Stability is not the same as never being disturbed

Ecosystem stability is a property of an ecosystem that allows its community structure and ecosystem processes to persist through time. A stable ecosystem isn’t sealed off like a museum exhibit. Individuals are born and die, seasons pass, and small fluctuations happen. What matters is that the overall pattern remains recognisable.

Some natural ecosystems show remarkable continuity. Tropical rainforests such as the Daintree in northern Australia and lowland rainforests in Borneo have lineages and community features associated with very ancient forests. Deserts can last for immense periods too: the Namib Desert has persisted for tens of millions of years, with fog supplying enough moisture for highly specialised organisms. Isolated systems, such as deep cave ecosystems, may also remain recognisably continuous over millions of years, although they can be extremely vulnerable to disturbance.

Resistance and resilience

Resistance is a property of a community or ecosystem that reduces change when a disturbance occurs. A highly resistant ecosystem changes little when disturbed.

Resilience is a property of a community or ecosystem that allows recovery after disturbance. A highly resilient ecosystem may be knocked away from its original state but returns towards it relatively quickly.

These two properties aren’t identical. A grassland may burn easily, showing low resistance to fire, but regrow rapidly, showing high resilience. An old forest may cope well with small changes, yet if it is pushed beyond recovery it may be slow to return. So “stable” needs careful wording in biology: stability can mean staying the same, recovering after change, or both.

The guiding idea for this topic is continuity and change. Some ecosystems maintain themselves over immense timescales, while human actions can alter them within decades. Both are biological truths; the question is what mechanisms keep change within limits.

What an ecosystem needs to keep going

For an ecosystem to stay stable, four requirements are especially important.

First, it needs a continuing supply of energy. In most ecosystems, that energy is light absorbed by photosynthetic producers. Energy is not recycled; it moves through food chains and eventually dissipates as heat.

Second, nutrients have to be recycled. Nutrient cycling is the repeated transfer of chemical elements between organisms and the abiotic environment. Carbon, nitrogen, phosphorus and other elements become part of biomass, move through food webs, leave organisms in wastes and dead organic matter, and return through decomposers. If nutrients are removed faster than they are replaced, productivity falls.

Third, populations need genetic diversity. Genetic diversity is variation in alleles among individuals in a population. It gives natural selection material to act on when conditions change. A genetically uniform population is often less able to withstand disease, climate stress or other selection pressures.

Fourth, climatic variables need to stay within tolerance limits. Tolerance limits are the upper and lower values of an abiotic factor within which an organism can survive, grow and reproduce. Rainfall, temperature, humidity and seasonal patterns affect whether forest, grassland, desert or wetland communities can persist.

Stability is weakened by leaks and shocks

Ecosystems lose stability when these requirements are disrupted. Harvesting timber, fish or plant products removes biomass and nutrients. Soil erosion takes away mineral nutrients and decomposer-rich topsoil. Eutrophication changes population balance in water. Poaching, disease or selective killing can remove species that hold the community together.

Interactions matter too. Intraspecific interactions are interactions between individuals of the same species, such as competition between seedlings of one tree species for light. Interspecific interactions are interactions between individuals of different species, such as predation, mutualism or competition between plant species for mineral ions. Competition is minimized when species use different resources, occupy different microhabitats, feed at different times, or are controlled by predators before one competitor excludes the others. This helps explain why diverse ecosystems often show stronger stability: not because every species does the same job, but because roles are spread and connected.

The Amazon makes part of its own climate

A tipping point is a threshold in a system beyond which positive feedback drives a rapid shift to a different stable state. The key part is “different stable state”: once the shift has happened, removing the original cause may still not bring the old ecosystem back.

The Amazon rainforest works well as an example because a wide area of forest helps create the conditions that keep forest there. Trees take up water from the soil and release water vapour through transpiration. That water vapour in the atmosphere helps with cloud formation, cooling, air movement and rainfall. As forest area falls, transpiration falls too. Rainfall can then fall, drought risk rises, fires become more likely, and more forest is lost. That is positive feedback.

