Ecosystem

Ecosystem – Structure & Function

What is an Ecosystem?

• An ecosystem is a functional unit of nature.
• It includes interactions between living organisms and their physical environment.
• Ecosystems can be small (pond) or large (forest, sea).
• The entire biosphere is considered a global ecosystem.
• Ecosystems are divided into:
  • Terrestrial: Forest, grassland, desert.
  • Aquatic: Pond, lake, wetland, river, estuary.
  • Man-made: Crop fields, aquarium.

Structure of an Ecosystem

• Ecosystems have both biotic (living) and abiotic (non-living) components.
• Physical Structure:
  • The physical structure is unique for each ecosystem.
  • Species Composition: Types of plants and animals in the ecosystem.
  • Stratification: Vertical layers of species (trees, shrubs, herbs, grasses).

Functions of an Ecosystem

• Ecosystems function as a unit through:
  • Productivity: Creation of organic material by autotrophs.
  • Decomposition: Breakdown of dead matter by decomposers.
  • Energy Flow: Transfer of energy through food chains/webs.
  • Nutrient Cycling: Recycling of nutrients in the ecosystem.

Example of an Aquatic Ecosystem: Pond

• Abiotic Components:
  • Water with dissolved substances.
  • Soil deposit at the bottom.
  • Solar input, temperature, day-length, and climatic conditions.
• Biotic Components:
  • Autotrophs: Phytoplankton, algae, floating and submerged plants.
  • Consumers: Zooplankton, free-swimming and bottom-dwelling animals.
  • Decomposers: Fungi, bacteria, flagellates.
• Processes:
  • Conversion: Sunlight helps autotrophs convert inorganic materials into organic material.
  • Consumption: Heterotrophs eat autotrophs.
  • Decomposition: Dead matter is broken down and minerals are released back to autotrophs.
  • Energy Flow: Energy moves in one direction through the food chain and is lost as heat.

Productivity

What is Productivity?

• Primary Production: Amount of biomass (organic matter) produced by plants during photosynthesis.
  • Measured in weight (g/m²) or energy (kcal/m²).
• Productivity: Rate of biomass production.
  • Measured in g/m²/year or kcal/m²/year.

Types of Productivity

• Gross Primary Productivity (GPP): Total rate of organic matter production during photosynthesis.
• Net Primary Productivity (NPP): Biomass available for consumption by heterotrophs (herbivores and decomposers).
  • NPP = GPP – Respiration losses (R).
• Secondary Productivity: Rate of new organic matter formation by consumers.

Factors Affecting Productivity

• Plant species in the area.
• Environmental factors.
• Availability of nutrients.
• Photosynthetic capacity of plants.

Global Productivity

• Annual net primary productivity of the biosphere: ~170 billion tons (dry weight) of organic matter.
• Oceans (70% of Earth’s surface): ~55 billion tons.
• Land: Remaining productivity.
• Discuss with your teacher why ocean productivity is low despite covering a large surface area.

Decomposition

What is Decomposition?

• Decomposition: Process where decomposers break down complex organic matter into inorganic substances like carbon dioxide, water, and nutrients.

What is Detritus?

• Detritus: Dead plant parts (leaves, bark, flowers) and animal remains, including fecal matter, which are the raw materials for decomposition.

Steps in Decomposition

1. Fragmentation: Detritivores (e.g., earthworms) break down detritus into smaller particles.
2. Leaching: Water-soluble nutrients seep into the soil and become unavailable salts.
3. Catabolism: Bacteria and fungi break down detritus into simpler inorganic substances.
4. Humification: Formation of humus, a dark, amorphous substance resistant to decomposition.
5. Mineralisation: Further breakdown of humus by microbes, releasing inorganic nutrients.

Important Points

• Humus:
  • Dark, amorphous substance.
  • Decomposes very slowly.
  • Acts as a nutrient reservoir.
• Decomposition: Mostly requires oxygen.

