Complete study notes for BEVAE-181 Block 1 Unit 2. Covers the definition and structure of ecosystems, biotic and abiotic components, food chains and food webs, energy flow through ecosystems, ecological pyramids (numbers, biomass, energy), biogeochemical cycles (carbon, nitrogen, water), producers, consumers and decomposers, and the concept of ecological balance. Includes labelled diagrams, SAQs, and model terminal question answers. Free PDF download.
Unit 2 · Index
An ecosystem is the basic functional unit of nature — where living and non-living components interact together. This unit covers what an ecosystem is, its components, how energy flows through trophic levels, how nutrients are cycled, and how ecosystems change over time through ecological succession.
In Unit 1 we learnt about the word "environment" and its definition. We know that organisms have both external and internal environments. The action and interaction of physical and living components of an organism's environment creates a system of relationships called an ecosystem.
☀️ Key Fact: The Sun is the only source of energy for all ecosystems on Earth. Without the Sun, the biosphere would collapse — no energy means no life!
For centuries humans treated the Earth as an unlimited resource. But gradual changes have altered our environment in many different ways — making it essential for us to understand and manage ecosystems wisely.
You and I live in a defined area where plants, animals, and we ourselves develop relationships with each other for life, food, water, shelter, and mates. This area has both living and non-living components that are interdependent and interrelated in terms of structure and functioning. Such a discrete unit is called an ecosystem.
📌 Did you know? The word ecosystem was coined by British ecologist Prof. Arthur Tansley in 1935. The prefix "eco" means environment.
An ecosystem is defined as "any unit (a biosystem) that includes all the organisms that function together (the biotic community) in a given area, interacting with the physical environment (abiotic component) so that the flow of energy leads to defined biotic structures and cycling of materials between living and nonliving parts."
In simple words: An ecosystem is a dynamic system where living and non-living things interact continuously — energy flows in, gets used, and matter cycles through the system.
Energy flows from the Sun → Producers → Consumers → Decomposers → Lost as heat. Matter cycles back through the inorganic nutrient pool.
A. Abiotic components:
B. Biotic components:
Ecosystems vary enormously in size — from the smallest puddle of water to the entire global biosphere. The hierarchy from smallest to largest:
The biosphere (also called ecosphere) is that part of the Earth, water, and atmosphere in which many smaller ecosystems exist and operate. It has three main subdivisions:
The land component of Earth — soil, rocks, mountains, deserts
The water component — oceans, lakes, rivers, groundwater, ice caps
The air (gaseous envelope) around Earth — extends up to 22.5 km height
🌍 Biosphere Facts:
• Extends from 11,000 m below ocean surface to 9,000 m above sea level
• Most densely populated region is just above and below sea level
• Life is absent at extreme poles, highest mountains, and deepest oceans
• The terrestrial biosphere is divided into large regions called biomes (characterized by climate, vegetation, animal life, and soil type)
• If Earth were the size of an apple, the biosphere would be as thin as its skin!
🌐 Biomes vs Aquatic Life Zones: The terrestrial biosphere is divided into biomes (forest, grassland, desert, tundra, etc.). The aquatic biosphere is divided into aquatic life zones (differentiated by salinity, nutrient levels, water temperature, and sunlight depth). The most important climatic factors determining biome boundaries are temperature and precipitation.
Biotic components: Primary producers, Consumers, Decomposers
Abiotic components: Energy, Environment, Inorganic elements, Soil
Each biome or aquatic zone is subdivided into smaller units called ecosystems. Any complete definition of an ecosystem includes both biotic and abiotic components AND their interaction.
🔑 Population → Community → Ecosystem:
• A population = a group of potentially interbreeding individuals of the SAME species in the same area and time
• A community = an aggregation of populations of different species in an area, living together with mutual tolerance and beneficial interactions
• Communities are usually named after the dominant plant (e.g., a grassland community is dominated by grasses)
The physical or abiotic components are the inorganic and non-living parts of the ecosystem — light, temperature, water, minerals, atmospheric gases, etc. Each factor influences and is influenced by all others.
The biological or biotic components of an ecosystem include three main groups:
Who they are: Chlorophyll-bearing green plants, green & purple bacteria, blue-green algae, and chemosynthetic bacteria.
What they do: They manufacture their own food from simple inorganic substances.
How: Through photosynthesis — using sunlight + CO₂ + water → organic food + O₂. Chemosynthetic bacteria use chemicals from inside the Earth instead of sunlight.
