D4.2 Stability and change

Ecosystems can appear unchanging for centuries — the same forest, the same coral reef — yet they are dynamic systems held in balance by countless interactions. The theme of D4.2 is the tension between stability and change: what allows a community to absorb disturbance and recover, and what pushes it past a point of no return. Understanding this matters more than ever, because human activity is now testing the limits of many ecosystems at once. The key insight is that stability is not the absence of change but the capacity to keep functioning despite it.

What makes an ecosystem stable

A stable ecosystem maintains its structure and function over time, recovering from disturbances rather than collapsing. The syllabus identifies several requirements for long-term stability:

• a steady supply of energy, ultimately from the Sun for most ecosystems;
• recycling of nutrients within the ecosystem, so that elements such as carbon and nitrogen are reused;
• genetic diversity within species, allowing populations to adapt to change;
• climatic conditions remaining within tolerable limits.

In general, ecosystems with greater biodiversity tend to be more stable, because complex food webs provide alternative pathways: if one species declines, others can take over its role, buffering the whole system. A simple ecosystem with few species offers no such backup, so the loss of a single species can cause cascading effects.

Keystone species and tipping points

Not all species matter equally to stability. A keystone species has a disproportionately large effect on its community relative to its abundance; removing it causes the structure of the ecosystem to change dramatically. The classic example is the sea otter: by eating sea urchins, otters prevent urchins from overgrazing kelp forests. Remove the otters and urchin numbers explode, the kelp is destroyed, and the many species that depend on the kelp forest disappear.

Disturbance does not always lead to gradual change. Many ecosystems can absorb stress up to a tipping point, beyond which they shift abruptly and often irreversibly into a different stable state. A coral reef may persist under rising temperatures until repeated bleaching pushes it past a threshold, after which it collapses into an algae-dominated state that does not readily recover. Recognising that change can be sudden and non-reversible — not always smooth — is a central D4.2 idea.

Human impact and loss of stability

Human activities repeatedly reduce the diversity and stability of ecosystems. The syllabus expects familiarity with examples such as:

• Deforestation, which removes habitat, breaks food webs and releases stored carbon;
• Introduction of invasive species, which lack natural predators and can outcompete or prey on native species;
• Overharvesting, such as overfishing, which removes species faster than they can reproduce;
• Pollution and eutrophication, where excess nutrients trigger algal blooms that deplete oxygen and kill aquatic life.

A useful case study is the mesocosm: a small, sealed model ecosystem used to investigate the conditions needed for sustainability and stability. Setting one up shows that even a simple closed system must balance producers, consumers and decomposers, with light and nutrient recycling, to persist — a microcosm of the principles that govern real ecosystems.

Conservation: protecting stability

Because stability depends on diversity, conserving biodiversity is the practical conclusion of D4.2. Strategies include protecting habitats through reserves and national parks, restoring degraded ecosystems, controlling invasive species, and managing harvesting sustainably so populations are not driven below recovery thresholds.

A key principle is the precautionary approach: where an action might push an ecosystem past a tipping point, it is wiser to avoid the risk than to wait for proof of harm, because some changes cannot be undone. Protecting keystone species and maintaining genetic diversity give ecosystems the resilience to absorb the disturbances that will inevitably come.