Factors Affecting Decomposition

• Detritus Composition:
  • Slow decomposition if rich in lignin and chitin.
  • Fast decomposition if rich in nitrogen and water-soluble substances.
• Climatic Factors:
  • Warm and moist conditions favor decomposition.
  • Low temperature and lack of oxygen (anaerobiosis) slow down decomposition, leading to organic material build-up.

Decomposition is essential for recycling nutrients in ecosystems, helping plants grow and maintaining soil health.

Energy Flow in Ecosystems

Source of Energy

• Sun: Primary source of energy for ecosystems (except deep-sea hydrothermal ecosystems).
• Photosynthetically Active Radiation (PAR): Less than 50% of solar radiation is usable by plants.

Primary Producers

• Autotrophs (Plants and Photosynthetic Bacteria): Capture 2-10% of PAR.
• Examples:
  • Terrestrial: Herbaceous and woody plants.
  • Aquatic: Phytoplankton, algae, higher plants.

Energy Flow Process

• Unidirectional Flow: Sun → Producers → Consumers.
• First Law of Thermodynamics: Energy is transferred.
• Second Law of Thermodynamics: Continuous energy supply needed to counteract disorder.

Food Chains and Webs

• Food Chain: Sequence of who eats whom.
• Producers: Plants, first trophic level.
• Primary Consumers: Herbivores (e.g., insects, birds, mammals), second trophic level.
• Secondary Consumers: Carnivores (e.g., primary carnivores), third trophic level.
• Tertiary Consumers: Higher level carnivores.

Example of Grazing Food Chain

• Grass (Producer) → Goat (Primary Consumer) → Man (Secondary Consumer)

Detritus Food Chain (DFC)

• Starts with Dead Organic Matter: Involves decomposers (fungi, bacteria).
• Decomposers: Break down dead matter into simpler substances.
• Importance: Larger energy flow through DFC in terrestrial ecosystems.

Food Webs

• Interconnected Food Chains: Complex interactions among organisms.
• Omnivores: Animals like cockroaches and crows feed on both plants and animals.

Trophic Levels

• Definition: Position in the food chain based on feeding relationships.
• Standing Crop: Mass of living material at each trophic level.
  • Measured as Biomass: Fresh or dry weight (dry weight is more accurate).

Energy Transfer Efficiency

• 10% Law: Only 10% of energy is transferred to the next trophic level.
• Limitation: Number of trophic levels in a grazing food chain is limited due to energy loss.

Understanding how energy flows through ecosystems helps us appreciate the delicate balance and interconnectedness of nature.

Ecological Pyramids

What are Ecological Pyramids?

• Shape: Broad base, narrows down at the top.
• Represents: Food or energy relationships at different trophic levels.
• Base: Producers (first trophic level).
• Apex: Tertiary or top-level consumers.

Types of Ecological Pyramids

1. Pyramid of Number
2. Pyramid of Biomass
3. Pyramid of Energy

Key Points

• Inclusions: Must include all organisms at each trophic level.
• Multiple Trophic Levels: An organism can occupy more than one trophic level.
  • Example: A sparrow eats seeds (primary consumer) and insects (secondary consumer).

Common Patterns

• Upright Pyramids: Common in most ecosystems.
  • Number: More producers than herbivores, more herbivores than carnivores.
  • Biomass: Producers have more biomass than herbivores, and herbivores more than carnivores.
  • Energy: Always more energy at lower trophic levels.

Exceptions and Examples

• Inverted Pyramids:
  • Number: A large tree with many insects, small birds eating insects, and larger birds eating small birds.
  • Biomass: In oceans, fish biomass exceeds phytoplankton biomass.

Pyramid of Energy

• Always Upright: Energy decreases as it moves up trophic levels.
• Energy Loss: Some energy is always lost as heat at each step.

Limitations of Ecological Pyramids

• Trophic Level Overlap: Doesn’t account for species at multiple levels.
• Simple Food Chains: Assumes simple food chains, but nature has complex food webs.
• Excludes Saprophytes: Doesn’t consider decomposers, which play a crucial role.

Ecological pyramids help us understand the structure and function of ecosystems, showing how energy and biomass are distributed among different organisms.

