Which type of ecological pyramid can never be inverted, and why?

The pyramid of energy, because energy is lost at each transfer and flows only one way

The pyramid of numbers, because there are always fewer individuals at higher levels

The pyramid of biomass, because biomass must always decrease upward

The pyramid of energy, because energy is lost at each transfer and flows only one way

All three pyramids are equally incapable of ever being inverted

The phosphorus cycle differs from the carbon and nitrogen cycles mainly in which respect?

It is a sedimentary cycle with no significant gaseous atmospheric phase

It is the only cycle in which living organisms play no part at all

It is driven entirely by human industrial activity rather than by natural processes

It moves far faster than the carbon and nitrogen cycles through a large air reservoir

Why does secondary succession on an abandoned farm field generally proceed much faster than primary succession on a fresh lava flow?

The soil, its seed bank and nutrients survive the disturbance, so soil-building is skipped

Abandoned fields receive far more rainfall and sunlight than any new lava flow does

The soil, its seed bank and nutrients survive the disturbance, so soil-building is skipped

Secondary succession skips the climax stage, so it reaches stability in fewer steps

Pioneer species are absent in secondary succession, which removes the slowest early stage

Which of the following statements about energy flow and nutrient cycling in ecosystems are correct?

Energy flows one way through the ecosystem and is ultimately lost as heat

Mineral nutrients are recycled repeatedly between organisms and the environment

Nitrogen fixation converts atmospheric N₂ into a form plants can assimilate

Energy flows one way through the ecosystem and is ultimately lost as heat

Mineral nutrients are recycled repeatedly between organisms and the environment

Nitrogen fixation converts atmospheric N₂ into a form plants can assimilate

Energy, like nutrients, is recycled back to producers by decomposers

Which of the following regions is widely regarded as one of India's richest centres of plant endemism?

The Indian Desert of the arid north-west

The Trans-Himalayan cold desert

A botanical garden maintains living collections of threatened plant species, and a seed bank stores their seeds under cold, dry conditions. These are examples of which conservation strategy?

Ex situ conservation, because the plants are kept away from their natural habitat

In situ conservation, because the plants are kept within natural ecosystems

Ex situ conservation, because the plants are kept away from their natural habitat

In situ conservation, because a seed bank protects whole habitats at once

Neither, because conservation applies only to animals and not to plants

Which of the following statements about biodiversity, hotspots and global change are correct as stated here?

A biodiversity hotspot combines exceptional endemism with a high degree of threat

On the Conservation International scheme, four hotspots extend into India

Simpson's diversity (1 − D) increases as a community becomes more diverse and even

A biodiversity hotspot combines exceptional endemism with a high degree of threat

On the Conservation International scheme, four hotspots extend into India

Simpson's diversity (1 − D) increases as a community becomes more diverse and even

Global warming is expected to shift species' ranges towards the equator and down mountains

An orchid grows perched on the branch of a forest tree, using it only for support and making its own food, while the tree is unaffected. This interaction is best described as:

Commensalism, as the orchid (an epiphyte) benefits while the tree is unaffected

Parasitism, with the orchid drawing nutrients from the tree through haustoria

Mutualism, since both the orchid and the tree gain from the association

Commensalism, as the orchid (an epiphyte) benefits while the tree is unaffected

Amensalism, because the orchid's presence harms the tree without benefiting itself

What distinguishes an endophyte from an epiphyte?

An endophyte lives inside host plant tissue; an epiphyte grows on another plant's outer surface

An endophyte is always a flowering plant, whereas an epiphyte is always a fungus

An endophyte is a parasite that always kills its host, whereas an epiphyte never contacts a host

An endophyte lives only in soil, whereas an epiphyte lives only in water

Which of the following are correct about ecological succession and ecosystem dynamics?

