How does human activity stress ecosystems?
Human-induced stress on ecosystems, including land clearing, pollution, overharvesting, invasive species, and climate change
A focused answer on human-induced stress. Land clearing (50 percent of original forest), pollution, overharvesting, invasive species (rabbits, cane toads, carp), and climate change as the major stressors with Australian quantitative data.
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What this dot point is asking
NESA expects you to identify the major human-induced stresses on ecosystems, quantify their scale in Australia, and recognise that they interact and compound. The HSC question typically asks you to assess relative importance or to apply the stresses to a specific ecosystem.
Land clearing
Australia has cleared around 50 percent of its original forest and woodland cover since European settlement in 1788. The Bureau of Statistics records continued clearing of around 400,000 hectares of woody vegetation per year.
The State of the Environment Report (2021) classified land-use change as the leading driver of Australian biodiversity decline, ahead of climate change and invasive species. Clearing drives:
- Direct habitat loss for native species.
- Edge effects on remaining habitat (sunlight, wind, predation, invasive species).
- Soil erosion and sediment delivery to waterways.
- Hydrological change (water tables rise as deep-rooted vegetation is removed; dryland salinity affects over 2 million ha).
- CO2 release (deforestation accounts for around 12-18 percent of Australia's total greenhouse emissions).
Queensland has the highest clearing rates (around 7.7 million ha cleared 2000-2020). NSW, Victoria, and Tasmania cleared most of their forests by 1900. WA has cleared around 14 million ha of wheatbelt since 1900.
Pollution
Agricultural runoff
Nitrogen and phosphorus from fertilisers and animal waste drive algal blooms in rivers, lakes, and coastal waters. The Great Barrier Reef catchments deliver around 14 Mt of sediment, 50,000 t of nitrogen, and 4,200 t of phosphorus annually to reef waters. Sugarcane and beef are the dominant sources. The Reef 2050 Plan includes targets for 60 percent reduction in nitrogen and 25 percent reduction in sediment.
Industrial pollution
Mine tailings, smelter emissions, refinery discharges. Acid mine drainage from old mines (Mount Lyell, Captains Flat) continues decades after closure. PFAS contamination from defence sites and firefighting foam affects groundwater across 90-plus Australian sites.
Plastic pollution
Around 130,000 t of plastic enters Australian waters each year. Microplastics are now ubiquitous in marine ecosystems and have been found in fish, seabirds, turtles, and human blood.
Atmospheric pollution
PM2.5 (fine particulate matter) from wood-fire heating, vehicle emissions, and bushfires causes around 1,700 premature deaths per year in Sydney (UNSW study 2022). Black Summer smoke produced PM2.5 levels 26 times above hazardous thresholds in Canberra for weeks.
Light and noise pollution
Light pollution affects 80 percent of the world's population. Sea turtle hatchlings disoriented by beach lighting. Noise pollution affects whale communication and reef fish behaviour.
Overharvesting
Removing species from an ecosystem faster than they reproduce drives population decline and ecosystem disruption.
Fisheries
Most Australian fisheries are now managed within sustainable limits, but historical overfishing has lasting impacts. Orange roughy off Tasmania collapsed in the 1990s and is only slowly recovering. School shark in southern Australian waters remains overfished. Tropical reef fish populations on the inshore Great Barrier Reef have declined from fishing pressure.
Southern bluefin tuna populations crashed from overfishing in the 1980s; international management (CCSBT) reduced total allowable catch to 12,400 t globally in 2024 to allow recovery to around 30 percent of unfished biomass by 2035.
Forestry
Native forest logging has been declining as states phase out: Victoria ended native forest logging in 2024, Western Australia in 2024, Queensland by 2025. NSW transitions remain contested. Tasmania continues with native forestry under managed plans.
Wildlife harvest
Bushmeat hunting is not a major issue in Australia (no legal hunting of native mammals). Crocodile and kangaroo harvest are managed under quotas.
Invasive species
Introduced species are the second largest driver of biodiversity loss in Australia after habitat clearing.
Mammalian predators and herbivores
- Feral cats. Around 2.1 million cats kill an estimated 1.7 billion native vertebrates per year (NESP 2019). Identified as a primary cause for at least 22 mammal extinctions.
- Foxes. Around 1.7 million foxes, primary cause of ground-dwelling mammal declines.
- Rabbits. Population peaked at around 300 million in the 1920s. Reduced by myxomatosis (1950s) and calicivirus (1995) but still cause major grazing damage.
