Describe ecosystem dynamics, including the role of keystone species and the processes of primary and secondary succession, and explain how species composition changes over time

What this dot point is asking

QCAA wants you to describe how communities change over time after disturbance and to recognise that some species have outsized influence on community structure. Succession appears as multi-mark stimulus questions almost every year; keystone species are a favourite for short response.

The answer

An ecosystem is not static. Species composition, abundance and structure change over ecological time through succession, and they are held in place at any moment by the interactions of all species, some of which (keystone species) carry disproportionate weight.

Primary succession

Primary succession begins on a substrate that has no existing soil or biological community. Examples of starting points: a fresh lava flow, glacial moraine, a newly formed coral cay, bare rock exposed by retreating ice.

Stages.

1. Pioneer community. Lichens and mosses colonise bare rock. Lichens are symbioses of fungi and algae or cyanobacteria. The fungal hyphae break the rock chemically (organic acids) and physically (expansion and contraction); the photobiont fixes carbon and (in cyanobacterial lichens) nitrogen.
2. Soil formation. Dead pioneers, dust and weathered rock accumulate. Over decades, a thin soil develops.
3. Herbs and grasses. Seeds blown in from neighbouring communities germinate in the new soil.
4. Shrubs and small trees. Deeper-rooted species displace the herbaceous layer as soil deepens.
5. Climax community. A stable, self-replacing community appropriate to the climate (rainforest, eucalypt woodland, alpine herbfield).

Primary succession typically takes hundreds to thousands of years because soil formation is slow.

Secondary succession

Secondary succession occurs after a disturbance that removes the existing community but leaves the soil intact. Examples: a bushfire, a cyclone, a cleared paddock left to regenerate.

Stages.

1. Resprouting and germination. Seeds in the soil seed bank germinate. Plants with lignotubers (mallee eucalypts), epicormic buds (most eucalypts) and serotinous fruits (banksia, hakea) regrow quickly.
2. Early colonisers. Fast-growing pioneers such as acacia and bracken dominate.
3. Mid-successional community. Slower-growing sclerophyll species establish under the pioneers.
4. Climax community. Returns over decades to a community similar to that present before the disturbance, provided the disturbance regime has not been altered.

Secondary succession is much faster than primary because the soil, microbiota and seed bank are already present.

Pioneer and climax communities

Pioneer species share traits that suit them to harsh, open conditions: tolerance of low nutrients, rapid life cycles, wind-dispersed seeds, high reproductive output.

Climax species are typically larger, slower-growing, shade-tolerant and competitive at the equilibrium state.

A climax community is not a fixed endpoint. It depends on the local climate, the disturbance regime and the species pool. In fire-adapted Australian landscapes, periodic fire is part of the system, and what looks like a "climax" eucalypt forest is actually a dynamic equilibrium maintained by fire.

Keystone species

A keystone species has a disproportionately large effect on its community relative to its abundance. Three common modes of keystone action:

Predator keystone. Top predators suppress mid-level consumers. Example: the dingo (Canis dingo) in arid Australia. Dingoes suppress red fox and feral cat populations, indirectly protecting small mammal diversity. On the cleared side of the dingo fence, foxes and cats are more abundant and small mammal richness drops.

Ecosystem engineer. A species that physically modifies the environment. Example: the bilby (Macrotis lagotis) and other digging marsupials turn over large volumes of soil each year, mixing organic matter, capturing leaf litter and creating microhabitats. Their loss across most of mainland Australia has reduced soil function in arid ecosystems.

Mutualist keystone. A species whose mutualistic role supports many others. Example: flying foxes (Pteropus species) pollinate and disperse seeds for dozens of eucalypt and rainforest tree species along the east coast. Their decline reduces seed dispersal distance and forest regeneration.

Worked example: starfish and the original keystone study

The term keystone species comes from Robert Paine's 1966 experimental removal of the starfish Pisaster ochraceus from a rocky intertidal community in Washington State. With the starfish removed, mussels (Mytilus) outcompeted other species and reduced the community from 15 species to about 8. Pisaster was less than 1 per cent of the biomass but maintained the diversity of the entire community.

The Australian analogue is the Crown of Thorns starfish (Acanthaster planci) in reverse: as a coral predator at outbreak densities it destroys hard coral cover and shifts reefs to algae-dominated states. At low densities it acts as a normal coral grazer.

Common traps

Confusing primary and secondary succession. The defining difference is the starting substrate. If soil exists at the start, it is secondary. If you are starting on bare rock or lava, it is primary.

Treating the climax community as a single fixed state. Climax communities depend on climate, soil and disturbance regime. Fire-regime change can shift the climax.

Calling any common species a keystone. Many abundant species are dominant but not keystone. A keystone species is influential beyond its abundance.

Forgetting that keystone effects cascade. The removal of one keystone often changes a chain of species, not just the next trophic level.

In one sentence

Ecosystem dynamics describe how communities change over time through primary succession on new substrates (lichens, then soil, then climax over centuries) and secondary succession after disturbance with an intact soil and seed bank (resprouting, then climax over decades), all held together by keystone species whose ecological influence is disproportionately large relative to their abundance.