Vulnerability and resilience of ecosystems including their adaptability, biodiversity, size, location, and the rate and magnitude of change

What this dot point is asking

NESA expects you to know why some ecosystems collapse under stress while others bounce back. The vulnerability and resilience framework is the conceptual core of the topic. Strong responses identify at least four factors, name the mechanisms by which each operates, and apply the framework to specific Australian ecosystems.

Vulnerability and resilience defined

Vulnerability is the susceptibility of an ecosystem to be harmed by stress. High-vulnerability ecosystems lose function or species when stressed.

Resilience is the capacity of an ecosystem to absorb stress and recover to the original state. Highly resilient ecosystems can experience disturbance and return.

The two concepts are not opposites. An ecosystem can be both vulnerable (suffer significant initial damage) and resilient (eventually recover). The opposite combination (low-vulnerability, low-resilience) is rare in nature.

The factors

1. Biodiversity

High biodiversity provides redundancy. If one species is lost, another can fill its functional role. Tropical rainforests with thousands of tree species lose less function from one species' decline than monocultures lose from the same pathogen.

The Wet Tropics of Queensland (the Daintree-Atherton bioregion) holds around 50 percent of Australia's biodiversity in 0.2 percent of its land area. The biodiversity supports complex food webs and rapid recovery from cyclones.

By contrast, Australian salt lakes have very low biodiversity (specialised salt-tolerant species only). Loss of one species can collapse the food web.

2. Ecosystem size and connectivity

Large ecosystems sustain larger populations, which are less vulnerable to extinction by chance events. Connectivity allows movement: species can shift range in response to climate change, recolonise after disturbance, and maintain gene flow.

The Australian wildlife corridor program (e.g., the Great Eastern Ranges initiative from Atherton to Grampians) recognises that fragmented habitat is more vulnerable than connected habitat of the same total area. Small habitat fragments suffer "edge effects" (more sunlight, wind, predators, invasive species at the boundary), which propagate inward and degrade the core.

3. Position in the geographical envelope

Ecosystems near the edge of their climatic, hydrological, or geological envelope are vulnerable to small environmental shifts. Examples:

• Alpine ecosystems at Mount Kosciuszko exist above the tree line (around 1,800 m) where freezing temperatures exclude lowland species. Warming compresses the alpine zone upward, and the mountain has no higher refuge.
• Tropical reefs like the Great Barrier Reef live at the warm edge of coral thermal tolerance. Marine heatwaves push corals above their thermal threshold and trigger bleaching.
• Arid-zone wetlands like the Macquarie Marshes depend on periodic flooding. Without flood pulses (now reduced by upstream dams) the wetlands dry out and the ecosystem transitions to a different state.
• Coastal ecosystems like mangrove forests and salt marshes are vulnerable to sea-level rise where landward retreat is blocked by human infrastructure.

4. Rate and magnitude of change

Ecosystems are adapted to historical patterns of disturbance. Slow change allows adaptation through migration, behavioural shift, or evolutionary change. Fast change exceeds adaptive capacity.

Past climate change (Pleistocene glaciations) occurred at rates of around 0.01 degrees C per decade. Current anthropogenic warming is 0.2-0.4 degrees C per decade, around 20-40 times faster. Most species cannot shift range fast enough.

Magnitude also matters. A small temperature shift is buffered by ecosystem homeostasis. A large shift, especially in conjunction with other stressors, can push the ecosystem past a tipping point.

5. Presence of keystone species

Some species have disproportionate roles in ecosystem function. Their removal triggers cascading change.

• Sea urchins on Tasmania's east coast. The long-spined sea urchin (Centrostephanus rodgersii) has expanded south as East Australian Current warming has shifted ranges. Urchin barrens now affect over 50 percent of Tasmania's eastern reefs, eliminating kelp and the species that depended on kelp.
• Apex predators on the GBR. Loss of sharks and predatory reef fish (through fishing) cascades down the food web. Crown-of-thorns starfish outbreaks are amplified.
• Pollinators. Loss of honey bees (introduced) or native bees from pesticide use threatens pollination services for both natural and agricultural ecosystems.

6. Stress history

Ecosystems with longer histories of disturbance often have higher resilience because the species present have evolved to tolerate stress. Australian eucalypt forests are highly resilient to fire because they have co-evolved with fire over millions of years. Rainforests, which have not, are not resilient to fire and may not recover.

7. Human pressure

Ecosystems with high prior human pressure (clearing, pollution, fishing) have reduced resilience to additional stress. The Murray-Darling Basin is more vulnerable to drought now than pre-1788 because river regulation, wetland clearing, and invasive species have reduced resilience.

Tipping points

Tipping points are thresholds beyond which an ecosystem shifts to a fundamentally different state and cannot easily return. The new state may be a different community with different species, lower biodiversity, and lower productivity.

Examples of identified or anticipated tipping points:

• Coral reef-to-algal-flat transition. When coral cover falls below around 20-30 percent and algae take over, recovery becomes increasingly difficult. Caribbean reefs have crossed this threshold; the Great Barrier Reef approaches it in worst-affected areas.
• Amazon rainforest-to-savanna. Drying and deforestation may convert parts of the Amazon to dry savanna. Past 2024, around 17 percent of the Amazon has been deforested, with the 20-25 percent threshold widely cited as the tipping point.
• Permafrost thaw releasing methane. Once initiated, hard to reverse on human timescales.
• Antarctic ice sheet collapse. Past around 2-3 degrees C of warming, the West Antarctic Ice Sheet may enter irreversible collapse.

Putting it together

The most vulnerable ecosystems combine multiple risk factors: low biodiversity, small size and fragmentation, edge position in the envelope, high rate and magnitude of stress, presence of keystone species under pressure, accumulated stress history, and high human pressure.

The Great Barrier Reef is highly biodiverse and large, which favours resilience. But it is at the warm edge of coral thermal tolerance, faces rapid temperature change, depends on keystone fish and herbivore species, and is under heavy human pressure. Net assessment: moderately resilient but rapidly losing resilience.

Macquarie Marshes is small, fragmented from upstream flood pulses, water-table limited, dependent on a few key plant species, and stressed by river regulation plus climate change. Net assessment: highly vulnerable.