Unit 12: Ecosystems

1. Ecosystem - Structure and Function

An ecosystem is a functional unit of nature where living organisms interact among themselves and with their surrounding physical environment. It can be as small as a drop of water or as large as a vast forest or ocean.

Structure of an Ecosystem

The structure of an ecosystem is characterized by the identification and enumeration of plant and animal species and the distribution of various species vertically. The two main components of an ecosystem are:

• Biotic Components: The living members of the ecosystem, including producers (autotrophs like plants), consumers (heterotrophs like animals), and decomposers (saprotrophs like fungi and bacteria).
• Abiotic Components: The non-living physical and chemical factors of the environment, such as temperature, light, water, soil, and minerals.

Function of an Ecosystem

An ecosystem functions as a single integrated unit through the following processes:

• Productivity: Incorporation of energy by plants.
• Decomposition: Breakdown of dead organic matter.
• Energy Flow: Unidirectional movement of energy through various trophic levels.
• Nutrient Cycling: Storage and movement of nutrient elements through the components of the ecosystem.

An ecosystem is a self-sustaining structural and functional unit consisting of a community of living organisms and their non-living environment, interacting as a system.

Exam-Oriented Note: Remember that biotic and abiotic components do not exist in isolation; they continuously interact, influencing each other to maintain ecological balance.

2. Productivity

A constant input of solar energy is the basic requirement for any ecosystem to function and sustain. Productivity refers to the rate of biomass production per unit area over a time period.

Primary Productivity

Primary productivity refers to 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-2 yr-1) or energy (kcal m-2 yr-1). Primary productivity is divided into two types:

• Gross Primary Productivity (GPP): The rate of total production of organic matter during photosynthesis. A considerable amount of GPP is utilized by plants for respiration.
• Net Primary Productivity (NPP): The available biomass for the consumption to heterotrophs (herbivores and decomposers). It is calculated as NPP = GPP - R, where R represents respiration losses.

Secondary Productivity

Secondary productivity is defined as the rate of assimilation of food energy by consumers. Since consumers do not make their own food, they build new biomass from the organic matter they ingest.

Common Mistake: Students often confuse GPP with NPP. Always remember that GPP is the total production, while NPP is what is actually left for the next trophic level after plants meet their own respiratory needs.

3. Decompositionချင်း

Decomposition is the process by which decomposers break down complex organic matter into inorganic substances like carbon dioxide, water, and nutrients. The dead plant remains such as leaves, bark, flowers, and dead remains of animals constitute detritus, which is the raw material for decomposition.

Steps in Decomposition

The important steps in the process of decomposition are:

• Fragmentation: Detritivores (such as earthworms) break down detritus into smaller particles, increasing the surface area for microbial action.
• Leaching: Water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
• Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
• Humification: Leads to accumulation of a dark-colored amorphous substance called humus. Humus is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature, it serves as a reservoir of nutrients.
• Mineralization: The humus is further degraded by some microbes and release of inorganic nutrients occurs.

Decomposition is the physical and chemical breakdown of complex dead organic matter into simpler inorganic constituents by detritivores and decomposers.

Important Observation: Temperature and soil moisture are the most important climatic factors that regulate decomposition through their effect on the activities of soil microbes.

4. Energy Flow

Except for the deep-sea hydrothermal ecosystem, sun is the only source of energy for all ecosystems on Earth. Of the incident solar radiation, less than 50 percent of it is photosynthetically active radiation (PAR).

Key Principles of Energy Flow

• Plants capture only 2 to 10 percent of the PAR and this small energy sustains the entire living world.
• Energy flow in an ecosystem is unidirectional. Energy trapped by autotrophs does not revert back to the sun; rather, it passes to herbivores, and then to carnivores.
• Ecosystems are governed by the Universal Laws of Thermodynamics. They need a constant supply of energy to synthesize the molecules they require to counter entropy and maintain their high state of order.
• Energy flows from producers to consumers. Green plants occupy the first trophic level (producer), herbivores occupy the second (primary consumer), primary carnivores occupy the third (secondary consumer), and large carnivores occupy the fourth (tertiary consumer).
• The amount of energy decreases at successive trophic levels. Only 10% of energy is transferred from one trophic level to the next (Lindeman's 10% Law).

Practical Example: If a producer level contains 10,000 joules of energy, the primary consumer level receives 1,000 joules, the secondary consumer level receives 100 joules, and the tertiary consumer level receives only 10 joules.

5. Ecological Pyramids

The trophic structure of an ecosystem is represented graphically in the form of an ecological pyramid. The base of a pyramid represents the producers or the first trophic level, while the apex represents tertiary or top-level consumers.

Types of Ecological Pyramids

Ecological pyramids are of three types:

• Pyramid of Number: Shows the total number of individuals at each trophic level. It can be upright (e.g., grassland ecosystem) or inverted (e.g., tree ecosystem where many birds depend on a single tree).
• Pyramid of Biomass: Represents the total dry weight of organisms at each trophic level. Generally upright in terrestrial ecosystems, but can be inverted in aquatic ecosystems where small standing crop of phytoplankton supports a large standing crop of zooplankton.
• Pyramid of Energy: Always upright, because energy flow from one trophic level to the next is always accompanied by a loss of energy as heat at each step.

Limitations of Ecological Pyramids:

• It does not take into account the same species belonging to two or more trophic levels simultaneously (e.g., a sparrow can be a primary consumer when eating seeds, and a secondary consumer when eating insects).
• It assumes a simple food chain, which almost never exists in nature; instead, food webs exist.
• Saprotrophs (decomposers) are not given any place in ecological pyramids, even though they play a vital role in the ecosystem.

Exam-Oriented Note: Always remember that the pyramid of energy is the only pyramid that is never inverted, because thermodynamic laws dictate that energy is lost as heat during transfer between trophic levels.