Ecological Succession

• Community Changes:
  • Communities change over time in response to environmental changes.
  • These changes are orderly and lead to a stable climax community in equilibrium with the environment.
  • This process is called ecological succession.
• Sere and Seral Stages:
  • The sequence of communities changing over time in a given area is called a sere.
  • The transitional communities are termed seral stages or seral communities.
  • Diversity, the number of species, and total biomass increase through these stages.
• Primary Succession:
  • Occurs in areas where no living organisms previously existed, such as bare rock or newly formed ponds.
  • Establishment is slow, taking hundreds to thousands of years to form fertile soil.
• Secondary Succession:
  • Occurs in areas where a natural biotic community has been destroyed, like abandoned farms or burned forests.
  • Faster than primary succession since soil or sediment is already present.
• Influence on Animals:
  • Vegetation changes during succession affect food and shelter for animals.
  • Animal and decomposer species change as succession progresses.
• Disturbances:
  • Natural or human-induced disturbances (like fire or deforestation) can revert succession to an earlier stage.
  • These disturbances create new conditions that favor some species and discourage others.

Succession of Plants

• Types of Succession:
  • Hydrarch Succession: Occurs in wet areas, starting from water (hydric) and progressing to medium water conditions (mesic).
  • Xerarch Succession: Occurs in dry areas, starting from dry conditions (xeric) and progressing to medium water conditions (mesic).
• Pioneer Species:
  • The first species to invade a bare area are called pioneer species.
  • On Rocks: Lichens are pioneers, secreting acids that dissolve rocks and help in soil formation. They are followed by small plants like bryophytes, then bigger plants, and ultimately a stable climax forest community.
  • In Water: Small phytoplanktons are pioneers, followed by rooted-submerged plants, rooted-floating angiosperms, free-floating plants, reed-swamp, marsh-meadow, scrub, and finally trees. This converts the water body into land over time.
• Secondary Succession:
  • Occurs in areas where the natural biotic community has been destroyed but soil is already present.
  • The species that invade depend on soil condition, water availability, environment, and presence of seeds or other propagules.
  • Succession is faster than primary succession, reaching the climax community more quickly.
• Climax Community:
  • The final, stable community in succession.
  • Both hydrarch and xerarch successions lead to a mesic climax community.
• Key Points:
  • Succession, especially primary succession, is very slow and can take thousands of years.
  • All successions, whether in water or on land, eventually lead to a mesic climax community.

Nutrient Cycling

• Importance of Nutrients:
  • Organisms need nutrients to grow, reproduce, and regulate body functions.
  • The amount of nutrients in the soil at any time is called the standing state.
  • Nutrients are recycled in ecosystems and are never permanently lost.
• Nutrient Cycling:
  • The movement of nutrients through an ecosystem is called nutrient cycling or biogeochemical cycles (bio: living organisms, geo: rocks, air, water).
• Types of Nutrient Cycles:
  • Gaseous Cycles: The main reservoir is the atmosphere (e.g., nitrogen and carbon cycles).
  • Sedimentary Cycles: The main reservoir is Earth’s crust (e.g., sulfur and phosphorus cycles).
• Factors Affecting Nutrient Release:
  • Environmental factors like soil, moisture, pH, and temperature regulate the rate at which nutrients are released into the atmosphere.
• Reservoir Function:
  • The reservoir helps balance the rate of nutrient input and output in an ecosystem.
• Examples of Nutrient Cycles:
  • Nitrogen Cycle: Studied in detail in Class XI.
  • Carbon Cycle: Involves the exchange of carbon between the atmosphere, organisms, and Earth’s crust.
  • Phosphorus Cycle: Involves the movement of phosphorus through the biosphere, lithosphere, and hydrosphere.