No one is certain about the minimum area of rainforest needed to keep these atmospheric processes going. This matters because the forest does not have to disappear completely before the system is at risk. If the remaining forest becomes too fragmented or too small, large regions could shift towards drier savanna-like ecosystems.

Calculating percentage change in forest area

Deforestation can be assessed by calculating percentage change from the original forest area:

percentage change = ((final amount − initial amount) ÷ initial amount) × 100%

A negative value means loss; a positive value means gain. For example, if rainforest area changes from an original area in hectares to a smaller final area in hectares, the calculation gives the percentage decrease relative to the starting area. Always divide by the initial amount, not by the final amount. That small detail is where students most often slip up.

Worked percentage change calculation for rainforest area loss.

Remote-sensing databases are useful here because they let forest cover be compared through time. The data may come from satellite imagery rather than direct field measurement, so definitions matter: “forest” may include planted stands in one database and exclude them in another.

Mesocosms are small ecosystems used as models

A model is a simplified representation of a system used to investigate or explain features of the real system. A mesocosm is a small experimental ecosystem set up to test ecological interactions under controlled conditions.

Mesocosms might be open tanks, fenced plots, sealed jars or glass vessels. In this topic, sealed glass vessels are especially useful: matter cannot enter or leave, but energy can still pass through as light and heat. They give a neat model of nutrient recycling and energy flow. Aquatic and microbial mesocosms often work better than terrestrial ones because small aquatic systems can fit producers, consumers and decomposers more evenly into the available space.

A sealed mesocosm needs autotrophs to fix carbon using light. It may also include consumers, if the design tests feeding interactions, and decomposers or detritivores to recycle nutrients from wastes and dead organic matter. Don’t pack the vessel with animals: oxygen can become limiting if respiration stays greater than photosynthesis over time. Possible variables to investigate include light intensity, temperature, pH, oxygen concentration, water level and nutrient supply.

Artificial and natural processes

An artificial process is a process directed or constrained by human design, while a natural process is a process that occurs through interactions among organisms and their environment without direct human control. A mesocosm sits between them. The container, species selection and starting conditions are artificial; the growth, feeding, decomposition and competition inside it are natural processes.

That mix is its strength, and also its limitation. Mesocosms allow replication and control, but they simplify space, migration, weather, rare disturbances and large food webs. They work best for focused questions, not for pretending that a jar is a whole rainforest.

Care and maintenance must follow IB experimental guidelines. Organisms should not be placed in conditions likely to cause suffering, and abiotic factors must stay within their tolerance limits. Planning with a teacher, using appropriate species, avoiding overcrowding and checking the mesocosm regularly are part of the science, not an optional kindness.

A small number of species can hold a community together

A keystone species is a species whose effect on community structure is disproportionately large compared with its abundance or biomass. Take it away, and the community may shift much more than its numbers would suggest.

Keystone species often keep diversity high by stopping one competitor from taking over. A predator may hold down a fast-growing prey species, which leaves space or resources for many other species. A large herbivore may make open patches that smaller plants can use. A mutualist such as a key pollinator may support many plant populations.

The danger is ecosystem collapse. Ecosystem collapse is a shift in which an ecosystem loses defining species, interactions and functions so that it no longer operates as the same type of ecosystem. Collapse doesn’t have to mean every organism dies; it means the old structure has broken down.

A classic marine example is the removal of a top predator from a rocky shore. Without that predator, a strong competitor such as a mussel can spread, crowding out algae and invertebrates and reducing community diversity. The connection with competition is the key point: the keystone predator minimizes interspecific competition by preventing one species from monopolising space.

Harvest must be slower than replacement

Sustainability is the capacity of a process to continue over time without depleting the resources or damaging the systems on which it depends. In natural ecosystems, the rule for harvesting sounds simple but is hard to apply: the harvesting rate must be lower than the replacement rate.

A renewable resource is a resource that can be replaced by natural growth or reproduction over a human-relevant timescale. Trees, fish and plant seeds can be renewable. Fossil fuels are not renewable on human timescales.

