D4.2 Stability and change

A stable ecosystem keeps the same broad structure, the same main species and the same processes over long periods. It may fluctuate from year to year, but it resists change or returns to its former state after a disturbance. Ecosystems have the potential to be sustainable: to continue indefinitely.

Evidence that ecosystems can last for millions of years

• The Daintree rainforest in north-east Queensland, Australia, is part of a rainforest that has persisted in the region, at least in refuges, through tens of millions of years of climate change. It contains many plant lineages that are very old, including some of the most ancient families of flowering plants, which survive almost nowhere else.
• The Namib Desert in south-west Africa has been arid or semi-arid for tens of millions of years, which is why so many of its organisms show extreme adaptations to drought, such as beetles that collect fog.

Evidence for long-term continuity comes from fossils, pollen preserved in sediments (which records the plants growing nearby layer by layer) and from endemic species whose lineages could only have survived if their habitat did.

Requirements for stability

High biodiversity also helps: if one species declines, others can take over its role in the food web.

A tipping point is a threshold beyond which a small extra change triggers positive feedback, so the ecosystem shifts quickly to a different state that may be very hard to reverse.

The forest makes its own rain

The Amazon rainforest is so large that it shapes its own climate:

1. Trees take up water from the soil and transpire huge amounts of water vapour.
2. Evaporation from leaves cools the forest, and the vapour rises and condenses into clouds. The resulting air flows help to draw moist air inland from the Atlantic Ocean (exactly how strongly is still debated).
3. The vapour falls as rain further inland, where more trees take it up and transpire it again. Much of the rain in the western Amazon has been recycled through the forest several times.

Deforestation and positive feedback

Clearing forest for cattle ranching, soya farming and logging means less transpiration, so less rainfall, longer dry seasons and higher temperatures downwind. Drought makes trees die and lets fires spread, which kills more trees and reduces transpiration further. Past a tipping point the remaining forest could not sustain itself and might turn into degraded, savanna-like vegetation.

Nobody knows the minimum area of forest needed to keep the cycle going. Some scientists have suggested a tipping point at around 20–25% loss of the original forest; the Amazon has already lost about 17%. The uncertainty is a strong argument for caution.

Calculating percentage change

percentage change = (final value − initial value) ÷ initial value × 100

Worked example: in the Brazilian Amazon, about 27,800 km² of forest was cleared in 2004 and about 4,600 km² in 2012. Change = (4,600 − 27,800) ÷ 27,800 × 100 = −83%, an 83% fall in the annual rate of clearance. In 2021 about 13,000 km² was cleared: (13,000 − 4,600) ÷ 4,600 × 100 = +183% compared with 2012.

Whole ecosystems are too big and complex for controlled experiments, so ecologists use mesocosms: small, enclosed experimental ecosystems that can be replicated and manipulated. They are models — simpler than nature, but useful for testing how one variable affects stability.

• Open tanks or fenced enclosures are easy to set up, but matter (gases, water, organisms) can enter and leave.
• Sealed glass vessels are better for studying stability, because they are closed systems: no matter enters or leaves, so nutrients must be recycled inside, but light energy enters through the glass and heat leaves.
• Aquatic or microbial mesocosms work better than terrestrial ones: they are easier to seal, keep moist and sample.

Setting up a mesocosm investigation

1. Fill several identical clear jars with pond water, a layer of pond mud (containing decomposers), aquatic plants or algae (producers) and, if wanted, a few small invertebrates such as snails (consumers). Leave an air space and seal each jar.
2. Choose one independent variable, e.g. light intensity, temperature, or nitrate added.
3. Set up at least three replicates at each level, plus a control.
4. Keep other variables constant: volumes of water and mud, number of each organism, temperature (avoid direct sunlight, which overheats sealed jars).
5. Measure dependent variables over weeks: dissolved oxygen (probe), pH (rises as CO₂ is removed by photosynthesis), turbidity or colour (algal growth), and survival and growth of organisms.

A keystone species has an effect on the structure of its community that is disproportionate to its abundance. Like the keystone at the top of an arch, removing it can make the whole structure collapse. Keystone species are often predators that control herbivores (top-down control, see C4.1 Populations and communities), but they can also be ecosystem engineers such as beavers, or species that many others depend on.

