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Quick recap: Ecology basics on one page
Both parts of Ecology basics condensed for quick revision: ecosystem structure, energy flow, pyramids and biomagnification; succession, interactions, special species roles and ecotones; the biogeochemical cycles; a one-table overview and eight mixed questions.
Ecology basics has two parts. Part 1 shows how an ecosystem is built and how energy moves through it; Part 2 shows how ecosystems change, how species affect one another and how matter is recycled. One line joins them: energy flows through once and leaves as heat, while nutrients go round in cycles. This recap gathers the facts most often tested.
Haeckel coined the word ecology in 1866 and Tansley the word ecosystem in 1935. A habitat is where a species lives; its niche is the role it plays there. Producers make food, mostly by photosynthesis, though bacteria at deep-sea vents use chemical energy; consumers eat others; decomposers such as bacteria and fungi break dead matter down chemically, while detritivores such as earthworms eat it. A grazing food chain starts with living plants and a detritus chain with dead matter, and chains interlock into food webs.
Energy flows one way, and on average only about a tenth of it passes from one trophic level to the next, the ten per cent law described by Lindeman in 1942; that is why chains are short and top predators few. Net primary productivity is gross productivity minus the producers' respiration; rainforests, reefs and estuaries are the most productive per unit area. Of Elton's three pyramids, those of number and biomass can be inverted but that of energy never is. Persistent, fat-soluble poisons such as DDT and mercury undergo biomagnification, reaching their highest levels in top predators.
ecological succession is the orderly replacement of communities in one place. Primary succession starts where there is no soil and is slow; secondary succession starts where soil survives and is faster. Lichens are the usual pioneers on rock (a xerosere) and floating algae in a pond (a hydrosere), and the series ends in a climax community in balance with the climate.
Interactions are sorted by gain and loss: mutualism (+/+, fig and fig wasp, lichens), commensalism (+/0, an orchid on a mango tree), parasitism and predation (+/−), competition (−/−, which Gause showed can drive one species out) and amensalism (0/−, Penicillium and bacteria). A keystone species has an effect far beyond its numbers; the tiger is both a flagship and an umbrella species; lichens indicate clean air; water hyacinth, Lantana and Parthenium are invasive species in India. An ecotone such as a mangrove belt often holds more species than either neighbouring community, the edge effect.
Each biogeochemical cycle is either gaseous, with its main store in the air or ocean (carbon, nitrogen), or sedimentary, with its main store in rocks (phosphorus, which has no significant gas phase, and sulphur, which does). Photosynthesis takes carbon out of the air and respiration returns it; burning fossil fuels and clearing forests add more than sinks absorb. Nitrogen is fixed by Rhizobium, Azotobacter, cyanobacteria, lightning and industry, then passes through ammonification, nitrification by Nitrosomonas and Nitrobacter, assimilation and denitrification. Excess nitrogen and phosphorus cause eutrophication; sulphur dioxide causes acid rain; the sun-driven water cycle links them all.
| Idea | Key point | Name or example |
|---|---|---|
| Energy flow | One way; about 10% passes up each level | Lindeman, 1942 |
| Pyramids | Energy never inverted; number and biomass can be | Elton, 1927 |
| Biomagnification | Highest at the top of the chain | DDT, mercury (Minamata) |
| Succession | Primary slow (no soil), secondary faster | Lichens on rock; climax community |
| Interactions | +/+, +/0, +/−, −/−, 0/− | Fig and wasp; orchid; Cuscuta; Penicillium |
| Species roles | Keystone, flagship, umbrella, indicator, invasive | Sea otter; tiger; lichens; water hyacinth |
| Cycles | Gaseous (C, N) vs sedimentary (P, S) | Phosphorus has no gas phase |
Exam traps
- Habitat is the address, niche the role; they are not the same thing.
- Earthworms are DETRITIVORES; bacteria and fungi are DECOMPOSERS.
- Only the pyramid of ENERGY is always upright.
- Energy is not recycled; nutrients are.
- Secondary succession is FASTER than primary because soil is already present.
- Lichen = mutualism; orchid on a tree = commensalism; Cuscuta = parasitism; Penicillium = amensalism.
- Phosphorus has no significant gaseous phase; sulphur, though sedimentary, has SO2 and H2S.
- Nitrosomonas: ammonia → nitrite; Nitrobacter: nitrite → nitrate. Do not swap them.
Recap
- Ecology (Haeckel, 1866); ecosystem (Tansley, 1935); habitat vs niche.
- Producers, consumers, decomposers and detritivores; grazing and detritus chains; food webs.
- Ten per cent law (Lindeman, 1942); NPP = GPP − respiration.