Take a long-lived rainforest tree, where people harvest nuts or fruits. Removing adults may seem sustainable for many years, since the old trees are still standing. But if nearly all seeds are taken, very few seedlings establish. A proper assessment would look at age structure: seedlings, saplings, young adults and old adults should all be present. If only old trees remain, the harvest is living on ecological savings.

For marine fish, sustainability depends on population size, breeding success and age structure. Stocks can collapse when adults are caught faster than young fish mature. Assessment can include catch per unit effort, surveys of stock biomass, age distribution, nursery protection, mesh sizes that allow juveniles to escape, closed seasons and catch quotas.

Maximum sustainable yield is often explained with a sigmoid growth curve. Populations grow fastest at intermediate population sizes, not when they are tiny or close to carrying capacity. If harvesting rises above the replacement rate, stock size falls. Future replacement then falls too, and the fishery can be pushed into collapse. That is why “more boats” can eventually mean “less fish”.

Agriculture is productive because inputs are added

Agriculture is the cultivation of plants, animals or other organisms for human use. It can give high yields, but whether it is sustainable depends on what the farm removes, what farmers add, and what damage occurs beyond the farm boundary.

Soil erosion is a major problem. Soil erosion is the removal of topsoil by wind, water or cultivation faster than it is replaced. Topsoil holds organic matter, mineral nutrients, roots, fungi, bacteria and soil animals. When that layer is lost, crop yields may depend more and more on external inputs.

Leaching reduces sustainability too. Leaching is the movement of dissolved substances through soil with draining water. Nitrate ions leach especially easily; phosphate can also enter waterways attached to soil particles or in solution. Losing nutrients lowers soil fertility and can cause eutrophication downstream.

Fertilizers and other inputs can keep yields up, but there are costs. Fertilizer is a substance added to soil or water to supply mineral nutrients for plant growth. Fertilizers require extraction, manufacture, transport and application. Irrigation, machinery, heating, animal feed, pesticides and herbicides are also inputs. Some are finite or energy-intensive.

Agrochemical pollution places another limit on sustainability. Agrochemicals are chemicals used in agriculture to increase yield or control pests, weeds and diseases. They may harm non-target species, reduce biodiversity, contaminate water or persist in food webs.

Agriculture also has a carbon footprint. A carbon footprint is the total greenhouse gas emission associated with a product, activity or process, usually expressed as carbon dioxide equivalents. Mechanized farming burns fuel; fertilizer production uses energy; livestock can release methane; drained soils and ploughed soils can lose stored carbon.

So sustainability in agriculture is not just yield per hectare. A high-yield system can still be unsustainable if it erodes soil, leaks nutrients, relies on large fossil-fuel inputs, pollutes water and reduces long-term fertility.

Fertilizer nutrients do not stay politely in fields

Eutrophication is nutrient enrichment of a water body that stimulates excessive growth of algae or photosynthetic bacteria. It often starts on land. Nitrogen and phosphate fertilizers dissolve in rainwater or irrigation water, then move by leaching and runoff into streams, lakes, estuaries and coastal waters. Manure and urine from livestock can cause the same problem when they’re stored or spread poorly.

In many aquatic systems, nitrate and phosphate are limiting nutrients, so extra nitrate or phosphate can trigger an algal bloom. Algal bloom is a rapid increase in the population of algae or photosynthetic bacteria in water. A bloom may shade submerged plants and stop photosynthesis below the surface. When the algae and shaded plants die, decomposers break down the dead organic matter.

Biochemical oxygen demand (BOD) is the amount of dissolved oxygen required by aerobic microorganisms to decompose organic matter in water. During eutrophication, BOD rises because bacteria have more dead organic matter to respire. Dissolved oxygen falls, fish and invertebrates may suffocate, and anaerobic conditions may develop.

The key chain is: fertilizer leaching → nutrient enrichment → algal bloom → light blocked → plant and algal death → bacterial decomposition → increased BOD → oxygen depletion → death of aerobic aquatic organisms. Explain that chain clearly and you’ve got the core of this statement.

