.g., disease, competition for food). [2]
• Density-independent: Impact is unrelated to density (e.g., volcanic eruption, severe frost). [2]

10. Inflection Point [3]

• The point where the growth rate is at its maximum. [1]
• Transition from the exponential growth phase to the decelerating growth phase. [1]
• Indicates that environmental resistance (limiting factors) is beginning to significantly slow growth. [1]

11. Coupled Oscillations [4]

• Prey population increases $\rightarrow$ more food for predators $\rightarrow$ predator population increases. [1]
• High predator density $\rightarrow$ over-predation $\rightarrow$ prey population crashes. [1]
• Lack of prey $\rightarrow$ predators starve $\rightarrow$ predator population crashes. [1]
• Reduced predation $\rightarrow$ prey population recovers, restarting the cycle. [1]

12. Carrying Capacity (K) [4]

• Definition: The maximum population size an environment can sustain indefinitely. [1]
• Factor 1: Food availability (e.g., seasonal fruiting). [1.5]
• Factor 2: Space/Nesting sites (e.g., deforestation). [1.5]

13. Intraspecific vs Interspecific Competition [3]

• Intraspecific: Between members of the same species; more intense because niches are identical. [1]
• Interspecific: Between different species; less intense if niches overlap only partially. [1]
• Intraspecific competition often leads to stronger selection for niche specialization. [1]

14. Competitive Exclusion & Niche Partitioning [4]

• Competitive Exclusion: Two species competing for the exact same resource cannot coexist; one will be eliminated. [2]
• Niche Partitioning: Species evolve to use different parts of the resource (e.g., different feeding heights in a tree). [1]
• This reduces competition and allows coexistence. [1]

15. Alpha vs Beta Diversity [3]

• Alpha: Diversity within a specific site or local area (e.g., number of species in one plot). [1]
• Beta: The variation in species composition between two different sites. [1]
• High beta diversity indicates high turnover of species across a landscape. [1]

16. Invasive Species [4]

• Lack of natural predators in the new environment allows rapid population growth. [1]
• Outcompete native species for food/space. [1]
• May prey on native species that have no evolved defenses. [1]
• Leads to extinction of native species and simplified food webs. [1]

17. Edge Effects [4]

• Fragmentation creates more "edges" relative to the "core" area. [1]
• Edges have different microclimates (more wind, light, lower humidity). [1]
• Interior species are sensitive to these changes and cannot survive at the edge. [1]
• Effective habitat size is reduced more than the actual physical area loss. [1]

18. Ocean Acidification [4]

• $\text{CO}_2$ dissolves in seawater $\rightarrow$ forms carbonic acid ($\text{H}_2\text{CO}_3$). [1]
• This dissociates, increasing $\text{H}^+$ concentration (lowering pH). [1]
• $\text{H}^+$ reacts with carbonate ions ($\text{CO}_3^{2-}$) to form bicarbonate ($\text{HCO}_3^-$). [1]
• Reduces availability of $\text{CO}_3^{2-}$, making it harder for corals/molluscs to build $\text{CaCO}_3$ shells. [1]

19. Monoculture vs Polyculture [3]

• Monoculture: Low genetic diversity; if a pest evolves to attack one plant, all are susceptible. [1]
• Polyculture: High diversity; some species may be resistant, slowing the spread of the pest. [1]
• Polyculture is more resilient and stable. [1]

20. Keystone Species [4]

• Definition: A species that has a disproportionately large effect on its environment relative to its abundance. [1]
• Example: Sea otters controlling sea urchin populations. [1]
• Removal: Urchins overgraze kelp forests $\rightarrow$ loss of habitat for many other species. [1]
• Result: Total collapse of the ecosystem structure/biodiversity. [1]