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Ecosystem

Ecosystem - Complete NEET Guide with Diagrams & Practice Questions

Table of Contents

  1. Introduction
  2. Key Concepts
  3. Important Formulas & Equations
  4. Memory Techniques (Mnemonics)
  5. Previous Year Questions (NEET)
  6. Key Takeaways for Quick Revision

Introduction

Welcome to the heart of Ecology! An ecosystem is the functional unit of nature where living organisms interact with each other and with their physical environment. This chapter builds upon the concepts of organisms and populations to explore how entire communities function as a whole. For the NEET exam, the Ecology unit is a high-weightage section, and this chapter, "Ecosystem," is fundamentally important, contributing 2-3 questions every year.

A solid understanding of concepts like productivity, decomposition, energy flow, and ecological pyramids is crucial for scoring well. Questions are often direct and concept-based, focusing on the 10% law, the shapes of ecological pyramids, and the processes of decomposition. This guide will simplify these core ecological processes, provide clear explanations with diagrams, and equip you with the knowledge to confidently tackle any related question.

Key Concepts

1. Ecosystem: Structure and Function

An ecosystem is a community of living organisms (biotic components) interacting with their physical environment (abiotic components). The size can range from a small pond to a vast forest or the entire biosphere.

  • Structural Components:

    1. Species Composition: The identification and enumeration of plant and animal species in an ecosystem.
    2. Stratification: The vertical distribution of different species occupying different levels. For example, in a forest, trees occupy the top layer, shrubs the second, and herbs/grasses the bottom layer.

  • Functional Components: The four key aspects of an ecosystem's function are:

    1. Productivity: The rate of biomass production.
    2. Decomposition: The breakdown of dead organic matter.
    3. Energy Flow: The unidirectional movement of energy through different trophic levels.
    4. Nutrient Cycling: The movement of nutrient elements through the various components of an ecosystem.

2. Productivity

The rate of biomass production is called productivity.

  • Primary Production: The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.
  • Gross Primary Productivity (GPP): The rate of production of organic matter during photosynthesis. It's the total energy captured.
  • Net Primary Productivity (NPP): This is the biomass available for consumption by heterotrophs (herbivores, decomposers). It is the GPP minus the energy lost by plants during respiration (R).
    • Formula: NPP = GPP - R
  • Secondary Productivity: The rate of formation of new organic matter by consumers.
  • The annual NPP of the entire biosphere is approximately 170 billion tons (dry weight). Oceans, despite covering 70% of the surface, account for only 55 billion tons, due to limiting factors like light and nutrients (especially nitrogen) in the deep sea.

3. Decomposition

Decomposition is the process by which decomposers (like bacteria and fungi) break down complex organic matter (detritus) into simpler inorganic substances like CO₂, water, and nutrients.

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

  • Steps of Decomposition:

    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 get precipitated as unavailable salts.
    3. Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
    4. Humification: Leads to the accumulation of humus, a dark-colored, amorphous substance that is highly resistant to microbial action and serves as a nutrient reservoir.
    5. Mineralisation: The process by which humus is further degraded by some microbes to release inorganic nutrients.

  • Factors Affecting Decomposition:

    • It is an oxygen-requiring process.
    • The rate is slower if detritus is rich in lignin and chitin.
    • The rate is quicker if detritus is rich in nitrogen and water-soluble substances like sugars.
    • Warm and moist environments favor decomposition.

Figure: Diagrammatic representation of the decomposition cycle in a terrestrial ecosystem, showing the flow from a living tree to leaf litter, and its breakdown by decomposers back into organic-rich soil.

4. Energy Flow

Energy flow in an ecosystem is always unidirectional, from the sun to producers and then to consumers. It follows the laws of thermodynamics.

  • Photosynthetically Active Radiation (PAR): Less than 50% of the incident solar radiation. Plants capture only 2-10% of PAR.
  • Trophic Levels: Organisms occupy a specific place in the food chain based on their source of nutrition.
    • Trophic Level 1 (T₁): Producers (Plants)
    • Trophic Level 2 (T₂): Primary Consumers (Herbivores)
    • Trophic Level 3 (T₃): Secondary Consumers (Carnivores)
    • Trophic Level 4 (T₄): Tertiary Consumers (Top Carnivores)
  • Food Chain: The transfer of food energy from producers through a series of organisms.
    • Grazing Food Chain (GFC): Starts with producers (e.g., Grass → Goat → Man). In an aquatic ecosystem, GFC is the major conduit for energy flow.
    • Detritus Food Chain (DFC): Starts with dead organic matter (detritus). In a terrestrial ecosystem, a much larger fraction of energy flows through the DFC.
  • Food Web: A network of interconnected food chains, which is more realistic for a natural ecosystem.
  • The 10% Law: Proposed by Lindeman. Only 10% of the energy is transferred from a particular trophic level to the next higher trophic level. The rest is lost as heat during metabolic activities. This limits the number of trophic levels in a GFC.

Figure: Energy flow through different trophic levels, illustrating the 10% law. At each step, a significant amount of energy is lost as heat.

  • Standing Crop: The total mass of living material at a particular trophic level at a particular time. It is measured as biomass (dry weight is more accurate) or number per unit area.

