12.1 Ecosystem - Structure and Function

An ecosystem is a dynamic functional unit of nature where **biotic components** (all living organisms like plants, animals, and microbes) and **abiotic components** (non-living elements like water, soil, air, and light) interact. This constant interaction results in the creation of a unique, self-sustaining physical structure.

A key aspect of an ecosystem's structure is its **species composition**, which refers to the different types of plants and animals found there. The vertical arrangement of these different species in a multi-layered ecosystem is called **stratification**. For example, in a forest, you can find different layers: the canopy layer with tall trees, a layer of shrubs, and finally a layer of herbs and grasses on the ground.

Ecosystems can be broadly classified into **terrestrial** (land-based) and **aquatic** (water-based) types. Examples of terrestrial ecosystems include vast **forests**, sprawling **grasslands**, and arid **deserts**. Aquatic ecosystems include **ponds**, **lakes**, **rivers**, and **estuaries**. Additionally, human-made environments like **crop fields** and **aquariums** also function as ecosystems, highlighting that the concept applies to both natural and artificial settings.

Functional Aspects of an Ecosystem

The core of an ecosystem's function lies in the continuous flow of energy and the cycling of matter. These processes are driven by the interactions between the biotic and abiotic components. The four primary functional aspects are:

• Productivity: The rate at which new organic material (biomass) is created by producers. This is the foundation of energy for the entire food chain.
• Decomposition: The crucial process of breaking down dead organic matter. This returns essential nutrients to the soil for producers to reuse, completing the cycle.
• Energy Flow: The unidirectional movement of energy, starting from the sun and moving through different **trophic levels**. This process is governed by the laws of thermodynamics.
• Nutrient Cycling: The continuous movement of essential inorganic nutrients like carbon, nitrogen, and phosphorus between living organisms and the physical environment.

These four functions are not isolated but are deeply interconnected. They work in harmony to maintain the stability and self-sustainability of the ecosystem, ensuring that life continues to thrive.

12.2. Productivity: Primary Production

The entire ecosystem is powered by a continuous input of **solar energy**. The amount of biomass or organic matter produced by plants (the **producers**) is known as **primary production**. This happens over a specific time period and is a result of **photosynthesis**. It is measured in units of mass (e.g., $g/m^2$) or energy (e.g., $kcal/m^2$).

The rate at which this biomass is produced is called **productivity**. It is a fundamental measure of an ecosystem's health and capacity. There are two main types:

• **Gross Primary Productivity (GPP):** This is the total rate of organic matter production during photosynthesis. Think of it as the total raw energy captured by the producers.
• **Net Primary Productivity (NPP):** This is the energy remaining after the producers have used some of the GPP for their own metabolic activities, such as **respiration (R)**. This is the energy that is actually available to the next trophic level (herbivores).

The relationship between these two is expressed by the equation: $$NPP = GPP - R$$

Therefore, **NPP** is the crucial metric that determines the amount of energy available for other organisms in the ecosystem, making it the foundation of the food web.

12.3 Decomposition: The Process

Decomposition is a vital process carried out by **decomposers** (mainly bacteria and fungi) that breaks down dead organic material, known as **detritus**, into simpler inorganic substances. This process is essential for recycling nutrients back into the ecosystem.

The entire process of decomposition involves a series of interconnected steps:

1. Fragmentation: This is the initial mechanical breakdown of detritus into smaller particles. It is primarily done by **detritivores**, such as earthworms, termites, and beetles.
2. Leaching: Water-soluble inorganic nutrients, such as sugar and salts, dissolve and percolate into the soil, where they may be lost.
3. Catabolism: The fragmented detritus is then broken down further by the enzymes released by bacteria and fungi. This converts complex organic molecules into simpler inorganic ones.
4. Humification: During this step, a dark, amorphous, and highly microbe-resistant substance called **humus** is formed. Humus is a valuable reservoir of nutrients, as its slow decomposition rate prevents nutrients from being lost quickly.
5. Mineralisation: This is the final step, where some microbes further break down the humus, releasing the stored inorganic nutrients back into the soil, making them available for plants to absorb and use again.

12.4 Energy Flow: Trophic Levels and the 10% Law

Energy flow in an ecosystem is **unidirectional** and follows the **first law of thermodynamics** (energy cannot be created or destroyed) and the **second law of thermodynamics** (energy conversion is always inefficient, with energy lost as heat). The sun is the ultimate source of energy for most ecosystems, but only a small fraction of the total incident solar radiation, less than 50%, is **photosynthetically active radiation (PAR)** that plants can actually use.

Organisms are classified into **trophic levels** based on their source of nutrition. Producers form the first trophic level, followed by primary consumers (herbivores), secondary consumers (carnivores), and so on. Energy is transferred from one trophic level to the next through a **food chain**. For example: a grasshopper (primary consumer) eats grass (producer), and a frog (secondary consumer) eats the grasshopper.

A crucial concept is the **10% law**, which states that only about 10% of the energy from one trophic level is transferred to the next. The remaining 90% is lost as heat during metabolic activities like respiration, or is lost as waste. This is why food chains are typically short, usually consisting of only 3 to 4 trophic levels, as there is not enough energy to support more.

12.5 Ecological Pyramids: A Visual Representation

An **ecological pyramid** is a graphical representation of the relationship between different trophic levels in an ecosystem. They can be upright, inverted, or spindle-shaped, depending on the ecosystem and the type of pyramid. There are three main types:

• **Pyramid of Number:** This represents the number of individual organisms at each trophic level. In a grassland, it is typically **upright** (many grasses, fewer rabbits, even fewer foxes). However, it can be **inverted**, for example, if one large tree supports a large population of insects and birds.
• **Pyramid of Biomass:** This represents the total dry weight of organisms at each trophic level. Terrestrial ecosystems usually have an **upright** pyramid of biomass (e.g., a large biomass of plants supporting a smaller biomass of herbivores). Aquatic ecosystems can have an **inverted** pyramid, where the total biomass of phytoplankton (producers) is less than that of the zooplankton (primary consumers) at any given time because the phytoplankton reproduce and are consumed at a very high rate.
• **Pyramid of Energy:** This represents the amount of energy available at each trophic level. Due to the 10% law, energy is always lost as heat as it moves up the trophic levels. Therefore, the pyramid of energy is **always upright**. It can never be inverted because there is always less energy available for each successive trophic level.