Australian outdoor environments face a range of serious threats — from the gradual creep of land degradation to the sudden violence of catastrophic fire and flood. Understanding these threats, their causes, and their impacts on both ecosystems and human communities is central to Unit 4 of OES. This note covers land degradation, introduced species, urbanisation, climate change, flood, and fire as required by the study design.
| Category | Threats |
|---|---|
| Chronic, gradual | Land degradation, introduced species |
| Structural | Urbanisation |
| Systemic/global | Climate change |
| Acute events | Flood, fire |
Most real threats are interconnected — climate change intensifies fire and flood; urbanisation accelerates land degradation; introduced species thrive in degraded environments.
Land degradation refers to the long-term decline in the capacity of land to support its natural biodiversity and human uses.
Key forms in Australia:
- Soil erosion (wind and water): Over 3 million ha of Australia are seriously affected. Western NSW, the Mallee, and semi-arid Queensland are particularly vulnerable.
- Salinity: Clearing native deep-rooted vegetation raises water tables, bringing salt to the surface. Over 5.7 million ha of agricultural land are salt-affected; the South-West WA Biodiversity Hotspot is severely impacted.
- Soil acidification: Affects >23 million ha; caused by nitrogen fertilisers, long-term cropping.
- Compaction and structural decline: Heavy machinery and overgrazing; reduces water infiltration, increases runoff.
- Loss of soil carbon: Clearing and cultivation releases stored carbon; reduces soil fertility and water-holding capacity.
Impacts on society:
- Reduced agricultural productivity: estimated \$9 billion/year economic cost (CSIRO estimates)
- Sedimentation of waterways and reservoirs reduces water supply security
- Dust storms affect human health in regional cities
- Loss of ecosystem services (carbon storage, water filtration, biodiversity habitat)
KEY TAKEAWAY: Land degradation is Australia’s most widespread and economically costly environmental problem. It is largely invisible compared to dramatic events like fire and flood, which is why it receives less attention despite its severity.
Australia’s isolated evolutionary history means its native species evolved without key predators, competitors, and diseases now present following European colonisation.
Introduced predators:
- Feral cats: Kill an estimated 2 billion native animals per year (federal government estimate, 2022). Single most significant driver of mammal extinction in Australia.
- Red foxes: Have caused or contributed to the extinction of numerous ground-nesting birds and small marsupials across mainland Australia. Managed in Victoria through the state-wide 1080 baiting program and Feral Cat Management Plan.
Introduced herbivores:
- Rabbits: Estimated 200 million individuals nationally. Cause severe vegetation loss, erosion, and competition with native herbivores. Spread of calicivirus (RHDV) and myxomatosis has reduced but not eliminated populations.
- Feral horses (brumbies): ~400,000 nationally. Severely degrade alpine bogs, riparian zones, and subalpine grasslands — especially in Victoria’s Alpine NP and Kosciuszko NP (NSW).
- Feral pigs: Rootle up and destroy wetlands, grasslands, and ground cover; spread weeds; kill ground-nesting birds.
Introduced plants (weeds):
- Over 2,700 introduced plant species are established in Australian environments.
- Key threats: blackberries (Rubus fruticosus agg.), willows (Salix spp.) in riparian zones, gorse, serrated tussock, Paterson’s curse.
- Weeds change fire behaviour, soil chemistry, and habitat structure.
Case study — Feral horses in the Alpine National Park, Victoria:
Population estimated at 10,000–13,000 horses. Research by CSIRO and ANU documents severe damage to sphagnum bogs (critical for water storage and biodiversity), erosion of stream banks, and trampling of rare alpine ash regrowth. Parks Victoria manages via mustering and rehoming; aerial culling proposed but politically contested.
EXAM TIP: For introduced species, distinguish between direct impacts (predation, competition, physical damage) and cascading ecological effects (changing fire behaviour, altering soil chemistry, enabling further invasion). The most compelling answers trace cause-and-effect chains.
Australia is one of the world’s most urbanised nations — over 85% of the population lives in urban areas, predominantly clustered in coastal cities.
Direct environmental impacts:
- Habitat loss and fragmentation: Urban expansion onto the urban fringe eliminates native vegetation and creates isolated habitat patches. Melbourne’s western and south-eastern growth corridors have destroyed extensive grasslands (nationally critically endangered ecological community).
- Impervious surfaces: Reduce infiltration, increase runoff, flashflood frequency, and stormwater pollution in urban creeks.
- Urban heat island: Cities are 1–5°C warmer than surrounding areas; affects local biodiversity and human health.
- Light pollution: Disrupts nocturnal fauna behaviour (migration, breeding).
- Noise pollution: Interferes with bird communication and breeding.
Impacts on outdoor environments specifically:
- Increased recreational pressure on peri-urban parks and reserves as cities grow
- Weed spread from gardens into adjacent native vegetation
- Feral animals (foxes, cats) that are fed and sheltered in urban areas predating on nearby bushland
Impacts on society:
- Loss of green space reduces physical and mental health benefits
- Urban heat kills: ~1,700 deaths/year in Australia attributed to heat (Climate Council, 2023)
- Flooding of urban areas: infrastructure damage, insurance costs, loss of life
Climate change is a threat multiplier — it intensifies nearly every other threat to outdoor environments.
Observed impacts in Australia:
- Average temperature increase of ~1.5°C since 1910; hottest decade on record is 2011–2020
- Southern Australia experiencing longer, hotter fire seasons
- Shifts in species distributions — alpine species migrating to higher elevations
- Bleaching of the Great Barrier Reef (mass bleaching events 2016, 2017, 2020, 2022, 2024)
- Reduced snowpack in Victorian and NSW alps
Cascading effects on specific environments:
| Environment | Climate Change Impact |
|---|---|
| Alpine | Reduced snowpack, vegetation zone shifting upward, increased fire risk |
| Coastal | Sea-level rise, increased storm surge, reef bleaching |
| Wetlands | Drying of southern wetlands; increased drought; altered flood regimes |
| Forests | Increased fire frequency and severity; drought stress |
Bushfire is a natural part of many Australian ecosystems — but climate-driven increases in fire frequency, severity, and season length are beyond what many ecosystems can recover from.
2019–20 Black Summer:
- ~19.4 million ha burned
- ~3 billion animals killed or displaced (University of Sydney)
- 33 people died directly; hundreds from smoke inhalation
- 3,000+ homes destroyed
Ecological impacts: Fires that are too frequent prevent plant regeneration; seed banks depleted. Hollow-dependent species (possums, parrots) lose nesting sites; recovery of complex habitat structure takes decades.
Australia’s flood-prone landscape is increasingly affected by the interaction of La Niña events and climate change.
2022 South-East Queensland and NSW floods:
- Largest recorded floods in many catchments
- Economic cost: ~\$6.5 billion; 23 deaths
- Ecological impacts: Introduced species spread into new areas post-flood; blackwater events in rivers killed fish
VCAA FOCUS: You need to describe the impact of two of these threats in depth. Choose two that you can connect to specific Australian environments you have studied — this is where knowledge of your fieldwork sites is valuable.
