Ecology & Environment – NDA Biology Notes

Every living organism exists in relation to other organisms and to the non-living world around it. Ecology is the study of these relationships: how living things interact with each other and with their environment. Understanding ecology means understanding how energy flows through living systems, how species depend on each other, and how human activities affect the natural world.

For the NDA exam, this chapter provides consistent, high-frequency questions on a precise set of topics: ecological unit definitions, food chain validity, the energy vs. nutrient flow contrast, the leather/plastic biodegradability confusion, eutrophication productivity, acid rain chemistry, and the biogas methane fact. Every section below marks exactly where exam questions originate.

1. What is Ecology?

Ecology is the branch of biology that studies the relationships between living organisms and their environment. It examines how organisms interact with one another (biotic interactions) and with the nonliving physical world (abiotic factors such as temperature, light, water, and soil).

The term ecology comes from the Greek words ‘oikos’ (home) and ‘logos’ (study), literally, the study of the home, or the living environment.

2. Ecological Units: Population, Community, Ecosystem

Three key terms describe different levels of ecological organisation. Their definitions must be precise. Exam questions frequently exploit the distinctions between them.

Population

A population is a group of organisms of one species living in the same area at the same time.

Not a multi-species group: that is a community. Not a biotic-abiotic unit: that is an ecosystem. Not organisms of one species dispersed across different areas in different seasons: that is dispersal, not a population.

Community

A community is all the populations of different species living together in the same area, the biotic (living) component of an ecosystem. It does not include the physical environment.

Ecosystem

An ecosystem is a biological community together with its physical (abiotic) environment: all the living and non-living components interacting together as a system.

A pond, an ocean, a forest, and an aquarium are all ecosystems. They include both the organisms (biotic) and the water, soil, temperature, light, and minerals (abiotic). A biome is a larger geographic unit spanning multiple ecosystems. A community is only the living part. Only an ecosystem includes both biotic and abiotic components.

Unit	Definition	What It Includes	Example
Population	One species, same area, same time	Only members of one species	All the frogs in a pond
Community	All species populations in the same area	All living organisms in an area	All plants, animals, and microbes in a pond
Ecosystem	Community + physical environment	Living + non-living components	Pond + water + light + temperature + minerals

3. Classification of Resources

Resources are classified by their origin into two categories:

• Biotic resources: of living origin: plants, animals, microorganisms, forest products.
• Abiotic resources: of non-living origin: water, minerals, sunlight, air, soil.

This origin-based classification is different from the renewable/non-renewable classification (based on replenishment rate) and the individual/community classification (based on access rights). When a question asks about classification by origin, the answer is biotic vs abiotic.

4. Food Chain: Structure and Valid Examples

A food chain is the sequential passage of food and energy from one organism to another, from producers through progressively higher consumer levels. Each organism in a food chain feeds on the one before it in a genuine, real-world feeding relationship.

The correct structure is always:

Producer  →  Primary Consumer (Herbivore)  →  Secondary Consumer  →  Tertiary Consumer

Valid Food Chain

Grass → Goat → Human [NDA 2018-I]

Grass is the producer. The goat is the primary consumer (herbivore, eats only grass). The human is the secondary consumer (eats the goat). Each link feeds on the link before it in a genuine feeding relationship. This is a valid food chain.

Invalid Food Chains: Common Exam Mistakes

“Grass, Human, and Fish”: invalid. Humans do not typically eat grass directly. The primary consumer level is skipped. This is not a linear feeding chain.  [NDA 2018-I]

“Tree, Tree Cutter, and Tiger”: invalid. A tree cutter is performing an economic activity, cutting wood, not eating it. A tiger does not eat tree cutters as part of a regular trophic relationship. There is no genuine producer → consumer feeding sequence here.  [NDA 2018-I]

[Image Suggestion: Diagram showing a valid food chain (Grass → Grasshopper → Frog → Snake → Hawk) with energy percentages decreasing at each step, next to the two invalid chains marked with ✗]

5. Trophic Levels

The position of an organism in the food chain is called its trophic level. Understanding which organism occupies which trophic level is essential for answering food chain identification questions.

Trophic Level	Name	Examples
1st	Producer (Autotroph): makes its own food	Grass, plants, algae, crabapple tree
2nd	Primary Consumer (Herbivore): eats producers directly	Caterpillar, goat, grasshopper [NDA 2019-I]
3rd	Secondary Consumer (Carnivore): eats herbivores	Frog, shrew, small bird
4th	Tertiary Consumer: eats secondary consumers	Snake, hawk, sparrowhawk

★ IMPORTANT The primary consumer is the organism that feeds directly on the producer (plant). A caterpillar feeds on plant leaves, making it the primary consumer.  [NDA 2019-I] A crabapple tree is a producer. A frog feeds on insects, making it a secondary consumer. A sparrowhawk is a higher-level carnivore. Among crabapple tree, caterpillar, frog, and sparrowhawk, only the caterpillar is the primary consumer.

