MEV 011: Unit 11 - Energy Flow and Material Cycling

 UNIT 11 – ENERGY FLOW AND MATERIAL CYCLING

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11.0 Introduction

All living organisms need energy to grow, reproduce, and perform essential functions. In an ecosystem, energy flows from one organism to another in a unidirectional way, mainly starting from the sun. Alongside energy flow, matter (nutrients) circulates within the ecosystem through biogeochemical cycles. This unit explains the models of energy flow and the cycling of essential elements like carbon, nitrogen, phosphorus, sulfur, and water, which help maintain ecosystem balance.

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11.1 Objectives

After studying this unit, you should be able to:

·         Understand the concept and importance of energy flow in ecosystems.

·         Explain the two major energy flow models: single-channel and Y-shaped.

·         Describe the biogeochemical cycles of essential nutrients.

·         Appreciate the significance of material cycling for ecosystem sustainability.

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11.2 Energy Flow in Ecosystems

Energy flow in an ecosystem refers to the passage of energy through trophic levels – from producers to herbivores, then carnivores and decomposers. This flow is governed by the laws of thermodynamics:

·         First Law: Energy can neither be created nor destroyed.

·         Second Law: In energy transfer, some energy is lost as heat.

The sun is the ultimate source of energy. Plants (producers) convert solar energy into chemical energy through photosynthesis, which is then passed on through various consumers.

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11.2.1 Single Channel Energy Model

The single-channel model represents a linear food chain:

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Sun → Producer → Primary Consumer → Secondary Consumer → Tertiary Consumer → Decomposer

·         This model shows a unidirectional flow of energy.

·         It is simple and easy to understand but does not account for the complex interactions in ecosystems.

·         At each level, about 90% of energy is lost as heat, and only 10% is transferred to the next level (10% Law by Lindeman).

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11.2.2 Y-Shaped Energy Model

The Y-shaped model combines grazing and detritus food chains:

·         Grazing food chain: Starts from green plants → herbivores → carnivores.

·         Detritus food chain: Begins from dead organic matter → decomposers → detritivores.

This model better reflects the dual pathways of energy flow in natural ecosystems and shows how decomposers play a vital role in recycling nutrients and maintaining ecosystem health.

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11.3 Biogeochemical Cycling in Ecosystems

Biogeochemical cycles refer to the recycling of nutrients and elements (such as carbon, nitrogen, phosphorus, sulfur, and water) within the ecosystem. These cycles connect the biotic (living) and abiotic (non-living) components of nature.

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11.3.1 Carbon Cycle

·         Source: CO₂ in the atmosphere.

·         Role: Used by plants in photosynthesis → passed to consumers → returned to the atmosphere via respiration, decomposition, combustion (burning of fossil fuels).

·         Human Impact: Excessive burning of fossil fuels increases CO₂ levels, contributing to global warming.

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11.3.2 Nitrogen Cycle

·         Importance: Nitrogen is essential for proteins and DNA.

·         Steps:

1.      Nitrogen fixation: Conversion of N₂ to usable forms (NH₃) by bacteria.

2.      Nitrification: NH₃ → NO₂⁻ → NO₃⁻ by nitrifying bacteria.

3.      Assimilation: Plants absorb NO₃⁻.

4.      Ammonification: Decomposition of organic nitrogen into NH₃.

5.      Denitrification: NO₃⁻ → N₂ by denitrifying bacteria (returns nitrogen to the atmosphere).

·         Human Impact: Use of nitrogen-rich fertilizers leads to water pollution and eutrophication.

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11.3.3 Phosphorus Cycle

·         Source: Found in rocks and soil minerals.

·         Role: Needed for ATP, DNA, bones.

·         Unlike carbon or nitrogen, phosphorus does not cycle through the atmosphere.

·         Weathering releases phosphate → taken up by plants → moves through food chain → returns via decomposition.

·         Human Impact: Overuse of phosphate fertilizers leads to algal blooms and aquatic ecosystem damage.

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11.3.4 Sulphur Cycle

·         Source: Rocks, volcanoes, and decay of organic matter.

·         Cycle: Sulphur released into air as SO₂ → reacts to form sulfuric acid (acid rain) → absorbed by plants → moves through food web → returns via decomposition.

·         Human Impact: Burning of fossil fuels increases SO₂ levels → contributes to acid rain.

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11.3.5 Hydrological (Water) Cycle

·         Processes:

o    Evaporation: Water turns to vapor from oceans, lakes.

o    Condensation: Vapor turns into clouds.

o    Precipitation: Rain, snow.

o    Infiltration and runoff: Water enters soil or runs off into bodies of water.

·         This cycle maintains moisture balance, supports all life forms, and influences climate.

·         Human Impact: Deforestation and urbanization disturb natural water balance, leading to floods and droughts.

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11.4 Let Us Sum Up

·         Energy flows in a unidirectional path through ecosystems, from the sun to producers to consumers and decomposers.

·         The Single Channel Model represents linear flow, while the Y-Shaped Model includes both grazing and detritus chains.

·         Biogeochemical cycles ensure the constant recycling of key elements: carbon, nitrogen, phosphorus, sulfur, and water.

·         These cycles are essential for ecosystem functioning and life sustainability.

·         Human activities have significantly altered these cycles, causing environmental challenges such as climate change, eutrophication, and acid rain.