Ecosystem — NEET notes

Ecosystem Structure

An ecosystem is a functional unit of nature where living organisms (biotic community) interact with each other and their physical environment (abiotic factors). Every ecosystem has structural components (producers, consumers, decomposers + abiotic factors) and functional components (energy flow, nutrient cycling, productivity). Ecosystems range from a small pond to an entire ocean — size doesn't matter, function does.

Ecosystem = Biotic Community + Abiotic Environment

What is an Ecosystem

An ecosystem includes all living organisms in an area plus the physical environment they interact with. The term was coined by A.G. Tansley (1935).

An ecosystem is a functional unit of nature where biotic components (all living organisms) interact among themselves and with the abiotic components (physical environment). The term was coined by A.G. Tansley in 1935. The key insight: an ecosystem is NOT just a collection of species — it's a system with inputs, outputs, energy flow, and nutrient cycling.

• Coined by A.G. Tansley (1935) — 'ecosystem' = ecological system
• An ecosystem can be as small as a drop of pond water or as large as an ocean
• Two types: Natural (forest, lake, desert, grassland) and Artificial/Man-made (crop field, aquarium, garden)
• Ecosystem = biotic community + abiotic environment interacting as a functional unit

Abiotic Components

Physical and chemical factors like temperature, light, water, soil, minerals, and gases form the abiotic foundation of every ecosystem.

Abiotic components are the non-living physical and chemical factors of an ecosystem. They determine what organisms can survive in a given habitat. Think of them as the stage — biotic organisms are the actors.

• Climatic factors — temperature, light, humidity, rainfall, wind
• Edaphic factors (soil) — texture, pH, mineral content, water-holding capacity
• Inorganic substances — C , N , P , S , H_2O that participate in nutrient cycling
• Organic substances — humus, proteins, carbohydrates, lipids (link abiotic to biotic)
• Physical factors — temperature, light are the most important for distribution of organisms

Biotic Components

All living organisms in an ecosystem are classified by their nutritional role: producers (autotrophs), consumers (heterotrophs at various trophic levels), and decomposers (saprotrophs).

Biotic components are classified based on how they obtain food. This classification directly maps to trophic levels and energy flow — the backbone of ecosystem ecology.

• Producers (Autotrophs) — Fix solar energy into chemical energy via photosynthesis. Green plants, algae, cyanobacteria. They form the base of every food chain
• Primary consumers (Herbivores) — Feed directly on producers. Deer, grasshopper, zooplankton
• Secondary consumers (Primary carnivores) — Feed on herbivores. Frog, small fish, fox
• Tertiary consumers (Top carnivores) — Feed on secondary consumers. Lion, hawk, shark
• Decomposers (Saprotrophs) — Break down dead organic matter. Fungi and bacteria. They recycle nutrients back to the abiotic pool

Stratification

Ecosystems show vertical stratification — distinct layers (strata) that provide different microhabitats. This is most visible in forests with emergent, canopy, understory, and forest floor layers.

Stratification refers to the vertical layering within an ecosystem. A forest has distinct layers — tall trees form the canopy, shorter trees form the understory, shrubs below them, herbs at ground level, and a litter/root layer underground. Each stratum has different light, temperature, and humidity conditions, creating distinct microhabitats.

• Tree layer (canopy) — Receives maximum sunlight, tallest plants, most photosynthesis
• Shrub layer — Moderate light, woody plants shorter than trees
• Herb/ground layer — Low light, grasses, ferns, mosses
• Forest floor (litter layer) — Decomposition hub, fungi, detritivores, burrowing animals
• Aquatic ecosystems also show stratification: photic zone (sunlit) → aphotic zone (dark)

Productivity

Productivity is the rate of biomass production in an ecosystem. Gross Primary Productivity (GPP) is the total rate of photosynthesis. Net Primary Productivity (NPP) = GPP minus respiration — it's the biomass available to consumers. Secondary productivity is the rate at which consumers form new biomass. The standing crop (biomass) at any time depends on the balance between production and consumption.

NPP = GPP - R

GPP & NPP

GPP is the total organic matter produced by photosynthesis per unit area per unit time. NPP is what remains after the plant uses some for its own respiration — this is the biomass available for the next trophic level.

Primary productivity is the amount of biomass (or energy) produced per unit area per unit time by producers through photosynthesis. It is expressed in g/m^2/year or kcal/m^2/year .