Some pollutants become more concentrated up food chains

Bioaccumulation is the increase in concentration of a substance in an organism’s tissues over its lifetime because uptake exceeds elimination. It is most serious for substances that are persistent, fat-soluble or not easily excreted.

Biomagnification is the increase in concentration of a substance in the tissues of organisms at successively higher trophic levels in a food chain or food web. A predator eats many prey items, so even a low concentration of pollutant in each prey organism can build up to a higher concentration in the predator.

DDT is the classic case. DDT is a synthetic insecticide that persists in the environment and can accumulate in fatty tissues. In birds of prey, DDT and related breakdown products were linked to eggshell thinning, so fewer young survived. It’s a useful example of a pollutant damaging reproduction, rather than simply poisoning adults outright.

Mercury is another required example. Mercury is a metallic element that can be converted by microorganisms into methylmercury, a neurotoxic organic compound that accumulates in aquatic food webs. For this reason, large predatory fish can contain higher mercury concentrations than the smaller fish and invertebrates they consume.

Not every pollutant biomagnifies to the same extent. The strongest biomagnification occurs in substances that persist, enter organisms readily, are stored in tissues, and are eliminated slowly. Position in the food web matters as well: top consumers face the greatest risk.

Plastic persists because most of it is not biodegradable

Plastic is a human-made polymer material that can be moulded into products and does not decompose quickly. Many plastics remain in natural environments for a long time because they are non-biodegradable, or because they degrade only very slowly.

Macroplastic is visible plastic debris that can be seen easily, such as bags, bottles, ropes, fishing nets and packaging. In oceans, macroplastic can entangle turtles, seals, seabirds and fish. Animals may drown, lose the ability to feed, or develop wounds and infections. Plastic bags and fragments may also be swallowed when animals mistake them for prey.

Microplastic is plastic debris less than 5 mm in diameter. Some is manufactured at that small size; some forms when larger plastic breaks into fragments through sunlight, wave action and abrasion. Plankton, filter feeders, fish, seabirds and marine mammals can ingest microplastics. These particles can block or irritate digestive systems, reduce feeding efficiency, and carry toxic substances on or within their surfaces.

The scientific story of plastic pollution is also a nature of science story. When scientists communicate evidence clearly, they can influence citizen behaviour and policy. Early reports of floating plastic fragments did not immediately change how the public behaved. Later media coverage of plastic-filled birds, entangled animals and ocean gyres made the problem visible, and easier to understand emotionally. Public perception shifted. That helped drive measures such as restrictions on single-use plastics, beach clean-ups and improved waste management.

There is a caution here. Simple phrases such as “garbage patch” help people notice the issue, but they can make plastic pollution sound like a floating island in one place. The harder truth is wider: microplastics are dispersed through marine ecosystems, including water, sediment and animal tissues.

Rewilding restores processes, not just scenery

Rewilding is an ecological restoration approach that aims to restore self-sustaining natural processes with minimal ongoing human control. It isn’t just planting a few trees and leaving the site alone. The aim is a working ecosystem, with interactions such as predation, grazing, seed dispersal, decomposition, succession and nutrient cycling.

One method is to reintroduce apex predators and other keystone species. An apex predator is a consumer at the top of a food web that has no regular natural predator as an adult. If wolves, big cats or other apex predators return, they can change herbivore behaviour and abundance, which can then allow vegetation and associated species to recover.

Connectivity matters too. Habitat connectivity is the degree to which organisms can move between suitable habitat patches. Corridors and large connected areas reduce isolation, allow gene flow and help populations recolonise after local loss.

A third method is to lower human impact through ecological management. This might mean stopping logging or intensive grazing, removing invasive species, controlling fires where appropriate, reducing pollution and allowing natural regeneration. In some places, people need to intervene at the start so that later intervention can decrease.

