Keystone Species, Ecological Succession & Ecosystem Ecology – Lecture 3

Mizoram Civil Service Exam Notes | Environment Series

Lecture 3 is content-rich and very frequently tested in UPSC and MPSC. We cover Keystone Species, Ecological Niche, Succession, Ecosystem Ecology, Food Chains, Food Webs, and Ecological Pyramids — all in one post.

Keystone Species

A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance.
It is NOT necessarily the strongest or most abundant species.
Such an organism plays a role in its ecosystem analogous to the role of a keystone in an arch — remove it and the whole structure collapses.
A small impact on a keystone species could cause major disruption to the whole ecosystem.

Examples:

Blue Jays (Cyanocitta cristata) in oak forests — they disperse seeds that give rise to oak forests. Without Blue Jays, oak forests cannot be naturally replenished, and without forests, all other species in the ecosystem cannot survive.
Tigers in grasslands/forests — check herbivore populations from overgrazing.

MPSC 2013: What is the name of a species that is highly connected to the entire ecosystem and whose loss results in ecosystem collapse?
Answer: (c) Keystone species

Ecological Niche

The Ecological Niche is the multi-dimensional ecological profile of a species in a community covering all measurable aspects of its ecological activities.
It is a term describing the relational position and functional role of a species or population in an ecosystem.
Includes how a population responds to the abundance of its resources and enemies, and how it affects those same factors.
Think of it this way: Habitat = address; Niche = job

UPSC 2013: Which term describes not only the physical space occupied by an organism, but also its functional role in the community of organisms?
Answer: (b) Ecological Niche

Principle of Competitive Exclusion (Gause’s Principle)

Two different species can have a partially overlapping niche in the same community.
But they can NEVER have a totally overlapping niche if they are competing for the exact same limiting resources.
In case of total overlap, one species will be eliminated or excluded from the community, unless they adapt via resource partitioning.
This is called the “Principle of Competitive Exclusion”.

Prelims Practice Question: With reference to ecological niche, consider the following statements:

Two different species can have totally overlapping niche in the same community.

In case of total overlap of niche, both species continue to survive through mutualism.

Answer: (d) Neither 1 nor 2 — Statement 1 is incorrect (total overlap leads to exclusion); Statement 2 is also incorrect (one species will eventually eliminate or exclude the other rather than forming a mutualistic bond).

Ecological Guild

A group of species in the same community which utilize the same class of environmental resources in a similar manner.
Example: Grazing animals in a grassland community (like cows and buffaloes competing for the same forage).

Ecological Equivalent

Two different species in different communities or geographic regions that play largely similar or the same role.
They occupy similar niches in separate geographical areas.
Example: Tiger in the Sunderbans and Lion in the Gir Forest — both are apex predators occupying similar ecological niches in distinct locations.

Ecological Succession

The continued presence of a plant community in a site changes the physical factors of the locality over time — often making conditions less suitable for themselves and more suitable for new arrivals.
The site is gradually invaded by members of another plant community which eventually replaces the former.
This orderly, predictable process of community change is called Ecological Succession.

Kinds of Succession

1. Primary Succession

Takes place on sites which have previously not borne vegetation and lack formed soil (e.g., bare rock, new lava flows, sand dunes).
The first species to colonize the area – Pioneer species.
Example: Lichens – a symbiotic association of Fungi + Algae where the fungus provides physical structure/anchorage and minerals, while the algae performs photosynthesis.
Progresses slowly through several stages until it reaches a stable climax stage.

UPSC 2014: Lichens, which are capable of initiating ecological succession even on a bare rock, are actually a symbiotic association of?
Answer: (b) Algae and Fungi

2. Secondary Succession

Takes place on a site after the destruction of the whole or part of the original vegetation.
It begins in areas where natural vegetation has been cleared but soil or sediment is already present (e.g., abandoned farm fields, burned forests).
Because soil is already intact, secondary succession progresses much faster than primary succession.
The intermediate communities that appear are called sere (or seral) communities.

Exam Question: When an area is burned in a forest fire and then plants and animals begin to return to the burned area, this is called?
Answer: (b) Secondary succession

📌 Autogenic vs. Allogenic Succession (Crucial Distinction)

Autogenic Succession: Driven by the resident biotic components themselves (e.g., the plants alter the soil nutrient levels or shade conditions, forcing their own replacement).
Allogenic Succession: Driven by external, abiotic forces (e.g., fires, storms, human land-clearing, or structural changes in the environment).

Key Succession Terms

Term	Definition
Sere / Seral Stage	The entire sequence of communities that successively replace one another in a given area; each individual stage is a seral stage.
Pioneer Species	The initial community or species that colonizes a bare, un-vegetated area (e.g., lichens on rock).
Climax Community	The final, relatively stable, and self-perpetuating stage of succession that exists in equilibrium with the regional climate.

Climax

If succession is allowed to progress without major external disturbance, a stage is reached when the vegetation is in equilibrium with the environment.
At this stage, the community stays unchanged indefinitely by reproducing itself and represents the culmination stage of ecological succession.

UPSC 2013: In the grasslands, trees do not replace the grasses as part of an ecological succession because of?
Answer: (c) Water limits and fire — periodic fires and moisture limitations keep grasslands at a stable climax state, preventing them from shifting into a forest ecosystem.

Ecosystem Ecology

What is an Ecosystem?

A naturally occurring structural and functional unit of the biosphere consisting of a biotic community interacting with its abiotic environment.
It is a relatively self-contained, dynamic system where energy flows and nutrients cycle.
The sequential positions that organisms occupy based on their feeding relationships within an ecosystem are called trophic levels.