GPP = Total rate of photosynthesis (total organic matter fixed)

NPP = GPP - R

Where R = respiratory losses by the producers themselves. Plants photosynthesize (GPP) but also burn some of that food for their own metabolism (R). What's left over (NPP) is the biomass available for herbivores and decomposers.

• GPP — Gross Primary Productivity = total photosynthesis by all producers. Measured per unit area per unit time
• NPP — Net Primary Productivity = GPP - R. This is the biomass AVAILABLE to consumers
• R — Respiratory loss by producers (typically 20-40% of GPP)
• Units: g/m^2/year (biomass) or kcal/m^2/year (energy)
• NPP is ALWAYS less than GPP (plants must respire to stay alive)

Factors Affecting Primary Productivity

Primary productivity varies enormously across ecosystems depending on plant species, environmental conditions, nutrient availability, and photosynthetic capacity.

Primary productivity depends on the type of plant species inhabiting the area, their photosynthetic capacity, and a range of environmental factors. This is why productivity varies from <1 to >3000 g/m^2/year across different ecosystems.

• Sunlight — The energy source for all photosynthesis. Tropical regions get more light → higher productivity
• Temperature — Enzymes work optimally at moderate temperatures. Too cold or too hot reduces productivity
• Water/Rainfall — Limits terrestrial productivity in deserts and grasslands
• Nutrient availability — N and P are most limiting. Oceans are nutrient-poor in open water → low productivity despite huge area
• Plant species — C4 plants (sugarcane, maize) have higher photosynthetic rates than C3 plants

Annual NPP of the whole biosphere is approximately 170 billion tonnes (dry weight) of organic matter. Despite oceans covering a much larger area, the contribution of oceanic and terrestrial productivity is roughly comparable.

Secondary Productivity & Standing Crop

Secondary productivity is the rate at which consumers convert ingested food into their own biomass. Standing crop is the total biomass present at any given time in an ecosystem.

Secondary productivity is the rate of formation of new organic matter by consumers (heterotrophs). When a deer eats grass and grows 5 kg in a month, that's secondary productivity. It depends on how efficiently animals assimilate and convert the food they eat.

• Secondary productivity = rate of new biomass formation by consumers. Includes all trophic levels above producers
• Standing crop = total amount of living biomass present at a given time. Measured as biomass ( g/m^2 ) or number of organisms per unit area
• Standing crop ≠ productivity — a forest may have a huge standing crop (biomass of trees) but moderate annual productivity
• Standing state = amount of inorganic nutrients (N, P, K, Ca) present in the soil at any given time. Varies seasonally — different in rainy vs dry season

Decomposition

Decomposition is the breakdown of dead organic matter (detritus) into simpler inorganic substances by decomposers (fungi, bacteria). It proceeds through fragmentation, leaching, catabolism, humification, and mineralization. The dark-colored amorphous substance humus is highly resistant to decomposition and serves as a nutrient reservoir. Rate of decomposition depends on temperature, moisture, oxygen, and the chemical composition of detritus.

Detritus Decomposers CO_2 + H_2O + Inorganic nutrients

Detritus & Decomposers

Detritus is dead organic matter (fallen leaves, dead animals, fecal matter). Decomposers (mainly fungi and bacteria) break it down, releasing inorganic nutrients back into the ecosystem.

Detritus includes dead plant remains (leaves, bark, flowers), dead animal remains, and fecal matter. This is the raw material for decomposition. The organisms that break it down — primarily fungi and bacteria — are called decomposers or saprotrophs. They secrete digestive enzymes onto the detritus and absorb the products.

• Detritus = dead organic matter (both plant and animal origin)
• Decomposers = fungi and bacteria that break down detritus by extracellular digestion
• Detritivores = organisms that physically fragment detritus by eating it — earthworms, dung beetles, millipedes
• Detritivores HELP decomposers by breaking detritus into smaller pieces (increases surface area for microbial action)
• The process returns locked-up nutrients ( C , N , P , S ) to the soil and atmosphere

Steps of Decomposition

Decomposition proceeds through five sequential steps: fragmentation, leaching, catabolism, humification, and mineralization. Each step converts complex organic matter into progressively simpler substances.

Decomposition is NOT a single event — it's a multi-step process. NCERT describes five steps that proceed somewhat simultaneously but have distinct roles.

