D4.2 Stability and change - UWCSEA - IB Biology 2025

D4.2.1—What is stability as a property of natural ecosystems?

Ecosystem stability refers to the ability of an ecosystem to maintain its structure and function;

over long periods of time;

Forests, deserts, and other ecosystems have shown continuity for millions of years;

providing evidence of their inherent stability.

D4.2.2—What are the requirements for stability in ecosystems?

Stability in ecosystems depends on several key factors:

A consistent supply of energy;

typically from the sun, which supports primary production;

Recycling of nutrients through processes like decomposition and nutrient cycling;

Genetic diversity within species, enhancing resilience to changes and disturbances;

Climatic variables like temperature, water levels and pH remaining within the tolerance levels of the species present.

D4.2.3—How does deforestation of the Amazon rainforest illustrate a possible tipping point in ecosystem stability?

The Amazon rainforest requires a large area to generate atmospheric water vapour;

through transpiration;

This process leads to cooling, air flows, and rainfall patterns essential for the ecosystem;

Deforestation reduces the forest area, risking disruption of these processes;

There is uncertainty about the minimum area needed to maintain them;

and continued deforestation could push the ecosystem past a tipping point from which it cannot recover;

Calculation of percentage change in forest area can be used to assess the impact

D4.2.4—How can a model be used to investigate the effect of variables on ecosystem stability?

Models like mesocosms can simulate ecosystems to study how different variables affect stability;

Sealed glass vessels are preferable because they prevent matter exchange;

while allowing energy transfer;

Aquatic or microbial ecosystems are often more successful in mesocosms than terrestrial ones;

benefits are they can be controlled; focussing on particular variables and allowing precise changes;

disadvantage is they do not replicate the complex interactions in real-life ecosystems;

D4.2.5—What is the role of keystone species in the stability of ecosystems?

Keystone species have a disproportionate (very large) impact on community structure;

and ecosystem function;

Their presence maintains biodiversity and ecological balance;

Removing a keystone species can lead to significant changes or collapse of the ecosystem;

e.g. removal of starfish as predators can result in collapse of biodiversity;

highlighting their critical role in maintaining stability;

D4.2.6—How is the sustainability of resource harvesting from natural ecosystems assessed?

Sustainability depends on harvesting rates being lower than the rates of natural replacement;

For example:

Terrestrial plant species like bamboo can be sustainably harvested if cut at a rate that allows regrowth;

Marine fish species like sardines must be fished at levels that do not deplete their populations;

D4.2.7—What factors affect the sustainability of agriculture?

Agricultural sustainability is impacted by:

Soil erosion:

Resulting from monocropping, and loss of vegetation, leading to soil degradation;

Use of agrochemicals:

which are synthetic fertilizers and pesticides; causing soil acidification and reduced soil organic matter;

water scarcity (lack of) due to excessive water use and pollution from water runoff;

Loss of biodiversity due to growing one crop reduces genetic diversity, increasing vulnerability to pests, diseases, and climate change;

Carbon footprint: Emissions of CO₂, CH₄, and N₂O from agricultural activities contribute to climate change, which adversely affects agriculture.

D4.2.8—What is eutrophication? How does it occur? Where?

Eutrophication occurs when excess nitrogen and phosphate fertilizers;

leach (run off) into water bodies;

stimulating overgrowth of algae;

This increases biochemical oxygen demand (BOD);

as decomposers break down dead algae;

leading to very low oxygen and killing aquatic species;

D4.2.9—What is biomagnification?Explain the process What are the effects of it?

Biomagnification is the increase in concentration of toxins in higher trophic levels of a food chain;

Pollutants like DDT and mercury are persistent and not easily broken down;

They accumulate in the tissues of organisms (bioaccumulation);

When predators eat contaminated prey, the toxins concentrate further;

DDT caused thinning of eggshells in birds of prey like eagles and falcons;

Mercury accumulates in fish, especially large predators like tuna;

Humans consuming these fish can suffer mercury poisoning.

D4.2.10—Effects of microplastic and macroplastic pollution of the oceans

Plastics are persistent in the natural environment due to their non-biodegradable nature;

Microplastics (small plastic particles) and macroplastics (larger plastic debris) pose significant threats to marine life;

For example, sea creatures such as turtles and seabirds often mistake plastic for food, leading to ingestion that causes blockages, starvation, or death;

Larger plastic debris can also cause entanglement of marine animals, impairing their mobility and leading to injury or death. Additionally, plastics can absorb and concentrate toxic chemicals;

which can then enter the marine food web and harm species at all levels.

D4.2.11—How can rewilding and other techniques be used to restore natural habitats

Rewilding aims to restore ecosystems to their natural state by reintroducing apex predators and keystone species;

Another key method is the re-establishment of habitat connectivity over large areas, allowing species to migrate freely and maintain healthy populations;

ecological management, which may include restricting land use, reducing pollution, and allowing natural processes to occur without interference;

An important example of rewilding is the Hinewai Reserve in New Zealand, where reforestation efforts and the reintroduction of native species have led to the recovery of biodiversity and natural processes.

D4.2.12—Ecological succession and its causes

Ecological succession refers to the gradual process by which ecosystems change and develop over time;

It can be triggered by changes in both abiotic factors (non-living elements, such as climate, soil, or water availability);

and biotic factors (living organisms, such as species interactions, competition, or the introduction of new species).

D4.2.13—Changes occurring during primary succession

During primary succession, an ecosystem gradually develops from bare rock or soil to a fully functioning community;

this process includes an increase in plant size;

Primary production also increases as more plant life establishes itself;

Over time, species diversity increases as different plants and animals colonise the area. The complexity of food webs grows through predator-prey relationships, competition, and mutualism;

nutrient cycling (the movement of nutrients such as nitrogen and phosphorus through the ecosystem) increases as more organisms contribute to processes such as decomposition.

D4.2.14—Cyclical succession in ecosystems

Cyclical succession is where different communities replace one another over time. For example, in a temperate grassland, cycles of fire or grazing may lead to periodic shifts between grasses and shrubs, preventing a permanent climax community from forming.;

These ecosystems experience a cycle of communities that is maintained by recurring disturbances, rather than progressing towards an unchanging climax state;

Another example of cyclical succession can be seen in savannah ecosystems, where fire cycles prevent tree dominance and maintain a balance between grasses and woody vegetation.

D4.2.15—Climax communities and arrested succession

In most ecosystems, ecological succession tends to lead to a stable, long-term climax community;

which is the final stage of succession and reflects the environmental conditions of the area;

However, human activities can interfere with this process and arrest succession, preventing the ecosystem from reaching its natural climax state;

Examples of this include grazing by farm livestock, which can stop the growth of shrubs and trees by feeding on young plants, and the drainage of wetlands, which can alter water levels and destroy the conditions necessary for the climax community to develop.