1The idea in one paragraph

Left alone, a natural ecosystem can last almost indefinitely, because it runs on sunlight, recycles its nutrients, holds enough genetic diversity to adapt, and sits in a climate its organisms can tolerate. Some species hold far more of that stability than their numbers suggest: remove a keystone species and the whole community can reorganise. Humans threaten stability in many ways: clearing so much forest that it can no longer make its own rain, harvesting faster than populations can replace themselves, farming in ways that wash soil and fertilizer into rivers, releasing toxins that concentrate up food chains, and filling the oceans with plastic. Some damage can be reversed by rewilding. At HL, ecosystems also change on their own, by succession, towards a climax community, unless something stops them.

2Stability as a property of natural ecosystems

An ecosystem is stable when its community and its processes persist over long periods: the same kinds of organisms, in similar proportions, carrying out the same energy flow and nutrient cycling, year after year. A stable ecosystem also resists disturbance and recovers from it. A forest that loses trees in a storm regrows the gap; a desert returns to its sparse pattern after a rare wet year.

The evidence for stability comes from records that span long periods: pollen preserved in lake and bog sediments, fossils, and dated layers of soil and rock. They show that some ecosystems have persisted, in recognisably the same form, for very long times. Pollen and fossil records show that rainforest with ancient plant lineages has survived in the Wet Tropics of north-east Australia for many millions of years, contracting and expanding with the climate but never disappearing. The Namib Desert in south-western Africa has been arid for tens of millions of years, and its specialised desert community has had that long to evolve. On shorter timescales, parts of old-growth forest in Europe and North America have kept their structure for thousands of years.

3What stability requires

Four conditions have to hold, and Figure 1 sets them around an ecosystem.

• A supply of energy. Energy flows through an ecosystem and is lost as heat at every trophic level, so it must be replaced continuously. Almost all ecosystems are supplied by sunlight, which has been available for billions of years and will be for billions more.
• Recycling of nutrients. Unlike energy, the supply of chemical elements such as carbon, nitrogen and phosphorus is finite. Decomposers break down dead organisms and wastes and release the nutrients so producers can use them again. If nutrients were not recycled, they would run out.
• Genetic diversity. Populations with many alleles contain individuals able to survive new conditions, such as a new disease or a hotter summer. Natural selection (D4.1) can then adapt the population. A population with little genetic diversity can be wiped out by a single change.
• Climatic variables within tolerance limits. Every species has a range of temperature, rainfall and other conditions it can survive. If the climate stays within those ranges, the community persists; if it moves outside them, species decline or disappear (D4.3).

4The Amazon rainforest: a possible tipping point

A tipping point is a threshold in a system beyond which a small further change causes a large, self-reinforcing change that is hard or impossible to reverse. The guide asks you to study the Amazon rainforest as a possible example.

How the rainforest makes its own rain. Trees pull water from the soil and lose it from their leaves as water vapour in transpiration. Evaporation absorbs heat, so a large forest cools the air and the land beneath it. The vapour rises, condenses and falls again as rain further along the prevailing winds, which in the Amazon blow from the Atlantic westwards towards the Andes. Water that falls as rain near the coast is transpired, carried inland, rained out, and transpired again, several times over, so rain in the western Amazon depends partly on forest further east. The moist air also drives air flows that draw more moist air in from the ocean. Figure 2 shows the chain.

Why that creates a tipping point. Clearing forest breaks links in the chain. Cleared land transpires far less, so less water vapour is passed westwards, rainfall downwind falls, and the dry season lengthens. Drier forest is more likely to burn and more trees die, which reduces transpiration further. Past some extent of deforestation, the remaining forest may no longer generate enough rainfall to sustain itself, and large areas could shift to a drier, savanna-like ecosystem.

The uncertainty. Nobody knows the minimum area of rainforest needed to keep the cycle going. Some scientists have estimated that the threshold could lie at around 20 to 25% loss of the original forest, when combined with climate change and fire; others think different regions will respond differently, or at different levels. The science is uncertain because a whole continental system cannot be tested by experiment and models depend on assumptions. Uncertainty is not a reason for delay here: by the time the threshold is known for certain, it may already have been crossed.