• Step 1: Fragmentation — Detritivores (earthworms, beetles) break detritus into smaller particles. Increases surface area for microbial attack
• Step 2: Leaching — Water-soluble inorganic nutrients are washed (leached) into the soil by rain. They dissolve and seep into deeper soil layers
• Step 3: Catabolism — Bacterial and fungal enzymes break down detritus into simpler inorganic substances. This is the main chemical breakdown step
• Step 4: Humification — Partially decomposed dark-colored amorphous substance called humus accumulates. Humus is HIGHLY resistant to further decomposition
• Step 5: Mineralization — Humus is further degraded by microbes, releasing inorganic nutrients ( CO_2 , NH_4^+ , PO_4^ 3- , SO_4^ 2- ) into the soil for plant uptake

Factors Affecting Decomposition

Rate of decomposition depends on chemical composition of detritus and climatic conditions — temperature, moisture, oxygen availability, and the lignin-to-nitrogen ratio.

The rate of decomposition is NOT constant — it varies dramatically across ecosystems. A dead leaf in a tropical forest may decompose in weeks, while in a cold tundra it may persist for years. Two main categories of factors control the rate.

• Temperature — Warm temperatures accelerate microbial enzyme activity. Decomposition is fastest in warm, humid tropics
• Moisture — Water is essential for microbial growth and enzyme action. Dry conditions slow decomposition dramatically
• Oxygen — Aerobic decomposition is much faster than anaerobic. Waterlogged soils → slow anaerobic decomposition → peat formation
• Chemical quality of detritus — High lignin content (wood) = slow decomposition. High nitrogen/sugar content (young leaves) = fast decomposition
• Lignin:Nitrogen ratio — Higher ratio = slower decomposition. Lignin is tough and resists microbial enzymes

Energy Flow

Energy flows unidirectionally through an ecosystem from producers to top consumers — it cannot be recycled. At each trophic level, only about 10% of energy is passed to the next level (Lindeman's 10% Law). Food chains (grazing and detritus) link trophic levels; food webs are interconnected food chains. The longer the food chain, the less energy reaches the top — which is why top predators are always rare.

Energy at T_ n+1 = 10 % Energy at T_n (Lindeman's 10 % Law)

Trophic Levels

Each step in a food chain is a trophic level. Producers occupy T1, primary consumers T2, secondary consumers T3, and so on. Energy decreases at each step.

A trophic level represents the position of an organism in a food chain based on its source of nutrition. Each level is a step away from the original energy source (sunlight).

• T_1 — Producers (plants, algae). Fix solar energy into organic molecules
• T_2 — Primary consumers (herbivores). Eat producers directly — deer, rabbit, grasshopper
• T_3 — Secondary consumers (primary carnivores). Eat herbivores — frog, snake, small fish
• T_4 — Tertiary consumers (top carnivores). Eat secondary consumers — hawk, lion, shark
• Decomposers don't fit neatly into one trophic level — they feed on dead matter from ALL levels

Grazing vs Detritus Food Chain

The grazing food chain starts with living plants and goes through herbivores to carnivores. The detritus food chain starts with dead organic matter and goes through decomposers and detritivores.

Every ecosystem has two types of food chains operating simultaneously. Energy enters through producers and then takes two routes.

• The GFC is the more familiar chain: Grass → Deer → Tiger
• The DFC handles the BULK of energy flow in most terrestrial ecosystems — more energy goes through dead matter than through live grazing
• In a forest, only about 10% of NPP is consumed by herbivores (GFC). The remaining 90% enters the DFC as dead organic matter
• The two food chains are connected — the waste/dead matter from GFC enters the DFC
• In aquatic ecosystems, GFC is the major pathway because phytoplankton are consumed quickly

Food Web

A food web is a network of interconnected food chains. Real ecosystems rarely have simple linear food chains — organisms feed at multiple trophic levels, creating complex webs.

In nature, a single organism is rarely part of just ONE food chain. A grasshopper may be eaten by a frog, a bird, or a lizard. Each of these predators may themselves be eaten by different higher predators. When you connect all these feeding relationships together, you get a food web — a network of interconnected food chains.

• A food web = interconnected network of food chains in an ecosystem
• Food webs provide stability — if one species declines, its predator can switch to alternative prey
• More complex food webs (more connections) = greater ecosystem stability
• The concept of food web was given by Charles Elton
• Food webs show why removing one species can have cascading effects on the entire ecosystem

10% Law (Lindeman's Law)

Lindeman (1942) stated that only about 10% of energy at one trophic level is transferred to the next. The remaining 90% is lost as heat through respiration. This explains why food chains are short and top predators are rare.

