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Succession, interactions and biogeochemical cycles

Ecology basics · Part 2 / 2 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

Primary and secondary succession, pioneer species, xerosere and hydrosere, the climax community; mutualism, commensalism, parasitism, predation, competition and amensalism; keystone, flagship, umbrella, indicator and invasive species; ecotones and the edge effect; gaseous and sedimentary cycles; the carbon, nitrogen, phosphorus, sulphur and water cycles, eutrophication and acid rain.

Part 1 looked at an ecosystem as if it were a photograph: who is present, who eats whom and how energy passes along. This part turns the photograph into a film. Ecosystems change with time, so a bare rock can end up as a forest. Species living in them help, use or fight one another. The material that bodies are made of, carbon, nitrogen, phosphorus, sulphur and water, is used again and again. One rule frames the whole part: energy flows through an ecosystem once and is lost as heat, but matter goes round and round.

ecological succession is the gradual, fairly predictable replacement of one community by another in the same place, until a community develops that stays roughly stable. It happens because every set of organisms alters its own surroundings, adding soil, shade or moisture, and in doing so makes room for newcomers better suited to the changed site. When the process begins on ground that has never carried life and has no soil, such as freshly cooled lava, bare rock or a new sand dune, it is called primary succession, and it is very slow because soil must first be made. When life returns to a place whose vegetation was removed but whose soil survived, such as an abandoned field, a forest after fire or a cleared plot, it is secondary succession, and it runs much faster. The whole series of communities is a sere, and each step along it is a seral stage.

The first colonisers of a bare site are pioneer species. On rock these are usually lichens, which release weak acids that eat into the surface and, together with their own dead remains, start a thin layer of soil; mosses follow, then herbs and grasses, then shrubs and finally trees. A sere that begins on dry, bare ground is a xerosere (the one on rock is also called a lithosere). A sere that begins in open water is a hydrosere: floating microscopic algae come first, then rooted plants under the water, then rooted plants with floating leaves, then reeds, then marsh sedges, and as silt and dead matter fill the pond it turns into wet meadow and at last woodland. The end point is the climax community, which is in balance with the local climate.

Ecologists sort the ways species affect one another by asking, for each partner: does it gain, lose or feel nothing? In mutualism both gain: a lichen is an alga that makes food and a fungus that shelters it and holds water; fig trees can be pollinated only by their own species of fig wasp, which in turn raises its young inside the fig. In commensalism one gains and the other is unaffected, as when an orchid grows on the branch of a mango tree for light and support without taking its food. In parasitism one gains at the other's cost, usually without killing it quickly: Cuscuta, a leafless climbing plant, draws its food from the hedge plants it wraps around. Predation also helps one and harms the other, but the prey is killed and eaten. In competition both lose, because they draw on the same limited resource. In amensalism one is harmed and the other is unaffected, as when the Penicillium mould releases a substance that kills nearby bacteria.

Two of these interactions shape whole communities. Competition is sharpest between species that need exactly the same thing; G. F. Gause, growing two kinds of Paramecium together in the laboratory, found that one eventually wiped out the other, which gave rise to the competitive exclusion principle: two species cannot share the very same niche indefinitely. In nature such rivals often survive by splitting the resource, feeding at different heights, hours or seasons. Predation, for its part, keeps prey numbers in check and so stops any one plant-eater from stripping its habitat bare. When prickly pear cactus, brought into Australia, spread over vast areas of grazing land, it was brought under control in the 1920s only after a cactus-eating moth from its home range was released. A special case is brood parasitism: the Asian koel lays its eggs in the nests of crows, which then rear the koel's chicks as their own.

Some species are singled out because of the part they play. A keystone species holds its community together out of all proportion to its abundance, so removing it causes many other changes: where sea otters vanished from the North Pacific coast, the sea urchins they eat multiplied and stripped away the kelp forests. A flagship species is a well-loved animal used to win public support and money for conservation, such as the tiger, chosen for Project Tiger in 1973. An umbrella species needs such a large area of habitat that protecting it also protects many smaller species living there; the tiger and the elephant both serve this way. An indicator species reveals the state of its environment by its presence or absence: many lichens disappear where the air carries sulphur dioxide. An invasive species is a species brought from elsewhere that spreads unchecked and crowds out native life, like water hyacinth on Indian lakes, or Lantana and Parthenium on farm and forest land.

Where two communities meet there is usually a band of transition called an ecotone: the strip between forest and grassland, the estuary where a river meets the sea, or the mangrove belt that lies between land and open water. An ecotone contains species from both neighbouring communities and often some that live only in the transition band itself. As a result the number of species, and sometimes the density of individuals, tends to be higher in the ecotone than in either community on its own; this rise is called the edge effect. Cutting a forest into many small patches multiplies its edges, letting in light, wind, weeds and predators that harm species needing deep forest interior.

Living bodies are built from a small set of chemical elements, and the Earth receives almost no fresh supply of them from space, so the same atoms must be reused. The paths by which an element moves from the air, water and rocks into organisms and back again are called biogeochemical cycles. They fall into two groups. In gaseous cycles, such as those of carbon, nitrogen and oxygen, the main store is the atmosphere or the ocean, so the element spreads widely. In sedimentary cycles, such as those of phosphorus and sulphur, the main store is the Earth's crust, and the element is released only slowly as rocks weather, so it is more easily locked away or lost. Every cycle also has two compartments: a large, slow-moving reservoir pool and a smaller, fast exchange or cycling pool, where the element passes quickly between living things and their immediate surroundings.