Primary succession begins on a surface with no pre-existing soil, such as a new lava flow

A hydrosere is the succession that proceeds from open water towards dry land

Pioneer species such as lichens help build soil in the early stages of primary succession

Primary succession begins on a surface with no pre-existing soil, such as a new lava flow

A hydrosere is the succession that proceeds from open water towards dry land

Pioneer species such as lichens help build soil in the early stages of primary succession

The climax community is the first, pioneer stage of a succession

Ecology and Environment: Ecosystems, Energy Flow, Biogeochemical Cycles, Succession, Biogeographic Zones, Biodiversity and Plant Interactions

This chapter answers Section XL2.9 of XL2 Botany, one of the five optional sections of the Life Sciences (XL) paper: “Ecology and Environment”. The heading names ecosystems and their types, dynamics, degradation, biogeochemical cycles and ecological succession; food webs and energy flow; vegetation types of the world, Indian vegetation types and biogeographical zones, climate and floral endemism; pollution and global climate change, speciation and extinction, biodiversity and conservation, ecological hotspots, afforestation and habitat restoration; and plant interactions with other organisms, including epiphytes, parasites and endophytes. Where a stem depends on a scheme that differs between sources, this chapter states the scheme it uses: the Rodgers and Panwar (Wildlife Institute of India) classification of 10 biogeographic zones, and the Conservation International hotspot scheme. The numericals are net primary productivity, ecological transfer efficiency, a diversity index and the Lincoln-Petersen population estimate.

1. The ecosystem: structure, types, dynamics and degradation

An ecosystem is a community of organisms together with its physical environment, interacting as a unit through energy flow and nutrient cycling. Its structure has two parts: abiotic (climate, soil, water, light and inorganic nutrients) and biotic — producers (autotrophs, mainly green plants), consumers (herbivores, carnivores, omnivores) and decomposers (detritivores and microbes that mineralise dead matter). Broad types are terrestrial (forest, grassland, desert, tundra) and aquatic (freshwater: lentic and lotic; marine; and brackish estuaries). Ecosystems are dynamic: they change through daily and seasonal rhythms, population fluctuations and succession, and show resilience up to a point.

Degradation is the loss of an ecosystem's structure, productivity and services through deforestation, overgrazing, soil erosion and salinisation, pollution, eutrophication and the spread of invasive species. It lowers biodiversity, breaks nutrient cycles and can push the system past a threshold from which it does not recover on its own, which is why restoration is harder than protection.

2. Food webs, energy flow and ecological efficiency

Energy enters as sunlight fixed by producers and flows one way along a food chain, the interlocking chains forming a food web. Each step is a trophic level. Gross primary productivity (GPP) is the total energy fixed; net primary productivity (NPP) is what is left for growth after the plants' own respiration, NPP = GPP − R. Energy is lost as heat at every transfer, so only a fraction passes up: Lindeman's ten-per-cent law is the rule of thumb that about 10% of the energy at one trophic level reaches the next. The transfer efficiency is (energy at level n+1 ÷ energy at level n) × 100.

Ecological pyramids show numbers, biomass or energy by trophic level. The pyramid of energy is always upright, because energy is always lost upward. Pyramids of number and biomass can be inverted: a single large tree supports many insects (inverted number pyramid), and in the open ocean a small standing biomass of fast-turning-over phytoplankton supports a larger biomass of zooplankton at one instant (inverted biomass pyramid). Because so much energy is lost per step, food chains are usually only four or five links long.

⚠️ Exam trap

The pyramid of energy cannot be inverted; only pyramids of number and biomass can. A claim that an energy pyramid is inverted contradicts the one-way, ever-decreasing flow of energy.

3. Biogeochemical cycles

Nutrients, unlike energy, are recycled between organisms and the environment. A gaseous cycle (carbon, nitrogen) has its main reservoir in the atmosphere or ocean and moves fast; a sedimentary cycle (phosphorus, sulphur) has its reservoir in rock and soil and moves slowly, with no large atmospheric pool.

Carbon: fixed by photosynthesis, returned by respiration, decomposition and combustion; burning fossil fuels moves ancient carbon into the atmosphere as CO₂ faster than it is removed.
Nitrogen: N₂ is fixed to ammonia (biologically by Rhizobium, Azotobacter and cyanobacteria, and industrially); ammonia is nitrified to nitrate, assimilated into proteins, and returned to N₂ by denitrification.
Phosphorus: a sedimentary cycle with no significant gaseous phase; released by the weathering of rock, taken up as phosphate, and lost to deep ocean sediments; it often limits productivity in fresh water.