Lindeman's 10% Law (1942) states that only about 10% of energy assimilated at one trophic level is passed to the next trophic level. The remaining 90% is lost as heat during metabolic processes (respiration). This is a fundamental law of ecosystem energetics.

If T_1 = 1000 kcal T_2 = 100 kcal T_3 = 10 kcal T_4 = 1 kcal

• Lindeman (1942) — proposed the 10% energy transfer law based on studies of Cedar Bog Lake, Minnesota
• At each trophic level, approximately 90% of energy is lost as heat through respiration
• Only 10% is converted to biomass and available to the next trophic level
• Energy flow is unidirectional — it flows from lower to higher trophic levels and CANNOT be recycled
• This is why food chains rarely have more than 4-5 trophic levels — too little energy reaches the top

Quick recall

If 1,000,000 J of energy is available at the producer level, how much energy reaches the tertiary consumer (T4)?

T_1 = 1,000,000 J → T_2 = 100,000 J → T_3 = 10,000 J → T_4 = 1,000 J. Only 0.1% of the original energy reaches T4.

Ecological Pyramids

Ecological pyramids graphically represent trophic level relationships in an ecosystem. There are three types: pyramid of number (number of organisms), pyramid of biomass (total dry weight), and pyramid of energy (energy at each level). Pyramid of energy is ALWAYS upright (10% law guarantees it). Pyramids of number and biomass CAN be inverted in specific cases — a single tree supporting thousands of insects (inverted number), or ocean where phytoplankton biomass < zooplankton biomass at any instant (inverted biomass).

Pyramid of Energy: always upright | Number/Biomass: can be inverted

Pyramid of Number

The pyramid of number shows the number of organisms at each trophic level. Usually upright (more grass plants than deer, more deer than tigers), but can be inverted when a single large organism supports many smaller ones.

A pyramid of number represents the number of individual organisms at each trophic level per unit area. The base represents producers ( T_1 ) and successive tiers represent consumers ( T_2 , T_3 , T_4 ).

• Upright in grassland ecosystem: Grasses (millions) → Grasshoppers (thousands) → Frogs (hundreds) → Snakes (tens) → Hawks (few)
• Inverted in a tree ecosystem: 1 large tree → thousands of herbivorous insects → hundreds of insectivorous birds → few parasites (hawks/raptors)
• Inverted in a parasitic food chain: 1 tree → birds → lice on birds → microbes on lice — number INCREASES going up
• The pyramid of number does NOT account for size differences — a single banyan tree 'counts' the same as a single grass blade

Pyramid of Biomass

The pyramid of biomass shows the total dry weight of organisms at each trophic level. Usually upright on land (trees weigh more than deer), but inverted in open ocean where phytoplankton biomass at any given time is less than zooplankton biomass.

A pyramid of biomass represents the total dry weight of all organisms at each trophic level per unit area at a particular time. It overcomes the size-blindness of the number pyramid — a tree weighs tonnes while an insect weighs milligrams.

• Upright in terrestrial ecosystems (forest, grassland): Producer biomass > herbivore biomass > carnivore biomass
• Inverted in open ocean (aquatic): Phytoplankton biomass (at any instant) < zooplankton biomass < fish biomass
• Why inverted in ocean? Phytoplankton reproduce extremely fast — their productivity is high but standing crop (biomass at any given moment) is low because they're consumed as fast as they're produced
• Biomass is measured as dry weight per unit area ( g/m^2 ) to eliminate variable water content

Pyramid of Energy

The pyramid of energy shows the amount of energy at each trophic level per unit area per unit time. It is ALWAYS upright because of the 10% law — energy is lost as heat at every step. This is the most accurate representation of trophic structure.

A pyramid of energy represents the amount of energy available at each trophic level per unit area per unit time. Unlike number and biomass pyramids, the energy pyramid is ALWAYS upright — it can NEVER be inverted. This makes it the most reliable ecological pyramid.

• ALWAYS upright — No exceptions. The 10% law (Lindeman) guarantees energy decreases at each successive trophic level
• Accounts for rate of energy production (per unit time), not just a snapshot — so it overcomes the standing crop problem of biomass pyramids
• Even in the ocean (where biomass pyramid is inverted), the energy pyramid is upright — phytoplankton fix MORE energy per year than zooplankton consume
• Units: kcal/m^2/year or J/m^2/year
• This is considered the most accurate and fundamental of all three pyramids

Quick recall

Which ecological pyramid is ALWAYS upright and why?

