Analyse spatial patterns of biodiversity at global scale, including biodiversity hotspots, centres of endemism, the latitudinal gradient, and the principles of island biogeography

Note: This page is part of the HSC Geography 11-12 (2022) syllabus, first examined in HSC 2025. The legacy 2009 syllabus "Ecosystems at Risk" content is preserved in sibling folders.

What this dot point is asking

This dot point asks you to describe and explain the spatial pattern of life on Earth. Biodiversity is not evenly distributed; some regions hold a disproportionate share of species, and many of those species are found nowhere else. You need to be able to define the kinds of biodiversity, use the biodiversity hotspots framework, refer to the latitudinal gradient, and apply island biogeography principles. Lean on the geographical concepts of spatial distribution, scale, and interconnection.

The answer

Three levels of biodiversity

Biodiversity is measured at three levels:

Species diversity

The number and relative abundance of different species in a defined area. Often the headline figure, but only one of three layers.

Genetic diversity

Variation within species (different populations, different alleles). Important because genetic variation provides the raw material for adaptation to environmental change. A population reduced to low numbers loses genetic diversity even if it recovers in count.

Ecosystem diversity

The variety of ecosystem types within a region. A landscape with rainforest, savanna, wetlands and reef has more ecosystem diversity than one of uniform plantation.

Strong responses use all three; a question about biodiversity loss is not just a question about lists of extinct species.

The latitudinal gradient

Across most groups of organisms, species richness peaks near the equator and declines toward the poles. Tropical rainforests, coral reefs and tropical savannas hold a disproportionate share of global biodiversity. Proposed explanations include greater available energy and productivity in the tropics, longer geological stability without ice-age glaciation, larger tropical land area in deep time, and faster speciation rates. No single explanation is fully agreed upon in the scientific literature, but the pattern itself is well-established.

This gradient is one of the strongest spatial patterns in geography and underpins why tropical conservation gets disproportionate global attention.

Biogeographic realms

The continents and oceans are divided into broad biogeographic realms with distinct evolutionary histories: Nearctic (North America), Neotropical (South America), Palaearctic (Eurasia and North Africa), Afrotropical (sub-Saharan Africa), Indomalayan (South and South-East Asia), Australasian, Oceanian and Antarctic. Australia and New Zealand together form an Australasian realm with deep evolutionary isolation since the breakup of Gondwana; this is why Australia has a fauna dominated by marsupials and monotremes found almost nowhere else.

Biodiversity hotspots (Conservation International framework)

The biodiversity hotspots framework was developed by Norman Myers in the late 1980s and is now maintained by Conservation International. A region qualifies as a hotspot if it meets two criteria:

1. It contains at least 1500 species of vascular plants that are endemic (found nowhere else).
2. It has lost at least 70 percent of its primary native vegetation.

Conservation International recognises 36 biodiversity hotspots globally (as of recent updates). Hotspots collectively cover a small share of Earth's land surface but hold a very large share of endemic plant and vertebrate species. The framework is influential because it directs conservation funding and attention to the places with the most to lose.

Notable hotspots include:

• Cape Floristic Region (South Africa). Exceptional plant endemism in fynbos vegetation; a globally significant centre of plant diversity in a small area.
• Madagascar and the Indian Ocean Islands. Deeply isolated continental island with extensive endemic lemurs, baobabs and reptiles.
• Forests of East Australia. Includes parts of the Wet Tropics of Queensland and the Gondwana Rainforests; recognised for high plant endemism and ancient evolutionary lineages.
• South-west Australia. Mediterranean-climate region with very high plant endemism on ancient nutrient-poor soils.

Centres of endemism

A centre of endemism is a region with an unusually high number of species found nowhere else. Endemism arises through long isolation, climatic stability, or unique substrates. Important examples for HSC reference:

• Madagascar. Separated from mainland Africa for tens of millions of years; lemurs, fossas and tenrecs are endemic; the baobab family includes Madagascar-endemic species.
• Australia. A continent-scale centre of endemism for marsupial mammals, eucalypts and acacias, reflecting long isolation since Gondwana broke up.
• Cape Floristic Region. Roughly 9000 plant species in a small area, the great majority endemic.
• Galapagos and the Hawaiian Islands. Oceanic islands with high endemism shaped by isolation and adaptive radiation.

Endemism matters because endemic species cannot be replaced by recolonisation from elsewhere; an extinction in a centre of endemism is a global loss.

Island biogeography

MacArthur and Wilson's theory of island biogeography (1967) is the canonical framework for understanding why isolated systems matter. Two principles drive the number of species an island supports:

• Island size. Larger islands support more species because they hold more habitat types and larger populations less prone to extinction.
• Distance from a mainland. Closer islands receive more colonising species; isolated islands receive fewer.

Equilibrium species richness on an island is set by the balance between immigration (slowing as more species are present) and extinction (rising as more species compete). The framework extends beyond literal islands: a national park surrounded by farmland, a remnant patch of native vegetation, or a coral bommie on a sandy seafloor all behave as ecological islands. This is why habitat fragmentation is a biodiversity threat: it converts continuous habitat into smaller, more isolated patches that follow the island rules.

Island biogeography is also why isolated systems are biodiversity-fragile. Species evolved without competition from continental mammals, large predators or aggressive weeds can be devastated by introductions; Australia's mammal extinction record is a national-scale example of this principle in action.

Examples in context

Example 1. Madagascar as a centre of endemism. Madagascar separated from mainland Africa and India in deep geological time, producing one of the most distinctive biotas on Earth. Lemurs, fossas, tenrecs and many endemic plant lineages evolved in isolation. Habitat loss from slash-and-burn agriculture, charcoal production and mining has stripped a large share of original forest cover (estimates vary widely across published assessments). Madagascar is recognised by Conservation International as a biodiversity hotspot. The case demonstrates how the spatial distribution concept connects deep evolutionary isolation to current conservation priority.

Example 2. The Wet Tropics of Queensland. The Wet Tropics World Heritage Area on Australia's north-east coast covers a relatively small area but holds an unusually high share of Australia's vertebrate species and ancient flowering-plant lineages with Gondwanan affinities. It is part of the recognised Forests of East Australia hotspot. Fragmentation from agriculture in surrounding lowlands has produced an "island" pattern: tropical forest blocks separated by cleared land, behaving according to island biogeography principles. The case demonstrates how hotspots, endemism and fragmentation interact within Australia and links directly to the dot point on threats to biodiversity.

Try this

Q1. Define the three levels of biodiversity and give one example of each. [3 marks]

• Cue. Species (number and abundance), genetic (variation within species), ecosystem (variety of ecosystem types). Examples for each.

Q2. Explain Conservation International's biodiversity hotspots framework and identify two hotspots, one outside Australia and one within. [6 marks]

• Cue. Two criteria (1500 endemic plant species; 70 percent habitat loss). 36 globally recognised hotspots. Outside: Madagascar / Cape Floristic Region / Atlantic Forest. Inside: Forests of East Australia / South-west Australia.

Q3. Analyse the principles of island biogeography and apply them to a fragmented landscape of your choice. [8 marks]

• Cue. MacArthur and Wilson (1967): island size and isolation determine species number; equilibrium between immigration and extinction. Apply to a remnant-vegetation landscape (e.g. cleared Cumberland Plain, Wet Tropics, urban bushland patches). Reference fieldwork, remote sensing and GIS as tools for measuring patch size and connectivity.