An Ecosystem is a functional unit of nature where living organisms interact among themselves and also with the surrounding physical environment. Structure of Ecosystem: Interaction of biotic and abiotic components results in a physical structure. Stratification is the vertical distribution of different species occupying different levels (e.g., trees occupy top vertical strata, shrubs the second, and herbs/grasses the bottom layers).

The four important functional aspects of an ecosystem are:

1. Productivity

2. Decomposition

3. Energy Flow

4. Nutrient Cycling

Productivity: A constant input of solar energy is the basic requirement for any ecosystem to function and sustain.

• Primary Production: Amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. Expressed in terms of weight (g⁻²) or energy (kcal • m⁻²).

• Gross Primary Productivity (GPP): Rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilized by plants in respiration (R).

• Net Primary Productivity (NPP): Gross primary productivity minus respiration losses (R). GPP – R = NPP. NPP is the available biomass for the consumption to heterotrophs (herbivores and decomposers).

• Secondary Productivity: Rate of formation of new organic matter by consumers.

Decomposition: Decomposers (e.g., earthworms, bacteria, fungi) break down complex organic matter into inorganic substances like carbon dioxide, water, and nutrients. The raw material for decomposition is Detritus (dead plant remains like leaves, bark, flowers, and dead animal remains).

Steps of Decomposition:

1. Fragmentation: Detritivores (e.g., earthworms) break down detritus into smaller particles.

2. Leaching: Water-soluble inorganic nutrients go 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-coloured amorphous substance called Humus (highly resistant to microbial action, undergoes decomposition at an extremely slow rate, acts as a reservoir of nutrients).

5. Mineralisation: Humus is further degraded by some microbes and release of inorganic nutrients occurs.

   Factors affecting decomposition: It is largely an oxygen-requiring process. Warm and moist environment favors decomposition; low temperature and anaerobiosis inhibit it.

Energy Flow: Sun is the only source of energy for all ecosystems on Earth (except deep sea hydro-thermal ecosystem). Of the incident solar radiation, less than 50% is Photosynthetically Active Radiation (PAR). Plants capture only 2-10% of the PAR.

Energy flow is unidirectional (Sun →  Producers →  Consumers).

• Trophic Levels: Organisms occupy a specific place in the food chain known as their trophic level. Producers (First), Herbivores (Second), Carnivores (Third).

• 10% Law: Proposed by Lindeman. Only 10% of the energy is transferred to each successive trophic level from the lower trophic level. The remaining 90% is lost as heat.

• Food Chains:

  • Grazing Food Chain (GFC): Begins with producers. (Grass →   Goat →   Man).

  • Detritus Food Chain (DFC): Begins with dead organic matter. Made up of decomposers (saprotrophs).

Ecological Pyramids: Graphical representation of the trophic structure.

1. Pyramid of Number: Usually upright (e.g., Grassland). Can be inverted (e.g., Single tree ecosystem with many parasites).

2. Pyramid of Biomass: Usually upright. Inverted in aquatic ecosystems (Small standing crop of phytoplankton supports a large standing crop of zooplankton).

3. Pyramid of Energy: Always Upright. Energy is always lost as heat at each step; it can never increase.

Ecological Succession: The gradual and fairly predictable change in the species composition of a given area. The entire sequence of communities that successively change in a given area are called Sere.

• Primary Succession: Occurs on newly cooled lava, bare rock, newly created pond (slow process).

• Secondary Succession: Occurs in areas where natural biotic communities have been destroyed (e.g., burned forests, flooded lands). Faster than primary succession.

• Types:

  • Hydrarch Succession: Takes place in wet areas (Pond →   →   →   Forest).

  • Xerarch Succession: Takes place in dry areas (Rock →   →   →   Forest).

  • Both successions lead to a medium water condition called Mesic condition.

  • Pioneer Species: The species that invade a bare area. Lichens on rocks (secrete acids to dissolve rock); Phytoplankton in water.

  • Climax Community: A community that is in near equilibrium with the environment.

Nutrient Cycling (Biogeochemical Cycles): The movement of nutrient elements through the various components of an ecosystem.

• Standing State: The amount of nutrients (C, N, P, Ca, etc.) present in the soil at any given time.

• Gaseous Cycles: Reservoir is the atmosphere or hydrosphere (e.g., Nitrogen, Carbon).

  • Carbon Cycle: 71% of global carbon is dissolved in oceans. Atmosphere contains only about 1%. Photosynthesis fixes 4 × 10¹³ kg of carbon annually. Decomposers also release CO₂.

• Sedimentary Cycles: Reservoir is Earth’s crust (e.g., Phosphorus, Sulphur).

  • Phosphorus Cycle: Phosphorus is a constituent of biological membranes, nucleic acids, and cellular energy transfer systems. The natural reservoir is rock (contains phosphorus as phosphates). weathering releases minute amounts into soil solution. There is no respiratory release of phosphorus into the atmosphere (unlike carbon).

Ecosystem Services: The products of ecosystem processes (e.g., purifying air/water, mitigating floods, generating fertile soil). Robert Costanza and colleagues have put an average price tag of US 33 trillion a year on these fundamental ecosystem services (nearly twice the global GNP). Soil formation accounts for about 50% of this cost.