Hinewai Reserve in New Zealand is a useful example. Formerly degraded farmland on Banks Peninsula has been allowed to regenerate into native forest and shrubland. Management has reduced damaging human activities, controlled invasive mammals and allowed native vegetation to recolonise. The reserve shows that restoration is not always tidy or quick; scrub and early regrowth may look messy, but they can be the nursery stage for forest recovery.

Rewilding also links back to artificial and natural processes. Humans may artificially set boundaries, remove pressures and reintroduce species, but the intended recovery depends on natural ecological interactions taking over.

Succession is directional community change

Ecological succession is the sequence of changes in community composition and abiotic conditions in an ecosystem over time. It isn’t just a list of species arriving. Organisms change the environment, and that changed environment then favours different organisms.

Abiotic change can start succession. Glacier retreat can expose bare rock or mineral deposits. A landslide, flood, volcanic eruption, fire or drying of a lake can reset conditions. Changes in light, temperature, water availability, soil depth and mineral content then affect which species can survive.

Biotic change can start it too. A new herbivore, pathogen, invasive plant, burrowing animal or dominant tree species can change competition, grazing pressure, soil structure, shade or nutrient cycling. Once the biotic community changes, the abiotic environment often changes with it.

For example, young trees growing in open ground reduce light at soil level, increase humidity, alter soil temperature, add leaf litter and change infiltration. Shade-tolerant plants may then replace sun-loving plants. This is succession as feedback: species affect abiotic variables, and abiotic variables affect species distribution.

The timescale varies enormously. Microbial communities can change within days. Ponds may infill over decades or centuries. Forest succession after glacier retreat may take hundreds of years. Ancient stable ecosystems may show continuity over millions of years while still having small-scale turnover within them.

Primary succession starts where there is no developed soil

Primary succession is ecological succession that begins on a newly exposed or newly formed substrate where soil is absent or extremely undeveloped. On land, this can happen on ground exposed by retreating glaciers, on lava flows, or on bare rock surfaces.

The first colonisers are often bacteria, cyanobacteria, lichens and mosses. They can cope with low nutrient availability, exposure and shallow anchorage. As these organisms grow and die, organic matter starts to build up. Weathering and biological activity slowly form a thin soil.

With time, larger plants can establish. Herbs, grasses and shrubs may appear, followed eventually by trees where the climate allows. Plant size, primary production, species diversity, food-web complexity and nutrient cycling generally increase. These are trends, not a magic staircase; local climate, disturbance and dispersal all influence the path.

Succession after glacier retreat is a good terrestrial example. In a newly deglaciated area, pioneer lichens and mosses are usually the first to colonise. Later, nitrogen-fixing shrubs or small plants may enrich the developing soil. Deciduous shrubs and trees follow, and under suitable conditions a coniferous or mixed forest may eventually develop.

Through this sequence, primary production rises because more leaf area captures light. Species diversity increases as more habitats and resources become available. Food webs become more complex because there are more producers, herbivores, predators, detritivores and decomposers. Nutrient cycling also increases, since more biomass is produced and more dead organic matter enters the soil.

Not every ecosystem moves towards one permanent endpoint

Cyclical succession means repeated succession: a community moves through a recurring sequence of states instead of settling into one unchanging climax community. Repeated is the key word here. The same broad stages come round again because of biological interactions or recurring disturbance.

One example occurs on rocky shores in parts of New Zealand. Bare rock may first be colonised by barnacles and crustose algae. Mussels then settle, forming dense mats over the earlier colonisers. Later, waves remove the mussels, or the mussels detach, exposing bare rock again. The cycle restarts.

A similar pattern can occur in heathland. Heather may dominate for a time, then become old and less vigorous. Lichens or other plants invade gaps, bare patches develop, and heather later recolonises. Burning, grazing or local dieback can affect the timing, but the main idea is that the ecosystem contains a cycle of communities.

This changes the older idea that succession always moves towards one final climax. Some ecosystems stay stable because the cycle itself persists. The species present at any one point in the cycle change, but the repeated pattern can be long-term and predictable.