Chapter 12: Ecosystem

Introduction

Concepts

• Ecosystem: Self-regulatory, self-sustaining unit of biosphere with living community and physical environment interacting.
• Term coined by A.G Tansley in 1935.
• Odum: Biotic and abiotic components influence each other.
• Size varies: small pond to large forest or sea.
• Natural ecosystems: Terrestrial (forest, grassland, desert) and Aquatic (pond, lake, wetland, river, estuary).
• Man-made ecosystems: Crop fields, aquariums.

Memory Aids

• Ecosystem = Community + Environment interaction.
• Tansley = Ecosystem term origin.
• Two types: Terrestrial & Aquatic.

Quick Checkpoints

• What defines an ecosystem?
• Who coined the term ecosystem?
• Types of natural ecosystems?

Ecosystem Structure and Function

Concepts

• Every ecosystem has structural (components) and functional (processes) parts.
• Open system: Solar energy input; energy flow is unidirectional from producers to consumers.
• Components: Abiotic (non-living) and Biotic (living).
• Cycling of matter occurs between biotic and abiotic parts.
• Biotic: Producers (green plants), Consumers (herbivores, carnivores), Decomposers (microbes).
• Abiotic: Light, temperature, wind, soil, water.

Memory Aids

• Energy flows one way: Sun → Producers → Consumers.
• Biotic = Living; Abiotic = Non-living.
• Matter cycles between biotic and abiotic.

Quick Checkpoints

• What are the two main ecosystem components?
• How does energy flow in an ecosystem?
• Examples of biotic components?

Biotic Components of Ecosystem

Concepts

• Producers (Autotrophs): Fix light energy, use inorganic substances, synthesize organic compounds.
• Convert solar energy to chemical energy.
• Include phytoplankton, some algae, submerged and marginal plants.
• Consumers (Heterotrophs): Consume organic food, rearrange and utilize complex organic materials.
• Include zooplankton, free-swimming and bottom-dwelling animals.
• Types: Herbivores and Carnivores.
• Decomposers (Reducers): Saprotrophs feeding on dead organisms and organic waste.
• Secrete enzymes to digest organic matter externally.
• Also called reducers and microconsumers.
• Include bacteria and fungi (moulds, mushrooms).

Memory Aids

• Producers = Energy converters.
• Consumers = Energy users.
• Decomposers = Nature’s recyclers.

Quick Checkpoints

• Role of producers in ecosystem?
• Difference between herbivores and carnivores?
• Why are decomposers called reducers?

Abiotic Components of Ecosystem

Concepts

• Light: Source of solar energy for heating and photosynthesis.
• Wind: Controls weather, transpiration, pollination, and seed dispersal.
• Soil: Influences vegetation, flora, fauna via composition, temperature, water retention, aeration, minerals.
• Temperature: Varies globally; extreme values cause inactivity or death.
• Water: Essential for plants and animals; absorbed from soil or water bodies.

Memory Aids

• Light = Energy source.
• Wind = Weather and dispersal agent.
• Soil = Growth medium.
• Temperature = Life regulator.
• Water = Vital resource.

Quick Checkpoints

• Functions of light in ecosystem?
• How does wind affect ecosystems?
• Importance of soil composition?

Patterns of Ecosystem

Concepts

• Physical structure results from biotic-abiotic interactions.
• Species composition: Identification and count of species; varies by geography, climate.
• Maximum species diversity in tropical rainforests and coral reefs; minimum in deserts and arctic.
• Stratification: Vertical layers of species; e.g., 5-7 layers in tropical rainforests (emergent, canopy, understory, shrub, herb layers).
• Key functional aspects: Productivity, decomposition, energy flow, nutrient cycling.

Memory Aids

• Species composition = Diversity measure.
• Stratification = Vertical layering.
• Four functions = Ecosystem life processes.

Quick Checkpoints

• What affects species composition?
• Define stratification in ecosystems.
• Name four key ecosystem functions.

