ECOSYSTEM

Ecosystem – Structure and Function

An ecosystem is a functional unit of nature where living organisms interact among themselves and with their surrounding physical environment. Ecosystems vary greatly in size, from a small pond to a large forest or a sea. The entire biosphere can be considered a global ecosystem. For convenience, it is divided into two basic categories: terrestrial and aquatic.

• Terrestrial Ecosystems: Forest, grassland, desert.
• Aquatic Ecosystems: Pond, lake, wetland, river, estuary.
• Man-made Ecosystems: Crop fields, aquarium.

To understand an ecosystem, one must appreciate its:

• Input: Productivity.
• Energy Transfer: Food chain/web, nutrient cycling.
• Output: Degradation and energy loss.
• Relationships: Cycles, chains, webs formed by energy flows.

Ecosystems consist of abiotic and biotic components. The interaction of these components creates a physical structure characteristic of each ecosystem type.

• Species Composition: Identification and enumeration of plant and animal species.
• Stratification: Vertical distribution of different species occupying different levels.
  • Example: In a forest, trees occupy the top layer, shrubs the second, and herbs and grasses the bottom layers.

The components of an ecosystem function as a unit through four key aspects:

1. Productivity.
2. Decomposition.
3. Energy Flow.
4. Nutrient Cycling.

Pond Ecosystem: A Self-Sustainable Unit

A small pond serves as a simple, self-sustainable example of an aquatic ecosystem where all four basic components are well-exhibited.

• Abiotic Components: Water with dissolved inorganic and organic substances, rich soil deposit at the bottom, solar input, temperature cycle, day-length, and other climatic conditions regulating the pond’s function.
• Autotrophic Components: Phytoplankton, some algae, and floating, submerged, and marginal plants.
• Consumers: Zooplankton, free-swimming forms, and bottom-dwelling forms.
• Decomposers: Fungi, bacteria, and flagellates, especially abundant at the bottom.

A pond ecosystem performs all functions of any ecosystem, including:

• Conversion of inorganic to organic material by autotrophs using solar energy.
• Consumption of autotrophs by heterotrophs.
• Decomposition and mineralisation of dead matter for reuse by autotrophs.
• Unidirectional movement of energy towards higher trophic levels and its dissipation as heat.

Productivity

A constant input of solar energy is fundamental for any ecosystem’s function and sustenance.

• Primary Production: The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.
  • Expressed in terms of weight (gm⁻²) or energy (kcal m⁻²).
• Productivity: The rate of biomass production.
  • Expressed in terms of gm⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹ for comparison.
• Gross Primary Productivity (GPP): The rate of production of organic matter during photosynthesis.
  • A significant portion of GPP is used by plants for respiration.
• Net Primary Productivity (NPP): The biomass remaining after accounting for respiration losses (R).
  • Formula: GPP – R = NPP.
  • NPP is the available biomass for consumption by heterotrophs (herbivores and decomposers).
• Secondary Productivity: The rate of formation of new organic matter by consumers.

Primary productivity is influenced by:

• The plant species inhabiting an area.
• Various environmental factors.
• Availability of nutrients.
• Photosynthetic capacity of plants.

It varies across different types of ecosystems. The annual net primary productivity of the entire biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, oceans, despite occupying about 70 percent of the surface, contribute only 55 billion tons.

Decomposition

Decomposition is the process where decomposers break down complex organic matter into inorganic substances like carbon dioxide, water, and nutrients. Earthworms are known as farmer’s friends for their role in breaking down organic matter and loosening soil.

• Detritus: The raw material for decomposition, consisting of dead plant remains (leaves, bark, flowers) and dead animal remains, including fecal matter.

The important steps in decomposition are:

1. Fragmentation: Detritivores (e.g., earthworms) break down detritus into smaller particles.
2. Leaching: Water-soluble inorganic nutrients seep down into the soil horizon and precipitate as unavailable salts.
3. Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. These steps occur simultaneously.
4. Humification: Leads to the accumulation of humus, a dark-colored amorphous substance.
   • Humus is highly resistant to microbial action and decomposes very slowly.
   • Being colloidal, it serves as a reservoir of nutrients.
5. Mineralisation: Humus is further degraded by some microbes, releasing inorganic nutrients.

Decomposition is largely an oxygen-requiring process. Its rate is controlled by:

• Chemical composition of detritus: Slower if rich in lignin and chitin; quicker if rich in nitrogen and water-soluble substances like sugars.
• Climatic factors: Temperature and soil moisture are most important, affecting soil microbe activity.
  • Warm and moist environment favor decomposition.
  • Low temperature and anaerobiosis inhibit decomposition, leading to organic material build-up.

