Which of the following statements about ecological pyramids are correct? Select all that apply.

The pyramid of numbers in a tree-based ecosystem can be inverted

The pyramid of energy is always upright

The pyramid of energy is inverted in the open ocean

The pyramid of numbers in a tree-based ecosystem can be inverted

The pyramid of energy is always upright

Assertion (A): Energy flow through an ecosystem is unidirectional. Reason (R): At each trophic level energy is lost as heat in respiration and cannot be passed back to the producers. Choose the correct option.

Both A and R are true, and R is the correct explanation of A

Both A and R are true, but R is not the correct explanation of A

Geography of Environment I: Ecosystems and Human Ecology, Energy Flow, Biogeochemical Cycles, Environmental Ethics and Deep Ecology

Unit IV opens with the ecosystem seen as geographers see it. The document names its components — the ecosystem, in a geographic classification, and human ecology; its functions — trophic levels, energy flows, the geochemical cycles of carbon, nitrogen and oxygen, food chains, food webs and ecological pyramids; human interaction and impacts; and environmental ethics and deep ecology. This platform's shared geography course introduces the food chain and the pyramids at survey level; this chapter works at postgraduate depth — Tansley and Lindeman, inverted pyramids, the microbiology of the nitrogen cycle, Barrows's human ecology, and the philosophical positions from Leopold to Naess.

1. Components: the ecosystem, its geographic classification and human ecology

The term ecosystem was coined by Arthur Tansley in 1935 for the whole system of organisms together with the physical factors of their environment, treated as one functional unit. Its abiotic components are energy, climate, the substrate and the pool of inorganic nutrients; its biotic components are producers (autotrophs, mainly green plants and phytoplankton), consumers (herbivores as primary consumers, carnivores as secondary and tertiary consumers, and omnivores) and decomposers (bacteria and fungi, which return nutrients to the abiotic pool). The transition zone between two ecosystems is an ecotone, and the tendency for it to hold more species than either neighbour is the edge effect.

The geographic classification of ecosystems follows the major environments. On land the units are biomes — regional ecosystems defined by their climax vegetation, which is controlled mainly by temperature and moisture (Robert Whittaker's biome diagram plots them on those two axes): tropical rainforest, tropical deciduous forest and savanna, hot desert, temperate grassland, Mediterranean scrub, temperate deciduous forest, boreal forest (taiga), tundra and the altitudinal zones of mountains. In water the division is between marine ecosystems (the open ocean, continental shelf seas, estuaries, mangroves and coral reefs) and freshwater ecosystems — lentic (still water: lakes, ponds), lotic (running water: streams, rivers) and wetlands.

Human ecology in geography means the study of the relationship between human societies and their environment. Harlan H. Barrows, in his 1923 presidential address to the Association of American Geographers, "Geography as Human Ecology", proposed that geography should be defined precisely as that — the study of human adjustment to the natural environment — rather than as a science of the physical Earth. The phrase was also used by the Chicago school of sociology for the competition of groups for urban space (see the urban chapter). In both senses human ecology treats people as part of the ecosystem, not outside it, which is the bridge from the ecology of this section to the ethics of the last.

2. Functions: trophic levels, energy flow, food chains, food webs and pyramids

Raymond Lindeman's "trophic-dynamic" paper of 1942, on a Minnesota lake, founded the study of energy flow. Producers capture solar energy as gross primary productivity (GPP); what remains after their own respiration is net primary productivity (NPP), the energy available to consumers. At each transfer from one trophic level to the next, most energy is lost as heat in respiration or passes unused to decomposers, and only about 10 per cent is stored in the next level — the rule of thumb known as the ten per cent law (Lindeman's efficiency). Two consequences follow: energy flow is unidirectional and cannot be recycled, and food chains are short, rarely beyond four or five links. Per unit area, NPP is highest in tropical rainforests, estuaries, swamps and coral reefs and lowest in deserts and the open ocean — though the open ocean's vast area makes its total production large.

A grazing food chain begins with living green plants (grass → grasshopper → frog → snake → hawk); a detritus food chain begins with dead organic matter consumed by detritivores and decomposers, and in forests it carries more energy than the grazing chain. Real feeding relationships interlock into food webs, whose complexity gives an ecosystem stability, because a consumer with several food sources survives the loss of one. Charles Elton (1927) described the ecological pyramids, which show the structure of trophic levels at a given time. Persistent toxins that are not broken down, such as DDT and mercury, become more concentrated at each higher level — biomagnification — which is why top predators suffer most.

Pyramid	Upright or inverted?	Why
Numbers	Usually upright; inverted in a tree-based system	One large tree can support thousands of insects, which support fewer birds
Biomass	Upright on land; can be inverted in the open ocean	Phytoplankton are few at any moment but reproduce so fast that they feed a larger standing crop of zooplankton
Energy	Always upright	Energy is lost at every transfer, so no level can hold more energy than the one below it

⚠️ Only one pyramid can never be inverted

Questions often offer "the pyramid of biomass is always upright". It is not — the open ocean inverts it. The pyramid of energy is the only one that is always upright, because it measures a flow over time rather than a standing stock at an instant.