Productivity

Types and Definitions

• Productivity is the rate of biomass production per unit area over time by plants.
• Measured in energy (Kcal m0B2 yr0B9) or weight (gm m0B2 yr0B9).
• Depends on solar energy input, plant species, environment, nutrients, and photosynthetic capacity.
• Two main types: Primary productivity and Secondary productivity.

Primary Productivity

• Amount of organic matter produced by plants via photosynthesis per unit area and time.
• Includes Gross Primary Productivity (GPP): total organic matter produced.
• Net Primary Productivity (NPP): GPP minus energy used in plant respiration (NPP = GPP - R).
• NPP is the biomass available for herbivores and decomposers.
• Annual global NPP: ~170 billion tons dry weight; 55 billion tons from oceans, 115 billion tons from land.

Secondary Productivity

• Rate of food energy assimilation by consumers.
• Represents energy available at consumer level for transfer to next trophic level.

Memory Aids

• Productivity = Biomass production rate.
• GPP = Total photosynthesis output.
• NPP = GPP minus plant respiration.
• Secondary productivity = Consumer energy assimilation.

Quick Checkpoints

• What is the difference between GPP and NPP?
• How is productivity measured?
• What factors affect primary productivity?
• Define secondary productivity.
• Why is NPP important for heterotrophs?

Decomposition

Overview and Types

• Breakdown of complex organic matter (detritus) into inorganic substances (CO₂, water, nutrients).
• Earthworms aid decomposition and soil aeration; called farmer's friend.
• Detritus includes dead leaves, barks, flowers, animal remains, and fecal matter.
• Key steps: fragmentation, leaching, catabolism, humification, mineralisation.
• Two types: above-ground (leaf litter, dried parts, animal remains) and below-ground (dead roots, underground animals).
• Decomposition rate depends on detritus composition and climate.
• Slower if detritus rich in lignin and chitin; faster if rich in nitrogen and water-soluble substances like sugars.

Memory Aids

• "Farmer's friend" = Earthworm helps decomposition and soil.
• Detritus = dead organic matter for decomposition.
• Slow decay = lignin + chitin; Fast decay = nitrogen + sugars.

Quick Checkpoints

• What is detritus?
• Types of detritus?
• Factors affecting decomposition rate?

Processes Involved in Decomposition

Key Processes

• Fragmentation: Detritus broken into smaller particles by detritivores (e.g., termites, earthworms), increasing surface area.
• Catabolism: Decomposers (bacteria, fungi) secrete enzymes to convert complex organic matter into simple soluble compounds and inorganic substances; some nutrients immobilised.
• Leaching: Soluble substances (sugars, nutrients) washed into deeper soil layers by water percolation.
• Humification: Formation of humus, a dark, colloidal, nutrient-rich, lignin and cellulose-rich substance resistant to microbial breakdown; maintains soil moisture and aeration.
• Mineralisation: Further breakdown of humus by microbes releasing inorganic nutrients; slow due to humus trapping and immobilisation.
• Factors affecting decomposition: temperature, moisture, pH.
• Aerobiosis: Oxygen is essential; aerobic conditions promote decomposition; anaerobic conditions slow it and cause detritus accumulation.

Memory Aids

• Fragmentation = detritivores break down particles.
• Catabolism = enzymes convert complex to simple.
• Leaching = nutrients washed down by water.
• Humus = stable, nutrient reservoir, colloidal.
• Mineralisation = humus to inorganic nutrients.
• Oxygen needed for decomposition.

Quick Checkpoints

• What role do detritivores play?
• Define catabolism in decomposition.
• What is humus and its importance?
• Why is oxygen essential for decomposition?

Decomposition Cycle

Cycle Explanation

• Tree produces green leaves in soil.
• Fallen leaves eaten by insects and animals, adding nutrients to food web.
• Nutrients leach into soil chemically.
• Fungi and bacteria partially decompose leaves into litter.
• Earthworms, bacteria, mites, fungi further decompose leaf material.
• Resulting organic-rich soil nourishes tree and supports new growth, completing cycle.

Memory Aids

• Leaves → eaten → leached → decomposed → soil enriched → tree growth.