Two classic examples

• The sea star Pisaster ochraceus on rocky shores of the Pacific coast of North America. In the 1960s Robert Paine removed it from experimental areas of shore. Without the sea star, mussels (Mytilus californianus) took over the rock, crowding out most other species of seaweeds and invertebrates, and the number of species fell sharply. Where Pisaster remained, the community stayed diverse.
• Sea otters in kelp forests along the same coast. Otters eat sea urchins. Where otters were hunted almost to extinction for their fur, urchin numbers exploded and the urchins grazed the kelp down to bare rock — an urchin barren — and the fish and invertebrates that live in kelp declined. Where otters returned, the urchins were kept scarce and the kelp forests regrew.

The model shows the difference: removing crabs, a common prey of otters, reduces otter numbers, but urchins stay scarce, so the kelp forest survives. Removing the otters changes every population. This is why losing a keystone species risks ecosystem collapse, and why conservation often targets keystone species (see A4.2 Conservation of biodiversity).

Wild plants and animals are renewable resources, but only if they are harvested sustainably: the rate of harvesting must not be greater than the rate of replacement by reproduction and growth.

Maximum sustainable yield

A population growing logistically (see C4.1 Populations and communities) adds the most new biomass per year when it is at half its carrying capacity (K/2): there are enough breeding adults, and competition for resources is not yet intense. At K, births and natural deaths balance and there is nothing spare. The largest catch that can be taken every year for ever is the maximum sustainable yield (MSY); in the logistic model MSY = rK/4, taken at N = K/2.

A marine fish: Atlantic cod (Gadus morhua)

• Grand Banks, off Newfoundland: cod were caught for centuries, but industrial trawlers in the 1960s–1980s took far more than the replacement rate. Catches were based on over-optimistic estimates of the stock. The stock collapsed, and Canada stopped commercial fishing in 1992. Decades later it has still not fully recovered.
• North Sea: after cod numbers fell sharply, measures were agreed internationally: catch quotas based on surveys of the numbers and ages of fish, larger net mesh sizes so young fish escape, closed areas to protect spawning and nursery grounds, and fewer fishing boats. The stock then increased, though it has fluctuated since.

Sustainability of a fishery is assessed by estimating stock size and age structure (survey catches, catch per unit effort, counts of young fish) and comparing the catch with the estimated rate of replacement.

A terrestrial plant: the Brazil nut (Bertholletia excelsa)

Brazil nuts are gathered from wild trees in the Amazon rainforest. The trees live for centuries, and seeds are spread and buried by agoutis. Harvesting is sustainable if enough nuts are left for new trees to establish; surveys of the age structure of the trees show whether young trees are replacing old ones. In heavily harvested areas there are few young trees, so harvesting is not sustainable in the long term. Logging of the forest also reduces the yield, and trees need their natural pollinators (large bees), so they crop poorly in plantations.

Most human food now comes from agriculture. Intensive farming produces high yields, but several factors determine whether it can continue indefinitely:

The carbon footprint is the total greenhouse gas emissions caused by an activity or product, expressed as CO₂ equivalent. Food production is a major source of greenhouse gases, contributing to climate change (see D4.3 Climate change).

Leaching is the loss of soluble ions such as nitrate (NO₃⁻) and phosphate (PO₄³⁻) from soil, carried away by water draining through it. Fertilizer that crops do not absorb, and manure or slurry that washes off fields, ends up in streams, rivers, lakes and coastal seas. Enrichment of water with nutrients is eutrophication.

The sequence

1. Nitrate and phosphate had been limiting factors for algae; with more of them, algae and cyanobacteria grow explosively: an algal bloom.
2. The bloom blocks light, so submerged plants die. Bloom organisms are short-lived and also die.
3. Saprotrophic bacteria decompose the dead organic matter. They multiply and respire aerobically, using up dissolved oxygen: the biochemical oxygen demand (BOD) rises.
4. Dissolved oxygen falls (hypoxia). Fish and other aerobic animals die, adding more dead matter, so the process feeds on itself.

BOD is the amount of dissolved oxygen that microorganisms use to break down the organic matter in a water sample, usually measured over 5 days at 20 °C in the dark. Clean water has a low BOD; water with sewage or dead algae has a high BOD.

Freshwater growth is usually limited by phosphate and coastal growth by nitrogen, so both cause problems. In the sea, nutrient run-off creates seasonal dead zones of hypoxic water, for example in the Gulf of Mexico at the mouth of the Mississippi. Some blooms of cyanobacteria also release toxins that harm animals and people.