Pyramid of energy — because energy transfer obeys the 10% law (Lindeman). At each trophic level, ~90% of energy is lost as heat. So energy at T_ n+1 is always less than at T_n . This is a thermodynamic constraint — no biological exception is possible.

Limitations of Ecological Pyramids

Ecological pyramids have significant limitations: they don't include decomposers and detritus food chain, they assume simple food chains (not webs), and they don't account for organisms occupying multiple trophic levels.

While ecological pyramids are powerful visual tools, they have important limitations that NCERT explicitly mentions.

• Pyramids assume a simple food chain — real ecosystems have complex food WEBS with organisms at multiple trophic levels
• A single species (like humans) can be T_2 , T_3 , or T_4 depending on what it eats — pyramids can't represent this
• Saprophytes/decomposers are NOT given any place in ecological pyramids despite being crucial for nutrient cycling
• Pyramids don't show seasonal variations — standing crop and productivity change with seasons
• Same species at different life stages may occupy different trophic levels (e.g., tadpole = herbivore, adult frog = carnivore)

Nutrient Cycling

Unlike energy (which flows unidirectionally), nutrients cycle between biotic and abiotic compartments — this is called biogeochemical cycling. Two main types: gaseous cycles (reservoir in atmosphere/ocean — carbon, nitrogen) and sedimentary cycles (reservoir in Earth's crust — phosphorus, sulphur). The carbon cycle involves photosynthesis, respiration, decomposition, and fossil fuel combustion. The phosphorus cycle has NO gaseous phase — it moves through rocks, soil, water, and organisms.

Nutrients: cycled (reused) | Energy: flows (not recycled)

Biogeochemical Cycles

Nutrients move cyclically between biotic (living) and abiotic (non-living) compartments. The amount of nutrient stays constant — only its location changes between pools. Two types: gaseous (reservoir = atmosphere) and sedimentary (reservoir = Earth's crust).

The movement of nutrient elements through various components of an ecosystem is called nutrient cycling or biogeochemical cycling (bio = living, geo = rocks/earth, chemical = elements). Unlike energy, nutrients are NOT used up — they are recycled between biotic and abiotic compartments.

• Nutrient cycling = biogeochemical cycle — movement of elements ( C , N , P , S ) between living organisms and the physical environment
• The total amount of a nutrient in the biosphere remains constant — it just moves between pools
• Two compartments: biotic (organisms) and abiotic (atmosphere, water, soil, rocks)
• Gaseous cycles — Reservoir is atmosphere or ocean. Examples: carbon cycle, nitrogen cycle. Global in scale, more perfect recycling
• Sedimentary cycles — Reservoir is Earth's crust (rocks, soil). Examples: phosphorus cycle, sulphur cycle. Local, less perfect — nutrients can get locked in deep sediments

Carbon Cycle

Carbon cycles through the atmosphere (as CO₂), biotic community (as organic compounds), oceans (as dissolved CO₂ and carbonates), and fossil fuels. Photosynthesis fixes it; respiration, decomposition, and combustion release it.

The carbon cycle is a gaseous biogeochemical cycle with the main reservoir being the atmosphere (as CO_2 , currently ~0.04% or 420 ppm) and the oceans (dissolved CO_2 and carbonates). Carbon is the backbone of all organic molecules — cycling it is essential for life.

• Photosynthesis — Removes CO_2 from atmosphere, fixes it into organic carbon (glucose). About 4 × 10¹³ kg of carbon fixed annually
• Respiration — All living organisms release CO_2 back by breaking down organic molecules. Plants, animals, and microbes all respire
• Decomposition — Decomposers break down dead organic matter, releasing CO_2 and CH_4
• Combustion — Burning fossil fuels (coal, petroleum, natural gas) releases ancient locked-up carbon as CO_2 . Major cause of rising atmospheric CO_2
• Ocean dissolution — Oceans absorb CO_2 (act as carbon sinks). Marine organisms use dissolved carbon to build shells ( CaCO_3 )
• Fossil fuel formation — Over millions of years, dead organisms buried under pressure → coal, petroleum. This LOCKS carbon out of the active cycle

Phosphorus Cycle

Phosphorus cycles through rocks, soil, water, and organisms — but never enters the atmosphere as a gas. Its reservoir is the Earth's crust (sedimentary cycle). Weathering of rocks releases phosphates into soil; plants absorb them; animals get phosphorus from plants; decomposition returns it to soil.

