Unit 5: Ecology and Environment - Chapter 2: Ecosystem

5.2 Ecosystem

Types and Components: Biotic, Abiotic

An ecosystem is a functional unit of nature, where living organisms (biotic components) interact among themselves and also with the surrounding physical environment (abiotic components).

Types of Ecosystems:

1. Natural Ecosystems: Exist without significant human intervention.
   • Terrestrial: Forests, grasslands, deserts, tundra.
   • Aquatic: Ponds, lakes, rivers, oceans, estuaries.
2. Artificial (Man-made) Ecosystems: Created and maintained by humans.
   • Crop fields, aquariums, gardens, dams.

Components of an Ecosystem:

1. Abiotic Components (Non-living):

   • Climatic Factors: Temperature, light, water, wind, humidity, rainfall.
   • Edaphic Factors: Soil (pH, mineral composition, topography).
   • Inorganic Substances: Carbon dioxide, oxygen, nitrogen, water, phosphorus, calcium.
   • Organic Substances: Proteins, carbohydrates, lipids, humic substances (link biotic and abiotic components).

2. Biotic Components (Living):

   • Producers (Autotrophs): Organisms that produce their own food, primarily through photosynthesis (e.g., green plants, algae, cyanobacteria). In deep-sea hydrothermal vents, chemosynthetic bacteria are producers.
   • Consumers (Heterotrophs): Organisms that depend on producers or other consumers for food.
     • Primary Consumers (Herbivores): Feed directly on producers (e.g., deer, rabbit, insects).
     • Secondary Consumers (Primary Carnivores): Feed on primary consumers (e.g., frog, small birds).
     • Tertiary Consumers (Secondary Carnivores): Feed on secondary consumers (e.g., snake, large fish).
     • Quaternary Consumers (Tertiary Carnivores): Feed on tertiary consumers (e.g., eagle, lion).
   • Decomposers (Saprotrophs): Organisms that break down dead organic matter (detritus) of producers and consumers into simpler inorganic substances (e.g., bacteria, fungi). They play a crucial role in nutrient cycling.

Pond Ecosystem

A pond is a shallow, freshwater aquatic ecosystem. It serves as a good example to understand the basic structure and function of an ecosystem.

Abiotic Components:

• Water: The medium for all life processes.
• Sunlight: Penetrates to varying depths, supporting photosynthesis.
• Dissolved Oxygen: Essential for aquatic respiration.
• Inorganic Salts: Nutrients like nitrates, phosphates, carbonates, sulfates.
• Organic Substances: Humus, amino acids, fatty acids from decaying matter.
• Temperature: Varies with depth and season.

Biotic Components:

• Producers:
  • Phytoplankton: Microscopic, free-floating algae (e.g., Spirogyra, Volvox, diatoms).
  • Floating Plants: (e.g., Lemna, Azolla, Pistia).
  • Submerged Plants: (e.g., Hydrilla, Vallisneria).
  • Marginal Plants: (e.g., Typha, Sagittaria).
• Consumers:
  • Primary Consumers (Zooplankton): Microscopic animals that feed on phytoplankton (e.g., Daphnia, Cyclops).
  • Secondary Consumers: Small fish, insects, frogs that feed on zooplankton and small aquatic animals.
  • Tertiary Consumers: Large fish, birds (e.g., kingfisher) that feed on smaller fish and frogs.
• Decomposers: Bacteria and fungi (e.g., Aspergillus, Penicillium) that break down dead organic matter at the bottom of the pond.

Productivity: GPP, NPP, Secondary Productivity

Productivity refers to the rate of biomass production or the rate at which energy is accumulated in an ecosystem.

1. Primary Productivity: The rate at which producers (plants) synthesize organic matter from inorganic substances, primarily through photosynthesis.

   • Gross Primary Productivity (GPP): The total rate of organic matter production during photosynthesis. It includes the organic matter used by producers for their own respiration.
   • Net Primary Productivity (NPP): The amount of organic matter remaining after producers have utilized some for their own respiration (R). It is the available biomass for heterotrophs. NPP = GPP - R
   • Units: g/m²/year (for biomass) or kcal/m²/year (for energy).

2. Secondary Productivity: The rate of formation of new organic matter by consumers (heterotrophs). It is the rate of assimilation of food by consumers.

Decomposition: Steps, Factors

Decomposition is the process by which decomposers (bacteria and fungi) break down complex organic matter (detritus) into simpler inorganic substances like CO2, water, and nutrients. It is a vital process for nutrient cycling.

Steps of Decomposition:

1. Fragmentation: Detritivores (e.g., earthworms) break down detritus into smaller particles, increasing the surface area for microbial action.
2. Leaching: Water-soluble inorganic nutrients seep down into the soil horizon and get precipitated as unavailable salts.
3. Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
4. Humification: Accumulation of a dark-colored, amorphous substance called humus. Humus is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. It is a reservoir of nutrients.
5. Mineralization: Humus is further degraded by some microbes, releasing inorganic nutrients (e.g., CO2, H2O, mineral ions) into the soil.