4. Ecological succession

Succession is the orderly change in a community over time at a site, ending in a relatively stable climax. Primary succession begins on bare ground with no soil (a new lava flow, a cooled rock, a glacial moraine), where pioneer species such as lichens and mosses build soil; it is slow. Secondary succession begins where a disturbance (fire, flood, abandonment of a field) has removed vegetation but left the soil, so it is much faster. The developing stages are seres; in water, a hydrosere fills and dries a pond from open water through reed and marsh stages to land.

🎯 Why it matters

Secondary succession is faster than primary because the soil, its seed bank and nutrients survive the disturbance, so the slow soil-building stage is skipped.

5. Vegetation types, Indian biogeographic zones and endemism

World vegetation falls into biomes set by climate, chiefly temperature and rainfall: tropical rainforest, savanna, desert, temperate grassland, temperate deciduous and coniferous (taiga) forest, and tundra. India's own vegetation ranges from tropical wet evergreen forest of the Western Ghats and the north-east, through moist and dry deciduous forest (including the sal and teak forests), thorn forest of the dry north-west, to alpine and temperate forest in the Himalaya, plus mangroves on the coasts.

Biogeographic zones divide a country by its distinctive assemblages of species. On the widely used Rodgers and Panwar (Wildlife Institute of India) classification, India has 10 biogeographic zones: Trans-Himalaya, Himalaya, Indian Desert, Semi-Arid, Western Ghats, Deccan Peninsula, Gangetic Plain, Coasts, North-East and Islands. Endemism is the restriction of a taxon to one area; the Western Ghats, the north-eastern hills and the islands are India's richest centres of plant endemism, a consequence of their isolation, stable wet climate and varied terrain.

6. Biodiversity, its measurement, hotspots, conservation and climate change

Biodiversity spans three levels: genetic, species and ecosystem diversity. Species diversity has two parts, richness (how many species) and evenness (how equally abundant). A diversity index combines both: Simpson's index D = Σ pᵢ², where pᵢ is a species' proportion, measures dominance, and Simpson's diversity (1 − D) rises with diversity; the Shannon index H = −Σ pᵢ ln pᵢ does the same. Population size is often estimated by mark-recapture: by the Lincoln-Petersen method, N = (M × C) ÷ R, where M are marked and released, C is the second sample and R the marked animals recaptured, assuming a closed population and that marks neither fall off nor affect recapture.

A biodiversity hotspot is a region with exceptional endemism that is also under severe threat. On the Conservation International scheme (of the order of three dozen hotspots worldwide as currently recognised), four extend into India: the Himalaya, the Western Ghats-Sri Lanka, Indo-Burma and Sundaland (which reaches the Nicobar Islands). Conservation is in situ (protecting species in their habitat: national parks, wildlife sanctuaries, biosphere reserves, sacred groves) or ex situ (away from it: botanical gardens, seed and gene banks, cryopreservation, tissue culture), backed by afforestation and habitat restoration.

Pollution and global climate change are major drivers of species loss. Rising atmospheric CO₂ and other greenhouse gases warm the climate, shift species' ranges towards the poles and up mountains, alter flowering and migration timing, and acidify the ocean. With overexploitation, habitat loss and invasive species, these push rates of extinction far above the background, while speciation, the origin of new species through reproductive isolation, is far too slow to keep pace.

7. Plant interactions: epiphytes, parasites and endophytes

Two-species interactions by their effect on each partner (+ benefit, − harm, 0 neutral)
Interaction	Effect	Plant example
Mutualism	+ / +	Mycorrhiza; pollination of flowers by insects
Commensalism	+ / 0	An epiphytic orchid on a tree, which it uses only for support
Parasitism	+ / −	Cuscuta (dodder) drawing sap from a host
Competition	− / −	Two plants competing for the same light or nutrients
Amensalism	− / 0	Allelopathy: one plant's chemicals suppress another

An epiphyte grows on another plant only for support, taking nothing from it, and feeds itself (many orchids, ferns and mosses, with aerial roots that absorb moisture); it is not a parasite. A parasite draws nourishment from its host through haustoria (Cuscuta, a holoparasite; mistletoe, a hemiparasite). An endophyte lives inside plant tissue, often symptomlessly, for part or all of its life; many are fungi or bacteria that can protect the host by producing deterrent compounds or inducing resistance, grading from mutualist to latent pathogen.