- Wild dogs. Hybridised with dingoes; livestock predator.
- Camels. Around 1 million feral camels in central Australia damage waterholes and vegetation.
- Pigs. Around 23 million feral pigs cause severe agricultural and ecological damage.
Amphibians
- Cane toads. Introduced 1935 to control cane beetles. Spread across 1.2 million km2 (NT, QLD, NSW, WA). Toxic to native predators (quolls, monitor lizards, snakes).
Fish
- Carp. European carp dominate 70-90 percent of fish biomass in the Murray-Darling. Disturb sediment, increase turbidity, outcompete natives. A National Carp Control Plan investigating biocontrol (a carp-specific herpesvirus) was paused over uncertainty.
- Tilapia. Established in northern Australia.
- Mosquitofish. Widespread; outcompete native rainbowfish and tadpoles.
Plants
Around 3,000 naturalised introduced plant species in Australia. Major weeds include lantana (4 million ha), prickly pear (still controlled by Cactoblastis moth), Mimosa pigra (NT wetlands), buffel grass (changing fire regimes across central Australia), and gamba grass (NT, drives fierce woodland fires).
Invertebrates
Yellow crazy ants on Christmas Island and Lord Howe Island. European honey bees outcompete native bees. Asian honey bees and Varroa mite (incursion in NSW 2022) threaten the apiculture industry.
Diseases
Phytophthora cinnamomi (water mould affecting WA jarrah). Chytrid fungus (frog declines). Myrtle rust (Myrtaceae plant family). Avian influenza variants spreading.
Climate change
Climate change is both a stress in its own right and an amplifier of all other stresses. Mass coral bleaching on the Great Barrier Reef (1998, 2002, 2016, 2017, 2020, 2022, 2024) is a direct climate impact. The Black Summer 2019-20 fires killed or displaced 3 billion vertebrate animals.
Climate change interacts with other stresses: drier conditions stress drought-vulnerable ecosystems; warmer oceans bleach reefs already stressed by sediment runoff; species struggling against invasive predators have less resilience to range-shifts.
How stresses interact
Single stresses are manageable; combinations cause collapse. The Great Barrier Reef faces sediment runoff, nutrient pollution, ocean warming, crown-of-thorns outbreaks (linked to nutrient enrichment), and tropical cyclones. The Murray-Darling Basin faces river regulation, water extraction, climate change, salinity, and invasive carp.
The strongest HSC answers map at least three interacting stresses onto one ecosystem and quantify each.
Past exam questions, worked
Real questions from past NESA papers on this dot point, with our answer explainer.
Practice (NESA)8 marksAnalyse the role of human activity as a source of stress on ecosystems.Show worked answer →
An 8-mark "analyse" needs at least four named human stresses, the mechanism, and Australian quantitative data.
- Land clearing
- Australia cleared around 7.7 million hectares in QLD alone between 2000 and 2020. Total since 1788 around 50 percent of original forests. Drives biodiversity loss, soil erosion, salinity, and CO2 release. The 2021 IPBES report identified land-use change as the leading global cause of biodiversity loss.
- Invasive species
- Around 3,000 introduced plant species and over 300 vertebrate animals naturalised. Cane toads have spread across 1.2 million km2 since 1935. European rabbits drive grazing pressure equivalent to around 80 million extra sheep. European carp dominate 70-90 percent of fish biomass in the Murray-Darling. Feral cats kill around 1.7 billion native animals per year (NESP 2019 estimate).
- Pollution
- Agricultural runoff to the GBR delivers around 14 Mt of sediment, 50,000 t of nitrogen, and 4,200 t of phosphorus annually. PFAS contamination at 90-plus Australian Defence sites. Plastic in marine ecosystems (about 130,000 t entering Australian waters annually).
- Overharvesting
- Many Australian fisheries face stress. Southern bluefin tuna catch reduced to 12,400 t globally (2024) to allow recovery. Orange roughy off Tasmania collapsed in the 1990s. Native forest logging declined after 2024 Victorian ban; NSW transitions ongoing.
- Climate change
- All other stresses are amplified by warming. Marine heatwaves bleached 67 percent of GBR northern reefs in 2016 and similar in 2024. Black Summer 2019-20 killed or displaced 3 billion vertebrates.
Markers reward (1) at least four stresses, (2) at least one statistic per stress, (3) recognition that stresses interact and compound, (4) explicit ecosystem-scale consequence.
Related dot points
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