Revision: Ecosystems and Energy Flow Biology HSC Science (General) 12th Standard Board Exam Maharashtra State Board

A consumer is an organism that obtains energy by feeding on other organisms. As it cannot produce its own food, consumers rely on plants or other animals for nourishment, which are a vital part of the food chain. Consumers are further classified into primary consumers, secondary consumers, and tertiary consumers.

The rate at which biomass is synthesised by a trophic level per unit area per unit time is called its productivity.

Decomposition is the process by which decomposers break down complex organic matter into simpler inorganic substances such as carbon dioxide, water, and nutrients.

The dead remains of plants and animals, along with faecal matter, that serve as raw material for decomposition are called detritus.

The enzymatic degradation of detritus into simpler inorganic substances by bacteria and fungi is called catabolism.

The formation and accumulation of a dark-coloured, amorphous, resistant substance during decomposition is called humification.

The dark-coloured, colloidal, nutrient-rich substance formed during decomposition that decomposes very slowly is called humus.

The conversion of humus into inorganic nutrients by microbial action is called mineralisation.

Decomposition is the process that involves the breakdown of complex organic matter or biomass from the bodies of dead plants and animals with the help of decomposers into inorganic raw materials such as carbon dioxide, water, and other nutrients.

An ecological pyramid is a graphical representation of various environmental parameters, such as the number of individuals present at each trophic level, the amount of energy, or the biomass present at each trophic level. Ecological pyramids represent producers at the base, while the apex represents the top-level consumers present in the ecosystem.

The circular movement of nutrients between living organisms and the physical environment of an ecosystem is called biogeochemical cycle.

The gradual and predictable changes in the species composition of a given area are called ecological succession.

Standing crop is the biomass or the amount of living matter (biotic components) present in an ecosystem at a given time. It represents the mass or number of organisms in a particular area at that moment.

Net Primary Productivity (NPP) = Gross Primary Productivity (GPP) - Respiratory loss by plants (R)

• Two Structural Features → Species Composition (identifying species) + Spatial Pattern (distribution of biotic/abiotic components).
• Two Spatial Patterns → Stratification (vertical, e.g., trees→shrubs→herbs) + Zonation (horizontal, e.g., intertidal, littoral zones).
• Types → Terrestrial (forests, grasslands, deserts) + Aquatic (lakes, rivers, seas, oceans).
• Classification → Natural (self-sustainable) + Artificial (needs human input, e.g., farmland, fish tank).
• Components → Biotic (living) + Abiotic (non-living).
• 4 Functions → Productivity + Decomposition + Energy Flow + Nutrient Cycling (PDEN).

• Productivity is the rate of biomass (organic matter) formation at any trophic level per unit area over time, primarily through photosynthesis or chemosynthesis. Unit: g m⁻² yr⁻¹ or g m⁻² day⁻¹.
• Primary productivity is the amount of biomass produced by plants; it includes Gross Primary Productivity (total production) and Net Primary Productivity (usable biomass left after plant respiration).
• Net Primary Productivity (NPP) is important because it represents the energy available to herbivores and other consumers in the ecosystem.
• Secondary productivity is the rate at which organic matter is formed by consumers (heterotrophs) and depends on primary productivity.
• Productivity varies across ecosystems depending on factors like plant species, nutrient availability, climate, and photosynthetic efficiency; globally, oceans contribute a significant share.

• Energy flow in an ecosystem is strictly unidirectional, moving from the sun through producers to consumers and decomposers.
• Unlike physical matter, energy within an ecosystem is never recycled and is progressively lost as heat at every trophic step.
• Less than 50% of incident solar radiation is Photosynthetically Active Radiation (PAR), and plants capture merely 2–10% of this PAR to sustain the living world.
• Grazing Food Chains (GFC) originate with living plants and dominate aquatic ecosystems, while Detritus Food Chains (DFC) originate with dead organic matter and dominate terrestrial ecosystems.
• Interconnected food chains create complex food webs that provide ecosystems with essential stability, resilience, and alternative energy pathways.
• Within a complex food web, a single organism has the ecological flexibility to occupy more than one trophic level simultaneously.
• Organisms are structured into sequential trophic levels, from producers (T1) to apex predators (T4), with available energy drastically decreasing at each successive tier.
• Decomposers act systematically across all trophic levels to break down dead organic matter, forming a critical biological link for continuous nutrient recycling.

