Ecosystem – Structure And Function

An ecosystem is a functional unit where living organisms interact with each other and their physical environment. Ecosystems vary in size, from small ponds to large forests or oceans. They are broadly categorized into terrestrial (forests, grasslands, deserts) and aquatic (ponds, lakes, rivers, oceans). Ecosystems function through the interplay of four basic components: productivity, decomposition, energy flow, and nutrient cycling.

The structure of an ecosystem is characterized by its species composition and vertical distribution of species, known as stratification. In a forest, for example, trees form the top layer, shrubs the middle, and herbs and grasses the bottom.

Pond Ecosystem: A pond serves as a simple, self-sustainable model for understanding ecosystem functions. Its abiotic components include water, dissolved nutrients, and bottom soil, regulated by solar input and climatic factors. Biotic components consist of producers (phytoplankton, algae, aquatic plants), consumers (zooplankton, fish), and decomposers (bacteria, fungi).

The fundamental processes are:

• Conversion of inorganic matter to organic matter by producers using solar energy.
• Consumption of producers by consumers.
• Decomposition and mineralisation of dead organic matter by decomposers, returning nutrients for reuse by producers.

Energy flows unidirectionally from producers to consumers and is dissipated as heat at each trophic level, in accordance with the laws of thermodynamics.

Productivity

Productivity is the rate of biomass or organic matter production per unit area over time. It's a fundamental requirement for ecosystem function, driven by solar energy.

• Primary Production: The rate at which autotrophs (producers) convert solar energy into organic matter through photosynthesis. It is expressed as dry weight (gm⁻²) or energy (kcal m⁻²).
• Gross Primary Productivity (GPP): The total rate of photosynthesis.
• Net Primary Productivity (NPP): GPP minus the energy lost through plant respiration (R). NPP represents the available biomass for consumers (heterotrophs and decomposers).
• Secondary Productivity: The rate of formation of new organic matter by consumers.

Primary productivity varies among ecosystems, influenced by plant species and environmental factors like nutrient availability and photosynthetic capacity. The Earth's biosphere has an annual NPP of about 170 billion tons of dry organic matter, with terrestrial ecosystems contributing more than oceanic ones, despite oceans covering a larger surface area.

Decomposition

Decomposition is the process by which decomposers (primarily bacteria and fungi) break down complex organic matter (detritus – dead plant and animal remains, fecal matter) into simpler inorganic substances like carbon dioxide, water, and nutrients. This process is crucial for nutrient cycling.

The key steps in decomposition are:

1. Fragmentation: Detritivores (like earthworms) break down detritus into smaller pieces.
2. Leaching: Water-soluble inorganic nutrients move down into the soil horizon.
3. Catabolism: Enzymes secreted by bacteria and fungi degrade detritus into simpler inorganic substances.
4. Humification: Formation of humus, a dark, amorphous, nutrient-rich substance highly resistant to microbial breakdown. Humus acts as a reservoir of nutrients.
5. Mineralisation: Further degradation of humus by microbes, releasing inorganic nutrients into the soil, making them available for producers.

Decomposition is largely an aerobic process. Its rate is influenced by the chemical composition of detritus (rich in lignin and chitin slows it down; rich in nitrogen and sugars speeds it up) and climatic factors like temperature and soil moisture (warm, moist conditions favour decomposition).

Energy Flow

The Sun is the primary energy source for most ecosystems. Producers (plants, algae, photosynthetic bacteria) capture solar energy (less than 50% of PAR) and convert it into organic matter through photosynthesis, with only 2-10% of PAR being utilized. Energy flows unidirectionally from producers to consumers and decomposers, with a significant loss of energy as heat at each transfer, adhering to the Second Law of Thermodynamics.

Food Chains and Food Webs: Organisms are linked by feeding relationships, forming food chains. A simple grazing food chain (GFC) starts with producers (e.g., Grass → Goat → Man). The detritus food chain (DFC) begins with dead organic matter, involving decomposers (bacteria, fungi) that break it down into inorganic nutrients.

Trophic Levels: Organisms occupy specific feeding positions called trophic levels: Producers (1st trophic level), Primary consumers/herbivores (2nd), Secondary consumers/carnivores (3rd), and Tertiary consumers/top carnivores (4th). Energy transfer between trophic levels follows the 10% Law, where only about 10% of the energy from one level is transferred to the next. The remaining energy is lost as heat. Food webs represent interconnected food chains.

Standing Crop: The biomass or number of organisms at a particular trophic level at a given time is called the standing crop. It is typically measured as dry weight for accuracy.

Ecological Pyramids

Ecological pyramids illustrate the relationship between different trophic levels in an ecosystem, typically showing a broad base (producers) narrowing towards the apex (top consumers).

There are three main types:

• Pyramid of Numbers: Represents the number of individuals at each trophic level. Usually upright (more producers than consumers), but can be inverted (e.g., a single large tree supporting numerous insects).
• Pyramid of Biomass: Represents the total dry weight of organisms at each trophic level. Generally upright, but can be inverted in aquatic ecosystems where the biomass of consumers (e.g., fish) can exceed that of producers (phytoplankton) due to the latter's rapid turnover rate.
• Pyramid of Energy: Always upright, as energy transfer between trophic levels is inefficient (only ~10% is transferred), with significant loss as heat at each step. It represents the energy available at each trophic level per unit area per unit time.

It's important to note that ecological pyramids represent functional trophic levels, not species themselves, as a species can occupy multiple trophic levels. Saprophytes (decomposers) are not typically included in these pyramids despite their vital role.

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