🌿 Chapter 01 · Topic 06 · Ecological Succession

Ecological Succession, Climax Community & Revision

Primary and secondary succession, seral stages, climax community, types of ecosystems quick facts, and complete Chapter 01 master revision checklist.

Ecological Succession

Ecological succession is the gradual, directional, and predictable change in the species composition of a community over time, eventually leading to a stable climax community.

Process is directional — always moves toward greater complexity and stability
Each stage (sere) modifies the environment, making it suitable for the next community
Begins with pioneer species — hardy organisms that colonise bare/disturbed areas first
Ends with climax community — stable, self-perpetuating community in equilibrium with climate
Biodiversity and biomass generally increase during succession

Key Terms: Sere = each stage of succession. Seral community = community at each stage. Pioneer species = first colonisers. Climax community = final stable stage.

Primary Succession

Occurs on bare, lifeless substrate where no community has existed before — no soil, no organic matter.

Starting point: Bare rock, newly formed volcanic island, sand dunes, glacial moraines
Speed: Very slow — may take hundreds to thousands of years
Pioneer species: Lichens (on bare rock) — they secrete acids that break down rock, forming primitive soil
Sequence on rock: Lichens → Mosses → Ferns/Grasses → Shrubs → Trees (climax forest)

Classic Example: Krakatoa volcanic island (1883) — after the eruption destroyed all life, primary succession began from bare rock. Within decades, a forest community re-established.

Stages of Primary Succession on Rock (Lithosere):

Bare rock stage — no life; weathering begins
Crustose lichen stage — first colonisers; break down rock surface
Foliose lichen stage — more complex lichens; more soil formation
Moss stage — mosses establish; soil deepens
Herb/grass stage — grasses and herbs colonise
Shrub stage — shrubs replace grasses
Climax forest — stable tree community

Secondary Succession

Occurs in areas where a community was previously present but was destroyed — soil and some organic matter remain.

Starting point: Abandoned farmland, burned forest, flooded land, cleared land
Speed: Much faster than primary succession — soil already present with seeds and nutrients
Pioneer species: Weeds, grasses, fast-growing herbs (not lichens)
Sequence: Weeds/grasses → Shrubs → Pioneer trees → Climax forest

Classic Example: Abandoned agricultural field — within years, weeds appear; within decades, shrubs and trees establish; eventually a forest climax community forms.

Feature	Primary Succession	Secondary Succession
Starting substrate	Bare rock / no soil	Disturbed area with soil
Speed	Very slow (centuries)	Faster (decades)
Pioneer species	Lichens, mosses	Weeds, grasses
Soil present?	No	Yes
Example	Volcanic island, bare rock	Abandoned farmland, burned forest

Seral Stages

A sere is a particular sequence of communities that follow one another in a given area during succession. Each community in the sequence is called a seral stage or seral community.

Type of Sere	Starting Habitat	Pioneer Species	Example
Hydrosere	Open water (pond/lake)	Phytoplankton, submerged plants	Pond succession → marsh → swamp → forest
Xerosere	Dry land (rock/sand)	Lichens (on rock), grasses (on sand)	Rock succession → grassland → forest
Lithosere	Bare rock	Crustose lichens	Volcanic rock → forest
Psammosere	Sand dunes	Marram grass	Sand dune → scrub → forest
Halosere	Saline/coastal areas	Salt-tolerant plants (halophytes)	Mangrove succession

Climax Community

The climax community is the final, stable, self-perpetuating community that develops at the end of succession. It is in dynamic equilibrium with the prevailing climate.

Determined primarily by climate (temperature and rainfall) — called climatic climax
High biodiversity, complex food webs, stable species composition
Biomass and species diversity are at their maximum
Net productivity approaches zero (GPP ≈ Respiration)
If disturbed, the community can recover through secondary succession

Monoclimax Theory (Clements): Each region has only one climax community determined by climate. Polyclimax Theory (Tansley): Multiple climax communities possible depending on local soil, topography, and other factors.