• Ecological pyramids graphically display trophic levels, placing producers at the broad base and top consumers at the apex.
• The pyramid of numbers is typically upright, though it can become inverted in a single-tree ecosystem that supports many insects and parasites.
• The pyramid of biomass is generally upright, except in oceanic ecosystems where it is inverted (small phytoplankton supporting larger zooplankton).
• The energy pyramid is always upright because only 10% of energy is successfully transferred upward to the next trophic level.
• As a general rule in standard terrestrial ecosystems, producers naturally exceed consumers in both population count and total biomass.
• These models have major limitations: they assume simple linear food chains, ignore complex food webs, exclude decomposers, and cannot handle species that occupy multiple trophic levels.

• Meaning → Movement of nutrient elements through the various components of an ecosystem is called Nutrient Cycling or Biogeochemical Cycles.
• Two Types → Nutrient cycles are of two types — Gaseous and Sedimentary.
• Gaseous Cycle → Reservoir is the atmosphere. Examples: Nitrogen cycle and Carbon cycle.
• Sedimentary Cycle → Reservoir is the Earth's crust. Examples: Sulphur cycle and Phosphorus cycle.
• Importance → Nutrient cycling ensures the continuous recycling of nutrients between the biotic (living) and abiotic (non-living) components of an ecosystem, unlike energy, which flows unidirectionally.

• Carbon is a key element of living organisms and is found in air, water, soil, rocks, and oceans (major reservoirs).
• The carbon cycle is the movement of carbon between the atmosphere, organisms, and the Earth through natural processes.
• Photosynthesis fixes CO₂ into organic matter, which passes through food chains.
• CO₂ returns to the atmosphere through respiration, decomposition, and combustion.
• Carbon sinks & humans: Fossil fuels and rocks store carbon, but human activities increase CO₂, causing global warming.

• Phosphorus is an essential element of DNA, ATP, proteins, bones, and teeth, and often acts as a limiting nutrient in ecosystems.
• The main reservoir of phosphorus is rocks, from which phosphates are released slowly by weathering.
• Plants absorb phosphates from soil, and phosphorus moves through the food chain to animals.
• Phosphorus returns to the environment through decomposition, excretion, runoff, and sedimentation, mainly with the help of microbes.
• The phosphorus cycle has no gaseous phase, and excess phosphates from human activities can cause eutrophication in water bodies.

• Ecological (biotic) succession is the gradual and predictable change in species composition and community structure over time, ending in a stable climax community.
• Succession starts with pioneer species, passes through a series of seral stages (sere), and finally reaches a climax community.
• The process follows steps like nudation, invasion, ecesis, aggregation, competition, reaction, and stabilisation.
• During succession, communities modify the environment, making it less suitable for themselves and more suitable for new species.
• Types of succession: Primary succession occurs on bare areas without life and is slow, while secondary succession occurs in previously inhabited areas and is faster.

• Plant succession is of two main types based on habitat: hydrarch (wet areas) and xerarch (dry areas).
• Hydrarch succession occurs in water bodies and progresses from hydric to mesic conditions, eventually converting water into land.
• The pioneer species in hydrarch succession are phytoplankton, followed by submerged plants, floating plants, reed swamp, marsh meadow, shrubs, and trees.
• Xerarch succession occurs in dry areas and progresses from xeric to mesic conditions, leading to a stable forest community.
• The pioneer species in xerarch succession are crustose lichens, followed by small plants, herbs, grasses, shrubs, and finally trees.
• Secondary succession occurs on existing soil, proceeds faster than primary succession, and reaches climax more quickly.

• Ecosystem services are the benefits humans obtain from ecosystems, supporting life and maintaining environmental balance.
• These services are grouped into supporting, provisioning, regulating, and cultural services, including nutrient cycling, food, climate regulation, and recreation.
• Ecosystems provide essential functions like oxygen production, carbon dioxide removal, rainfall generation, and climate control.
• Pollination, seed dispersal, decomposition, and nutrient cycling by organisms help maintain soil fertility, crop production, and waste recycling.
• Ecosystem services have economic, ecological, and survival value, but human activities can disturb them, making conservation of biodiversity important.