Ecology Notes (Year 7)

Photosynthesis

• Photosynthesis: The process by which plants convert light energy into chemical energy in the form of glucose.

• Reactants:

  • Carbon dioxide ($CO_2$): Obtained from the air through small openings called stomata usually present on the lower surface of leaves.
  • Water ($H_2O$): Absorbed from the soil through the roots and transported to the leaves.
  • Light: The energy source, typically sunlight, absorbed by chlorophyll.

• Equation:

  $6CO<em>2 + 6H</em>2O + Light Energy \rightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$

Leaf Structure

• Leaf Structure:
  • Epidermis: The outer layer of cells that protects the leaf. It is transparent to allow light to enter.
  • Mesophyll: The middle layer of the leaf where photosynthesis occurs. It contains:
    • Palisade mesophyll: Tightly packed cells near the upper surface, containing many chloroplasts for maximum light absorption.
    • Spongy mesophyll: Loosely packed cells with air spaces to facilitate gas exchange.
  • Vascular bundles (Veins): Contain xylem and phloem for transporting water and nutrients.
  • Stomata: Small openings, usually on the lower surface, that allow carbon dioxide to enter and oxygen to exit. Guard cells control the opening and closing of stomata.

Factors Affecting Photosynthesis

• Light Intensity:
  • As light intensity increases, the rate of photosynthesis generally increases until a saturation point is reached.
• Carbon Dioxide Concentration:
  • Increasing carbon dioxide concentration generally increases the rate of photosynthesis until another factor becomes limiting.
• Temperature:
  • Photosynthesis is an enzyme-controlled process. As temperature increases, the rate of photosynthesis increases up to an optimum temperature. Beyond this, the enzymes denature, and the rate decreases.
• Water Availability:
  • Lack of water can limit photosynthesis, as water is a reactant. It also affects the opening and closing of stomata, limiting $CO_2$ intake.

Food Chains and Food Webs

• Food Chain: A linear sequence of organisms through which nutrients and energy pass as one organism eats another.
  • Example: Grass → Grasshopper → Frog → Snake → Hawk
• Food Web: A network of interconnected food chains, showing the complex feeding relationships in an ecosystem.
  • Food webs illustrate that many organisms have multiple food sources and roles.
• Trophic Levels:
  • Producers: Organisms that make their own food through photosynthesis (e.g., plants).
  • Primary Consumers: Herbivores that eat producers (e.g., grasshoppers).
  • Secondary Consumers: Carnivores that eat primary consumers (e.g., frogs).
  • Tertiary Consumers: Carnivores that eat secondary consumers (e.g., snakes).
  • Apex Predators: Top-level predators with no natural predators (e.g., hawks).
  • Decomposers: Organisms that break down dead plants and animals, returning nutrients to the soil (e.g., bacteria and fungi).

Quadrats

• Quadrats: Square or rectangular frames used to sample and measure the abundance and distribution of organisms in a habitat.
• Method:
  • Place the quadrat randomly or systematically in the area of study.
  • Count the number of individuals of each species within the quadrat.
  • Repeat the process multiple times in different locations.
  • Calculate the average number of individuals per quadrat to estimate population density.
• Uses:
  • Estimating plant population density
  • Measuring species frequency and percentage cover
  • Comparing species distribution in different areas

Line Transects

• Line Transects: A straight line along which organisms are sampled to study their distribution and abundance.
• Method:
  • Lay a measuring tape or rope along the ground in a straight line.
  • Record the species present at regular intervals along the line.
  • Alternatively, record the distance of each individual organism from the start of the line.
• Uses:
  • Studying changes in species distribution along an environmental gradient (e.g., elevation, moisture levels).
  • Assessing the impact of human activities on vegetation.
  • Measuring the abundance of sessile organisms (e.g., plants, barnacles).

Pyramids of Number and Biomass

• Pyramid of Number: A graphical representation of the number of organisms at each trophic level in an ecosystem.
  • Typically, the number of organisms decreases as you go up the trophic levels.
  • Producers (e.g., plants) are at the base and are most numerous.
  • Apex predators are at the top and are least numerous.
• Pyramid of Biomass: A graphical representation of the total mass of living organisms at each trophic level in an ecosystem.
  • Biomass generally decreases as you go up the trophic levels.
  • Producers have the highest biomass, while apex predators have the lowest.
• Differences and Implications:
  • Pyramids of number can be inverted if, for example, many insects feed on a single tree.
  • Pyramids of biomass provide a more accurate representation of the energy available at each trophic level, as biomass accounts for the size and mass of organisms.