Unit 4: Ecosystem Ecology

Ecosystem Ecology studies the flow of energy and the cycling of materials through the biotic and abiotic components of an ecosystem. It focuses on function (what it does) rather than just structure (what is there).

Ecosystem: Components

An ecosystem is composed of two main types of components:

1. Abiotic Components (Non-living):
   • Climatic Factors: Sunlight (energy), temperature, water (precipitation), humidity, wind.
   • Edaphic Factors (Soil): Soil type, pH, mineral composition, soil moisture.
   • Inorganic Substances: Carbon, nitrogen, phosphorus, water, CO2.
2. Biotic Components (Living):
   • Producers (Autotrophs): "Self-feeders." They create their own food from abiotic substances, usually via photosynthesis (using sunlight).
     • Example: Plants, algae, cyanobacteria.
   • Consumers (Heterotrophs): "Other-feeders." They obtain energy by feeding on other organisms.
     • Primary Consumers (Herbivores): Eat producers (e.g., rabbits, cows, grasshoppers).
     • Secondary Consumers (Carnivores): Eat primary consumers (e.g., foxes, snakes).
     • Tertiary Consumers (Carnivores): Eat secondary consumers (e.g., eagles, lions).
     • Omnivores: Eat both producers and consumers (e.g., bears, humans, raccoons).
   • Decomposers (Saprotrophs): They break down dead organic matter (dead plants, animals, waste products) and return essential nutrients to the soil, making them available for producers again.
     • Example: Bacteria and fungi.

Ecosystem: Types

Terrestrial Ecosystems

Ecosystems found on land (e.g., forests, grasslands, deserts).

• Abiotic: Sunlight availability (can be limited by canopy), temperature (variable), water (often a limiting factor), soil type, wind.
• Biotic:
  • Producers: Plants (trees, shrubs, grasses).
  • Consumers: Insects, birds, mammals, reptiles.
  • Decomposers: Fungi and bacteria are dominant (Detritus food chain is very important).

Lentic and Lotic Ecosystems

These are the two main types of freshwater ecosystems.

• Lentic (Standing Water):
  • Examples: Lakes, ponds, marshes.
  • Abiotic: Characterized by thermal stratification (layers: epilimnion, hypolimnion). Light decreases with depth (zones: littoral, limnetic, profundal, benthic).
  • Biotic: Producers are algae (phytoplankton) and rooted plants (macrophytes) in the littoral (shallow) zone. Consumers include zooplankton, fish, amphibians.
• Lotic (Running Water):
  • Examples: Rivers, streams, creeks.
  • Abiotic: Characterized by unidirectional current. Water is generally well-mixed and oxygenated. The substrate (rocky vs. silty) is critical.
  • Biotic: Producers are often attached algae (periphyton) on rocks. Many organisms have adaptations to avoid being washed away (e.g., flattened bodies, suction cups). Fish are often strong swimmers (e.g., trout).

Estuarine and Marine Ecosystems

• Estuarine Ecosystems (Estuaries):
  • Definition: An ecotone where a freshwater river meets the saltwater ocean.
  • Abiotic: Characterized by fluctuating salinity (salt content) due to tides. Water is often murky (turbid) and nutrient-rich (from river runoff).
  • Biotic: Very high productivity ("nutrient trap"). Dominated by euryhaline organisms (those that can tolerate a wide range of salinity). Examples: Mangrove swamps, salt marshes. They are critical "nurseries" for many marine fish and shellfish.
• Marine Ecosystems (Oceans):
  • Definition: Saltwater ecosystems, covering >70% of Earth.
  • Abiotic: High salinity, vast, deep. Zoned by light (photic, aphotic) and location (intertidal, neritic, oceanic, benthic).
  • Biotic: Producers are almost exclusively phytoplankton (algae) in the photic (sunlit) zone. Supports a huge diversity from zooplankton to large whales. Deep-sea (aphotic) ecosystems rely on "marine snow" (falling detritus) or chemosynthesis (at hydrothermal vents).

Ecosystem Structure and Function

Abiotic and Biotic Components (Recap)

Ecosystem structure refers to its components: both the abiotic (non-living) factors and the biotic (living) factors, including their distribution and organization (e.g., stratification, species diversity).

Ecosystem function refers to the processes that occur within it, primarily energy flow and nutrient cycling.

Trophic Level (Functional Level): A trophic level is the position an organism occupies in a food chain. It represents a step in the transfer of energy.

• Trophic Level 1 (T1): Producers (Plants, algae).
• Trophic Level 2 (T2): Primary Consumers (Herbivores).
• Trophic Level 3 (T3): Secondary Consumers (Carnivores).
• Trophic Level 4 (T4): Tertiary Consumers (Carnivores).