6. Energy Flow and Nutrient Flow: A Critical Distinction

In any ecosystem, two things move through the food chain: energy and nutrients (matter). They behave in completely opposite ways, and this contrast is a direct and repeated exam target.

Energy Flow: Unidirectional

Energy flows in one direction only: from sunlight to producers to consumers. At each step, energy is lost as heat. It cannot be recycled back. Once heat escapes to the environment, it is gone.

The 10% Law

At each trophic level, approximately 90% of the energy is lost as heat. Only about 10% is passed to the next level. This is why food chains have limited length. By the time three or four trophic levels are traversed, so little energy remains that sustaining another level becomes impossible.

Example: 1000 units of energy enter the grass. Goat gets 100. Human gets 10. A hypothetical next consumer gets only 1. Eventually there is too little energy to support another trophic level.

Nutrient Flow: Cyclic

Nutrients flow in cycles. Carbon, nitrogen, phosphorus, water: these materials cycle continuously between organisms and the abiotic environment. They are used by one organism, released back to the environment, and taken up by another. Nothing is permanently lost.

★ IMPORTANT Energy flow = UNIDIRECTIONAL (sunlight in → lost as heat: cannot be recycled) Nutrient flow = CYCLIC (reused repeatedly between organisms and abiotic environment) These two are consistently confused in exam options. They are complete opposites. Energy is NOT cyclic. Nutrients are NOT unidirectional.

7. Bioaccumulation and Biomagnification

Two related but distinct concepts, must not be confused with each other.

Concept	Definition	Scale	Example
Bioaccumulation	Progressive build-up of heavy metals, persistent pesticides, and non-biodegradable toxins in the tissues of a single organism over its lifetime	Within one organism	A fish accumulates DDT in its fat tissue over years
Biomagnification	Increase in concentration of toxins at each successive trophic level	Across the food chain	Top predator has far higher DDT concentration than plankton at base

Result: large fish at the top of a pond food chain contain significantly higher concentrations of pesticides than the plankton at the base. The fish has eaten thousands of small organisms, each carrying a small amount of toxin, concentrating it in its own tissues.

8. Ecological Succession

A forest fire burns an area to bare ground. Left alone, life slowly returns: first simple organisms, then more complex ones, until eventually a stable ecosystem develops. This predictable, directional process is called ecological succession.

The Four Stages: In Fixed Order

Nudation  →  Migration  →  Reaction  →  Stabilisation

Stage	What Happens	Example
Nudation	A bare, unvegetated area is created	Fire, flood, volcanic eruption, or human clearing destroys existing ecosystem
Migration	Pioneer organisms arrive and establish	Seeds blow in; spores settle; mosses, lichens, and bacteria colonise
Reaction	Pioneer organisms modify the habitat, enabling new species to arrive	Organisms break down rock into soil; add organic matter; create conditions for complex species
Stabilisation	A stable, mature climax community is established	Mature forest or grassland that can maintain itself indefinitely

Mnemonic: N-M-R-S: No More Rough Spots. The order is fixed. Reaction does NOT precede Migration.

9. Biodegradable vs Non-Biodegradable Waste

This is the frequently asked:

Category	Materials
Biodegradable (broken down by microorganisms)	Wood, Grass, Leather, Jute, Wool, Fruit peels, Cow dung, Paddy husk, Vegetable waste, Paper, Sewage, Cotton
Non-biodegradable (resist microbial breakdown; persist for centuries)	Plastic, Glass, Silver foil, Polystyrene, China clay (ceramic), Iron scrap

★ IMPORTANT Leather is BIODEGRADABLE: not non-biodegradable. This is the most commonly misclassified material in the chapter. Students assume leather is non-biodegradable because it is processed and durable. But leather is an animal-derived organic material (treated skin) that decomposes under microbial action. It belongs in the same category as wool, jute, and wood, all of which are biodegradable. Plastic, glass, silver foil, and China clay are non-biodegradable; leather is not.

Thermal power plants generate the largest volume of non-biodegradable industrial waste, primarily fly ash and heavy-metal-laden bottom ash (silica, alumina, and iron oxide residues). Food processing, textile mills, and paper mills generate predominantly biodegradable organic waste.

Plastics are non-biodegradable. They resist microbial decomposition and persist for hundreds of years. They DO leach into water and food. Microplastics have been found in drinking water, food, and human blood. They are NOT harmless to humans.

10. Environmental Pollution

Acid Rain

Acid rain is caused by the dissolution of sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) in rainwater. These gases react with water to form sulphuric acid and nitric acid.

These gases are emitted by burning fossil fuels, industrial processes, and vehicle exhaust. Ordinary minerals, dust, or soil dissolved in rain do NOT produce acid rain.

Photochemical Smog

Photochemical smog forms under warm, dry, and sunny climatic conditions. Intense ultraviolet radiation drives photochemical reactions between nitrogen oxides and hydrocarbons from vehicle exhaust, producing ozone and PAN (peroxyacetyl nitrate).

