Ecology and Environment | Unit -X: Chapter 02 - Ecosystems
- UniDrill
- Mar 2
- 4 min read
Updated: Mar 3
CUET (UG) Biology Notes: Ecosystem
1. Ecosystem: Structure and Function
An ecosystem is a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment.
Stratification: The vertical distribution of different species occupying different levels in an ecosystem.
Example in a forest: Trees occupy the top vertical strata or layer, shrubs the second, and herbs and grasses occupy the bottom layers.
Components of Function: To understand the ethos of an aquatic or terrestrial ecosystem, we study it through four basic functional aspects:
Productivity
Decomposition
Energy flow
Nutrient cycling
2. Productivity
A constant input of solar energy is the basic requirement for any ecosystem to function and sustain.
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 (g/m^2) or energy (kcal/m^2).
Gross Primary Productivity (GPP): The total rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilized by plants in respiration.
Net Primary Productivity (NPP): The gross primary productivity minus respiration losses (R). This is the available biomass for the consumption of heterotrophs (herbivores and decomposers)
Equation: NPP = GPP - R
Secondary Productivity: Defined as the rate of formation of new organic matter by consumers (heterotrophs).
3. Decomposition
Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water, and nutrients.
Detritus: Dead plant remains (leaves, bark, flowers) and dead remains of animals, including fecal matter. This is the raw material for decomposition.
Steps of Decomposition:
Fragmentation: Detritivores (e.g., earthworms) break down detritus into smaller particles.
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: Accumulation of a dark-colored amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. It serves as a reservoir of nutrients.
Mineralization: The humus is further degraded by some microbes, releasing inorganic nutrients into the soil.
Factors Affecting Decomposition: It is largely an oxygen-requiring process.
Slower if detritus is rich in lignin and chitin.
Quicker if detritus is rich in nitrogen and water-soluble substances like sugars. Warm and moist environments favor decomposition.
4. Energy Flow, Food Chains, and Food Web
Except for the deep sea hydro-thermal ecosystem, the sun is the only source of energy for all ecosystems on Earth.
Photosynthetically Active Radiation (PAR): Plants capture only 2-10% of the PAR. This small amount of energy sustains the entire living world.
Unidirectional Flow: Energy flows from the sun to producers and then to consumers. It never flows backward.
The 10% Law: Formulated by Lindeman. Only 10% of the energy is transferred to each trophic level from the lower trophic level. The rest is lost as heat (respiration).
Food Chains
Grazing Food Chain (GFC): Starts with producers (plants). In an aquatic ecosystem, GFC is the major conduit for energy flow.
Example: Grass -> Goat -> Man
Detritus Food Chain (DFC): Starts with dead organic matter. It is made up of decomposers (saprotrophs, mostly fungi and bacteria). In a terrestrial ecosystem, a much larger fraction of energy flows through the DFC than the GFC.
Food Web & Trophic Levels
Food Web: Natural interconnections of food chains form a food web, providing stability to the ecosystem.
Trophic Level: Organisms occupy a specific place in the food chain based on their source of food.
First Trophic Level (T1): Producers (Plants, Phytoplankton)
Second Trophic Level (T2): Primary Consumers (Herbivores, Zooplankton)
Third Trophic Level (T3): Secondary Consumers (Carnivores)
Fourth Trophic Level (T4): Tertiary Consumers (Top Carnivores)
Standing Crop: Each trophic level has a certain mass of living material at a particular time. It is measured as the biomass of living organisms or their number in a unit area.
5. Ecological Pyramids
The graphical representation of ecological parameters (number, biomass, energy) at different trophic levels. The base represents the producers, and the apex represents the top-level consumers.
Type of Pyramid | Shape & NCERT Examples |
Pyramid of Number | Usually Upright: (e.g., Grassland ecosystem, where the number of grasses is far greater than the number of top carnivores). Inverted: Tree ecosystem (One single tree supports many birds, which in turn support even more parasites). |
Pyramid of Biomass | Usually Upright: (e.g., Forest ecosystem, where the biomass of trees is huge compared to the animals). Inverted: Aquatic ecosystems (e.g., Sea/Pond). The biomass of fishes far exceeds that of phytoplankton, as phytoplankton have a very rapid turnover rate. |
Pyramid of Energy | ALWAYS Upright: It can never be inverted. Because when energy flows from a particular trophic level to the next, some energy is always lost as heat at each step (the 10% Law). |
Limitations of Ecological Pyramids
They assume a simple, straight food chain, which almost never exists in nature.
They do not accommodate a food web.
Saprophytes (decomposers) are not given any place in ecological pyramids, even though they play a vital role in the ecosystem.
They do not take into account the same species belonging to two or more trophic levels (e.g., a bird eating seeds is a primary consumer, but eating insects makes it a secondary consumer).
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