9.1 Community and Ecosystem Structure
Basic Ecological Terms
- Habitat: The natural environment or living place of an organism.
- Species: A group of similar organisms that can interbreed to produce fertile offspring.
- Population: A group of organisms of the same species living in the same habitat at the same time.
- Community: All populations of different species living and interacting with one another in a specific habitat.
- Ecosystem: A dynamic system of living organisms (biotic components) interacting with each other and their non-living physical environment (abiotic components).
- Niche: The role or function of an organism in its ecosystem, including its interactions with biotic and abiotic factors.
Abiotic Components
- pH Value: Affects distribution of soil organisms and aquatic life.
- Temperature: Influences physiological enzyme rates in organisms.
- Light Intensity: Determines the rate of photosynthesis in autotrophs.
- Topography: Physical features of land (altitude, gradient, aspect).
- Microclimate: Climate conditions of a small, localized area (e.g., under a fallen log).
Biotic Components & Feeding Relationships
- Producers (Autotrophs): Green plants and photosynthetic organisms that synthesize organic food from inorganic substances.
- Consumers (Heterotrophs):
- Primary Consumers: Herbivores feeding directly on producers.
- Secondary Consumers: Carnivores or omnivores feeding on primary consumers.
- Tertiary Consumers: Top carnivores feeding on secondary consumers.
- Decomposers: Microorganisms (saprophytic bacteria and fungi) that break down dead organic matter into simpler inorganic nutrients.
Energy Flow & Ecological Pyramids
- Energy Flow: Sunlight is captured by producers and transferred through trophic levels via food chains and food webs. Approximately 90% of energy is lost at each trophic level as heat via respiration, excretion, and unabsorbed waste; only 10% of energy is passed to the next level.
- Pyramid of Numbers: Displays the total number of organisms at each trophic level.
- Pyramid of Biomass: Represents the total dry mass of living organic matter per unit area at each trophic level.
- Pyramid of Energy: Displays the total energy content per unit area per unit time at each trophic level (always upright/pyramidal).
9.2 Ecosystem Dynamics
Nutrient Cycles
- Carbon Cycle: Carbon dioxide ($CO_2$) is removed from the atmosphere via photosynthesis by producers, and returned to the atmosphere via respiration, decomposition, and combustion of fossil fuels.
- Oxygen Cycle: Oxygen ($O_2$) is produced by photosynthesis and consumed by respiration, combustion, and decomposition.
- Water Cycle: Water moves via evaporation, transpiration, condensation, precipitation, percolation, and runoff.
Interspecific and Intraspecific Interactions
- Intraspecific Competition: Competition between organisms of the same species for limited resources (e.g., mates, territory, food).
- Interspecific Competition: Competition between organisms of different species for shared resources (e.g., sunlight, nutrients, space).
- Symbiosis: Close physical relationship between two different species:
- Mutualism: Both species benefit (+/+) (e.g., sea anemone and clownfish, nitrogen-fixing bacteria in root nodules).
- Commensalism: One species benefits while the other is unaffected (+/0) (e.g., remora fish and shark, epiphytes on trees).
- Parasitism: One species benefits (parasite) while harming the other host (+/-) (e.g., tapeworm in human intestine, Rafflesia sp.).
- Predation: Predator hunts, kills, and eats prey (+/-) (e.g., owl and rat).
- Saprophytism: Organisms absorb nutrients from dead organic matter via external enzymatic digestion (+/0) (e.g., Mucor sp.).
Ecological Succession
The gradual process by which ecosystems change and develop over time:
- Primary Succession: Occurs in bare, lifeless areas with no pre-existing soil (e.g., volcanic rock, newly formed sand dunes, mangrove estuaries).
- Pioneer Species: The first organisms to colonize a barren area (e.g., lichens, mosses, Avicennia sp.). They weather rock/soil and add organic matter upon dying.
- Successor/Climax Community: Reaches a stable, mature stage with balanced species diversity (e.g., tropical rainforest, terrestrial forest after mangrove succession).
- Secondary Succession: Occurs in areas where an existing community was disturbed or destroyed, but soil remains intact (e.g., abandoned farmland, burnt forest). Proceeds much faster than primary succession.
9.3 Population Ecology
Quadrat Sampling Method
Used to estimate population size and distribution of stationary/slow-moving organisms (e.g., plants, barnacles):
$$\text{Frequency (\%)} = \frac{\text{Number of quadrats containing the species}}{\text{Total number of quadrats sampled}} \times 100\%$$
$$\text{Density} = \frac{\text{Total number of individuals of species in all quadrats}}{\text{Total number of quadratssampled} \times \text{Area of a quadrat}}$$
$$\text{Percentage Cover (\%)} = \frac{\text{Total aerial coverage area of species in all quadrats}}{\text{Total number of quadrats sampled} \times \text{Area of a quadrat}} \times 100\%$$
Capture-Mark-Release-Recapture (CMRR) Method
Used to estimate population size of mobile animals (e.g., woodlice, snails, fish):
$$\text{Estimated Population Size} = \frac{\text{Number of animals in 1st sample} \times \text{Number of animals in 2nd sample}}{\text{Number of marked animals in 2nd sample}}$$
Assumptions of CMRR Method:
- The population remains closed (no immigration, emigration, births, or deaths between samples).
- The mark does not affect the animal's survival, mobility, or vulnerability to predation.
- Marked individuals mix randomly back into the general population before second capture.