The phosphorus cycle is a sedimentary biogeochemical cycle — its main reservoir is rocks and natural phosphate deposits in the Earth's crust. Unlike carbon and nitrogen, phosphorus has NO gaseous phase — it does not enter the atmosphere. This makes it cycle more slowly and locally.

• Reservoir — Rocks and phosphate deposits in Earth's crust (NOT atmosphere)
• Weathering releases phosphates ( PO_4^ 3- ) from rocks into soil and water
• Plants absorb dissolved phosphates from soil through roots
• Animals get phosphorus by eating plants or other animals
• Decomposers break down dead organisms and return phosphorus to the soil
• Phosphorus is a key component of DNA, RNA, ATP, cell membranes (phospholipids), and bones/teeth (calcium phosphate)
• No gaseous phase — this is the CRITICAL distinguishing feature from carbon and nitrogen cycles

Ecological Succession

Ecological succession is the sequential, gradual, and predictable change in species composition of an area over time. Primary succession starts on bare/lifeless substrate (lava, bare rock) where no soil exists. Secondary succession starts on land where soil and seed banks already exist (abandoned farmland, burned forest). Succession proceeds through seral stages until a stable climax community is reached. Hydrarch succession (in water → land) and xerarch succession (on dry rock → mesic community) are the two classic sequences.

Pioneer species seral stages Climax community

Ecological Succession

Succession is the orderly process of community change over time in a given area. Each community modifies the environment, making it suitable for the next community until a stable climax is reached.

Ecological succession is the gradual and fairly predictable change in the species composition of a given area. It's not random — communities change in an orderly sequence. Each stage (called a seral stage or seral community) modifies the environment, making it suitable for the next stage. The entire sequence from pioneer to climax is called a sere.

• Succession — sequential change in species composition over time in an area
• Sere — the entire sequence of communities from pioneer to climax
• Seral stage/community — each intermediate community in the succession sequence
• Pioneer species — the first species to colonize a new/disturbed habitat (lichens on bare rock, phytoplankton in water)
• Climax community — the final, stable, self-perpetuating community in equilibrium with the environment
• Each seral community modifies the environment (adds soil, changes pH, creates shade) making it suitable for the NEXT community

Primary vs Secondary Succession

Primary succession begins on totally new, lifeless substrate with no soil. Secondary succession occurs where soil and biological legacy (seeds, roots) already exist, so it proceeds much faster.

The critical difference between primary and secondary succession is the starting substrate — whether soil exists or not.

• Primary succession: no prior biological community, no soil → very slow (centuries to millennia)
• Secondary succession: prior community destroyed but soil/seeds remain → faster (decades to centuries)
• In primary succession, lichens (pioneer) secrete acids that weather rock → create thin soil → mosses grow → more soil forms → grasses → shrubs → trees
• In secondary succession, the process skips the soil-building stage entirely

Hydrarch & Xerarch Succession

Hydrarch succession starts in water bodies and progresses toward mesic (moderately moist) conditions. Xerarch succession starts on dry bare rock and also progresses toward mesic conditions. Both converge on a mesic climax.

Depending on whether succession begins in a wet or dry environment, it follows different pathways — but both ultimately converge on a mesic (moderately moist) climax community.

• Hydrarch succession — starts in aquatic (water) habitat → becomes progressively DRIER → ends at mesic climax
• Xerarch succession — starts on dry bare rock/sand → becomes progressively WETTER → ends at mesic climax
• Both converge on a mesophytic/mesic climax — a community adapted to moderate moisture conditions (typically a forest)
• Hydrarch sequence: Phytoplankton → Submerged plants → Free-floating plants → Reed swamp → Sedge meadow → Woodland → Forest
• Xerarch sequence: Crustose lichen → Foliose lichen → Mosses → Grasses/herbs → Shrubs → Trees/Forest

Quick recall

What is the pioneer species in xerarch succession on bare rock?

Crustose lichens — they can grow on bare rock with no soil. They secrete acids that slowly weather the rock surface, creating tiny particles. When lichens die, their organic matter mixes with rock particles to form the first thin layer of soil. Mosses follow, then grasses, then shrubs, then trees.