Cool, humid conditions inhibit the formation of this type of smog. The classic example is Los Angeles, where warm, sunny conditions promote photochemical smog.

Eutrophication

Eutrophication is the excessive enrichment of a water body with nutrients, typically nitrogen and phosphorus from agricultural runoff and sewage.

Feature of Eutrophic Lakes	Correct / Incorrect
High plant nutrient flux	Correct
Frequent algal blooms (especially blue-green algae)	Correct
HIGH primary productivity	Correct: NOT low. This is the most tested misconception.
Low dissolved oxygen (from decomposing algae)	Correct
Low primary productivity	WRONG: this describes oligotrophic lakes (opposite)

★ IMPORTANT Primary productivity is HIGH in eutrophic lakes: NOT low. Excess nutrients cause explosive algal growth, leading to very high productivity. It is the subsequent decomposition of this massive biomass that depletes oxygen, causing fish kills. Low primary productivity describes oligotrophic (nutrient-poor) lakes, the complete opposite.

Endosulfan

Endosulfan is a pesticide, specifically a highly toxic organochlorine pesticide. It is NOT a fertiliser, NOT a sulpha drug, NOT an antibiotic.

Its use has been banned in many countries due to its persistence in the environment and bioaccumulation in food chains. It was widely used in cashew plantations in Kerala and caused serious health impacts in local communities.

11. Bioremediation

Bioremediation is the use of microorganisms or their enzymes to restore a contaminated environment to its original, uncontaminated condition.

It can target specific pollutants. Certain bacteria can break down chlorinated pesticides (such as DDT and lindane) that persist in soil and water. Bioremediation uses microorganisms to restore contaminated environments; specific bacteria can degrade chlorinated pesticides.

12. Biogas

Biogas is produced by the anaerobic microbial digestion of organic matter: cow dung, agricultural waste, and sewage. Its major constituent is methane (CH₄), comprising approximately 55–65% of biogas. Carbon dioxide is the second component (35–45%).

Hydrogen and nitrogen dioxide are NOT major constituents of biogas. Biogas is a renewable, clean-burning fuel used for cooking and electricity generation.

13. Red Data Book

The Red Data Book is maintained by the IUCN (International Union for Conservation of Nature). It is a comprehensive list of all endangered species of plants and animals worldwide, along with their conservation status and threat levels.

It is NOT a record of pollutant levels. NOT a record of nuclear event consequences. NOT a sociological study of GMOs. It lists endangered species. That is its sole purpose.

14. Carbon and Nitrogen Cycles

Carbon and nitrogen are the two most important nutrient cycles in ecology. Both are examples of cyclic nutrient flow. The matter is continuously reused between organisms and the abiotic environment.

Carbon cycle: Carbon dioxide is absorbed by plants through photosynthesis (building organic matter). Respiration by organisms releases CO₂ back to the atmosphere. Decomposition of dead organisms also releases CO₂. Burning of fossil fuels adds extra CO₂ to the atmosphere. This is the main driver of the enhanced greenhouse effect.

Nitrogen cycle: Plants cannot use atmospheric nitrogen (N₂) directly. Nitrogen-fixing bacteria (such as Rhizobium in legume root nodules) convert N₂ into ammonia and nitrates, forms plants can absorb. Denitrifying bacteria return nitrogen to the atmosphere by converting nitrates back to N₂.

15. Greenhouse Effect and Global Warming

Greenhouse gases (CO₂, methane, nitrous oxide, and water vapour) trap heat in the atmosphere by absorbing and re-emitting infrared radiation. This natural process keeps Earth warm enough for life.

Increasing concentrations of greenhouse gases, primarily from burning fossil fuels and deforestation, enhance this effect, raising global temperatures. This is global warming. Consequences include rising sea levels, more extreme weather events, and shifts in species distributions.

16. Conservation: In-Situ and Ex-Situ

Conservation Type	Definition	Examples
In-situ conservation	Conservation of species in their natural habitat	National parks, Wildlife sanctuaries, Biosphere reserves
Ex-situ conservation	Conservation of species outside their natural habitat	Zoos, Botanical gardens, Seed banks, Gene banks

17. Ecological Pyramids

Ecological pyramids represent the relationship between trophic levels graphically. Three types exist.

Pyramid Type	Measures	Shape	Key Feature
Pyramid of numbers	Number of organisms at each trophic level	Varies: can be inverted	A single tree can support many caterpillars: makes it inverted
Pyramid of biomass	Total mass of organisms at each level	Usually, upright	Total plant biomass > herbivore biomass > carnivore biomass
Pyramid of energy	Total energy at each level	Always upright	Energy always decreases upward: the 10% law means it can never be inverted

The pyramid of energy is always upright. Energy at each level is always less than the level below it, following the 10% law. It can never be inverted.

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