Calculating the extent of deforestation. The extent is expressed as a percentage change from the original area. For an invented region whose original forest covered 4.0 million km² and which now has 3.3 million km²:

Always divide by the original area, and keep the minus sign, or say "a decrease of".

5Investigating stability with a mesocosm

A mesocosm is a small, enclosed, experimental ecosystem used to investigate how variables affect ecosystem processes. It can be set up in an open tank, but a sealed glass vessel is better, because it prevents matter from entering or leaving while still letting energy in, as light, and out, as heat. That makes it a closed system for matter, which is exactly the situation of an ecosystem that must recycle its nutrients. Aquatic or microbial mesocosms are more likely to succeed than terrestrial ones, because they establish balance faster and are easier to keep. Figure 3 shows a typical setup.

A worked plan. Fill several identical sealed jars with pond water, a layer of pond sediment containing decomposers, and pieces of the same aquatic plant; add one or two small invertebrates such as water snails to some. Choose one independent variable, for example light intensity (distance from a lamp), the presence or absence of consumers, or the temperature. Keep the others the same. Measure dependent variables over several weeks: dissolved oxygen concentration with a probe, pH as an indicator of carbon dioxide, the growth or colour of the plants, and the survival of animals. A stable mesocosm keeps these roughly constant; an unstable one shows oxygen falling, plants dying, or water clouding with bacteria.

Replicate each treatment, because a single jar can fail for reasons unrelated to the variable.

Nature of science: care of living things. Mesocosm work must follow the IB's experimental guidelines on the ethical use of animals: no experiment may cause pain or unnecessary harm to animals, invertebrates included. Choose variables that will not kill the animals, check them regularly, remove any that are in distress, and return organisms to a suitable habitat, or dispose of materials responsibly, at the end.

6Keystone species

A keystone species is a species that has a disproportionately large effect on the structure of its community, relative to its abundance. The name comes from the central stone at the top of an arch: it is one stone among many, but take it out and the arch collapses. Removing a keystone species can cause a cascade of changes and even the collapse of an ecosystem.

The standard example is the sea otter in the kelp forests of the North Pacific. Figure 4 shows how.

Sea otters eat sea urchins. Sea urchins graze on kelp, the giant brown seaweed that forms underwater forests sheltering hundreds of other species. Where otters are present, urchin numbers are kept low and the kelp forest thrives. Where otters were hunted almost to extinction for their fur, urchin populations grew unchecked and grazed the kelp down to bare rock, leaving urchin barrens with far fewer species. When otters returned or were reintroduced, the kelp recovered. Otters are never very numerous, yet the whole community depends on them.

Other examples: large predators such as wolves (section 12), beavers, which build dams that create wetlands, and elephants, which keep savanna open by pushing over trees.

7Sustainable harvesting of natural resources

Many natural resources are renewable: a population of fish or trees replaces what is taken, by reproduction and growth. Harvesting is sustainable when the rate of harvesting is lower than the rate of replacement, so the population is not depleted over time. Figure 5 models the difference.

In this model, a population that grows by reproduction is harvested at a fixed amount each year. At 100 per year, the harvest is less than the population can replace, and it settles at a stable size. At 150 per year, the harvest exceeds the maximum possible replacement, and the population collapses to zero within about twenty years. The two harvests differ by only 50 per year.

Assessing sustainability means measuring both rates.

A marine fish: Atlantic cod. Fisheries scientists estimate the size and age structure of a fish stock from research surveys at sea (trawls at fixed sites, echo-sounding) and from landings data: the number, size and age of fish caught. Age is read from growth rings in the ear bones. From this they estimate the breeding population, the rate at which young fish join it, and the death rate from fishing. They then set a catch limit, a quota, below the rate of replacement. The warning is the cod off Newfoundland, fished heavily for decades until the stock collapsed around 1990; Canada closed the fishery in 1992 and the stock has recovered only partly since. Signs of unsustainable fishing include falling catches despite more effort, and a stock made up of younger and smaller fish, because the older breeding fish have been removed.

