Unit 4: Ecosystem structure and function

Types of Ecosystem

An ecosystem is a community plus its abiotic environment. They are broadly classified into two categories:

1. Terrestrial Ecosystems: Land-based ecosystems.
   • Examples: Forest, Grassland, Desert, Tundra.
   • Limiting factors are often water and temperature.
2. Aquatic Ecosystems: Water-based ecosystems.
   • Freshwater:
     • Lentic (Standing): Ponds, Lakes.
     • Lotic (Flowing): Rivers, Streams.
   • Marine: Oceans, Coral Reefs, Estuaries (where fresh and salt water mix).
   • Limiting factors are often light and nutrients (N, P).

Components of Ecosystem

Every ecosystem has two fundamental components:

1. Abiotic (Non-Living) Components

The physical and chemical factors of the environment.

• Climatic: Sunlight (energy), temperature, precipitation, wind.
• Edaphic (Soil): Soil pH, minerals, soil texture.
• Chemical: Water, oxygen, CO₂, nutrient levels (N, P, K).

2. Biotic (Living) Components

All the living organisms, categorized by their feeding level (trophic level).

• Producers (Autotrophs): "Self-feeders." They create their own food from abiotic sources.
  • Photoautotrophs: Use sunlight (e.g., plants, algae).
  • Chemoautotrophs: Use chemical energy (e.g., bacteria in deep-sea vents).
• Consumers (Heterotrophs): "Other-feeders." They get energy by eating other organisms.
  • Primary Consumers (Herbivores): Eat producers (e.g., grasshopper, deer).
  • Secondary Consumers (Carnivores): Eat primary consumers (e.g., frog, wolf).
  • Tertiary Consumers: Eat secondary consumers (e.g., snake, hawk).
• Decomposers (Saprotrophs): Break down dead organic matter (detritus) and waste products, returning nutrients to the soil. (e.g., bacteria, fungi).

Energy Flow (Food Chain and Food Web)

The First Law of Thermodynamics states energy cannot be created or destroyed, only transferred. In an ecosystem, energy flows in one direction.

Food Chain

A simple, linear pathway of energy transfer between trophic levels.

Example: Grass (Producer) → Grasshopper (Primary Consumer) → Frog (Secondary Consumer) → Snake (Tertiary Consumer)

Food Web

A more realistic model showing the complex, interconnected network of many different food chains in an ecosystem. Most organisms eat more than one thing and are eaten by more than one thing.

[Image of a diagram of a simple food web]

Ecological Efficiencies

Ecological efficiency describes the efficiency with which energy is transferred from one trophic level to the next. This transfer is always inefficient due to the Second Law of Thermodynamics.

The 10% Rule (Trophic Efficiency)

As a general rule, only about 10% of the energy from one trophic level is converted into biomass at the next trophic level.

Where does the other 90% go?

• Lost as metabolic heat during respiration.
• Used for movement, reproduction, etc.
• Not consumed (e.g., bones, wood).
• Lost as waste (feces, urine).

Ecological Pyramids

A graphical representation of the trophic structure of an ecosystem. The base is always the producers (Trophic Level 1), followed by primary consumers (TL2), secondary (TL3), etc.

1. Pyramid of Numbers

• What it shows: The total number of individuals at each trophic level.
• Shape: Usually "upright" (e.g., 1,000,000 grasses support 100 rabbits, which support 1 fox).
• Inverted Pyramid: Can be inverted. For example, one large tree (producer) can support thousands of insects (primary consumers).

2. Pyramid of Biomass

• What it shows: The total dry weight (biomass) of all organisms at each trophic level.
• Shape: Usually upright, as biomass decreases at higher levels.
• Inverted Pyramid: Can be inverted in some aquatic ecosystems. The producers are phytoplankton, which have a very short lifespan and reproduce rapidly. At any given moment, their total biomass is small, but it can support a larger biomass of zooplankton (primary consumers) that live longer.

3. Pyramid of Energy

• What it shows: The rate of energy flow (productivity) through each trophic level.
• Shape: ALWAYS UPRIGHT.
• Why: Due to the Second Law of Thermodynamics and the 10% rule, energy is always lost at each transfer. A higher trophic level can never have more energy than the level below it.