Why important: They are the FOUNDATION of all food chains. All energy for other organisms comes from producers.
Organisms that CANNOT make their own food and depend on other organisms for energy. Also called phagotrophs or heterotrophs.
| Level | Common Name | What They Eat | Examples |
|---|---|---|---|
| Primary (Trophic II) | Herbivores | Plants directly | Goat, cow, deer, rabbit, grasshopper |
| Secondary (Trophic III) | Carnivores | Herbivores | Frog (eats grasshopper), birds |
| Tertiary (Trophic IV) | Carnivores | Secondary consumers | Cat (eats birds), snake (eats frog) |
| Top Carnivores (Trophic V) | Apex predators | Not eaten by others | Eagle, lion, tiger, vulture |
👤 Humans are omnivores — we eat both plants and animals, so we may belong to more than one trophic level!
Organisms that break down dead organic matter and waste from all organisms. Mostly microscopic — bacteria and fungi.
Why they are ESSENTIAL: Without decomposers, nutrients locked in dead matter would never return to the soil. No decomposers = no recycling of nutrients = no life!
Detritus = fragments of decomposing organic matter (the "raw material" for decomposers)
Examples: Bacteria (Pseudomonas, Clostridium), Fungi (Aspergillus, Neurospora)
In an ecosystem, organisms obtain food through one, two, three, or four steps from producers. These steps are called trophic levels (trophe = nourishment in Greek). A trophic level = a position occupied by organisms with similar feeding habits.
Energy flows from lower trophic levels (producers) to higher trophic levels (consumers). It NEVER flows in reverse. Some energy is lost as heat at each level — so usually there are only 4–5 trophic levels and rarely more than 6.
Energy decreases at each level (lost as heat). Arrows show direction of energy flow and food.
| Trophic Level | Name | Type | Example |
|---|---|---|---|
| Level I | Producers | Autotrophs | Green plants, algae |
| Level II | Primary Consumers | Herbivores | Goat, cow, deer, grasshopper |
| Level III | Secondary Consumers | Carnivores | Frog, small fish, birds |
| Level IV | Tertiary Consumers | Carnivores | Snake, large fish |
| Level V | Top Carnivores | Apex Predators | Eagle, lion, shark |
A food chain is a "linear sequence of links of organisms in which an organism becomes food for the next organism." Arrows in food chains show the direction of energy flow — from producer to consumer.
🌐 Food Web: In nature, food chains are interconnected at many points and form a complex network called a food web. All food webs begin with autotrophs (producers) and end with decomposers.
In nature, three main types of food chains exist:
Starts from green plants (producers). Primary consumers are herbivores that eat plants. This is the most common type in nature.
Starts from dead organic matter (detritus) — decaying bodies and metabolic wastes. Energy comes from the energy stored in dead matter. In forests, a large portion of energy flows through this chain.
📊 Did You Know? In a shallow sea community, about 30% of total energy flows through detritus chains! In forests with large plant biomass and small animal biomass, the proportion is even larger.
Starts with green plants, goes to the herbivores, then to parasites that live on and feed from them. Unlike predators, parasites do NOT kill the host.
First Trophic Level (Producers): Wheat, Corn (grass, algae)
Second Trophic Level (Primary Consumers/Herbivores): Goat, Rat (rabbit, grasshopper)
Third Trophic Level (Secondary Consumers/Carnivores): Lion, Cat (frog, snake)
A Grizzly Bear can occupy multiple trophic levels:
This is why bears (and humans) are called omnivores — they operate at multiple trophic levels.
The two fundamental processes of any ecosystem are:
Energy enters from the Sun, flows through producers → consumers → decomposers, and is lost as heat at each step. Energy flow is UNIDIRECTIONAL — it cannot be recycled.
Nutrients (carbon, nitrogen, water, minerals) cycle from non-living to living and back again. Unlike energy, nutrients are RECYCLED — they are never lost, just transformed.
Ecosystems are "open systems" — they need a net inflow of energy from the Sun to continue. Without the Sun, the biosphere would collapse. But the same nutrients are recycled over and over again indefinitely.
🔑 Energy vs Nutrients: Energy flows one-way (cannot be recycled — lost as heat). Nutrients cycle continuously (carbon, nitrogen, water all cycle back). This is a critical distinction!