⚠️ Exam trap

An epiphyte is not a parasite: it only perches on the host for support and makes its own food. A parasite feeds from the host, and an endophyte lives inside host tissue.

Key takeaways

An ecosystem couples abiotic factors with producers, consumers and decomposers; nutrients are recycled but energy flows one way. Degradation can push a system past a recovery threshold.
NPP = GPP − respiration. About 10% of energy passes to the next trophic level (Lindeman); the pyramid of energy is always upright, though number and biomass pyramids can be inverted.
Carbon and nitrogen have fast gaseous cycles; phosphorus has a slow sedimentary cycle with no gaseous phase. Primary succession builds soil from bare rock and is slow; secondary succession keeps the soil and is fast.
On the Rodgers and Panwar scheme India has 10 biogeographic zones; the Western Ghats, north-east and islands are the richest centres of plant endemism.
Simpson's diversity is 1 − Σpᵢ²; Lincoln-Petersen estimates N = (M × C) ÷ R. Four Conservation International hotspots extend into India; conservation is in situ or ex situ.
An epiphyte perches on a host for support only and feeds itself; a parasite feeds from the host through haustoria; an endophyte lives inside host tissue, often without symptoms.

Practice questions (16)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

Which group carries out the mineralization of dead organic matter, returning inorganic nutrients to the soil for reuse by plants?
1. Producers
2. Primary consumers
3. Top carnivores
4. Decomposers
Show answer
Answer: D — Decomposers
Decomposers (detritivores and microbes) break down dead matter and release inorganic nutrients back to the soil, closing the nutrient cycle. Producers fix energy and build organic matter, and consumers eat other organisms; none of these performs the mineralization step.
In a grassland the gross primary productivity is 1000 g m⁻² yr⁻¹ (dry mass) and the producers' respiration is 400 g m⁻² yr⁻¹. Calculate the net primary productivity, in g m⁻² yr⁻¹.
Numerical answer — type the value.
Show answer
Answer: 600
NPP = GPP − respiration = 1000 − 400 = 600 g m⁻² yr⁻¹. NPP is the energy left for growth and for the next trophic level after the plants meet their own respiratory needs.
The producers of a food chain fix 8000 kJ m⁻² yr⁻¹ of net energy. Assuming exactly 10% of the energy at each trophic level is passed on to the next, how many kJ m⁻² yr⁻¹ reach the carnivores at the third trophic level (producers → herbivores → carnivores)?
Numerical answer — type the value.
Show answer
Answer: 80
Each step transfers 10%. Herbivores (second level) receive 8000 × 0.10 = 800 kJ; carnivores (third level) receive 800 × 0.10 = 80 kJ m⁻² yr⁻¹. Applying 10% only once (giving 800) forgets that the carnivore is two steps above the producer.
Which type of ecological pyramid can never be inverted, and why?
1. The pyramid of numbers, because there are always fewer individuals at higher levels
2. The pyramid of biomass, because biomass must always decrease upward
3. The pyramid of energy, because energy is lost at each transfer and flows only one way
4. All three pyramids are equally incapable of ever being inverted
Show answer
Answer: C — The pyramid of energy, because energy is lost at each transfer and flows only one way
The pyramid of energy is always upright because energy is lost as heat at each transfer and cannot increase at a higher level. Pyramids of number (a tree with many insects) and biomass (ocean phytoplankton supporting more zooplankton at an instant) can be inverted, so neither they nor "all three" is the correct answer.
The phosphorus cycle differs from the carbon and nitrogen cycles mainly in which respect?
1. It is a sedimentary cycle with no significant gaseous atmospheric phase
2. It is the only cycle in which living organisms play no part at all
3. It is driven entirely by human industrial activity rather than by natural processes
4. It moves far faster than the carbon and nitrogen cycles through a large air reservoir
Show answer
Answer: A — It is a sedimentary cycle with no significant gaseous atmospheric phase
Phosphorus has a sedimentary cycle: its reservoir is rock and soil and it has no significant gaseous phase, so it moves slowly, not faster, than the gaseous carbon and nitrogen cycles. Living organisms are central to it (uptake and release), and it is a natural cycle, though mining and fertilizer use now perturb it.