3. Biogeochemical cycles: carbon, nitrogen and oxygen

Unlike energy, matter is recycled. Biogeochemical cycles are either gaseous, with the main reservoir in the atmosphere or ocean (carbon, nitrogen, oxygen), or sedimentary, with the main reservoir in the Earth's crust (phosphorus, and in most textbooks sulphur). In the carbon cycle, photosynthesis draws carbon dioxide from the air and respiration, decomposition and combustion return it. The largest store by far is the lithosphere — carbonate rocks and fossil fuels; the deep ocean holds about fifty times as much carbon as the atmosphere and exchanges it slowly by dissolution; soils hold more than vegetation and the atmosphere together. Burning fossil fuels moves carbon from the slow geological store into the fast atmospheric one, which is the root of the present imbalance. The oxygen cycle is coupled to it: photosynthesis releases oxygen, respiration and combustion consume it, and in the stratosphere ultraviolet light converts some oxygen to ozone.

Step of the nitrogen cycle	What happens	Agents
Fixation	Atmospheric N2 converted to ammonia or nitrate	Rhizobium in legume root nodules; free-living Azotobacter; cyanobacteria (Anabaena, Nostoc); lightning; the industrial Haber–Bosch process
Ammonification	Organic nitrogen in dead matter released as ammonium	Decomposer bacteria and fungi
Nitrification	Ammonium oxidised to nitrite, then nitrite to nitrate	Nitrosomonas (to nitrite), then Nitrobacter (to nitrate)
Assimilation	Plants take up nitrate and ammonium and build proteins	Green plants, then consumers
Denitrification	Nitrate reduced back to N2 gas in waterlogged, oxygen-poor soils	Denitrifying bacteria such as Pseudomonas

🎯 Why 78 per cent nitrogen still leaves plants hungry

The triple bond of the N2 molecule is so strong that plants cannot use atmospheric nitrogen directly; only fixation breaks it. That is why legumes enrich soil in crop rotations, and why the Haber–Bosch process — now fixing roughly as much nitrogen as all natural land processes — both fed the Green Revolution and caused nitrate pollution and eutrophication.

4. Human interaction and impacts; environmental ethics and deep ecology

Human impact on ecosystems has grown with each stage of technology — from fire and hunting through agriculture and irrigation to industrial energy use. George Perkins Marsh's Man and Nature (1864) was the first systematic account of how human action degrades the land; the Princeton symposium Man's Role in Changing the Face of the Earth (1956), with Carl Sauer among its organisers, brought the theme into geography. The main impacts are deforestation, the loss of biodiversity, soil erosion and desertification, the alteration of the carbon and nitrogen cycles, and pollution; their scale has led Paul Crutzen and Eugene Stoermer (2000) to propose that the Earth has entered a new epoch, the Anthropocene. Garrett Hardin's "The Tragedy of the Commons" (1968) argued that freely accessible resources are over-used because each user gains the whole benefit of extra use while the cost is shared.

Environmental ethics asks whether nature has value only as a means to human ends. Anthropocentrism says yes; biocentrism extends moral standing to all living beings; ecocentrism extends it to whole ecosystems. Aldo Leopold's land ethic, in A Sand County Almanac (1949), is the classic ecocentric statement: "A thing is right when it tends to preserve the integrity, stability, and beauty of the biotic community. It is wrong when it tends otherwise." Rachel Carson's Silent Spring (1962), on pesticides, launched the modern environmental movement, and Lynn White's "The Historical Roots of Our Ecologic Crisis" (1967) blamed the dominion attitude of Western Christianity. Deep ecology was named by the Norwegian philosopher Arne Naess in 1973. He contrasted a shallow ecology, which fights pollution and resource depletion chiefly for the health and affluence of people in developed countries, with a deep ecology based on the intrinsic value of all life, biospherical egalitarianism and self-realisation through identification with the wider natural world; with George Sessions he set out an eight-point platform in 1984. Its critics include Murray Bookchin's social ecology, which traces ecological crisis to social hierarchy, and ecofeminism (Françoise d'Eaubonne coined the word in 1974; Vandana Shiva's Staying Alive, 1988, is the Indian landmark).

Key takeaways

Tansley coined "ecosystem" (1935); producers, consumers and decomposers plus abiotic factors; biomes are the geographic units on land, and marine, lentic and lotic systems in water; Barrows (1923) defined geography as human ecology.
Lindeman (1942): energy flows one way and about 10 per cent passes to each higher trophic level, so chains are short; NPP is highest in rainforests, estuaries, swamps and reefs.
Elton's pyramids: numbers can invert (a tree), biomass can invert (the open ocean), energy never inverts; persistent toxins biomagnify up the chain.
Carbon, nitrogen and oxygen cycles are gaseous, phosphorus sedimentary; nitrogen is fixed by Rhizobium, Azotobacter, cyanobacteria, lightning and Haber–Bosch, nitrified by Nitrosomonas then Nitrobacter, and returned to the air by denitrifiers.
Marsh (1864) and the 1956 symposium framed human impact; Hardin's commons, Leopold's land ethic, Carson and White shaped environmental ethics; Naess (1973) set deep, intrinsic-value ecology against shallow, human-centred ecology.