A terrestrial plant: big-leaf mahogany. This tropical tree of Central and South America yields valuable timber and was logged so heavily that in many areas few large trees remained. It grows slowly and needs large trees to produce seed. Sustainability is assessed by forest inventories: counting and measuring the trees of each size class in sample plots, measuring growth rates, and checking that seedlings and young trees are regenerating. Rules can then limit felling to trees above a minimum diameter, keep enough seed trees standing, and set a cutting cycle long enough for the forest to regrow. International trade in mahogany now requires evidence that the harvest does not harm the survival of the species.

8Is agriculture sustainable?

Agriculture replaces a natural ecosystem with a managed one, and several factors decide whether it can continue on the same land indefinitely. An extended-response answer should weigh them.

• Soil erosion. Bare or ploughed soil is washed away by rain and blown by wind, faster than new soil forms, which takes centuries. Contour ploughing, cover crops, keeping crop residues on the surface, and hedgerows and terraces all reduce it.
• Leaching of nutrients. Soluble nutrients such as nitrates are washed down through the soil by rain, out of reach of roots and into groundwater and rivers. Fields lose fertility, and the leached nutrients cause eutrophication (section 9).
• Supply of fertilizers and other inputs. Harvested crops remove nutrients that must be replaced. Nitrogen fertilizer is made by an energy-intensive industrial process that uses fossil fuels; phosphate fertilizer comes from rock deposits that are finite. Water for irrigation, pesticides, fuel and machinery are all inputs that may not be available forever.
• Pollution by agrochemicals. Pesticides can kill non-target organisms, including pollinators and the predators of pests, and some persist and biomagnify (section 10). Herbicides reduce the diversity of wild plants around fields.
• Carbon footprint. The total greenhouse gas emissions of producing food include carbon dioxide from machinery and fertilizer manufacture, methane from cattle and rice paddies, and nitrous oxide from fertilized soils. Clearing forest for farmland releases stored carbon too.

Practices that reduce these impacts include crop rotation with legumes, which add nitrogen through their root nodules, organic matter such as manure and compost, integrated pest management, reduced ploughing, and planting trees along field margins. Whether a farm is sustainable is a matter of degree, and a good answer says what is sustained and at what cost.

9Eutrophication

Eutrophication is the enrichment of a body of water with nutrients, especially nitrates and phosphates. Its main human cause is fertilizer leached from farmland, with sewage and animal waste adding to it. The sequence is set out in Figure 6.

1. Nitrates and phosphates leach from fields into streams, rivers, lakes or the sea.
2. Nutrients that were in short supply now are not, so algae and cyanobacteria grow rapidly: an algal bloom.
3. The bloom forms a layer at the surface that blocks light from reaching plants below, which die.
4. Algae also die in great numbers as the bloom ends.
5. Bacteria that decompose the dead organic matter multiply and respire aerobically, using large amounts of dissolved oxygen. The biochemical oxygen demand (BOD) rises. BOD is the amount of dissolved oxygen used by microorganisms to break down the organic matter in a sample of water over a fixed time, and a high BOD means heavy organic pollution.
6. Dissolved oxygen falls, often to levels too low for fish and many invertebrates, which die or leave. In the worst cases whole areas become dead zones where little except anaerobic bacteria survives.

Note that the direct cause of death is lack of oxygen, not the nutrients, and the oxygen is used by decomposers, not by the algae, which release oxygen in daylight.

10Biomagnification

Biomagnification is the increase in concentration of a pollutant in the tissues of organisms at successively higher trophic levels of a food chain. It happens when a pollutant has three properties:

• it is persistent: not broken down in the environment or in organisms;
• it is fat-soluble, so it is stored in fatty tissue rather than excreted;
• it is taken in with food.

Each organism eats many organisms from the level below over its lifetime and keeps nearly all the pollutant they contained, but uses up most of their biomass in respiration. So the pollutant becomes more concentrated at every step. Figure 7 shows illustrative concentrations.

DDT. DDT is an insecticide used widely from the 1940s, in farming and to kill mosquitoes that transmit malaria. Its concentration in water was tiny, but in fish-eating birds such as ospreys, peregrine falcons and bald eagles it reached levels that caused their eggshells to be thin. The shells broke under the parents during incubation, and breeding failed. Populations of these birds fell steeply. After DDT was banned for agricultural use in many countries from the 1970s, the birds recovered. It is still used in some places, in limited amounts, against malaria mosquitoes.

