Edexcel Biology GCSE Topic 9: Ecosystems and Material Cycles

9.1 - Levels of Organisation

  • Individual: A single living organism.

  • Species: A group of similar organisms that can interbreed to produce fertile offspring.

  • Habitat: The specific environment where an organism lives.

  • Population: All individuals of the same species living in a particular habitat.

  • Communities: Multiple populations of different species interacting within a habitat.

  • Ecosystem: A biological community of interacting organisms (biotic) and their physical, non-living environment (abiotic).

  • Competition: Organisms compete for limited resources:

    • Plants: Light, space, water, and mineral ions.

    • Animals: Food, mates, and territory.

9.2 - Abiotic and Biotic Factors Affecting Communities

Abiotic Factors (Non-living):

  1. Light intensity: Direct impact on the rate of photosynthesis; limits plant distribution.

  2. Temperature: Influences enzyme-controlled reactions (e.g., photosynthesis and metabolism).

  3. Moisture levels: Crucial for plant turgidity and animal hydration.

  4. Soil pH and minerals: Nitrate levels affect protein synthesis in plants; pH affects enzyme activity in soil microbes.

  5. Wind intensity: Affects transpiration rates and physical damage to flora.

  6. CO_2 levels: A limiting factor for photosynthesis in producers.

  7. Oxygen levels: Vital for aerobic respiration in aquatic organisms; levels drop in polluted water.

Biotic Factors (Living):

  1. Food availability: Determines the carrying capacity of a population.

  2. New predators: Can lead to a rapid decline in prey species that lack defense mechanisms.

  3. New pathogens: Spread quickly through populations without immunity, potentially wiping out a community.

  4. Competition: If two species compete for the same niche, the less adapted species may be displaced (Interspecific competition).

9.3 - Interdependence

  • Species rely on each other for food (predation/herbivory), shelter (nesting material), and reproduction (pollination/seed dispersal).

  • A stable community is one where all species and environmental factors are in balance, such that population sizes remain fairly constant over time.

9.4 - Parasitism and Mutualism

  • Parasitism: A symbiotic relationship where the parasite benefits at the expense of the host (e.g., fleas, tapeworms, or leeches).

  • Mutualism: A relationship where both organisms benefit (e.g., bees and flowers: bee gets food, flower gets pollinated).

  • Commensalism: One benefits, the other is neutral (e.g., barnacles on a whale).

9.6 - Fieldwork and Counting Organisms

  • Quadrats: Square frames used to sample stationary organisms. Sampling must be random (using a coordinate grid and random number generator) to avoid bias.

  • Transects: A line (tape measure) used to investigate the change in distribution of species across an environmental gradient (e.g., moving up a beach shore).

  • Calculation: \text{Estimated Population} = \frac{\text{Total Area}}{\text{Sampled Area}} \times \text{Counted Number of Organisms}

9.7B - Trophic Levels and Pyramids of Biomass

  • Trophic levels: Represent the feed positions (Level 1: Producers, Level 2: Primary Consumers, etc.).

  • Pyramids of Biomass: Show the total dry mass of organisms at each level; they are always pyramid-shaped because biomass is lost.

  • Energy Transfer: Only approximately 10\% of energy is transferred to the next level. Energy is lost via:

    • Heat from respiration.

    • Undigested material (egestion/feces).

    • Excretion (urea/urine).

    • Movement and maintaining body temperature.

9.9 - Human Interactions with Ecosystems

  • Positive:

    • Reforestation and habitat restoration.

    • Captive breeding programs for endangered species.

    • Protection of SSSIs (Sites of Special Scientific Interest).

  • Negative:

    • Global warming due to excess CO2 and CH4 (methane).

    • Eutrophication from fertilizer runoff into water sources.

    • Habitat destruction for grazing or monocultures.

9.11B - Factors Affecting Food Security

  • Food Security: Having enough food to feed a population reliably.

  • Factors:

    • Increasing population growth rates.

    • Pests and pathogens destroying crop yields.

    • Climate change causing droughts or flooding.

    • Conflicts/War disrupting food distribution.

9.12 - Cycling of Materials

  • Carbon Cycle: Photosynthesis (6CO2 + 6H2O \rightarrow C6H{12}O6 + 6O2) takes in carbon. Respiration, combustion, and decay release it back into the atmosphere.

  • Water Cycle: Involves Evaporation, Transpiration, Condensation, and Precipitation.

9.15 - The Nitrogen Cycle

  • Nitrogen-fixing bacteria: Convert atmospheric N_2 into nitrates in the soil/root nodules.

  • Nitrifying bacteria: Convert ammonia from decayed matter into nitrates (NO_3^-) for plant uptake.

  • Denitrifying bacteria: Convert nitrates back into gas (N_2) in waterlogged, anaerobic conditions.

9.17B and 9.18B - Decomposition

  • Temperature: Warmer conditions increase enzyme activity, but temperatures above 45^\circ C denature the enzymes of decomposers.

  • Oxygen: Faster decay occurs in aerobic conditions; anaerobic decay (lack of oxygen) is slower and produces biogas (methane).