Practice questions (10)

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.

The term "ecosystem" was coined in 1935 by:
1. Ernst Haeckel
2. Arthur Tansley
3. Raymond Lindeman
4. Charles Elton
Show answer
Answer: B — Arthur Tansley
Tansley introduced "ecosystem" in 1935 for organisms and their physical environment as one system. Haeckel coined "ecology" in 1866, Elton described ecological pyramids and Lindeman founded the trophic-dynamic study of energy flow.
The generalisation that only about one-tenth of the energy at one trophic level is stored at the next is associated with:
1. Tansley
2. Whittaker
3. Lindeman
4. Hardin
Show answer
Answer: C — Lindeman
Lindeman's 1942 trophic-dynamic study of Cedar Bog Lake gave rise to the ten per cent law (Lindeman's efficiency). Whittaker is known for the biome diagram and Hardin for the tragedy of the commons.
Which ecological pyramid is always upright?
1. Pyramid of numbers
2. Pyramid of standing crop
3. Pyramid of biomass
4. Pyramid of energy
Show answer
Answer: D — Pyramid of energy
Energy is lost at every transfer, so no trophic level can contain more energy than the one feeding it. The pyramid of numbers inverts in a tree-based system and the pyramid of biomass (standing crop) in the open ocean.
Producers in an ecosystem fix 10,000 kcal of energy. Applying the ten per cent law, how many kcal reach the tertiary consumers? Type a number.
Numerical answer — type the value.
Show answer
Answer: 10
Producers 10,000 → primary consumers 1,000 → secondary consumers 100 → tertiary consumers 10 kcal. Three transfers at 10 per cent each multiply by 0.1 three times, which is why food chains rarely exceed four or five levels.
In the nitrogen cycle, the oxidation of ammonium to nitrite is carried out by:
1. Pseudomonas
2. Nitrosomonas
3. Rhizobium
4. Nitrobacter
Show answer
Answer: B — Nitrosomonas
Nitrification is two steps: Nitrosomonas oxidises ammonium to nitrite, then Nitrobacter oxidises nitrite to nitrate. Rhizobium fixes atmospheric nitrogen in legume nodules, and Pseudomonas is among the denitrifiers.
Which of the following statements about ecological pyramids are correct? Select all that apply.
1. The pyramid of energy is inverted in the open ocean
2. The pyramid of biomass is always upright
3. The pyramid of numbers in a tree-based ecosystem can be inverted
4. The pyramid of energy is always upright
Show answer
Answer: C — The pyramid of numbers in a tree-based ecosystem can be inverted; D — The pyramid of energy is always upright
Energy pyramids never invert, and one tree supporting many insects inverts the pyramid of numbers. The biomass pyramid is inverted in the open ocean, where a small, fast-reproducing stock of phytoplankton supports a larger stock of zooplankton; it is the biomass pyramid, not the energy pyramid, that inverts there.
Assertion (A): Energy flow through an ecosystem is unidirectional. Reason (R): At each trophic level energy is lost as heat in respiration and cannot be passed back to the producers. Choose the correct option.
1. Both A and R are true, and R is the correct explanation of A
2. Both A and R are true, but R is not the correct explanation of A
3. A is true, but R is false
4. A is false, but R is true
Show answer
Answer: A — Both A and R are true, and R is the correct explanation of A
Both are true and R explains A. Nutrients cycle, but energy degrades to heat at every step and leaves the system, so it must be continually resupplied by the Sun.
The distinction between "shallow" and "deep" ecology was drawn in 1973 by:
1. Murray Bookchin
2. Garrett Hardin
3. Arne Naess
4. Aldo Leopold
Show answer
Answer: C — Arne Naess
Naess contrasted shallow ecology — anti-pollution and resource conservation for human benefit — with deep ecology, grounded in the intrinsic value of all life. Bookchin developed social ecology, a critique of deep ecology.
"Geography as Human Ecology" was the title of the 1923 presidential address to the Association of American Geographers by:
1. Harlan H. Barrows
2. Ellen Churchill Semple
3. Carl Sauer
4. Richard Hartshorne
Show answer
Answer: A — Harlan H. Barrows
Barrows argued that geography should be the study of human adjustment to the natural environment. Sauer led the Berkeley school of cultural landscape, Semple popularised environmental determinism, and Hartshorne wrote The Nature of Geography (1939).
"A thing is right when it tends to preserve the integrity, stability, and beauty of the biotic community" is the core of:
1. Garrett Hardin's tragedy of the commons
2. Aldo Leopold's land ethic
3. The Brundtland definition of sustainable development
4. Rachel Carson's Silent Spring
Show answer
Answer: B — Aldo Leopold's land ethic
The sentence comes from A Sand County Almanac (1949) and is the classic ecocentric principle, assigning moral weight to the biotic community as a whole. The Brundtland definition speaks of meeting present needs without compromising future generations.