Quick Checkpoints

• What happens to fallen leaves?
• Which organisms decompose leaf litter?
• How does decomposition support plant growth?

Energy Flow

Key Points

• Sun is the primary energy source for all ecosystems except deep sea hydro-thermal vents.
• Less than 50% of solar radiation is photosynthetically active radiation (PAR).
• Autotrophs capture only 2-10% of PAR to sustain life.
• Energy flow is unidirectional: Sun → Producers → Consumers, following the first law of thermodynamics.
• Ecosystems require constant energy input to maintain order, per the second law of thermodynamics.
• Producers: Green plants on land (herbaceous and woody) and phytoplankton, algae in aquatic ecosystems.

Memory Aids

• Energy flows one way: Sun to plants to animals.
• Only a small fraction of sunlight is usable energy.
• Producers start the energy chain.

Quick Checkpoints

• What is the main energy source for ecosystems?
• Define photosynthetically active radiation (PAR).
• Why is energy flow unidirectional?
• Who are producers in terrestrial and aquatic ecosystems?

Food Chain

Key Points

• Food chain: sequence of nutrient and energy transfer through organisms by eating.
• Involves repeated eating: each group eats and is eaten.
• Usually straight with 3-6 trophic levels; some organisms occupy multiple levels.
• Energy, biomass, and number of individuals decrease at higher trophic levels.
• Major energy loss as heat at each trophic level.
• Trophic level: position in food chain based on feeding method.
• Standing crop: biomass or number of organisms per unit area at a trophic level (dry weight preferred).
• Parasites feed across multiple trophic levels, not fixed.
• Energy passes to higher levels or decomposers after organism death.
• Lindeman's 10% law: only 10% energy transfers to next trophic level.
• Shorter food chains support more biomass efficiently.

Memory Aids

• Energy decreases by 90% at each trophic level.
• Standing crop = biomass at each level.
• Parasites span multiple levels.
• Short food chains are more efficient.

Quick Checkpoints

• What defines a trophic level?
• Explain Lindeman's 10% law.
• Why is energy lost at each trophic level?
• What is standing crop?
• How do parasites fit in food chains?

Types of Food Chain

Key Points

• Grazing Food Chain (GFC): Most common, involves predation at every step. Example: Grass → Goat → Man.
• Parasitic Food Chain (PFC): Starts with host, ends with parasite; involves parasites and scavengers.
• Detritus Food Chain (DFC): Begins with dead organic matter (detritus). Detritivores and decomposers (saprotrophs) feed on detritus. Example: Detritus → Earthworm → Sparrow → Falcon; with alternate path Frog → Snake → Peacock.

Memory Aids

• GFC = predator chain.
• PFC = parasite chain.
• DFC = decomposer chain.

Quick Checkpoints

• What organisms start each type of food chain?
• Which food chain involves parasites?
• Define saprotrophs.

Food Web

Key Points

• Food web: interconnected food chains forming multiple feeding connections.
• Provides alternative feeding paths, enhancing ecosystem stability.
• Example: Grass eaten by rabbit or mouse; mouse eaten by hawk directly or via snake.

Memory Aids

• Food web = network of food chains.
• Alternative feeding paths maintain balance.

Quick Checkpoints

• What is a food web?
• How does a food web differ from a food chain?
• Why are food webs important for ecosystems?

Ecological Pyramids

Definition and Basics

• Graphical representation of relationships between individuals at different trophic levels in a food chain.
• Measured in terms of number, biomass, or energy.
• Base represents producers; apex represents tertiary or top consumers.
• Developed by Charles Elton (1927), also called Eltonian pyramids.
• All organisms at a trophic level must be included for accurate calculations.
• Trophic level is functional, not species-specific; species can occupy multiple levels.

Types of Ecological Pyramids

• Pyramid of Numbers: Shows number of individuals per unit area at each trophic level; usually upright but can be inverted (e.g., parasitic food chains) or spindle-shaped (forest ecosystem).
• Pyramid of Biomass: Shows total biomass per unit area at each trophic level; upright in terrestrial habitats, inverted in aquatic ecosystems.
• Pyramid of Energy: Shows rate of energy flow/productivity per unit time and area; always upright due to energy loss at each level.

