NEET Biology - Unit 10- Ecosystem- Study Notes - New Syllabus

Energy Flow in Ecosystems

🌱 Introduction

Energy is the driving force of ecosystems.
All living organisms require energy to survive, grow, and reproduce.
Source of energy: Sun → captured by producers (autotrophs) via photosynthesis.
Energy flows unidirectionally from one trophic level to the next, unlike nutrients which cycle.
Energy is eventually lost as heat due to metabolic processes (Second Law of Thermodynamics).

🧬 Trophic Levels

Definition: Trophic levels are the hierarchical positions organisms occupy in a food chain based on energy flow and feeding relationships.

🔄 Food Chain and Food Web

1. Food Chain

Simple linear sequence showing who eats whom.

• Grazing food chain: Starts with green plants → herbivores → carnivores.
  Example: Grass → Grasshopper → Frog → Snake → Hawk
• Detritus food chain: Starts with dead organic matter → detritivores → decomposers → carnivores.
  Example: Dead leaves → Earthworms → Beetles → Birds

2. Food Web

Network of interlinked food chains showing complex feeding relationships in an ecosystem.
Provides stability: If one species is lost, energy can still flow through alternate pathways.

📊 Energy Flow Principles

Unidirectional Flow: Sun → producers → consumers → decomposers. Energy cannot be recycled.

Energy Loss: At each trophic level, 10% of energy is transferred to the next level (Lindeman’s 10% law).
Remaining 90% is lost as heat via respiration, feces, and metabolic processes.

Pyramids of Energy

Graphical representation of energy at each trophic level. Always upright because energy decreases at higher levels.

Observation: Only a small fraction of energy reaches top carnivores → limits number of trophic levels in an ecosystem.

🌿 Productivity and Energy Flow

• Gross Primary Productivity (GPP): Total energy captured by producers.
• Net Primary Productivity (NPP): Energy available to herbivores.
  \( \mathrm{NPP} = \mathrm{GPP} – \mathrm{Respiration\ by\ producers\ (R)} \)
• Secondary productivity: Energy accumulated by consumers.
• Tertiary productivity: Energy accumulated by carnivores.
• Energy Transfer Efficiency:
  \( \mathrm{Efficiency} = \frac{\text{Energy at higher trophic level}}{\text{Energy at lower trophic level}} \times 100 \)
  Usually ~10% between successive levels.

🔹 Detritus and Energy Recycling

• Decomposers (bacteria, fungi) recycle energy and nutrients from dead organisms.
• Energy is not recycled, but nutrients like N, P, C are returned to soil → used by producers.
• Detritus-based food chains are important in forests and aquatic ecosystems.

🖼 Example: Pond Ecosystem Energy Flow

• Producers: Phytoplankton, algae
• Primary consumers: Zooplankton, insects
• Secondary consumers: Small fish
• Tertiary consumers: Large fish, birds
• Decomposers: Bacteria, fungi

Energy Flow Path:

Sun → Phytoplankton (Producers) → Zooplankton (Primary) → Small Fish (Secondary) → Large Fish/Birds (Tertiary) → Decomposers

Energy decreases at each level → explains why top predators are fewer.

🔹 Key Points

• Energy flows, does not cycle in ecosystems.
• Only ~10% energy is transferred to the next trophic level.
• Energy loss limits number of trophic levels.
• Producers are the foundation of all energy flow.
• Decomposers are essential for nutrient recycling but not energy recycling.

📝 Quick Recap 
– Source of energy: Sun → captured by producers → flows through consumers → lost as heat.
– Trophic levels: Producers → Herbivores → Carnivores → Top carnivores → Decomposers
– Energy transfer: 10% from one level to next (Lindeman’s Law)
– Pyramids of energy: Always upright
– Food chains: Grazing & detritus
– Food webs: Interlinked chains → ecosystem stability
– Detritus role: Recycles nutrients for producers
– Mnemonic: “Sun Produces Energy, Herbivores Consume, Carnivores Top, Decomposers Recycle”