15.1 Ecosystem

What is an Ecosystem?

An ecosystem is a self-sustaining structural and functional unit of the biosphere. It consists of a community of living organisms (biotic factors) interacting with their non-living physical environment (abiotic factors). The term was coined by A.G. Tansley in 1935.

Ecosystems can be as small as a pond or as large as a tropical rainforest. They represent the level of biological organization above a community.

Ecological Hierarchy

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Components of an Ecosystem

Every ecosystem has two major components:

1. Abiotic Components (Non-living)

These are the physical and chemical factors of the environment:

• Light — primary energy source for photosynthesis
• Temperature — affects metabolic rates
• Water — essential for all life processes
• Soil — provides minerals and anchorage
• Atmosphere — supplies $C{O}_2$, $O_2$, and $N_2$
• pH and salinity

2. Biotic Components (Living)

Organisms are classified by their feeding relationships:

Producers (Autotrophs)

• Synthesize their own food from inorganic raw materials using light energy (photosynthesis) or chemical energy (chemosynthesis).
• Examples: green plants, algae, cyanobacteria.
• Occupy the first trophic level.

Consumers (Heterotrophs)

Cannot synthesize their own food; depend on producers directly or indirectly.

Decomposers (Saprotrophs / Detritivores)

• Break down complex organic matter from dead organisms into simple inorganic nutrients.
• Return minerals to the soil and atmosphere, enabling nutrient cycling.
• Examples: bacteria, fungi.
• Without decomposers, nutrients would remain locked in dead organic matter and become unavailable to producers.

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Habitat vs. Niche

• Joseph Grinnell (1917) first used the term niche to describe the microhabitat of a species.
• Charles Elton (1927) redefined niche as the functional role of an organism — what it eats, what eats it, and how it interacts with other species.

No two species can occupy the same niche in the same ecosystem at the same time — this is the Competitive Exclusion Principle (Gause's Law).

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Types of Ecosystems

Natural Ecosystems

• Terrestrial: forests, grasslands, deserts, tundra
• Aquatic (Freshwater): ponds, lakes, rivers, wetlands
• Aquatic (Marine): oceans, coral reefs, estuaries

Artificial Ecosystems

• Created and maintained by humans
• Examples: croplands, aquariums, gardens

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Key Features of a Healthy Ecosystem

1. Energy flow — unidirectional, from producers through consumers
2. Nutrient cycling — elements cycle between biotic and abiotic components
3. Biodiversity — variety of species ensures stability
4. Self-regulation — feedback mechanisms maintain balance (homeostasis)

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