The circulation of nutrients includes basic inorganic elements: carbon (C), hydrogen (H), oxygen (O), nitrogen (N) as well as sodium, calcium, potassium; and compounds like water, carbonates, and phosphates.
In an ecosystem, producers (plants) capture solar energy through photosynthesis and store it in the form of carbohydrates (food). The plant tissues containing stored solar energy then become a source of energy for herbivores (primary consumers). Herbivores pass on the energy to carnivores (secondary consumers), and so on up the food chain.
At each step, some energy is lost as heat. Eventually all energy originally captured from the Sun is dissipated. Without a continuous supply of solar energy, the entire food chain would collapse. Therefore, the Sun is the ultimate and ONLY source of energy for all life on our planet.
Nutrients essential for life are distributed in air (atmosphere), soil/rock (lithosphere), water (hydrosphere), and living beings. Over time, elements move from one sphere to another through biogeochemical cycles.
📊 Macro vs Micronutrients:
• Macronutrients = needed in large amounts: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P) — make up 97% of body mass
• Micronutrients = needed in small/trace amounts: Iron, Manganese, Zinc, Copper, etc.
| Type | Reservoir | Nature | Examples |
|---|---|---|---|
| Gaseous Cycle | Atmosphere or Hydrosphere | Perfect/complete — nutrients replaced as fast as used | Water cycle, Carbon cycle, Nitrogen cycle |
| Sedimentary Cycle | Earth's crust (rocks, soil) | Imperfect — some nutrients get locked in sediments | Phosphorus, Sulphur, Calcium, Magnesium |
Water is one of the most important substances for life — it constitutes about 70% of body weight of most organisms. It also provides the medium for transport of all other elements.
Water moves: Ocean → Evaporation → Atmosphere → Condensation → Precipitation → Runoff/Rivers + Infiltration → Groundwater → Ocean
Key Water Facts: Oceans contain 97% of all water on Earth. Less than 1% is fresh water in rivers, lakes, and aquifers — yet this tiny amount is crucial for all terrestrial life. Water covers 75% of Earth's surface. About one-third of all solar energy is used in driving the water cycle.
Carbon is a minor constituent of the atmosphere but is the backbone of all organic substances — from coal and oil to DNA. Without carbon, life cannot exist. Carbon exists in both organic and inorganic forms.
Carbon moves through: Atmosphere → Photosynthesis → Plants → Animals → Decomposition → Back to atmosphere. Fossil fuel combustion adds extra CO₂.
Carbon Cycle Key Points:
• Oceans contain about 50 times more CO₂ than the atmosphere — they are a major carbon sink
• Carbon regulates atmospheric CO₂ level to about 0.032%
• Too little carbon → atmosphere cools. Too much carbon → atmosphere warms (global warming!)
• Fossil fuels (coal, petroleum, natural gas) store ancient carbon — burning them releases it back as CO₂
• The Global Carbon Cycle includes long-term storage in peat, limestone, dolomite (millions of years)
Nitrogen is an essential constituent of protein — the building block of all living tissue. It constitutes nearly 16% by weight of all proteins. The atmosphere has an inexhaustible supply of N₂, but most organisms cannot use it directly — it must first be "fixed" (converted to NH₃, nitrites, or nitrates).
Nitrogen Fixation happens in THREE ways:
1. Biological fixation: Free-living bacteria (Azotobacter, Clostridium) and symbiotic bacteria (Rhizobium in root nodules of legumes like beans, peas) + Blue-green algae (Anabaena, Spirulina)
2. Industrial fixation: Fertilizer factories — humans using industrial processes
3. Atmospheric phenomena: Thunder and lightning — converts N₂ to nitrates through rain
Nitrogen is returned to atmosphere by denitrifying bacteria (e.g., Pseudomonas) and as nitrogen oxides in automobile exhaust.
The sulphur cycle is mostly sedimentary with a short gaseous phase. It is a good example of the linkage between air, water, and the Earth's crust. Phosphorus, calcium, and magnesium also circulate by sedimentary cycles.
How sulphur moves:
• Large reservoir: In soil/sediments as sulphates, sulphides, and organic sulphur (coal, oil, pyrite rock)
• Released by: Weathering of rocks, erosional runoff, decomposition by bacteria and fungi, volcanic eruptions, combustion of fossil fuels
• Enters atmosphere as: Hydrogen sulphide (H₂S) and sulphur dioxide (SO₂)
• Returns to earth: Atmospheric SO₂ dissolves in rainwater as weak sulphuric acid (H₂SO₄) — this is what causes ACID RAIN!