5. Ecological Pyramids

An ecological pyramid is a graphical representation of the relationship between different organisms in an ecosystem at different trophic levels (number, biomass, or energy).

Type of PyramidDescriptionUpright ExampleInverted Example
Pyramid of NumberRepresents the number of individuals at each trophic level.Grassland Ecosystem (Producers > Herbivores > Carnivores)Tree Ecosystem (One large tree supports many insects, which support fewer birds - Spindle-shaped)
Pyramid of BiomassRepresents the total dry weight (biomass) of organisms at each trophic level.Terrestrial Ecosystems (e.g., Forest)Aquatic Ecosystems (e.g., Sea). The biomass of phytoplankton (producers) is much smaller than that of zooplankton and fish.
Pyramid of EnergyRepresents the amount of energy at each trophic level.All EcosystemsNEVER Inverted. This is because energy is always lost as heat at each successive trophic level, in accordance with the 10% law.

Figure: The three types of ecological pyramids: (a) Pyramid of numbers in a grassland (upright), (b) Pyramid of biomass in a forest (upright), (c) Inverted pyramid of biomass in a sea.

  • Limitations of Ecological Pyramids:
    • They assume a simple food chain, which rarely exists in nature (a food web is more accurate).
    • They do not account for a species belonging to two or more trophic levels.
    • Saprophytes (decomposers) are not given any place in ecological pyramids, even though they play a vital role.

Important Formulas & Equations

  • Net Primary Productivity (NPP): NPP = GPP - R
    • GPP: Gross Primary Productivity
    • R: Respiration loss
  • 10% Law of Energy Transfer: Energy at trophic level n+1 = 10% of Energy at trophic level n.

Memory Techniques (Mnemonics)

  • Decomposition Steps: "Funny Little Cats Hunt Mice"
    • Fragmentation, Leaching, Catabolism, Humification, Mineralisation.
  • Ecological Pyramids:
    • "Energy is always upright and righteous." -> Pyramid of Energy is always upright.
    • Think of a single large tree supporting thousands of tiny insects. This easily helps you remember the spindle-shaped pyramid of number.
    • Think of the ocean: tiny, short-lived phytoplankton supporting massive, long-lived fish. This explains the inverted pyramid of biomass in the sea.
  • GFC vs. DFC:
    • GFC starts with Green plants (Grazing).
    • DFC starts with Dead organic matter (Detritus).

Previous Year Questions (NEET)

Q1. In an ecosystem, the rate of production of organic matter during photosynthesis is termed as: (NEET 2020) a) Net primary productivity b) Gross primary productivity c) Secondary productivity d) Net productivity

Explanation: Gross Primary Productivity (GPP) is defined as the rate of production of organic matter during photosynthesis. Net Primary Productivity (NPP) is GPP minus the energy lost in respiration. Answer: (b) Gross primary productivity

Q2. Which of the following ecological pyramids is generally inverted? (NEET 2019) a) Pyramid of biomass in a forest b) Pyramid of number in a grassland c) Pyramid of energy d) Pyramid of biomass in a sea

Explanation: In a marine ecosystem, the biomass of producers (phytoplankton) at any given time is much lower than the biomass of consumers (zooplankton, fish). This results in an inverted pyramid of biomass. The pyramid of energy is always upright. Answer: (d) Pyramid of biomass in a sea

Q3. What type of ecological pyramid would be obtained with the following data? Secondary consumer: 120 g, Primary consumer: 60 g, Primary producer: 10 g. (NEET 2018) a) Inverted pyramid of biomass b) Pyramid of energy c) Upright pyramid of numbers d) Upright pyramid of biomass

Explanation: The data shows biomass at T3 (120 g) > T2 (60 g) > T1 (10 g). This does not represent a natural ecosystem, but if we follow the data, the biomass is decreasing from top to bottom, which would be an inverted pyramid (producers at the bottom with the lowest mass). Answer: (a) Inverted pyramid of biomass (based on the structure the data implies).

Q4. The primary producers of the deep-sea hydrothermal vent ecosystem are: (NEET-II 2016) a) Blue-green algae b) Coral reefs c) Green algae d) Chemosynthetic bacteria

Explanation: In deep-sea hydrothermal vents, there is no sunlight for photosynthesis. The producers are chemosynthetic bacteria that derive energy by oxidizing chemical compounds like hydrogen sulfide. Answer: (d) Chemosynthetic bacteria

Key Takeaways for Quick Revision

  • An ecosystem has structural (species composition, stratification) and functional (productivity, decomposition, energy flow, nutrient cycling) components.
  • NPP = GPP - R. NPP is the energy available to the next trophic level.
  • Decomposition involves Fragmentation, Leaching, Catabolism, Humification, and Mineralisation.
  • Energy flow is unidirectional, and only 10% of energy is transferred between successive trophic levels.
  • The Pyramid of Energy is always upright.
  • The Pyramid of Biomass can be inverted in aquatic ecosystems.
  • The Pyramid of Number can be inverted/spindle-shaped in a tree ecosystem.
  • Decomposers (saprophytes) are not given a place in ecological pyramids.
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