Grade 8 Chapter 18: Ecosystems

Introduction

Our planet Earth is home to an incredible diversity of life, from microscopic bacteria to giant trees and whales. All living organisms interact with each other and with their physical surroundings. An 'ecosystem' is a fundamental concept in ecology that describes these interactions. This chapter will introduce you to the components of an ecosystem, different types of ecosystems, how energy flows through them, and the vital biogeochemical cycles that sustain life. We will also explore the importance of maintaining environmental balance and the need for ecosystem conservation.

Ecosystems: Definition and Components

An ecosystem is a functional unit of nature where living organisms (biotic components) interact among themselves and also with their non-living physical and chemical environment (abiotic components).

Components of an Ecosystem

Ecosystems consist of two main types of components:

1. Biotic Components (Living Components)

These are the living organisms within an ecosystem.

• Producers (Autotrophs): Organisms that produce their own food, mainly through photosynthesis (using sunlight, CO₂, water). They form the base of the food chain.
  • Examples: Green plants, algae.
• Consumers (Heterotrophs): Organisms that depend on producers or other consumers for their food.
  • Primary Consumers (Herbivores): Feed directly on producers (plants). Examples: Deer, cow, rabbit.
  • Secondary Consumers (Carnivores/Omnivores): Feed on primary consumers. Examples: Lion (carnivore), human (omnivore).
  • Tertiary Consumers (Top Carnivores/Omnivores): Feed on secondary consumers. Examples: Tiger, eagle.
• Decomposers (Saprotrophs): Organisms that break down dead organic matter (dead plants and animals) into simpler substances, returning nutrients to the soil. They play a crucial role in nutrient cycling.
  • Examples: Bacteria, fungi.

2. Abiotic Components (Non-living Components)

These are the non-living physical and chemical factors that influence the living organisms.

• Physical Factors: Sunlight, temperature, rainfall, wind, humidity, atmospheric pressure, soil type, topography.
• Chemical Factors: Water, oxygen, carbon dioxide, nitrogen, minerals (e.g., phosphorus, potassium), pH of soil/water.

Types of Ecosystems

Ecosystems can be broadly classified into natural and artificial (man-made) types:

• Natural Ecosystems: Exist naturally without significant human interference.
  • Terrestrial Ecosystems: Forest ecosystem, Grassland ecosystem, Desert ecosystem.
  • Aquatic Ecosystems: Pond ecosystem, Lake ecosystem, River ecosystem, Ocean ecosystem.
• Artificial (Man-made) Ecosystems: Created and maintained by humans.
  • Examples: Crop land (farm), Aquarium, Garden, Dam.

Food Chain and Food Web

• Food Chain: A sequence showing how energy is transferred from one organism to another through feeding relationships. It always starts with a producer.
  Example: Grass → Deer → Tiger
  (Producer → Primary Consumer → Secondary Consumer)
• Food Web: A network of interconnected food chains in an ecosystem. It represents more realistic feeding relationships, as most organisms feed on more than one type of organism and are eaten by more than one type of organism.

Energy Flow in an Ecosystem

Energy flows unidirectionally (in one direction) through an ecosystem, from producers to consumers. Only about 10% of the energy is transferred from one trophic level to the next; the rest is lost as heat.

• Pyramid of Energy: A graphical representation showing the amount of energy at each trophic level. It is always upright, as energy decreases at successive trophic levels.
  Producers (most energy) → Primary Consumers → Secondary Consumers → Tertiary Consumers (least energy)

Biogeochemical Cycles

The continuous cycling of various chemical elements (like carbon, oxygen, nitrogen, water) between the living (biotic) and non-living (abiotic) components of the ecosystem is called a biogeochemical cycle. These cycles ensure the availability of essential nutrients for life.

1. Carbon Cycle

• Carbon is exchanged between the atmosphere, oceans, land, and living organisms.
• Processes: Photosynthesis (CO₂ from atmosphere to plants), Respiration (CO₂ from organisms to atmosphere), Decomposition (CO₂ from dead organisms), Combustion (CO₂ from burning fossil fuels).

2. Oxygen Cycle

• Oxygen is exchanged between the atmosphere, oceans, land, and living organisms.
• Processes: Photosynthesis (O₂ released by plants), Respiration (O₂ consumed by organisms), Combustion (O₂ consumed).

3. Nitrogen Cycle

• Nitrogen (N₂) is abundant in the atmosphere but cannot be directly used by most organisms.
• Processes:
  • Nitrogen Fixation: Atmospheric N₂ converted to usable forms (ammonia/nitrates) by nitrogen-fixing bacteria (e.g., Rhizobium in leguminous plants, blue-green algae) or lightning.
  • Nitrification: Ammonia converted to nitrites and then nitrates by nitrifying bacteria.
  • Assimilation: Plants absorb nitrates and convert them into proteins.
  • Ammonification: Decomposers convert nitrogenous waste/dead organisms into ammonia.
  • Denitrification: Nitrates converted back to atmospheric N₂ by denitrifying bacteria.

4. Water Cycle (Hydrological Cycle)

• The continuous movement of water on, above, and below the surface of the Earth.
• Processes: Evaporation, Transpiration, Condensation, Precipitation, Runoff, Infiltration.

Environmental Balance and Human Interference

Environmental balance is the equilibrium between the biotic and abiotic components of an ecosystem. Human activities often disturb this balance:

• Deforestation: Leads to soil erosion, loss of biodiversity, climate change.
• Pollution: Air, water, soil pollution harms organisms and disrupts cycles.
• Over-exploitation of resources: Depletes natural resources.
• Introduction of invasive species: Disrupts native ecosystems.

Conservation of Ecosystems

Protecting and managing ecosystems is vital for sustainable development and the well-being of all life forms.

• Measures:
  • Afforestation (planting trees) and preventing deforestation.
  • Controlling pollution (industrial, vehicular, domestic).
  • Promoting sustainable use of resources.
  • Protecting endangered species and their habitats (national parks, wildlife sanctuaries).
  • Awareness and education.

Summary

• Ecosystem: Functional unit where biotic (living) and abiotic (non-living) components interact.
• Biotic Components: Producers (plants), Consumers (herbivores, carnivores, omnivores), Decomposers (bacteria, fungi).
• Abiotic Components: Physical (light, temp, water) and Chemical (gases, minerals).
• Types of Ecosystems: Natural (forest, pond, ocean) and Artificial (cropland, aquarium).
• Food Chain: Linear energy flow (Producer → Consumer). Food Web: Interconnected food chains.
• Energy Flow: Unidirectional, decreases at each trophic level (Pyramid of Energy is always upright).
• Biogeochemical Cycles: Cycling of elements (Carbon, Oxygen, Nitrogen, Water) between living and non-living components.
• Human Interference: Deforestation, pollution, over-exploitation cause ecological imbalance.
• Conservation: Essential for maintaining environmental balance and biodiversity.

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 18: Ecosystems.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.