Some pollutants become more concentrated at each step of a food chain. This biomagnification happens to substances that are:

• persistent — not broken down by organisms or the environment;
• fat-soluble — stored in fatty tissue rather than dissolved in blood and urine, so they are not excreted.

Within one organism the pollutant builds up over its lifetime (bioaccumulation). Then, because only about 10% of energy passes from one trophic level to the next (see C4.2 Transfers of energy and matter), each consumer must eat a large biomass of food, and it retains the pollutant from all of it. The pollutant is concentrated at every step, so top predators receive the highest doses.

DDT

DDT is an insecticide widely sprayed from the 1940s. It, and its breakdown product DDE, built up in birds of prey and fish-eating birds such as peregrine falcons, ospreys, bald eagles and brown pelicans. DDE interferes with calcium deposition in eggshells, so shells were thin and broke during incubation, and populations crashed. Rachel Carson’s book Silent Spring (1962) publicized the problem; most countries banned DDT for farm use from the 1970s, and the bird populations recovered. It is still used in a few countries, inside houses, to control malaria mosquitoes.

Mercury

Mercury released by burning coal, gold mining and industry is converted by bacteria in sediments into methylmercury, which is fat-soluble and biomagnifies. From the 1930s to the 1960s a chemical factory discharged methylmercury into Minamata Bay, Japan. People who ate fish and shellfish from the bay suffered severe nerve damage — Minamata disease, identified in 1956 — and children were born with brain damage. Today pregnant women are advised to limit large predatory fish such as swordfish, shark and some tuna, which have the highest mercury levels.

Plastics are synthetic polymers, mostly made from oil. They are persistent because they are not biodegradable: very few decomposers have enzymes that can digest them. Instead plastics are broken into ever smaller fragments by UV light, waves and abrasion, and they build up in the oceans, on beaches and in sediments.

Macroplastics (larger than 5 mm)

• Entanglement: discarded fishing nets (“ghost nets”), ropes and packaging bands trap seals, turtles, whales and seabirds, which drown, starve or are injured.
• Ingestion: sea turtles swallow plastic bags that look like jellyfish. Albatrosses on Pacific islands such as Midway Atoll pick up floating plastic and feed it to their chicks; the plastic fills the stomach, causing blockage and starvation.

Microplastics (smaller than 5 mm)

Microplastics include microbeads from cosmetics, plastic pellets, fibres shed from synthetic clothing, tyre dust and fragments of larger items. They are eaten by zooplankton, filter feeders such as mussels and oysters, and fish, where they can block guts and reduce feeding. Toxic chemicals such as PCBs stick to their surfaces, and these can pass up food chains. Even smaller nanoplastics can cross membranes into cells; their effects are still being researched.

Rewilding aims to restore natural processes in a degraded area so that the ecosystem becomes self-sustaining, with as little human management as possible. Methods include:

• Reintroducing apex predators and other keystone species, which restore top-down control and engineer habitats. Grey wolves returned to Yellowstone National Park in 1995 reduced overgrazing by elk; Eurasian beavers released in Scotland create ponds and wetlands.
• Re-establishing connectivity: linking fragments of habitat with corridors over large areas so populations can move, interbreed and recolonize.
• Minimizing human impact: stopping farming, logging or fishing in the area, controlling invasive species, and otherwise allowing natural succession to proceed (ecological management).

Hinewai Reserve, New Zealand

Hinewai Reserve on Banks Peninsula, in the South Island of New Zealand, is former sheep farmland that has been allowed to return to native forest since 1987. It now covers well over 1,000 hectares.

• Grazing farm animals were removed, and introduced browsing mammals such as goats, deer and brushtail possums are controlled, because they eat native seedlings.
• Otherwise there is minimal interference: no burning, no herbicide spraying and little planting. Birds disperse seeds of native trees into the area.
• The surprising strategy concerns gorse (Ulex europaeus), an invasive shrub that farmers fight. At Hinewai gorse is left alone: it acts as a nurse canopy, sheltering native tree seedlings, which grow up through it, overtop it and shade it out, because gorse cannot grow in shade.

This is secondary succession allowed to run its course. The result is regenerating native forest, with native birds and insects returning.