Biology: Class 12 — Chapter 12: Ecosystem

Stratification is the vertical distribution of different species occupying different levels in an ecosystem. For example, in a forest ecosystem:

• Top vertical strata/layer: Trees
• Second layer: Shrubs
• Bottom layers: Herbs and grasses

To function as a self-sustaining unit, an ecosystem relies on four key aspects:

1. Productivity: Rate of biomass production.
2. Decomposition: Breakdown of complex organic matter into inorganic substances.
3. Energy flow: Unidirectional movement of energy from producers to higher trophic levels.
4. Nutrient cycling: Storage and movement of nutrient elements through ecosystem components.

A pond is a self-sustainable, shallow aquatic ecosystem exhibiting all four basic components:

• Abiotic components: Water with dissolved inorganic and organic substances, solar input, temperature cycle, day-length, and rich soil deposit at the bottom.
• Autotrophic components (Producers): Phytoplankton, some algae, and floating, submerged, and marginal plants.
• Consumers: Zooplankton, free-swimming, and bottom-dwelling forms.
• Decomposers: Fungi, bacteria, and flagellates (abundant at the bottom).

• Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight ($g/m^2$) or energy ($kcal/m^2$).
• Productivity is the rate of biomass production, expressed in $g\cdot m^{-2}\cdot yr^{-1}$ or $kcal\cdot m^{-2}\cdot yr^{-1}$ to compare different ecosystems.

• Gross Primary Productivity (GPP): The rate of production of organic matter during photosynthesis in an ecosystem.
• Net Primary Productivity (NPP): The biomass available for consumption by heterotrophs (herbivores and decomposers). It is GPP minus respiration losses ($R$).

Equation:

Secondary productivity is defined as the rate of formation of new organic matter by consumers.

• The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter.
• Despite occupying about 70% of the Earth's surface, the productivity of the oceans is only 55 billion tons, while the rest is produced on land.

Detritus is the raw material for decomposition. It consists of dead plant remains (leaves, bark, flowers) and dead remains of animals, including fecal matter.

Decomposition involves five simultaneous steps:

1. Fragmentation: Detritivores (e.g., earthworms) break down detritus into smaller particles.
2. Leaching: Water-soluble inorganic nutrients migrate down into the soil horizon and precipitate as unavailable salts.
3. Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
4. Humification: Leads to the accumulation of humus, a dark-coloured amorphous substance highly resistant to microbial action that decomposes extremely slowly and acts as a nutrient reservoir.
5. Mineralisation: Humus is further degraded by microbes to release inorganic nutrients.

• Decomposition is largely an oxygen-requiring process.
• Chemical composition: Decomposition is slower if detritus is rich in lignin and chitin, and quicker if rich in nitrogen and water-soluble substances like sugars.
• Climatic factors: Warm and moist environments favour decomposition, whereas low temperature and anaerobiosis (absence of oxygen) inhibit it, leading to a build-up of organic materials.

• Of the incident solar radiation, less than 50% is Photosynthetically Active Radiation (PAR).
• Plants and photosynthetic bacteria (autotrophs) capture only 2-10% of this PAR to synthesize food. This small amount of energy sustains the entire living world.

Energy flows unidirectionally from producers to consumers. Organisms occupy a specific place in a food chain based on their source of nutrition, known as their trophic level:

• First Trophic Level (Producers): Phytoplankton, grass, trees.
• Second Trophic Level (Primary Consumers / Herbivores): Zooplankton, grasshopper, cow.
• Third Trophic Level (Secondary Consumers / Primary Carnivores): Birds, fishes, wolf.
• Fourth Trophic Level (Tertiary Consumers / Top Carnivores): Man, lion.

• Grazing Food Chain (GFC): Begins with green plants (producers) at the first trophic level.

  Example: Grass (Producer) → Goat (Primary Consumer) → Man (Secondary Consumer)

• Detritus Food Chain (DFC): Begins with dead organic matter. It is made up of decomposers (saprotrophs like fungi and bacteria) which secrete digestive enzymes to break down dead matter and absorb the nutrients.

In aquatic ecosystems, GFC is the major conduit for energy flow. In terrestrial ecosystems, a much larger fraction of energy flows through the DFC than the GFC.

• Standing crop is the mass of living material (biomass) or the number of organisms in a unit area at a particular time at each trophic level. Biomass is more accurately measured in terms of dry weight.
• 10 Per Cent Law: During energy transfer from a lower to a higher trophic level, only 10% of the energy is stored as biomass; the remaining 90% is lost as heat to the environment. This limits the number of trophic levels in a GFC.

• Pyramid of Number: Shows the number of individual organisms at each trophic level. Usually upright (e.g., grassland ecosystem), but can be inverted (e.g., many insects feeding on a single big tree).
• Pyramid of Biomass: Shows the total dry weight of living matter at each level. Usually upright in terrestrial ecosystems, but inverted in aquatic ecosystems (e.g., a small standing crop of phytoplankton supports a much larger biomass of zooplankton and fish).
• Pyramid of Energy: Shows the energy content at each level. It is always upright and can never be inverted, because energy is lost as heat at each transfer step.

1. They do not accommodate species that belong to two or more trophic levels simultaneously (e.g., sparrows eating both seeds and insects).
2. They assume a simple, linear food chain, which almost never exists in nature; they do not accommodate food webs.
3. Saprophytes and decomposers are not given any place in ecological pyramids, despite playing a vital role in the ecosystem.