A Level H2 Biology Ecology Quiz

Answer Key - A-Level Biology H2 Quiz: Ecology

1. Ecosystem: A biological community of interacting organisms and their physical environment. (1) Population: Group of organisms of the same species living in the same area at the same time. (1) Community: All the populations of different species living and interacting in a particular area. (1)

2. Energy is lost at each trophic level (1). Loss occurs via heat during respiration (1), excretion of undigested material (faeces), or death of organisms without being consumed (1).

3. Trophic efficiency is typically low (~10%) (1). Only a small fraction of energy is incorporated into the biomass of the next level (1). Eventually, the energy remaining is insufficient to support another viable population of tertiary/quaternary consumers (1).

4. Runoff leads to high nutrient levels (nitrates/phosphates) in water (1) $\rightarrow$ Algal bloom (rapid growth of algae) (1) $\rightarrow$ Algae block sunlight, killing submerged plants $\rightarrow$ Decomposition of dead plants/algae by aerobic bacteria (1) $\rightarrow$ Depletion of dissolved oxygen (hypoxia) $\rightarrow$ Death of fish and other aquatic animals (1).

5. Detritivores: Animals that ingest dead organic matter (e.g., earthworms) (1). Saprobes: Organisms (fungi/bacteria) that secrete enzymes externally to digest organic matter (1). Both break down complex organic molecules into simpler inorganic nutrients (mineralization) (1).

6. Decomposers break down organic nitrogen (proteins/amino acids) from dead matter (1) into inorganic ammonium ions ($\text{NH}_4^+$), which are then nitrified into nitrates ($\text{NO}_3^-$) for plant uptake (1).

7. Primary Producers: Biomass increases because there is less grazing pressure/herbivory (2). Tertiary Consumers: Biomass decreases because the food source (secondary consumers, who rely on primary consumers) declines, leading to starvation or migration (2).

8. Density-dependent: Factors whose effect varies with population density (e.g., competition for food, disease, predation) (2). Density-independent: Factors that affect populations regardless of density (e.g., volcanic eruption, sudden frost, floods) (2).

9. Logistic Growth: Growth starts exponentially but slows as the population approaches the carrying capacity (1). Carrying Capacity ($K$): The maximum population size that a particular environment can sustain indefinitely given the available resources (2).

10. Abundant resources (food, space) (1) and absence of limiting factors such as predators or disease (1).

11. Interspecific: Competition between different species (1). Intraspecific: Competition between individuals of the same species (1). Intraspecific is more intense because individuals have identical niches and requirements for the same limited resources (1).

12. Commensalism: Remora benefits (transport, food scraps), shark is neither helped nor harmed (2). Mutualism: Both species benefit from the interaction (1).

13. Predator population depends on prey density (1). When prey increases, predators have more food, leading to higher birth rates (1). However, as predators increase, they over-consume prey, causing the prey population to crash, followed by a crash in the predator population due to starvation (1).

14. Niche: The specific role and set of environmental conditions a species requires to survive (2). Competitive Exclusion: Two species competing for the same limiting resource cannot coexist; the more efficient competitor will outcompete the other, leading to the extinction or migration of the weaker species (2).

15. Biodiversity: The variety of life in a particular habitat or ecosystem (1). Genetic Diversity: Provides a wider range of alleles/phenotypes (1). This increases the probability that some individuals possess traits that allow them to survive a new disease or changing climate, preventing total population extinction (2).

16. Geographic isolation prevents gene flow between populations (1). Different selective pressures in the two environments lead to different mutations/adaptations (1). Over time, genetic divergence occurs (1). Eventually, the populations become reproductively isolated and cannot interbreed even if reunited (1).

17. Higher $\text{CO}_2$ may initially increase photosynthetic rates (1). However, warming temperatures allow lower-altitude species to migrate upwards (1), potentially outcompeting specialized alpine species or shrinking their available habitat (1).

18. Invasive species often lack natural predators in the new environment (1). They may outcompete native species for resources or prey directly on them (1), leading to the decline or extinction of native species and reduced overall biodiversity (1).

19. Fragmentation: Breaking large habitats into smaller patches (1). Edge Effect: Changes in abiotic factors (light, wind, humidity) at the boundary of the patch (1). Impact: Interior species are sensitive to these changes and lose "core" habitat, increasing vulnerability to predators or dehydration (2).

20. Ex-situ: Effective for preventing immediate extinction of critically endangered species; allows controlled breeding (2). However, it removes species from their natural evolutionary context and may lead to loss of behavioral traits (1). In-situ: Maintains the entire ecosystem and ecological interactions; allows continued co-evolution (2). However, it is harder to protect from poaching or sudden disasters (1).