Natural stress on ecosystems, including drought, fire, flood, cyclones, ENSO, and disease, and the role of stress in ecosystem dynamics

What this dot point is asking

NESA wants you to recognise that natural stress is not always negative; it can be a regular and necessary part of ecosystem dynamics. Australian ecosystems have evolved with fire, drought, cyclones, and climate variability. Problems arise when natural stress combines with human-induced stress or when the rate and magnitude of natural stress exceeds historic norms (typically because of climate change).

Drought

The dominant natural stress in Australian ecosystems. Caused by:

• ENSO El Nino phases. Weak trade winds shift warm Pacific surface water eastward, suppressing rainfall over eastern Australia.
• Positive Indian Ocean Dipole. Warm western Indian Ocean reduces moisture flow.
• Negative Southern Annular Mode. Westerly winds shift northward, reducing southern Australian winter rainfall.

Native species are adapted. Spinifex (Triodia) has roots reaching 4 m and resinous leaves that minimise water loss. Marsupials produce concentrated urine to conserve water. Eucalypts shed leaves and bark during drought to reduce transpiration. River red gums survive years of dry, sprouting from epicormic buds when wet conditions return.

Drought has a positive ecological role: it concentrates organic matter, clears excess vegetation, and creates conditions for fire that recycle nutrients. The boom-bust cycle of inland ecosystems depends on drought.

Problem: anthropogenic climate change is making drought more frequent and severe. The Millennium Drought (1997-2009) was the longest in recorded history. Black Summer (2019-20) followed three years of drought. Native species adaptations evolved for occasional drought are not adequate for chronic drought.

Fire

Australian eucalypt forests are one of the world's fire-adapted ecosystems. Adaptations:

• Serotiny. Banksia, hakea, and eucalypt seeds are stored in woody cones or capsules that open only after fire.
• Lignotubers. Underground swellings that allow many eucalypts to resprout from base after fire damage.
• Epicormic buds. Eucalypts produce new leaves directly from trunk bark after fire defoliates the canopy.
• Fire-stimulated germination. Many native species germinate only after fire-related cues (smoke compounds, heat shock, soil disturbance).

Fire frequency matters. Some ecosystems require frequent low-intensity fire (every 3-10 years) to maintain species diversity. Other ecosystems (rainforests, alpine bogs) have not co-evolved with fire and are damaged by it.

Aboriginal land management used systematic low-intensity fire (cultural burning, "cool burns", fire-stick farming) for over 60,000 years. The pattern shaped the vegetation, fauna, and soils that European settlers encountered.

Problem: post-1788 fire regimes are not the historical pattern. Suppression of fire allowed fuel to accumulate. Climate change extends fire season. Black Summer fires were larger and more intense than historical fires, burning ecosystems (alpine bogs, World Heritage Gondwana rainforests) that do not normally burn.

Tropical cyclones

Northern Australia averages 3-5 cyclones per year. Strong cyclones (Category 3-5) cause:

• Coastal flooding and storm surge.
• Mangrove die-off where surge exceeds salt tolerance.
• Reef damage to coral and seagrass.
• Forest tree-fall and defoliation.

Notable cyclones:

• Cyclone Tracy (December 1974). Category 4 hit Darwin; 71 deaths, 70 percent of houses destroyed.
• Cyclone Yasi (February 2011). Category 5 over Tropical North QLD; damaged 17 percent of the Great Barrier Reef but reef recovered most function within 5-10 years.
• Cyclone Debbie (March 2017). Category 4 hit central Queensland; major flooding.

Cyclones can have a positive ecological role: storm surge replenishes coastal wetlands and salt marshes; cyclone-driven mixing brings deeper nutrients to surface ocean, fuelling productivity.

Problem: climate change is projected to reduce cyclone frequency but increase the intensity of those that form. Mangroves and reefs may suffer more severe damage even with fewer events.

ENSO and the Indian Ocean Dipole

Australian ecosystems are tuned to climate variability. The El Nino-Southern Oscillation drives multi-year drought-flood cycles. The Indian Ocean Dipole adds an Indian Ocean component to rainfall variability.

Inland ecosystems require infrequent flooding to function. The Murray-Darling Basin had natural floods every 2-3 years pre-regulation. Lake Eyre fills around once every 8 years on average. The Sturt's desert pea germinates only after major flood pulses.

Problem: climate change is shifting the frequency and intensity of ENSO and IOD events. Reduced inland flooding (from drier conditions plus dam capture) has cut wetland productivity and bird breeding events.

Disease

Mostly an introduced rather than truly native stress in modern Australia, but worth recognising.

• Phytophthora dieback. Phytophthora cinnamomi (water mould) affects around 14,000 km2 of WA jarrah forest and around 7 species of threatened plants per affected area.
• Chytridiomycosis. Batrachochytrium dendrobatidis (chytrid fungus) has caused declines or extinction of 7 Australian frog species.
• Myrtle rust. Austropuccinia psidii arrived in Australia in 2010; threatens around 350 native plant species in the Myrtaceae family.

Disease is often a force multiplier with other stresses. Drought-weakened plants are more susceptible to phytophthora. Frogs in degraded habitat suffer chytrid more severely.

Floods

Australian ecosystems are adapted to periodic flooding. River red gum forests need flooding every 2-5 years to recruit new trees. Wetlands need flood pulses to maintain seasonal aquatic phases. Inland lakes and chains of ponds depend on flooding to refresh.

Notable Australian floods include the 1974 Brisbane flood, 2011 Brisbane and Queensland floods, 2019-20 Townsville flood, 2022 NSW Northern Rivers flood (Lismore), and the 2022 widespread eastern Australian flooding.

Excessive flooding (climate change driven extreme rainfall events) can be destructive. Sediment plumes from the 2019 Townsville flood deposited a mud blanket on inshore Great Barrier Reef. Lismore's 2022 flood damaged 4,000 homes and disrupted local ecosystems.

How natural stress becomes ecosystem risk

Natural stress alone rarely puts Australian ecosystems at risk in the long term. Native ecosystems have co-evolved with drought, fire, cyclones, and floods over millions of years. Problems arise when:

• Climate change shifts stress beyond historical envelopes.
• Human-induced stress (clearing, pollution, fishing) reduces resilience.
• Invasive species exploit disturbance windows.
• Stresses interact (drought plus heatwave plus fire plus disease).

The HSC examiner expects you to recognise both the constructive role of natural stress and the compounding problems when stress patterns shift.