Mercury. Mercury is released by burning coal, by some industries and by small-scale gold mining. In water, bacteria convert it to methylmercury, which is absorbed and retained by organisms and biomagnifies. Large, long-lived predatory fish such as tuna and swordfish contain the highest concentrations, which is why pregnant women are advised to limit how much of them they eat. Methylmercury damages the nervous system. In the 1950s, industrial waste containing mercury poisoned fish in Minamata Bay in Japan, and people who ate the fish suffered severe neurological damage; the condition is now called Minamata disease.

11Plastic pollution of the oceans

Plastics are persistent in the natural environment because they are not biodegradable: decomposers have no enzymes that break them down at a useful rate. They fragment into smaller and smaller pieces instead, and a plastic item in the sea may last for decades or centuries.

Macroplastics are large pieces: bags, bottles, fishing nets, packaging.

• Entanglement. Seals, turtles, seabirds and whales become caught in discarded fishing gear and loops of plastic, and drown or are injured as they grow.
• Ingestion. Sea turtles mistake plastic bags for jellyfish. Albatross parents pick up floating plastic and feed it to their chicks. Plastic fills the stomach, gives a false sense of fullness, and animals starve.

Microplastics are pieces smaller than 5 mm, from broken-down larger items, synthetic clothing fibres and, formerly, microbeads in cosmetics.

• They are eaten by filter feeders such as mussels and by zooplankton, and pass up food chains.
• Toxic chemicals can bind to their surfaces, so microplastics may carry pollutants into animals.
• Plastic particles in the gut can reduce feeding and growth.

Nature of science: science and public action. Scientists had described plastic pollution for decades, but public attention changed sharply when popular media showed its effects on marine life, most famously in 2017, when the BBC documentary series Blue Planet II showed albatrosses feeding plastic to their chicks and other scenes. Public concern helped drive measures such as charges on single-use bags and bans on microbeads in cosmetics. Scientists can influence the actions of citizens and governments when they provide clear, accessible information about what their research has found.

12Rewilding

Rewilding is the large-scale restoration of an ecosystem so that its natural processes run themselves again, with as little continuing human management as possible. The guide names three methods.

• Reintroduction of apex predators and other keystone species. When grey wolves were reintroduced to Yellowstone National Park in the United States in 1995, they preyed on elk and changed where elk grazed; willows and aspens recovered along some rivers, with knock-on effects for other species, including beavers. Scientists still debate how large and widespread these effects are.
• Re-establishing connectivity. Habitats broken into isolated fragments by roads and farmland are reconnected by wildlife corridors, so that animals can move, populations can interbreed and species can recolonise.
• Minimising human impact, including by ecological management. This can mean removing grazing livestock, controlling invasive species, stopping drainage, or blocking old ditches, and then leaving the land alone.

The guide's example: Hinewai Reserve, New Zealand. Hinewai is a reserve of over a thousand hectares on Banks Peninsula, on the east coast of the South Island, on land that had been cleared of native forest for sheep farming and had become covered in gorse, an invasive shrub. Since 1987 it has been managed by leaving natural regeneration to proceed with minimal interference. Grazing animals were removed, and, unusually, the gorse was not sprayed or burned. It was allowed to act as a nurse crop: native seedlings grow up through the gorse, which shelters them, and as the native trees overtop it the light-demanding gorse is shaded out and dies. Native forest has returned over much of the reserve, along with native birds and invertebrates. The management that is done is minimal: controlling feral goats and deer, and keeping out fire.

16Linking questions

What is the distinction between artificial and natural processes? Natural processes, such as succession or natural selection, proceed without human intention; artificial ones are directed by humans. Many topics on this page sit between the two: eutrophication is a natural response to nutrients, triggered by artificial fertilizer; rewilding uses human action to restore natural processes, then steps back.

Over what timescales do things change in different biological systems? An algal bloom takes days; eutrophication of a lake, years; a cod stock collapses in decades; primary succession takes centuries; stable ecosystems persist for millions of years. The mismatch between human timescales and ecosystem ones is why tipping points are dangerous: damage done in decades can take millennia to repair.