Types of Ecosystems — Quick Revision

Ecosystem	Key Features	Examples / Notes
Tropical Rainforest	Highest biodiversity; highest NPP; dense canopy; high rainfall (>200 cm/yr)	Amazon, Western Ghats, Congo
Temperate Forest	Deciduous trees; seasonal; moderate biodiversity	Oak, maple, beech forests
Grassland	Dominated by grasses; few trees; moderate rainfall; high soil fertility	Savanna (tropical), Prairie (temperate), Steppe
Desert	Low rainfall (<25 cm/yr); extreme temperatures; low biodiversity; xerophytes	Sahara, Thar, Atacama; cacti, camels
Wetlands	Transitional; highest productivity; carbon sinks; flood control; Ramsar sites	Mangroves, marshes, swamps, bogs
Coral Reef	“Rainforest of the sea”; highest marine biodiversity; built by coral polyps	Great Barrier Reef; threatened by bleaching
Freshwater	Lentic (still: lakes, ponds) vs Lotic (flowing: rivers, streams)	Supports ~10% of all species; only ~0.01% of water
Marine/Ocean	Largest ecosystem; covers 71% of Earth; low NPP per unit area but huge total	Pelagic, benthic, abyssal zones

Complete Chapter 01 Master Revision Checklist

📌 Topic 01 — Ecology Basics
✅ Ecology coined by Ernst Haeckel (1866) — oikos + logos
✅ Autecology = single species; Synecology = community
✅ Levels: Organism → Population → Community → Ecosystem → Biome → Biosphere
✅ Biotic = producers, consumers, decomposers; Abiotic = temperature, light, water, soil
✅ Habitat = address; Niche = profession; two species cannot share same niche (Gause)
✅ Ecotone = transition zone; Edge effect = higher biodiversity at ecotones
✅ Keystone species — removal causes ecosystem collapse

📌 Topic 02 — Food Chain & Food Web
✅ Grazing food chain starts with living plants; Detritus food chain starts with dead matter
✅ T1=Producers, T2=Primary consumers, T3=Secondary, T4=Tertiary
✅ Food web = more stable than food chain; greater complexity = greater stability
✅ Detritivore = physically breaks detritus; Decomposer = chemically breaks it
✅ Food chains limited to 4–5 levels due to 10% energy rule

📌 Topic 03 — Ecological Pyramids
✅ Introduced by Charles Elton (1927) — Eltonian Pyramids
✅ Pyramid of Numbers: upright (grassland), inverted (tree), spindle (parasitic)
✅ Pyramid of Biomass: upright (terrestrial), inverted (aquatic/pond)
✅ Pyramid of Energy: ALWAYS upright; NEVER inverted; most accurate
✅ Lindeman’s 10% Law (1942) — 10% energy transferred; 90% lost as heat

📌 Topic 04 — Energy Flow
✅ Energy flow = unidirectional and non-cyclic; nutrients = cyclic
✅ NPP = GPP − Respiration; NPP = energy available to consumers
✅ Highest NPP: Tropical rainforest; Lowest NPP: Desert
✅ Standing Crop = living biomass; Standing State = soil nutrients
✅ Most terrestrial energy flows through detritus food chain

📌 Topic 05 — Biogeochemical Cycles
✅ Gaseous cycles: Carbon, Nitrogen, Water (atmospheric reservoir)
✅ Sedimentary cycles: Phosphorus, Sulphur (rock/soil reservoir)
✅ Nitrogen fixation: Rhizobium (symbiotic), Azotobacter (free-living)
✅ Nitrification: Nitrosomonas (NH₃→NO₂⁻), Nitrobacter (NO₂⁻→NO₃⁻)
✅ Denitrification: Pseudomonas (NO₃⁻→N₂)
✅ Phosphorus cycle = sedimentary; NO gaseous phase; limiting nutrient

📌 Topic 06 — Ecological Succession
✅ Succession = directional change in species composition over time
✅ Primary succession: bare rock; very slow; lichens as pioneers
✅ Secondary succession: disturbed area with soil; faster; weeds as pioneers
✅ Hydrosere = succession in water; Xerosere = succession on dry land
✅ Climax community = final stable stage; determined by climate
✅ Monoclimax (Clements) vs Polyclimax (Tansley) theories
✅ Wetlands = highest productivity; carbon sinks; Ramsar sites
✅ Coral reefs = “rainforest of the sea”; highest marine biodiversity

🎯 Most Frequently Asked in Exams: Lindeman’s 10% Law · Pyramid of Energy (always upright) · Rhizobium/Nitrosomonas/Nitrobacter/Pseudomonas · NPP = GPP − R · Ecotone & Edge Effect · Primary vs Secondary Succession · Keystone Species · Phosphorus = sedimentary cycle (no gaseous phase)