- Pyramids (Elton, 1927): energy never inverted; biomagnification of DDT and mercury.
- Succession: pioneers → seral stages → climax; xerosere and hydrosere.
- Interactions: mutualism, commensalism, parasitism, predation, competition, amensalism.
- Keystone, flagship, umbrella, indicator and invasive species; ecotone and edge effect.
- Cycles: gaseous (C, N) and sedimentary (P, S); nitrogen fixation to denitrification; eutrophication; acid rain.
Practice questions
Q1. Which one of the following is a detritivore rather than a decomposer?
- Bacteria
- Fungi
- Earthworm
- Actinomycetes
Show answer
Answer: C. Earthworm
The earthworm eats dead matter and breaks it into pieces, so it is a detritivore. Bacteria, fungi and actinomycetes are microbes that break dead matter down chemically, so they are decomposers.
Q2. Consider the following statements:
1. The pyramid of energy can be inverted in an open-sea ecosystem.
2. The pyramid of number can be inverted when one tree supports many insects.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Show answer
Answer: B. 2 only
Statement 1 is wrong: the energy pyramid is always upright (the open sea inverts the biomass pyramid). Statement 2 is correct. So '1 only', 'both' and 'neither' are wrong.
Q3. If the producers of an ecosystem fix 20,000 units of energy, about how much will reach the second carnivores according to the ten per cent law?
- 2,000 units
- 200 units
- 20 units
- 2 units
Show answer
Answer: C. 20 units
Herbivores get about 2,000, first carnivores about 200 and second carnivores about 20. 2,000 and 200 stop too early in the chain, and 2 is one step too far.
Q4. Which one of the following is most likely to show the highest concentration of DDT in a lake ecosystem?
- Phytoplankton
- Zooplankton
- Small fish
- Fish-eating birds
Show answer
Answer: D. Fish-eating birds
DDT builds up at each step of the chain, so the top predator, the fish-eating bird, carries the most. Phytoplankton, zooplankton and small fish sit lower in the chain and hold less.
Q5. Consider the following statements:
1. Secondary succession is generally faster than primary succession.
2. The climax community is the first community to colonise a bare area.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Show answer
Answer: A. 1 only
Statement 1 is correct because soil already exists. Statement 2 is wrong: the first colonisers are pioneers; the climax is the final, stable stage. So '2 only', 'both' and 'neither' are wrong.
Q6. Match the species roles with the examples:
A. Keystone 1. Lichens in clean air
B. Indicator 2. Water hyacinth
C. Invasive 3. Sea otter
Select the correct answer using the code given below:
- A-3, B-1, C-2
- A-1, B-3, C-2
- A-3, B-2, C-1
- A-2, B-1, C-3
Show answer
Answer: A. A-3, B-1, C-2
The sea otter is the classic keystone, lichens indicate air quality, and water hyacinth is invasive. The other codes swap keystone with indicator, indicator with invasive, or keystone with invasive.
Q7. Consider the following statements:
1. The carbon cycle is a gaseous cycle.
2. The phosphorus cycle is a sedimentary cycle.
3. The sulphur cycle has no gaseous stage.
Which of the statements given above is/are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer
Answer: A. 1 and 2 only
Statements 1 and 2 are correct. Statement 3 is wrong: sulphur enters the air as sulphur dioxide and hydrogen sulphide. So every option containing 3 is wrong.
Q8. The conversion of nitrate back into nitrogen gas, which occurs mainly in waterlogged soils, is called
- nitrification
- ammonification
- denitrification
- nitrogen fixation
Show answer
Answer: C. denitrification
Denitrification returns nitrate to the air as N2. Nitrification makes nitrate from ammonia, ammonification makes ammonia from dead matter, and fixation turns N2 into usable compounds, the opposite direction.
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- Ministry of Environment, Forest and Climate Change (ENVIS resource material): ecological succession, species interactions, keystone, flagship and invasive species, ecotones — https://moef.gov.in
- NOAA Ocean Service and NOAA Research: estuaries and mangroves, the ocean carbon cycle and ocean acidification, nutrient pollution, hypoxia and dead zones — https://oceanservice.noaa.gov
- NASA Earth Observatory: the carbon cycle, the nitrogen cycle and the global cycles of matter — https://earthobservatory.nasa.gov
- US Geological Survey Water Science School: the water cycle, evapotranspiration, infiltration and groundwater — https://www.usgs.gov
- Indian Council of Agricultural Research: biological nitrogen fixation, Rhizobium, Azotobacter and blue-green algae biofertilisers — https://icar.org.in
- US Environmental Protection Agency: nutrient pollution and harmful algal blooms; sulphur dioxide and acid rain — https://www.epa.gov