A Level H1 Biology Ecology Quiz

Answer Key - A-Level Biology H1 Quiz: Ecology

Note: This content is syllabus-aligned and inferred from general A-Level assessment patterns.

Section A: Ecosystems and Energy Flow

1. Ecosystem: A biological community of interacting organisms and their physical (abiotic) environment (1). Population: A 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 the same area (1).
2. Energy is lost at each trophic level (1). Losses occur as heat during respiration, undigested materials (faeces), and parts of the organism not consumed (1). Eventually, there is insufficient energy to support another viable population at a higher level (1).
3. $\text{Respiration} = \text{GPP} - \text{NPP}$ (1). $8000 - 5000 = 3000\text{ kJ m}^{-2}\text{ yr}^{-1}$ (1).
4. Grazing food web: Energy flows from living primary producers to herbivores (1). Detritus food web: Energy flows from dead organic matter (detritus) to decomposers/detritivores (1).
5. Decomposers break down complex organic molecules (proteins, nucleic acids) from dead matter (1). Mineralization is the conversion of these organic forms into inorganic ions (e.g., $\text{NH}_4^+$) (1). This returns nutrients to the soil for uptake by plants (1).
6. Efficiency = $(\text{Energy at level } n / \text{Energy at level } n-1) \times 100$ (1). Typically low (~10%) because much energy is lost as heat via cellular respiration (1). Some energy is lost in excretion/egestion (1). Not all biomass is consumed by the predator (1).
7. In a forest, a pyramid of numbers is inverted or narrow at the base because one large tree supports many insects (1). However, the pyramid of biomass remains upright (1) because the total dry mass of the tree is far greater than the total mass of the insects (1).

Section B: Nutrient Cycling and Sustainability

8. Nitrosomonas bacteria convert ammonium ($\text{NH}_4^+$) into nitrites ($\text{NO}_2^-$) (1). Nitrobacter bacteria then convert nitrites into nitrates ($\text{NO}_3^-$) (1). This process requires oxygen (aerobic) (1).
9. Legumes have root nodules containing Rhizobium bacteria (1). These bacteria fix atmospheric nitrogen ($\text{N}_2$) into ammonia/ammonium (1), which is then converted to nitrates available for plant uptake (1).
10. Nitrate runoff leads to algal blooms (1). As algae die, aerobic decomposers break them down, consuming dissolved oxygen (1). This leads to hypoxia/anoxia (1), causing the death of fish and other aquatic organisms (1).
11. Phosphorus does not have a gaseous phase in the atmosphere (1). It is primarily locked in sedimentary rocks (1). Weathering is a slow process, making its release into the soil much slower than the nitrogen cycle (1).
12. Denitrifying bacteria (e.g., Pseudomonas) operate in anaerobic conditions (1). They convert nitrates ($\text{NO}_3^-$) back into atmospheric nitrogen gas ($\text{N}_2$) (1).
13. Fossil fuels store carbon from ancient organisms (1). Combustion releases this stored carbon as $\text{CO}_2$ into the atmosphere (1). This exceeds the rate at which photosynthesis/carbon sinks can remove it, leading to an increase in the greenhouse effect (1).
14. Crop Rotation: Planting legumes to naturally fix nitrogen (2). Organic Manure/Compost: Using decomposed organic matter to recycle nutrients slowly (2).

Section C: Population Dynamics and Interactions

15. Density-dependent: Factors whose impact varies with population density (e.g., competition for food, disease) (2). Density-independent: Factors that affect population regardless of density (e.g., volcanic eruption, flood) (2).
16. Logistic growth starts exponentially but slows as it approaches the carrying capacity (1). The curve is S-shaped (sigmoidal) (1). Carrying capacity ($K$) is the maximum population size the environment can sustain indefinitely (1) based on available resources (1).
17. Mutualism: Both species benefit (e.g., mycorrhizae and plant roots; fungus provides minerals, plant provides sugars) (2). Commensalism: One species benefits, the other is unaffected (e.g., epiphytes growing on tall trees for light) (2).
18. Competitive exclusion states that two species competing for the exact same resources cannot coexist (1). One will eventually outcompete the other (1). To survive, species evolve to use different resources or different parts of the habitat, known as niche partitioning (2).
19. Predator and prey populations fluctuate in cycles (1). An increase in prey leads to an increase in predators (1). The increased predation then causes the prey population to crash, followed by a crash in the predator population due to starvation (1).
20. A keystone species has a disproportionately large effect on its environment relative to its abundance (1). Its removal disrupts the balance of the food web (1). This can lead to a trophic cascade, where the loss of a top predator allows herbivores to overpopulate and overgraze the primary producers, collapsing the ecosystem structure (2).