Food Chain and Food Web

• Food Chain: A simple, linear pathway of energy transfer.
  • Example: Grass → Grasshopper → Frog → Snake → Eagle
• Food Web: A more realistic and complex network of interconnected food chains. It shows that most organisms eat, and are eaten by, multiple other species. A complex food web leads to a more stable ecosystem.

Types of Food Chains:

1. Grazing Food Chain (GFC): Starts with producers (plants) and goes to herbivores and then carnivores. This is the dominant food chain in many aquatic ecosystems.
   • (e.g., Phytoplankton → Zooplankton → Small Fish → Large Fish)
2. Detritus Food Chain (DFC): Starts with dead organic matter (detritus) which is consumed by decomposers (bacteria, fungi) and detritivores (worms, millipedes), which are then eaten by small carnivores. This is the dominant food chain in many terrestrial (forest) ecosystems.

Ecological Pyramids

Graphical representations of the trophic structure of an ecosystem, with producers at the base and successive trophic levels forming the top.

1. Pyramid of Numbers:
   • Shows the number of individual organisms at each trophic level.
   • Upright: Usually upright, as many small producers support fewer herbivores, which support even fewer carnivores (e.g., Grassland).
   • Inverted: Can be inverted. Example: A single large tree (1 producer) supports thousands of insects (many primary consumers).
2. Pyramid of Biomass:
   • Shows the total dry weight (biomass) of all organisms at each trophic level.
   • Upright: Usually upright for terrestrial ecosystems (e.g., forest).
   • Inverted: Often inverted for aquatic ecosystems (e.g., a pond). The producers (phytoplankton) have a very small biomass at any given time because they reproduce and are eaten extremely quickly by the zooplankton, which have a larger standing biomass.
3. Pyramid of Energy:
   • Shows the amount of energy (or energy flow) at each trophic level over a period of time.
   • Always Upright: Due to the 10% rule and the 2nd Law of Thermodynamics, energy is always lost at each transfer. It can never be inverted.

Energy Flow

The flow of energy in an ecosystem is unidirectional (one-way) and follows the laws of thermodynamics.

• 1st Law of Thermodynamics: Energy cannot be created or destroyed, only transformed. (e.g., solar energy is transformed into chemical energy by plants).
• 2nd Law of Thermodynamics: During any energy transfer, some energy is lost as heat, and entropy (disorder) increases.

The 10% Rule (Lindeman's Rule of Trophic Efficiency):

On average, only about 10% of the energy from one trophic level is incorporated into the biomass of the next trophic level.

Where does the other 90% go?

• Lost as heat during metabolic processes (respiration).
• Not consumed (e.g., bones, fur, wood).
• Not assimilated (passed through as waste/feces).

Example: 10,000 J of sunlight → 1,000 J in plants (T1) → 100 J in grasshoppers (T2) → 10 J in frogs (T3) → 1 J in a snake (T4).
This massive loss of energy at each step is why food chains are typically short (rarely more than 4-5 levels) and why there is less biomass at higher trophic levels.

Decomposition

Definition: Decomposition is the physical and chemical breakdown of dead organic matter (detritus) into simpler inorganic substances (like CO2, water, and minerals).

This process is critical for nutrient cycling. It is carried out by decomposers (bacteria, fungi) and detritivores (earthworms, millipedes).

The process involves several steps:

1. Fragmentation: Detritivores (e.g., earthworms) break down detritus into smaller pieces. This increases the surface area for microbial (bacterial and fungal) action.
2. Leaching: Water-soluble inorganic nutrients dissolve in water and seep into the soil, where they may be lost from the system or absorbed by plants.
3. Catabolism: The "chemical" breakdown. Bacteria and fungi secrete digestive enzymes onto the detritus, breaking down complex organic molecules (like cellulose, lignin) into simpler ones.
4. Humification: The formation of humus, a dark, amorphous, stable, and nutrient-rich organic substance. Humus decomposes very slowly and acts as a reservoir of nutrients in the soil.
5. Mineralization: The final step, where humus and other organic matter are fully degraded, releasing inorganic nutrients (like Ca²⁺, Mg²⁺, NH₄⁺) into the soil, where they can be absorbed by plant roots.

Factors affecting decomposition:

• Oxygen: Decomposition is fastest in aerobic (oxygen-rich) conditions.
• Temperature: Faster at warm temperatures (optimal for microbial activity).
• Moisture: Requires moisture, but waterlogging (which creates anaerobic conditions) slows it down.
• Detritus Quality: Decomposes faster if rich in nitrogen and sugars; slower if rich in lignin (wood) or chitin.