Natural Changes That Influence Biodiversity

Biodiversity is shaped by natural processes operating across vastly different time scales — from hours (fire) to millions of years (tectonic plate movement). Understanding these processes is essential for distinguishing natural fluctuation from human-induced change.

Volcanic Eruptions

Time scale: Hours to decades

Volcanoes affect biodiversity through multiple pathways:
- Short-term destruction: Lava flows, pyroclastic flows and ashfall destroy habitats and directly kill organisms
- Climate effects: Volcanic aerosols (sulfur dioxide) reflect solar radiation, causing temporary global cooling (‘volcanic winter’)
- Long-term benefits: Volcanic soils are mineral-rich and highly productive; volcanic islands provide new land surfaces for colonisation

Example: The eruption of Krakatoa (1883) devastated its local biota but was followed by remarkable recolonisation — documenting the processes of ecological succession and island biogeography.

Mass extinction link: The End Permian and End Triassic extinctions are both linked to massive flood basalt eruptions (Siberian Traps and Central Atlantic Magmatic Province respectively), which released vast quantities of $CO_2$ and $SO_2$.

Fire

Time scale: Hours to centuries

• Natural fire (lightning-ignited) is a major ecological process in Australian landscapes
• Fire-adapted ecosystems have evolved over millions of years of fire disturbance
• Fire promotes diversity at the landscape scale by creating a mosaic of habitats at different successional stages
• Obligate seeders (e.g. many banksias) require heat or smoke to germinate
• Species endemism in fire-prone ecosystems is often high because fire selects for specialised traits

Example: Victoria’s mountain ash (Eucalyptus regnans) forests are fire-dependent — regeneration occurs from seeds released after fire. But very high-frequency fires prevent trees from reaching reproductive maturity.

El Niño–Southern Oscillation (ENSO)

Time scale: Years to decades (cycles of 3–7 years)

ENSO describes irregular variation in sea surface temperatures in the Pacific Ocean:
- El Niño years: Warmer Pacific, increased rainfall in central Pacific, drought in Australia, Indonesia, parts of Africa
- La Niña years: Cooler Pacific, increased rainfall in Australia

Biodiversity effects:
- Drought and heat stress drive mass coral bleaching events (Great Barrier Reef)
- Extended droughts increase fire frequency, altering plant community composition
- Flood years drive boom-bust population cycles in arid species
- Species endemism can increase in isolated areas subject to repeated El Niño cycles, as populations adapt to local boom-bust conditions

Tectonic Plate Movement

Time scale: Millions of years

• The movement of tectonic plates over geological time creates and destroys ocean barriers, connects and separates land masses
• Australia separated from Gondwana ~45 million years ago → isolated evolution → extremely high endemism (~80% of mammals, ~90% of reptiles are endemic)
• Continental collision creates mountain ranges → new alpine habitats → diversification
• Sea level changes driven by tectonics alter habitat connectivity and drive speciation

Diversity hotspots often arise where tectonically complex topography creates many different microhabitats and environmental gradients.

Evolution

Time scale: Thousands to millions of years

• Speciation adds to biodiversity; extinction reduces it
• Key speciation mechanisms:
• Allopatric speciation: Geographic isolation (e.g. by tectonic rifting) leads to divergence
• Adaptive radiation: A lineage diversifies rapidly into many ecological niches (e.g. Darwin’s finches)

• Sympatric speciation: Speciation without geographic separation, often driven by ecological specialisation

• Evolution also drives the formation of diversity hotspots: areas of high endemism often represent ancient, stable refugia where species accumulated over long periods without periodic extinction events

Diversity Hotspots

A biodiversity hotspot (Conservation International definition) must have:
- ≥1,500 endemic vascular plant species
- ≤30% of original natural habitat remaining

There are 36 recognised hotspots globally, including:
- The Southwest Australian Floristic Region (WA)
- The Cape Floristic Region (South Africa)
- The Indo-Burma region
- The Cerrado (Brazil)

Hotspots typically form in areas with:
- Long periods of climatic stability (refugia)
- Complex topography
- Geographic isolation
- High habitat diversity

VCAA FOCUS: Be able to match each natural process to its approximate time scale and explain the mechanism by which it influences ecosystem diversity, endemism or extinction rate. VCAA often provides data on a specific natural event (e.g. a graph of species diversity after an El Niño event) and asks for interpretation.