Why does secondary succession on an abandoned farm field generally proceed much faster than primary succession on a fresh lava flow?
1. Abandoned fields receive far more rainfall and sunlight than any new lava flow does
2. The soil, its seed bank and nutrients survive the disturbance, so soil-building is skipped
3. Secondary succession skips the climax stage, so it reaches stability in fewer steps
4. Pioneer species are absent in secondary succession, which removes the slowest early stage
Show answer
Answer: B — The soil, its seed bank and nutrients survive the disturbance, so soil-building is skipped
Secondary succession begins where soil, its seed bank and nutrients already exist, so the slow pioneer stage of building soil from bare rock is bypassed, and recolonization is rapid. It is not driven by more rainfall, it still ends in a climax, and it does have early colonizers; the key difference is the pre-existing soil.
Which of the following statements about energy flow and nutrient cycling in ecosystems are correct?
1. Energy flows one way through the ecosystem and is ultimately lost as heat
2. Mineral nutrients are recycled repeatedly between organisms and the environment
3. Nitrogen fixation converts atmospheric N₂ into a form plants can assimilate
4. Energy, like nutrients, is recycled back to producers by decomposers
Show answer
Answer: A — Energy flows one way through the ecosystem and is ultimately lost as heat; B — Mineral nutrients are recycled repeatedly between organisms and the environment; C — Nitrogen fixation converts atmospheric N₂ into a form plants can assimilate
Energy flows one way and is lost as heat, nutrients are recycled, and nitrogen fixation makes N₂ available to plants, all correct. The false statement claims energy is recycled: decomposers recycle nutrients, but the energy they release is lost as heat and must be replenished continually by sunlight.
On the Rodgers and Panwar (Wildlife Institute of India) classification used here, into how many biogeographic zones is India divided?
1. 4 biogeographic zones
2. 7 biogeographic zones
3. 10 biogeographic zones
4. 25 biogeographic zones
Show answer
Answer: C — 10 biogeographic zones
The Rodgers and Panwar scheme recognises 10 biogeographic zones of India (Trans-Himalaya, Himalaya, Indian Desert, Semi-Arid, Western Ghats, Deccan Peninsula, Gangetic Plain, Coasts, North-East and Islands). The figure 4 is the number of global biodiversity hotspots reaching India, not the zone count, and 25 was an early count of world hotspots.
Which of the following regions is widely regarded as one of India's richest centres of plant endemism?
1. The Indian Desert of the arid north-west
2. The Western Ghats
3. The Gangetic Plain
4. The Trans-Himalayan cold desert
Show answer
Answer: B — The Western Ghats
The Western Ghats, with their isolation, stable wet climate and varied terrain, are among India's richest centres of plant endemism (and a global biodiversity hotspot). The arid desert, the intensively farmed Gangetic Plain and the sparsely vegetated trans-Himalayan cold desert all have far lower endemic plant richness.
A plant community has four species with 40, 30, 20 and 10 individuals, a total of 100. Using Simpson's index D = Σ(nᵢ/N)², compute Simpson's diversity (1 − D) to two decimal places.
Numerical answer — type the value.
Show answer
Answer: 0.70
The proportions are 0.4, 0.3, 0.2 and 0.1. D = 0.4² + 0.3² + 0.2² + 0.1² = 0.16 + 0.09 + 0.04 + 0.01 = 0.30. Simpson's diversity = 1 − D = 1 − 0.30 = 0.70. Reporting D itself (0.30) instead of 1 − D is the usual error; 1 − D rises with diversity.
To estimate a plant-dwelling beetle population, 50 beetles are marked and released. In a later sample of 40 beetles, 8 carry marks. Using the Lincoln-Petersen estimate N = (M × C) ÷ R, estimate the population size.
Numerical answer — type the value.
Show answer
Answer: 250
N = (M × C) ÷ R = (50 × 40) ÷ 8 = 2000 ÷ 8 = 250 beetles. The marked fraction in the second sample (8/40 = 0.2) is taken to equal the marked fraction of the whole population (50/N), giving N = 250.
A botanical garden maintains living collections of threatened plant species, and a seed bank stores their seeds under cold, dry conditions. These are examples of which conservation strategy?
1. In situ conservation, because the plants are kept within natural ecosystems
2. Ex situ conservation, because the plants are kept away from their natural habitat