Food Chain

A linear sequence of organisms through which nutrients and energy pass as one organism eats another.
A simplified sequence of “who eats whom”.
A food chain is primarily a conceptual tool — what actually occurs in nature is an interconnected network called a food web.

Types of Food Chains

1. Parasitic Food Chain (Auxiliary Food Chain)

Begins with a larger host organism and moves toward smaller parasites.
Energy moves from larger organisms to smaller ones.
Example: Tree (Host) → Herbivorous Birds → Parasites (Lice/Nematodes) → Hyperparasites (Bacteria/Viruses).

2. Detritus Food Chain

Begins with detritus (dead organic matter/waste) rather than living green plants.
Energy passes from dead matter to detritivorous organisms (decomposers/scavengers) → consumed by small carnivores → large carnivores.
Example: Dead leaves → Woodlouse → Blackbird.
Example: Organic Waste → Earthworm → Frog → Snake.

3. Grazing Food Chain (Predator Food Chain)

The most highly discussed type; moves from primary producers (autotrophs) to herbivores and then to carnivores.
Example: Grass → Grasshopper → Frog → Snake → Hawk.

Trophic levels in a grazing food chain:

Producer (T1) →Primary Consumer (T2) →Secondary Consumer (T3) → Tertiary Consumer (T4)

Exam Question: The food chain that starts with dead organic matter is?
Answer: (b) Detritus food chain

UPSC 2013: With reference to food chains in ecosystems, consider the following statements:

A food chain illustrates the order in which organisms feed upon each other. ✅

Food chains are found within populations of a species. ❌

A food chain illustrates the numbers of each organism eaten by others. ❌

Answer: (a) 1 only

Food Web

A complex network of interconnected food chains operating within a biotic community.
Reflects real-world conditions where animals have multiple alternative choices for food, providing stability to the ecosystem.

Prelims Practice Question: With reference to trophic levels and energy flow:

Only about 10% of energy is transferred to each higher trophic level from the lower level. ✅ (Lindeman’s 10% Law)

The flow of energy takes place unidirectionally in an ecosystem. ✅

The natural cross-connecting network of multiple food chains forms a food web. ✅

Which of the statements given above are correct?
Answer: All 1, 2, and 3

Ecological Pyramids

An ecological pyramid is a graphical representation of the trophic structure and function of an ecosystem, measuring numbers, biomass, or energy at each trophic level.
Producers always form the base, while top carnivores occupy the apex.
Decomposers/Saprophytes are typically not assigned a place in traditional ecological pyramids, which is a key limitation.
First conceptualized by Charles Elton (1927), hence they are also known as Eltonian Pyramids.

1. Pyramid of Numbers

Represents the total number of individual organisms at each trophic level per unit area.

2. Pyramid of Biomass

Represents the total dry weight of living organic matter at each trophic level at a given time.

3. Pyramid of Energy

Represents the amount of energy accumulation/productivity at each trophic level over a fixed time period. This pyramid is ALWAYS upright in all ecosystems due to the Second Law of Thermodynamics.

Key Pyramid Shapes by Ecosystem:

Ecosystem Type	Type of Pyramid	Shape	Key Reason / UPSC Trap
Grassland	Numbers	Upright	Thousands of grasses support fewer herbivores, supporting very few apex predators.
Single Tree Forest	Numbers	Inverted / Spindle	A single large tree (producer) supports thousands of insects (herbivores), which support a smaller number of birds (carnivores).
Parasitic Chain	Numbers	Inverted	One host plant supports hundreds of parasites, which host thousands of hyper-parasites.
Aquatic (Pond/Ocean)	Numbers	Upright	Millions of microscopic phytoplankton support fewer zooplankton, supporting even fewer fish.
Aquatic (Pond/Ocean)	Biomass	INVERTED	High-Yield UPSC Catch: The standing crop biomass of phytoplankton at any single moment is very low due to their rapid turnover rates and short lifespans, yet they support a much larger mass of long-lived fish.

Quick Revision Summary

Topic	Key Point
Keystone species	Disproportionately large ecological impact relative to abundance (Cyanocitta cristata).
Ecological Niche	Functional role/profession of a species; includes space, resources, and interactions.
Competitive Exclusion	Gause’s Principle: Two species cannot occupy the exact same niche indefinitely.
Ecological Guild	Group of species in the same community exploiting the same resource similarly.
Ecological Equivalent	Species in different regions playing identical functional roles (e.g., Tiger vs. Lion).
Primary Succession	Occurs on brand new, sterile surfaces; lacks soil; driven initially by pioneer lichens.
Secondary Succession	Occurs on previously vegetated soil after a disturbance; progresses much faster.
Autogenic vs. Allogenic	Autogenic is modified by the organisms themselves; Allogenic is driven by external disturbances.
Detritus Food Chain	Starts explicitly with dead organic material, vital for nutrient recycling.
Pyramid of Energy	Always upright across all ecosystems without exception.
Aquatic Ecosystems	Pyramid of Numbers is Upright, but Pyramid of Biomass is Inverted.

These notes are based on personal coaching class notes for Mizoram Civil Services Exam preparation.
← Read Lecture 2 | Stay tuned for Lecture 4 — Energy Flow & Biogeochemical Cycles!

Keystone Species, Ecological Succession & Ecosystem Ecology – Lecture 3 | Mizoram MPSC Exam Notes