Ecosystem

Learning Outcomes

1. Comprehend the structure and function of ecosystems.
2. Understand productivity and energy flow within ecosystems.
3. Identify the steps and significance of decomposition.
4. Analyze the formation of ecological pyramids.
5. Recognize the interconnectedness between biotic and abiotic components.

An ecosystem is a structural and functional unit of nature where living organisms interact with each other and with the non-living environment. These interactions form a system where energy flows and nutrients cycle continuously. Ecosystems vary in size, ranging from small ponds to large forests or oceans. Some ecologists consider the entire biosphere as a global ecosystem. However, due to its immense complexity, it is easier to study ecosystems by dividing them into two broad categories: terrestrial ecosystems like forests and deserts, and aquatic ecosystems such as ponds, lakes, rivers, and seas. Man-made ecosystems like crop fields and aquariums also function under similar principles.

Ecosystem Structure and Function

The ecosystem comprises biotic (living organisms) and abiotic (non-living elements like air, water, and soil) components. The interaction between these components results in a specific physical structure that is characteristic of each ecosystem. The species composition of an ecosystem is determined by identifying and counting the different plant and animal species it contains. Stratification refers to the vertical distribution of species in an ecosystem, where trees form the uppermost layer, followed by shrubs, and finally herbs and grasses at the bottom.

Important Note:
The structure of ecosystems is interlinked with four main functions: productivity, decomposition, energy flow, and nutrient cycling.

Pond Ecosystem Example

A pond serves as a self-sustaining aquatic ecosystem that exhibits the four essential components. The abiotic component includes water and soil at the bottom, rich in dissolved organic and inorganic substances. The autotrophic organisms, such as phytoplankton, algae, and plants, use solar energy to produce organic material through photosynthesis. Consumers in the pond include zooplankton and fish, while decomposers like fungi and bacteria break down dead organisms. The energy flow in the pond is unidirectional: from the sun to autotrophs, and then to consumers, with energy being dissipated as heat at each level.

Productivity

Productivity refers to the rate at which biomass or organic matter is produced by plants in an ecosystem. This process is driven by the constant input of solar energy. Primary production is measured in terms of weight (gm–²) or energy (kcal m–²) over time and is a crucial factor for ecosystem functioning. There are two types of productivity:

1. Gross Primary Productivity (GPP): The total rate at which plants capture solar energy and produce organic matter through photosynthesis.
2. Net Primary Productivity (NPP): The energy or biomass that remains after subtracting the energy used by plants during respiration. It is available for consumption by heterotrophs (e.g., herbivores and decomposers).

GPP – R = NPP

3. Secondary Productivity: The rate at which consumers produce organic matter by assimilating food.

Primary productivity is influenced by several factors, including plant species, environmental conditions, and nutrient availability. On a global scale, the annual net primary productivity of the biosphere is approximately 170 billion tons, with oceans contributing about 55 billion tons despite covering 70% of the Earth’s surface.

Decomposition

Decomposition is the process by which detritus (dead plant and animal remains) is broken down into simpler inorganic substances like carbon dioxide, water, and nutrients. Decomposers, such as bacteria and fungi, play a crucial role in recycling nutrients back into the ecosystem. The decomposition process includes five key steps:

1. Fragmentation: Detritivores like earthworms break down large detritus into smaller pieces.
2. Leaching: Water carries soluble inorganic nutrients into the soil, where they precipitate as salts.
3. Catabolism: Microbes such as bacteria and fungi further break down detritus into simpler inorganic substances through enzymatic activity.
4. Humification: This results in the formation of humus, a dark-colored, resistant substance that decomposes slowly and serves as a nutrient reservoir.
5. Mineralization: Humus is degraded by microbes, releasing inorganic nutrients into the soil.

Decomposition is an oxygen-dependent process and is influenced by factors like detritus composition and climate. For example, decomposition rates are slower in detritus rich in lignin and faster in detritus with high nitrogen content. Temperature and moisture also regulate microbial activity, with warm and moist environments accelerating decomposition.

Important Note:
Decomposition rates are slower in cold or anaerobic environments, leading to the buildup of organic material.

Energy Flow

The sun is the primary energy source for nearly all ecosystems. Plants, or producers, capture solar energy through photosynthetically active radiation (PAR), converting it into food. Plants only capture 2-10% of the total solar energy, but this small fraction sustains the entire living world. The energy captured by producers is transferred to consumers, and the flow of energy is always unidirectional, moving from lower trophic levels (producers) to higher ones (consumers).

Consumers can be classified based on their feeding relationships:

1. Primary consumers (herbivores) feed on plants.
2. Secondary consumers (carnivores) feed on herbivores.
3. Tertiary consumers feed on secondary consumers.

When an organism dies, it is decomposed by detritivores and decomposers, starting the detritus food chain. In ecosystems, the grazing food chain (GFC) and the detritus food chain (DFC) are interconnected, forming food webs that illustrate the complexity of energy flow.

Trophic Levels

Each organism occupies a specific trophic level in a food chain. Producers form the first trophic level, herbivores the second, and carnivores the third. The energy transfer between trophic levels follows the 10% law, meaning only 10% of the energy at one trophic level is passed on to the next.

Trophic levels and energy flow:

The standing crop at each trophic level refers to the biomass or number of organisms present at a given time. Biomass is usually measured in dry weight, which provides a more accurate representation than fresh weight.

MCQ:
Which of the following best explains why biomass is often measured in dry weight rather than fresh weight?
(a) Dry weight excludes the water content, making it a more stable measurement.
Answer: (a)

Ecological Pyramids

Ecological pyramids represent the energy or biomass relationships between organisms at different trophic levels. These pyramids can depict numbers, biomass, or energy, and are typically upright (with a broad base representing producers and a narrow top representing consumers). There are three types of pyramids:

1. Pyramid of Numbers: Represents the number of organisms at each trophic level. For example, in a forest, a large number of trees may support fewer herbivores and even fewer carnivores.
2. Pyramid of Biomass: Depicts the total biomass at each trophic level. In terrestrial ecosystems, biomass decreases at higher trophic levels. However, in aquatic ecosystems, this pyramid is often inverted because small phytoplankton support larger masses of zooplankton.
3. Pyramid of Energy: Shows the flow of energy at each trophic level. Unlike pyramids of numbers or biomass, the pyramid of energy is always upright because energy decreases at each successive trophic level.

Important Note:
The pyramid of energy can never be inverted because energy is lost as heat at every trophic level.

Pyramid Comparison

Limitations of Ecological Pyramids:

• Ecological pyramids do not account for organisms that occupy more than one trophic level.
• They assume simple food chains rather than the more complex food webs.
• Decomposers and saprophytes are not included, despite their critical role in nutrient cycling.

Nutrient Cycling

Nutrient cycling refers to the movement of nutrient elements through different ecosystem components. There are two main types of nutrient cycles:

1. Gaseous Cycles: The atmosphere or hydrosphere serves as the nutrient reservoir. The carbon cycle is an example.
2. Sedimentary Cycles: Nutrients are stored in the Earth’s crust. The phosphorus cycle is a typical example.

Nutrients are continuously recycled within ecosystems, ensuring their availability for all organisms.

MCQ:
In an aquatic ecosystem, what limits productivity?
(a) Nutrient availability
Answer: (a)

Summary of Ecosystem Processes

Each of these processes plays a vital role in maintaining the balance and sustainability of ecosystems.