Carbon Cycle

• Importance of Carbon:
  • Carbon makes up 49% of the dry weight of living organisms, second only to water.
• Carbon Reservoirs:
  • 71% of carbon is dissolved in oceans, which regulate atmospheric CO2.
  • The atmosphere contains only about 1% of global carbon.
  • Fossil fuels are also significant carbon reservoirs.
• Carbon Cycling:
  • Carbon moves through the atmosphere, ocean, and living/dead organisms.
  • Annually, 4 × 10¹³ kg of carbon is fixed in the biosphere via photosynthesis.
  • Carbon returns to the atmosphere as CO2 through respiration by producers and consumers.
  • Decomposers add CO2 by breaking down waste and dead matter.
  • Some fixed carbon is lost to sediments, removing it from circulation.
  • Additional CO2 sources include burning wood, forest fires, fossil fuel combustion, and volcanic activity.
• Human Impact:
  • Activities like deforestation and burning fossil fuels increase atmospheric CO2, contributing to the greenhouse effect.

Phosphorus Cycle

• Importance of Phosphorus:
  • Key component of biological membranes, nucleic acids, and energy transfer systems.
  • Essential for making shells, bones, and teeth in animals.
• Phosphorus Reservoir:
  • Found in rocks as phosphates.
  • Weathering of rocks releases phosphates into the soil.
  • Plants absorb phosphates from the soil.
• Phosphorus in the Food Chain:
  • Herbivores and other animals obtain phosphorus by eating plants.
  • Decomposers break down waste and dead organisms, releasing phosphorus back into the soil.
• Differences from Carbon Cycle:
  • No respiratory release of phosphorus into the atmosphere.
  • Atmospheric inputs of phosphorus (e.g., through rainfall) are much smaller than carbon inputs.
  • Negligible gaseous exchange of phosphorus between organisms and the environment.

Ecosystem Services

• What Are Ecosystem Services?:
  • Benefits provided by healthy ecosystems.
  • Examples: purified air and water, drought and flood mitigation, nutrient cycling, fertile soil generation, wildlife habitats, biodiversity maintenance, crop pollination, carbon storage, and cultural values.
• Importance of Ecosystem Services:
  • Essential for economic, environmental, and aesthetic well-being.
  • Often taken for granted because they are free.
• Valuing Ecosystem Services:
  • Researchers, like Robert Constanza, estimate these services to be worth about $33 trillion per year.
  • This value is nearly double the global gross national product (GNP) of $18 trillion.
• Cost Breakdown:
  • Soil formation: 50% of the total value.
  • Recreation and nutrient cycling: Less than 10% each.
  • Climate regulation and wildlife habitat: About 6% each.

Chapter Summary:

• An ecosystem is a functional unit of nature.
• It has abiotic and biotic components.
  • Abiotic components: inorganic materials like air, water, and soil.
  • Biotic components: producers, consumers, and decomposers.
• Each ecosystem has a characteristic physical structure.
• Interaction between abiotic and biotic components shapes this structure.
• Main structural features: species composition and stratification.
• Every organism occupies a place in an ecosystem based on its nutrition source.
• Important components of an ecosystem:
  • Productivity
    • Primary productivity: rate of solar energy capture or biomass production by producers.
      • Gross primary productivity (GPP): total production of organic matter.
      • Net primary productivity (NPP): remaining biomass after producer utilization.
    • Secondary productivity: rate of food energy assimilation by consumers.
  • Decomposition
    • Decomposers convert complex organic compounds into carbon dioxide, water, and inorganic nutrients.
    • Decomposition processes: fragmentation of detritus, leaching, and catabolism.
  • Energy flow
    • Energy flow is unidirectional: from plants (producers) to decomposers.
    • Organisms form food chains based on food or energy relationships.
  • Nutrient cycling
    • Movement and storage of nutrients in an ecosystem.
    • Two types of nutrient cycling: gaseous (e.g., carbon cycle) and sedimentary (e.g., phosphorus cycle).
    • Reservoirs: atmosphere or hydrosphere for gaseous cycle, Earth’s crust for sedimentary cycle.
• Ecosystem services are products of ecosystem processes.
  • Examples: purification of air and water by forests.
• Biotic communities are dynamic and change over time.
• These changes are called ecological succession.
  • Succession begins with pioneers in a lifeless area.
  • Pioneers are replaced by successors, leading to a stable climax community.
  • Climax community remains stable if the environment is unchanged.