3. In situ conservation, because a seed bank protects whole habitats at once
4. Neither, because conservation applies only to animals and not to plants
Show answer
Answer: B — Ex situ conservation, because the plants are kept away from their natural habitat
Botanical gardens, seed banks, gene banks and tissue culture keep organisms away from their natural habitat, which is ex situ conservation. In situ conservation keeps species within their habitats (national parks, sanctuaries, biosphere reserves), and conservation applies fully to plants.
Which of the following statements about biodiversity, hotspots and global change are correct as stated here?
1. A biodiversity hotspot combines exceptional endemism with a high degree of threat
2. On the Conservation International scheme, four hotspots extend into India
3. Simpson's diversity (1 − D) increases as a community becomes more diverse and even
4. Global warming is expected to shift species' ranges towards the equator and down mountains
Show answer
Answer: A — A biodiversity hotspot combines exceptional endemism with a high degree of threat; B — On the Conservation International scheme, four hotspots extend into India; C — Simpson's diversity (1 − D) increases as a community becomes more diverse and even
A hotspot pairs high endemism with high threat, four Conservation International hotspots reach India, and Simpson's diversity (1 − D) rises with richness and evenness, all correct. The false statement reverses the direction of climate-driven range shifts: warming generally pushes species towards the poles and up mountains, not towards the equator and downhill.
An orchid grows perched on the branch of a forest tree, using it only for support and making its own food, while the tree is unaffected. This interaction is best described as:
1. Parasitism, with the orchid drawing nutrients from the tree through haustoria
2. Mutualism, since both the orchid and the tree gain from the association
3. Commensalism, as the orchid (an epiphyte) benefits while the tree is unaffected
4. Amensalism, because the orchid's presence harms the tree without benefiting itself
Show answer
Answer: C — Commensalism, as the orchid (an epiphyte) benefits while the tree is unaffected
An epiphyte such as this orchid gains support (a benefit) while the host tree is neither helped nor harmed, which is commensalism (+/0). It is not parasitism because the orchid takes nothing from the tree and makes its own food; it is not mutualism because the tree gains nothing; and it is not amensalism because the tree is unharmed.
What distinguishes an endophyte from an epiphyte?
1. An endophyte lives inside host plant tissue; an epiphyte grows on another plant's outer surface
2. An endophyte is always a flowering plant, whereas an epiphyte is always a fungus
3. An endophyte is a parasite that always kills its host, whereas an epiphyte never contacts a host
4. An endophyte lives only in soil, whereas an epiphyte lives only in water
Show answer
Answer: A — An endophyte lives inside host plant tissue; an epiphyte grows on another plant's outer surface
An endophyte lives inside host tissue (often a fungus or bacterium, frequently symptomless), while an epiphyte grows on the external surface of another plant for support. Endophytes are not always flowering plants nor always lethal parasites, and neither is defined by living only in soil or water.
Which of the following are correct about ecological succession and ecosystem dynamics?
1. Primary succession begins on a surface with no pre-existing soil, such as a new lava flow
2. A hydrosere is the succession that proceeds from open water towards dry land
3. Pioneer species such as lichens help build soil in the early stages of primary succession
4. The climax community is the first, pioneer stage of a succession
Show answer
Answer: A — Primary succession begins on a surface with no pre-existing soil, such as a new lava flow; B — A hydrosere is the succession that proceeds from open water towards dry land; C — Pioneer species such as lichens help build soil in the early stages of primary succession
Primary succession starts on soil-free surfaces, a hydrosere runs from open water to land, and lichen pioneers build soil early, all correct. The false statement inverts the sequence: the climax is the relatively stable community at the end of succession, not the pioneer stage at the start.