Factors Affecting Decomposition:

• Chemical Composition of Detritus: Decomposition rate is slower if detritus is rich in lignin and chitin, and quicker if rich in nitrogen and water-soluble sugars.
• Temperature: Optimal temperature (warm) favors decomposition. Low temperatures inhibit it.
• Moisture: Adequate moisture is essential. Too dry or too wet conditions inhibit decomposition.
• Aeration: Oxygen is required for aerobic decomposition. Anaerobic conditions slow down the process.
• pH: Neutral to slightly alkaline pH is generally favorable.

Energy Flow: Food Chains (Grazing, Detritus), Food Webs, Trophic Levels

Energy Flow in an ecosystem is unidirectional and follows the laws of thermodynamics. Energy enters the ecosystem from the sun, is captured by producers, and then flows through various trophic levels.

1. Food Chain: A sequence of organisms through which energy is transferred from one trophic level to the next by feeding.

   • Grazing Food Chain (GFC): Starts with producers (plants) and moves to herbivores and then to carnivores.
     • Example: Grass → Deer → Tiger
   • Detritus Food Chain (DFC): Starts with dead organic matter (detritus) and moves to decomposers (bacteria, fungi) and then to detritivores.
     • Example: Dead leaves → Fungi → Earthworm → Bird
     • In aquatic ecosystems, GFC is the major conduit for energy flow. In terrestrial ecosystems, a large fraction of energy flows through the DFC.

2. Food Web: A network of interconnected food chains, showing the complex feeding relationships within an ecosystem. It is more realistic than a simple food chain as organisms often feed on multiple types of prey and are themselves preyed upon by multiple predators.

3. Trophic Levels: The position an organism occupies in a food chain, based on its feeding habits.

   • Trophic Level 1: Producers (Autotrophs) - e.g., Plants
   • Trophic Level 2: Primary Consumers (Herbivores) - e.g., Deer, Rabbit
   • Trophic Level 3: Secondary Consumers (Primary Carnivores) - e.g., Fox, Snake
   • Trophic Level 4: Tertiary Consumers (Secondary Carnivores) - e.g., Lion, Eagle
   • Decomposers occupy all trophic levels as they break down dead organic matter from all levels.

Ecological Pyramids: Energy, Number, Biomass

Ecological Pyramids are graphical representations of the relationship between different trophic levels in an ecosystem. The base of the pyramid represents the producers, and successive levels represent consumers.

1. Pyramid of Energy:

   • Always upright.
   • Represents the total amount of energy at each trophic level.
   • Energy decreases progressively at successive trophic levels because a large portion of energy is lost as heat during metabolic activities (10% Law).
   • Units: kcal/m²/year or J/m²/year.

2. Pyramid of Number:

   • Represents the number of individual organisms at each trophic level.
   • Can be upright, inverted, or spindle-shaped.
     • Upright: In grassland ecosystem (e.g., Grass > Grasshopper > Frog > Snake > Eagle).
     • Inverted: In a parasitic food chain (e.g., Single tree > Birds > Lice).
     • Spindle-shaped: In a forest ecosystem (e.g., Large trees < Deer > Fox).

3. Pyramid of Biomass:

   • Represents the total dry weight (biomass) of organisms at each trophic level.
   • Can be upright or inverted.
     • Upright: In most terrestrial ecosystems (e.g., Grassland: Producers > Herbivores > Carnivores).
     • Inverted: In aquatic ecosystems (e.g., Pond: Phytoplankton < Zooplankton < Small fish < Large fish). This is because phytoplankton have a very short lifespan and reproduce rapidly, so at any given time, their biomass is less than that of the zooplankton they support.

PAR, 10% Law, Standing Crop/State

1. Photosynthetically Active Radiation (PAR):

   • The portion of the electromagnetic spectrum (wavelengths 400-700 nm) that photosynthetic organisms are able to use in the process of photosynthesis.
   • Of the total incident solar radiation, less than 50% is PAR.
   • Plants capture only 2-10% of the PAR.

2. 10% Law (Lindeman's Law):

   • Proposed by Raymond Lindeman in 1942.
   • States that only about 10% of the energy from one trophic level is transferred to the next trophic level.
   • The remaining 90% is lost as heat during metabolic processes or remains unconsumed/undigested.
   • This explains why food chains rarely consist of more than 4-5 trophic levels.

3. Standing Crop/State:

   • Standing Crop: The amount of living biomass (or the number of organisms) present at a particular trophic level at a given time.
   • Standing State: The amount of inorganic nutrients (e.g., carbon, nitrogen, phosphorus, calcium) present in the soil or water at a given time. It is also known as the nutrient pool.