Stability and change

📚The biology

Stability in natural ecosystems

Stability is the ability of an ecosystem to persist, maintaining its structure and functioning over long periods and resisting or recovering from disturbance. Evidence shows that some ecosystems have shown continuity over immense periods:

• the tropical rainforests of Borneo and the Daintree in Australia are thought to have existed in some form for tens of millions of years;
• the Namib Desert in southern Africa is thought to have been arid for tens of millions of years, and has many species adapted to it found nowhere else.

Requirements for stability

The Amazon: a possible tipping point

A tipping point is a threshold beyond which a system changes rapidly and possibly irreversibly into a different state.

The Amazon rainforest partly makes its own rain. Trees draw water from the soil and release huge quantities of water vapour by transpiration. This vapour cools the surface, drives air flows that carry moisture westwards across the basin, and condenses to fall as rainfall further inland, where it is transpired again. This recycling depends on a large area of continuous forest.

If deforestation removes enough forest, less water vapour is produced, rainfall decreases, the dry season lengthens, and the remaining forest becomes drier and more prone to fire. Beyond some threshold, large areas could change irreversibly from rainforest to savanna. There is uncertainty over where that threshold lies: estimates of the minimum area of forest needed to maintain the rainfall cycle vary, with some scientists suggesting that loss of 20–25% of the original forest could be enough, especially combined with climate change.

The extent of deforestation is assessed by percentage change from the original area:

Mesocosms

A mesocosm is a small experimental ecosystem used as a model to investigate how variables affect stability — for example the effect of light, temperature, nutrient addition or the removal of a species. Sealed glass vessels are preferable to open tanks, because entry and exit of matter are prevented while energy (light and heat) can still pass in and out, so the system is close to a closed system. Aquatic or microbial ecosystems — pond water with algae, small invertebrates and plants — are more likely to succeed than terrestrial ones. A typical set-up has replicate vessels for each condition, monitored for dissolved oxygen, pH and numbers of organisms over weeks. Living organisms in mesocosms must be cared for and maintained in line with IB experimental guidelines.

Keystone species

A keystone species has a disproportionately large impact on the structure of its community relative to its abundance. Removing it can cause dramatic changes and even ecosystem collapse.

• Sea otters in kelp forests eat sea urchins. Where otters were hunted out, urchin numbers exploded and grazed the kelp away, turning rich forests into bare “urchin barrens”.
• Grey wolves in Yellowstone reduce elk browsing; after their reintroduction in 1995, willows and aspens recovered along rivers, with knock-on effects on beavers and birds.
• Fig trees in tropical forests fruit year-round, feeding many animals during times when little else is available.

Sustainable harvesting

Harvesting a renewable resource is sustainable only if the rate of harvesting is lower than the rate of replacement (by growth and reproduction). Otherwise the population declines.

Sustainability of agriculture

Agriculture is sustainable if it can continue indefinitely without degrading the land or the wider environment. Factors to consider:

Eutrophication

Eutrophication is the enrichment of water with nutrients, causing excessive growth of algae. When nitrogen and phosphate fertilizers are leached from farmland into rivers, lakes and coastal seas:

1. Algae and other producers grow rapidly (an algal bloom), turning the water green.
2. The bloom blocks light from reaching plants below, which die.
3. When the algae themselves die, decomposers (aerobic bacteria) multiply and break down the dead organic matter, using large amounts of oxygen.
4. This greatly increases the biochemical oxygen demand (BOD) — the amount of oxygen used by microorganisms to decompose organic matter in the water.
5. Dissolved oxygen falls so low that fish and other aerobic animals die, creating “dead zones”.

Biomagnification

Some pollutants are not broken down or excreted, and accumulate in the fatty tissues of organisms. At each trophic level, consumers eat many organisms from the level below, retaining the pollutant from all of them. The concentration therefore increases at each successive trophic level: biomagnification. Top predators receive the highest doses.

Plastic pollution in the oceans

Plastics are persistent in the environment because they are not biodegradable: decomposers lack enzymes that break down their polymers. They break up physically into smaller and smaller pieces but remain as plastic for hundreds of years.

Scientists can influence the actions of citizens when they provide clear information about their findings. Popular media coverage of plastic pollution — especially images of affected marine animals in nature documentaries — changed public perception globally and drove measures such as bans on single-use plastic bags and microbeads.

Rewilding

Rewilding is the restoration of natural processes in ecosystems, allowing them to become largely self-sustaining. Methods include:

• reintroducing apex predators and other keystone species (wolves in Yellowstone, beavers in parts of Europe);
• re-establishing connectivity between fragments of habitat over large areas, with corridors so populations can move and interbreed;
• minimizing human impact, including by ecological management such as removing invasive species or livestock, then stepping back.

Hinewai Reserve on Banks Peninsula, New Zealand, is a well-known example. From 1987, about 1500 hectares of former farmland, much of it covered in gorse (an invasive shrub regarded as a weed), were managed with minimal intervention: grazing animals were removed, and the gorse was left in place rather than cleared or sprayed. The gorse acted as a nurse crop, sheltering native seedlings, which grew up through it and eventually shaded it out. Native forest has regenerated over most of the reserve, and native birds and insects have returned.

Ecological succession AHL

Ecological succession is the gradual, directional change in the species composition of a community over time. It can be triggered by changes in the abiotic environment (for example new land exposed by a retreating glacier, a volcanic eruption or a landslide) and by biotic factors (the organisms themselves change conditions, making the environment more suitable for new species and less suitable for existing ones).

Primary succession AHL

Primary succession begins on bare ground with no soil, such as newly cooled lava or rock exposed by a glacier. A terrestrial example:

1. Pioneer species such as lichens and mosses colonize bare rock. They tolerate harsh conditions and begin to break down rock, and when they die they add organic matter.
2. A thin soil forms, allowing grasses and small herbaceous plants to grow.
3. As soil deepens and holds more water and nutrients, shrubs establish, shading out the smaller plants.
4. Eventually trees grow and form a woodland.

General principles, which you should be able to illustrate with any suitable example, are that during primary succession there are increases in:

Cyclical succession AHL

In some ecosystems there is not a single, unchanging endpoint, but a cycle of communities that repeats. On heathland in northern Europe, for example, heather plants go through a cycle: young plants grow into dense, mature bushes; as they age, the centres die and open up; lichens and grasses colonize the gaps; then heather seedlings establish again and the cycle restarts. Kelp forests and some grasslands maintained by periodic fire show similar patterns.

Climax communities and arrested succession AHL

For any specific set of environmental conditions, succession tends to lead to a particular type of climax community, which is relatively stable and changes little until disturbed — in much of the lowland tropics, rainforest; in temperate lowland Europe, deciduous woodland.

Human influences can prevent the climax community from developing: arrested succession.

• Grazing by farm livestock eats tree and shrub seedlings, so pasture remains as grassland instead of becoming woodland.
• Drainage of wetlands changes the abiotic conditions so that the natural wetland community, and its succession, cannot proceed; the land is held at an earlier stage or turned to farmland.