Organisms and the Environment Notes

Definitions

  • Species - A group of organisms that can interbreed and produce fertile offspring.
  • Individual - A single organism within a species.
  • Population - A group of individuals of the same species living in the same habitat at the same time.
  • Community - Different populations living and interacting with one another.
  • Ecosystem - A community of organisms (biotic factors) interacting with their physical environment (abiotic factors).
  • Biotic Factors - The living components of an ecosystem (e.g., predators, prey, plants, bacteria).
  • Abiotic Factors - The non-living physical and chemical components of an ecosystem (e.g., temperature, water, sunlight, soil).
  • Habitat - The natural environment where an organism lives.

Food Chains

  • A food chain shows the flow of energy from one organism to another.
  • It consists of producers (plants) and different levels of consumers.
  • Example of a Simple Food Chain: Grass → Caterpillar → Bird → Hawk
  • Producer - Makes its own food via photosynthesis (e.g., grass).
  • Primary Consumer - Herbivore that eats the producer (e.g., caterpillar).
  • Secondary Consumer - Carnivore/omnivore that eats the primary consumer (e.g., bird).
  • Tertiary Consumer - Eats the secondary consumer (e.g., hawk).
  • Decomposers - Bacteria & fungi that break down dead organisms, returning nutrients to the soil.

Food Webs

  • A food web is a more complex version of a food chain.
  • Defined as the interaction between multiple food chains.
  • It demonstrates how organisms depend on multiple food sources for survival.

Trophic Levels

  • Trophic levels represent the amount of energy and position of an organism in a food chain/web:
    • Trophic Level 1: Producers (plants & algae)
    • Trophic Level 2: Primary consumers (herbivores)
    • Trophic Level 3: Secondary consumers (carnivores/omnivores)
    • Trophic Level 4: Tertiary consumers
    • Trophic Level 5: Quaternary consumers (final consumers)
  • Decomposers – Break down dead organisms & recycle nutrients.

Energy Levels in Food Chains

  • Only 10% of energy is passed to the next trophic level because:
    • Energy is lost as heat through respiration.
    • Some parts of the organism are not eaten (bones, fur, shells).
    • Not all food is digested (waste in feces).
    • Energy used during growth, reproduction, and movement.
  • This energy is not enough to sustain many quaternary consumers.
  • Producers have the most energy, and quaternary consumers have the least energy.

Pyramid of Energy

  • Definition: Shows the flow of energy at each trophic level, measured in kJ/m^2/year.
  • Shape: Always a pyramid because energy decreases as it moves up the food chain.
  • Example (Grassland Ecosystem): Plants (10,000 kJ) → Herbivores (1,000 kJ) → Carnivores (100 kJ) → Top Predator (10 kJ)

Pyramid of Biomass

  • Definition: Shows the total dry mass of living organisms at each trophic level, measured in g/m^2.
  • Shape: Usually a pyramid but can be inverted in aquatic ecosystems.
  • Why Biomass Decreases?:
    • Energy loss at each level leads to less biomass available for the next level.
    • In aquatic systems, phytoplankton reproduce quickly but have a small individual mass, leading to an inverted pyramid (small producer biomass but large consumer biomass).
  • Trees (Large biomass) → Herbivores (Smaller biomass) → Carnivores (Even smaller biomass)

Pyramid of Numbers

  • Definition: Shows the number of organisms at each trophic level.
  • Shape: Can be a true pyramid, inverted, or irregular depending on ecosystem type.
  • Why Shape Varies?
    • In a grassland, many small producers (grass) support fewer herbivores (insects), and even fewer predators (birds).
    • In a forest, one large tree (producer) supports many herbivores (caterpillars), leading to an inverted pyramid.
  • Grassland: Many grasses → Fewer herbivores → Even fewer carnivores
  • Forest: Few trees → Many insects → Fewer birds

Comparison Between the Three Pyramids

  • Pyramid of Energy
    • Advantages:
      • Shows actual energy transfer between trophic levels.
      • Always pyramid-shaped (follows the 10% Rule).
      • No inverted pyramids, making comparisons easy.
    • Disadvantages:
      • Difficult to measure energy accurately.
      • Requires complex calculations over time.
      • Does not show population size.
  • Pyramid of Biomass
    • Advantages:
      • More accurate than a pyramid of numbers (accounts for size differences in organisms).
      • Shows how much living material is available at each level.
      • Helps understand food availability in ecosystems.
    • Disadvantages:
      • Difficult to measure dry mass without killing organisms.
      • Can be inverted (e.g., in aquatic ecosystems, where phytoplankton have less mass but reproduce quickly).
      • Does not show energy flow over time.
  • Pyramid of Numbers
    • Advantages:
      • Simple and easy to count organisms.
      • Gives a clear view of population structure.
      • No need for complex calculations.
    • Disadvantages:
      • Does not account for organism size (e.g., one tree can support many insects).
      • Can be inverted or irregular, making interpretation difficult.
      • Does not show the amount of energy or biomass at each level.

Prey-Predator Relationship

  • Predator population follows prey population in cycles.
  • If prey numbers increase, predators have more food and their population grows.
  • As predator numbers increase, they eat more prey, causing the prey population to decrease.
  • This leads to a fluctuating cycle of predator-prey populations.
  • Example:
    • More rabbits → More foxes
    • Too many foxes eat too many rabbits → Rabbit numbers drop
    • Less rabbits → Foxes starve → Fox numbers drop
    • Fewer foxes → Rabbits increase again

S-Curve or Population Graph

  • The S-Curve (or Population curve) represents the population growth of an organism in a habitat with limited resources.

Lag Phase

  • Description:
    • The population starts with a small number of organisms, so growth is slow.
    • Individuals are adapting to the new environment.
  • Reasons for Slow Growth:
    • Time Needed for Maturity – Organisms need time to grow and reach reproductive age.
    • Low Reproductive Rate – Few individuals mean limited mating opportunities, reducing the number of offspring.
    • Adaptation to the Environment – Organisms need time to adjust to new food sources, shelter, and climatic conditions.

Log Phase

  • Description:
    • Population size doubles rapidly due to abundant resources (food, space, mates).
    • Birth rate exceeds death rate.
  • Reasons for Rapid Growth:
    • No Competition – Plenty of food and space allow organisms to grow and reproduce freely.
    • Few Predators or Diseases – Low predator populations and minimal disease spread enable high survival rates.
    • Favorable Environmental Conditions – Optimal temperature, oxygen, and nutrients allow faster metabolism and reproduction.

Stationary Phase

  • Description: Population growth slows down and fluctuates around the carrying capacity. Resources become limited, leading to competition.
  • Reasons for Growth Slowing Down:
    • Resource Depletion – Food, water, and space become scarce, limiting further growth.
    • Increased Competition – Organisms compete for food, mates, and shelter.
    • Predation & Disease – High population density leads to higher spread of diseases and attracts more predators, increasing the death rate.

Decline Phase

  • Definition: The decline phase occurs when a population size decreases because the death rate exceeds the birth rate. This happens due to environmental pressures, resource depletion, and other limiting factors.
  • Reasons for Population Decline:
    • Severe Resource Depletion – Overconsumption of food, water, and shelter leads to starvation and malnutrition, reducing reproduction rates and increasing mortality.
    • Increased Competition – As population density rises, intense competition for limited resources causes weaker individuals to die out, lowering overall population numbers.
    • Predation and Disease – Higher population densities attract more predators and make disease transmission easier, leading to mass mortality.
    • Toxic Waste Accumulation – In some environments (e.g., bacteria in a closed system), waste products accumulate, creating harmful conditions that kill organisms.