Memory Aids

• Eltonian pyramids = Ecological pyramids.
• Number pyramid: Upright except parasitic chains (inverted).
• Biomass pyramid: Upright on land, inverted in water.
• Energy pyramid: Always upright.

Quick Checkpoints

• What does the base and apex represent in ecological pyramids?
• Can a species occupy multiple trophic levels?
• What are the three main types of ecological pyramids?

Pyramid of Numbers

Characteristics and Examples

• Shows number of individual organisms per unit area at each trophic level.
• Usually upright in grassland and pond ecosystems.
• In parasitic food chains, pyramid is inverted due to increasing number of parasites at higher levels.
• Spindle-shaped in forest ecosystems.
• Example: Grassland ecosystem pyramid shows decreasing numbers from producers to tertiary consumers.

Memory Aids

• Number pyramid = Count of individuals at each level.
• Inverted shape occurs in parasitic chains.
• Spindle shape in forests.

Quick Checkpoints

• What causes inversion in pyramid of numbers?
• How does the pyramid shape differ in forest ecosystems?
• Why do numbers decrease at higher trophic levels?

Pyramid of Biomass

Characteristics and Examples

• Shows total biomass per unit area at each trophic level.
• Upright in terrestrial habitats.
• Inverted in aquatic ecosystems due to small producer biomass (phytoplankton) supporting larger consumer biomass (zooplankton).
• Example: Dry weight decreases sharply from producers to tertiary consumers in terrestrial ecosystem.

Memory Aids

• Biomass = Total mass of living organisms.
• Upright on land, inverted in water.
• Phytoplankton reproduce fast, supporting more zooplankton biomass.

Quick Checkpoints

• Why is the biomass pyramid inverted in aquatic ecosystems?
• How does biomass change from producers to consumers?
• What causes the sharp decrease in biomass at higher levels?

Pyramid of Energy

Characteristics and Examples

• Shows rate of energy flow or productivity per unit time and area at each trophic level.
• Always upright due to energy loss at each level.
• Energy decreases from producers to tertiary consumers.
• Example: Energy flow from sunlight to top consumers shows sharp decrease at each level.

Memory Aids

• Energy pyramid always upright.
• Energy loss due to metabolism and heat.
• Only a small fraction of energy passes to next level.

Quick Checkpoints

• Why is the energy pyramid always upright?
• What causes energy loss at each trophic level?
• How much energy is typically transferred to the next level?

Limitations of Ecological Pyramids

Key Limitations

• Species occupying multiple trophic levels are not accounted for.
• Assumes a simple food chain, which is rare in nature.
• Saprophytes are excluded.

Memory Aids

• Multiple trophic levels ignored.
• Food chains are complex, not simple.
• No place for saprophytes.

Quick Checkpoints

• What are the main limitations of ecological pyramids?
• Why are saprophytes excluded?
• How does complexity of food chains affect pyramid accuracy?

Ecological Efficiency

Definitions and Types

• Percentage of energy converted into biomass at a trophic level over energy available at the lower level.
• Formula: Ecological efficiency = (Energy converted into biomass at trophic level / Energy present in biomass at lower trophic level) × 100
• Assimilation Efficiency: Percentage of food energy assimilated for body building from total food ingested.
• Photosynthetic Efficiency: Percentage of light energy converted into chemical energy by producers during photosynthesis (ratio of gross primary productivity to incident solar radiation).
• Net Production Efficiency: Percentage ratio of net primary productivity to gross primary productivity.

Memory Aids

• Efficiency = Energy converted ÷ Energy available × 100.
• Assimilation = Food energy used for growth.
• Photosynthetic = Light to chemical energy conversion.
• Net production = Net vs gross productivity.

Quick Checkpoints

• What is ecological efficiency?
• How is assimilation efficiency defined?
• What does photosynthetic efficiency measure?
• Difference between net and gross primary productivity?