D4.2 Stability and change

Ecosystem stability is the capacity to maintain characteristic structure and function over time or recover after disturbance.

Resistance limits the immediate effect of disturbance, while resilience is the capacity to recover. Evidence from forests, deserts and other natural ecosystems shows that some recognizable systems have persisted for millions of years.

A forest can undergo seasonal population changes and recover from storms while retaining its nutrient cycling, food-web structure and dominant vegetation over long periods.

Stability means continuity of key properties, not a frozen species count or absence of all change.

Long-term ecosystem stability requires continuing energy supply, nutrient recycling, genetic diversity and climatic variables within organismal tolerance limits.

If prolonged drought pushes precipitation outside tree tolerances, producer biomass falls and both energy input and habitat complexity decline.

The requirements interact; meeting one cannot compensate indefinitely for failure of another.

A large Amazon forest area is needed to recycle atmospheric water by transpiration, causing cooling, air movement and rainfall that help maintain the forest.

Deforestation lowers transpiration and rainfall, increases drying and fire risk, and fragments habitat; further forest loss can then reinforce the original change. The minimum area needed to maintain these processes remains uncertain.

The mapped local textbook reports 3,399,308 km² in 2017 and 3,390,835 km² in 2018: ((3,390,835 − 3,399,308) ÷ 3,399,308) × 100 = −0.25%, so estimated cover fell by 0.25%.

A proposed tipping range is uncertain, so a calculated percentage loss must not be presented as proof that an irreversible threshold has already been crossed.

A mesocosm is a contained ecosystem model used to test how a controlled variable affects stability.

Replicated sealed aquatic mesocosms can receive different light treatments while temperature, starting organisms and nutrient quantities are held constant.

A mesocosm supports causal inference about its model conditions but does not reproduce every migration, weather event or interaction in a natural ecosystem; it also requires ethical care and maintenance.

A keystone species has an effect on community structure much larger than its abundance would suggest.

Its predation, grazing, habitat engineering or other interaction controls competitors or resources. Removing it can trigger a trophic cascade and reduce diversity.

Predict the community change after removal by identifying the interaction the species controls.; separate state, pressure, control loop and time

Removing sea otters can allow sea urchins to increase and overgraze kelp forests, changing habitat for many species. This gives a concrete prediction from the stated ecosystem.

Keystone status is context-dependent; abundance alone does not identify a keystone species. Interpret the result within the stated model and evidence limits.

A renewable-resource harvest is sustainable only when long-term removal remains below replacement and leaves a viable reproducing population.

Replacement rates vary with age structure, habitat and climate, so monitoring must update quotas or harvest methods rather than treating one limit as permanent.

A renewable species is not automatically harvested sustainably; incomplete stock data and illegal or unreported removal increase uncertainty.

Sustainable agriculture maintains food production without reducing the soil, water, biodiversity and climate conditions needed by future production.

Crop rotation, soil cover and nutrient matching may reduce erosion, fertilizer demand and leaching, but yield and labour trade-offs must still be measured.

No single practice proves a farm sustainable; assessment must include outputs, inputs, pollution and long-term soil condition.

Eutrophication occurs when leached nitrogen and phosphate fertilizers enrich aquatic or marine water and stimulate excessive primary production.

Nitrate/phosphate leaching → algal or plant growth → shading and biomass death → decomposer respiration rises → biochemical oxygen demand (BOD) rises → dissolved oxygen falls → hypoxia and organism death.

After fertilizer runoff causes a bloom, bacteria decomposing dead algae consume oxygen; fish may die when oxygen demand exceeds reaeration and photosynthetic supply.

BOD measures oxygen demanded by biological decomposition; it is not the same as dissolved oxygen, and a high BOD predicts stronger oxygen depletion.

Biomagnification is increasing tissue concentration of a persistent pollutant in consumers at successively higher trophic levels.

DDT and mercury are retained or eliminated slowly. Predators consume many contaminated prey, so their total intake produces a higher tissue concentration than in organisms below them.

Mercury can be low in water or plankton, higher in fish and highest in fish-eating birds or mammals; DDT similarly reached damaging concentrations in top predators.

Bioaccumulation is increase within one organism over time; biomagnification is increase between trophic levels. Not every pollutant does either.

Ocean plastics persist because they are non-biodegradable; large macroplastics and small microplastics expose organisms in different ways.

Weathering fragments plastic but does not mineralize it, while rivers, wind, fishing and currents continually redistribute material.

Clear scientific communication and popular-media coverage changed public perception and helped drive measures to reduce plastic pollution.

Detection alone does not quantify biological effect; particle size, polymer, dose and exposure duration must be evaluated.

Rewilding restores self-sustaining ecosystem processes by reconnecting habitats, reintroducing apex predators or other keystone species, and minimizing human impact through ecological management.

Large connected areas allow movement and gene flow; keystone interactions can restore food-web regulation; reducing intensive intervention lets succession and natural disturbance rebuild habitat complexity.

At Hinewai Reserve in New Zealand, management supports natural regeneration of native forest, removes alien trees and vines, and otherwise uses minimal intervention so endemic flora and fauna can re-establish.

Rewilding is not simply abandoning land. Connectivity, invasive-species control, community effects and monitoring determine whether natural processes can recover safely.

Ecological succession is an orderly change in community composition through time, triggered by changes in abiotic conditions, biotic interactions, or both.

Pioneer organisms modify light, soil, nutrients and moisture; competition, facilitation, herbivory and dispersal then change which species can establish next.

In an abandoned quarry, lichens and mosses alter bare substrate and add organic matter, allowing grasses, shrubs and later woodland species to colonize.

Succession has a directional mechanism but not one universal fixed sequence; climate, starting conditions and disturbance can redirect it.

Primary succession begins on a surface without developed soil, so pioneer organisms must help create the conditions needed by later communities.

On fresh volcanic rock, lichens and mosses weather substrate and add organic matter; deeper soil later supports herbs, shrubs and trees.

If soil and a biological legacy remain after disturbance, recovery is secondary rather than primary succession.

Cyclical succession occurs when recurring disturbance repeatedly shifts a community through a predictable sequence without a single permanent endpoint.

Fire, flooding, grazing or seasonal conditions reset some species while survivors and propagules restart the pathway. Frequency and intensity set the cycle.

Compare disturbance interval with species life histories before predicting the recurring community.; separate state, pressure, control loop and time

A grassland burned every few years returns to fire-tolerant herbs rather than developing into forest. This gives a concrete prediction from the stated ecosystem.

A cycle is not random change; it requires a recurring driver and repeatable response. Interpret the result within the stated model and evidence limits.

Under specified environmental conditions, succession tends toward a relatively stable climax community; human activity can arrest it at an earlier or alternative state.

Removing the pressure may allow succession to resume if propagules, soil and hydrology remain recoverable; persistent change can instead hold an alternative stable state.

A climax is conditional on climate, soil and disturbance regime, not a universal permanent endpoint.