• Plants absorb: Sulphates (SO₄²⁻) from soil → incorporated into proteins
• Decomposition: Under aerobic conditions by fungi (Aspergillus, Neurospora); under anaerobic conditions by bacteria (Escherichia, Proteus)
🧠 Remember: Nutrient cycles do NOT operate independently — they interact with each other. For example, the water cycle supports the movement of carbon, nitrogen, and sulphur through the ecosystem.
iv) All of the above — Respiration (organisms release CO₂), Photosynthesis (plants absorb CO₂), and fossil fuel combustion (releases stored carbon as CO₂) all contribute to carbon cycling.
i) Atmosphere — About 78% of the atmosphere is nitrogen gas (N₂). It is by far the largest reservoir of nitrogen.
A biotic community = a group of interacting populations living in a given area. Communities exhibit progressive change as part of their normal development. This orderly process of species replacement over time is called ecological succession (also called community development).
📌 Key Terms:
• Seral stage (Sere) = an intermediate community stage in succession progressing toward the climax
• Climax community = the final, stable community — does not change unless disturbed
• Pioneer community = the very first community to colonize a new area
• The time scale for succession ranges from decades (after a forest fire) to millions of years (after a mass extinction)
Bare Rock → Lichens/Mosses (Pioneer) → Grasses → Shrubs → Small Trees → Climax Forest (Stable)
Primary succession occurs where NO community has ever existed before — bare, lifeless areas. Examples: newly exposed rock from landslides or lava flows, newly formed volcanic islands, sand dunes, emerging deltas.
Lichens are the "pioneer community." Their fungal component clings to rocks. They produce weak acids that slowly erode the rock surface — building the very first thin layer of soil.
As organic matter and sand accumulate in rock fissures, mosses take hold, followed by grasses and herbs. The soil layer thickens.
Larger, more nutrient-demanding plants like shrubs succeed. The earlier pioneers (lichens, mosses) can no longer compete for light, water, and minerals.
Eventually a stable climax forest forms. The final community is self-maintaining and resistant to change unless disturbed.
Secondary succession occurs when an existing community is drastically disturbed — destroyed — and a new community replaces it. The soil is already present, so succession proceeds faster than primary succession.
| Feature | Primary Succession | Secondary Succession |
|---|---|---|
| Starting point | Bare, lifeless area (no soil) | Disturbed area (soil present) |
| Soil availability | Not present — must form | Already present |
| Speed | Very slow (centuries) | Faster (decades) |
| Causes | Volcanic eruptions, new lava flows, sand dunes | Fire, flood, storm, logging, farming |
| Pioneer organisms | Lichens and mosses | Grasses and herbs (soil organisms survive) |
| Examples | New volcanic island, glacier retreat | Abandoned farmland, after forest fire |
📊 Succession Speed Examples:
• Grassland secondary succession: 20–40 years
• Disturbed tundra: hundreds of years (may never fully recover!)
• Climax communities change slowly even when climate is constant, but change rapidly if disturbed
Humans can and do change natural communities — both accidentally and deliberately. Today, approximately 40% of Earth's photosynthetic productivity is used or influenced by human activities.
⚠️ The Problem: We often try to correct past environmental mistakes with well-intentioned but poorly-informed measures. Our efforts fail because of lack of basic ecological knowledge. Our technology has far outpaced our understanding of the environment.
Each of us must be aware of the consequences of disturbing the delicate balances of ecosystems — and avoid being a contributor to ecosystem damage or degradation. Ecologists will play an increasingly important role in changing how we interact with the natural world.
🌿 The Way Forward: We have not yet learned to live in harmony with ecosystems of which we are a part. Sustainable ecosystem management requires:
• Understanding basic ecological principles
• Applying wise and restrained use of natural resources
• Monitoring and restoring damaged ecosystems
• Reducing our ecological footprint through better technology and behaviour
Exam-style questions from the IGNOU textbook with complete model answers.
There are two types of biogeochemical cycles:
Key difference: Gaseous cycles have a large atmospheric reservoir making them more complete and efficient; sedimentary cycles risk "leaking" nutrients into long-term geological storage.
Three pathways of NITROGEN FIXATION (N₂ → usable forms):
Two pathways of returning fixed nitrogen to the atmosphere:
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