In the carbon cycle, green plants, algae and some bacteria take carbon dioxide from the air or water and fix it into sugars by photosynthesis. It then passes along food chains and returns to the air as carbon dioxide through the respiration of plants, animals and decomposers. The ocean exchanges carbon dioxide with the air all the time and holds far more carbon than the atmosphere does, most of it in dissolved form. Carbon locked in the remains of ancient forests and plankton became coal, oil and natural gas. Human activity has sped the cycle up in one direction: burning fossil fuels and clearing forests add carbon dioxide faster than plants and the sea can take it up, which strengthens the greenhouse effect, and the extra gas dissolving in seawater makes the ocean more acidic. Forests, soils and the ocean that absorb more carbon than they release act as a carbon sink.

Nitrogen makes up about 78 per cent of the air, yet most living things cannot use the gas directly. It must first be converted into ammonia or nitrate, a step called nitrogen fixation. Most natural fixation is done by microbes: Rhizobium bacteria in the root nodules of pulses and other legumes, free-living soil bacteria such as Azotobacter, and cyanobacteria such as Anabaena and Nostoc, which are useful in flooded rice fields. Lightning fixes a smaller amount in the air, and factories fix a great deal by the Haber–Bosch process to make fertiliser. Decomposers turn the nitrogen of dead bodies and wastes into ammonia (ammonification); Nitrosomonas bacteria oxidise ammonia to nitrite and Nitrobacter oxidise nitrite to nitrate (nitrification); plants absorb nitrate and build it into proteins (assimilation); and in waterlogged, oxygen-poor soils bacteria such as Pseudomonas turn nitrate back into nitrogen gas (denitrification), closing the loop.

Phosphorus has no significant gas phase. Its cycle starts with rocks: weathering frees phosphate, plants take it up from the soil, animals get it by eating plants, and decomposers return it to the soil. Some is washed into rivers and the sea, where it settles into sediments and is lost to living things until geological uplift raises those beds again. When fertiliser and sewage carry extra phosphorus and nitrogen into lakes and coastal waters, the result is eutrophication: algae bloom, then die, and the bacteria decomposing them use up the dissolved oxygen, so fish suffocate and low-oxygen dead zones form. Sulphur is stored mainly in rocks and sediments but does have a gaseous stage: it reaches the air as sulphur dioxide from volcanoes and from burning coal and oil, and as hydrogen sulphide from decay in waterlogged places. Sulphur dioxide turns into sulphuric acid in the air, a main cause of acid rain.

The water cycle, powered by the sun, links all the others, because water carries dissolved nutrients from rock to soil and from land to sea. Most evaporation takes place from the oceans; plants add more by transpiration, losing water vapour through the pores of their leaves, and the two together are called evapotranspiration. The vapour cools as it rises, condenses into cloud and falls as rain or snow. On land part of it runs off into streams, part soaks in to recharge groundwater, and part is taken up by plants. Taken together: ecosystems change through succession because organisms modify their surroundings; species interact as partners, users or rivals; certain species hold communities together or show their health; and nutrients circulate while energy passes through only once.

Exam traps

• PRIMARY succession starts where there is NO soil (bare rock, new lava, sand dune) and is slow; SECONDARY succession starts where soil remains (abandoned field, burnt forest) and is faster.
• Lichens are the usual PIONEERS on bare rock; phytoplankton are the pioneers of a hydrosere. The final stable stage is the CLIMAX community.
• A lichen is MUTUALISM (alga + fungus), not parasitism; an orchid on a mango tree is COMMENSALISM, not parasitism; Cuscuta is a PARASITE.
• AMENSALISM is 0/− (Penicillium and bacteria); COMMENSALISM is +/0. Do not swap them.
• FLAGSHIP is about public appeal, UMBRELLA about habitat size, KEYSTONE about effect on the community; the tiger is flagship and umbrella at once.
• Phosphorus has NO significant gaseous phase; sulphur's main store is sedimentary but it DOES have gases (SO2, H2S). Carbon and nitrogen cycles are gaseous.
• NITRIFICATION is ammonia → nitrite (Nitrosomonas) → nitrate (Nitrobacter); DENITRIFICATION returns nitrate to N2 gas (e.g. Pseudomonas) in oxygen-poor soil.
• Eutrophication kills fish by REMOVING oxygen, not by poisoning; the cause is excess nutrients (nitrogen and phosphorus).

Recap

• Energy flows through an ecosystem once; matter (carbon, nitrogen, phosphorus, sulphur, water) cycles.
• Succession: primary (no soil, slow) vs secondary (soil left, faster); pioneers → seral stages → climax community.
• Xerosere on dry ground (lichens → mosses → herbs → shrubs → trees); hydrosere in water (phytoplankton → submerged → floating → reeds → sedges → woodland).
• Interactions: mutualism +/+, commensalism +/0, parasitism and predation +/−, competition −/−, amensalism 0/−.
• Gause: competitive exclusion; prickly pear in Australia controlled by a moth; the koel is a brood parasite of crows.
• Keystone (sea otter), flagship and umbrella (tiger), indicator (lichens for SO2), invasive (water hyacinth, Lantana, Parthenium).
• Ecotone = transition zone (mangroves, estuaries); edge effect = more species at the boundary.
• Gaseous cycles: carbon, nitrogen; sedimentary: phosphorus (no gas phase), sulphur (with SO2, H2S); reservoir vs exchange pool.
• Nitrogen: fixation (Rhizobium, Azotobacter, cyanobacteria, lightning, Haber–Bosch) → ammonification → nitrification → assimilation → denitrification.
• Excess N and P cause eutrophication and dead zones; SO2 causes acid rain; burning fossil fuels raises CO2 and acidifies the ocean.

Practice questions

Sources

• 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