Energy Flow

The sun is the only source of energy for all ecosystems on Earth, except for deep-sea hydrothermal ecosystems.

• Photosynthetically Active Radiation (PAR): Less than 50% of incident solar radiation is PAR.
• Energy Capture: Plants and photosynthetic bacteria (autotrophs) fix the sun’s radiant energy to produce food from simple inorganic materials. Plants capture only 2-10% of PAR, yet this sustains the entire living world.

Energy flows unidirectionally from the sun to producers and then to consumers. Ecosystems adhere to the Second Law of Thermodynamics, requiring a constant energy supply to counteract increasing disorderliness.

• Producers: Green plants that synthesise their own food.
  • Terrestrial: Herbaceous and woody plants.
  • Aquatic: Phytoplankton, algae, higher plants.
• Consumers (Heterotrophs): Organisms that depend on plants (directly or indirectly) for their food needs.
  • Primary Consumers (Herbivores): Feed on producers (e.g., insects, birds, mammals in terrestrial; molluscs in aquatic).
  • Secondary Consumers (Primary Carnivores): Feed on primary consumers.
  • Tertiary Consumers: Feed on secondary consumers.

Food Chains and Food Webs

Organisms are interconnected by feeding relationships, forming food chains or food webs. When an organism dies, its energy is transferred to decomposers.

• Grazing Food Chain (GFC): Starts with plants (producers).
  • Example: Grass (Producer) → Goat (Primary Consumer) → Man (Secondary Consumer).
  • Major conduit for energy flow in aquatic ecosystems.
• Detritus Food Chain (DFC): Begins with dead organic matter.
  • Composed of decomposers, mainly fungi and bacteria, also known as saprotrophs.
  • Decomposers secrete digestive enzymes to break down dead and waste materials into simple inorganic substances, which they then absorb.
  • In terrestrial ecosystems, a much larger fraction of energy flows through the DFC than the GFC.
• Interconnection: DFC can be connected to GFC; some DFC organisms are prey for GFC animals.
• Food Web: The natural interconnection of multiple food chains.
• Omnivores: Animals like cockroaches, crows, and humans, which consume both plants and animals.

Trophic Levels and Energy Transfer

An organism’s trophic level is its specific place in the food chain based on its source of nutrition.

• First Trophic Level: Producers.
• Second Trophic Level: Herbivores (primary consumers).
• Third Trophic Level: Carnivores (secondary consumers).
• Energy decreases at successive trophic levels.

Standing Crop: The certain mass of living material at a particular time at each trophic level.

• Measured as biomass (mass of living organisms) or number in a unit area.
• Dry weight is a more accurate measure of biomass.

10 Percent Law: Only 10 percent of the energy is transferred to each trophic level from the lower trophic level. The remaining 90% is lost, primarily as heat. This law restricts the number of trophic levels in a grazing food chain.

Ecological Pyramids

Ecological pyramids graphically represent the food or energy relationship between organisms at different trophic levels, resembling a pyramid shape.

• The base represents producers (first trophic level).
• The apex represents tertiary or top-level consumers.

The three types of ecological pyramids are:

1. Pyramid of Number: Represents the number of individual organisms at each trophic level.
2. Pyramid of Biomass: Represents the total mass of living organisms (biomass) at each trophic level.
3. Pyramid of Energy: Represents the amount of energy available at each trophic level.

Characteristics of Ecological Pyramids

• Calculations must include all organisms at that trophic level.
• A given organism may occupy more than one trophic level simultaneously.
  • A trophic level represents a functional level, not a species.
  • Example: A sparrow is a primary consumer when eating seeds and a secondary consumer when eating insects.

In most ecosystems, all pyramids (number, energy, biomass) are upright:

• Producers are greater in number and biomass than herbivores.
• Herbivores are greater in number and biomass than carnivores.
• Energy at a lower trophic level is always more than at a higher level.

Exceptions and Considerations

• Pyramid of Numbers - Inverted: If you count insects feeding on a large tree and then birds feeding on insects, the pyramid of numbers can be inverted (one tree supports many insects).
• Pyramid of Biomass - Inverted: In the sea, the biomass of fishes often exceeds that of phytoplankton. This is because phytoplankton reproduce and are consumed very rapidly, having a small standing crop at any given time but high productivity over time.
• Pyramid of Energy - Always Upright: A pyramid of energy can never be inverted. This is because energy is always lost as heat at each step when it flows from one trophic level to the next, adhering to the 10 percent law.

Limitations of Ecological Pyramids

Ecological pyramids have certain limitations:

• They do not account for species belonging to two or more trophic levels.
• They assume a simple food chain, which rarely exists in nature, and do not accommodate a food web.
• Saprophytes (decomposers) are not given any place in ecological pyramids, despite their vital role.