Describe biodiversity as the variety of all life forms on Earth, including the different plants, animals, micro-organisms, the genes they contain and the ecosystems they form, recognising biodiversity at the genetic, species and ecosystem levels

What this dot point is asking

QCAA wants you to define biodiversity and to recognise that it operates at three distinct but linked levels: genetic, species and ecosystem. This appears in multiple choice every paper and is the launchpad for short response items on measuring biodiversity or evaluating conservation strategies.

The answer

Biodiversity is the variety of all life forms on Earth, including the different plants, animals and micro-organisms, the genes they contain and the ecosystems they form. Biologists describe biodiversity at three levels.

Genetic biodiversity

Genetic biodiversity is the variation of alleles within a species, both within a single population and between populations.

Why it matters. Allelic variation is the raw material for natural selection. A species with high genetic biodiversity contains individuals with different combinations of alleles, so when the environment changes (a new pathogen, a heatwave, a salinity shift) at least some individuals carry alleles that confer resistance and survive to reproduce.

Named example. The cheetah (Acinonyx jubatus) is the classic case of low genetic biodiversity. A population bottleneck around 10 000 years ago left cheetahs so genetically uniform that skin grafts between unrelated individuals are not rejected. The species is vulnerable to disease outbreaks for this reason. In Australia, the Tasmanian devil shows the same pattern, and devil facial tumour disease has spread between hosts because most devils share MHC alleles and do not recognise the tumour cells as foreign.

Species biodiversity

Species biodiversity is the number and relative abundance of species in a defined area.

Two components.

• Species richness: the count of species present.
• Species evenness: how evenly individuals are distributed across those species. A community with 100 individuals split 50/50 across two species is more even, and more biodiverse, than one with 99 individuals of species A and 1 of species B.

Named example. A 1 hectare plot in the Daintree rainforest may contain more than 200 tree species. The same plot in a wheat monoculture in the Darling Downs contains one. Both have a defined area; only the rainforest has high species biodiversity.

Ecosystem biodiversity

Ecosystem biodiversity is the variety of habitats, communities and ecological processes within a region.

Why it matters. Different ecosystems support different species pools and provide different services (water filtration in wetlands, carbon storage in mangroves, pollinator habitat in heathland). A region with several ecosystem types buffers species loss when one ecosystem is disturbed.

Named example. Queensland contains tropical rainforest, mangrove forest, seagrass meadow, coral reef, dry sclerophyll woodland, freshwater wetland and arid grassland, all within the one state. The Wet Tropics World Heritage Area alone spans 20 distinct vegetation communities along an altitudinal gradient.

How the three levels connect

The levels are nested. Genetic biodiversity sits inside species biodiversity, and species biodiversity sits inside ecosystem biodiversity. Loss at one level cascades.

• Low genetic variation reduces the capacity of a species to adapt, which can lead to extinction.
• Loss of a species, especially a keystone or foundation species, alters community structure and can shift an ecosystem to a new state.
• Loss of an ecosystem removes the habitat that supported many species, reducing both species and genetic biodiversity in one step.

Worked example: the Great Barrier Reef

Genetic level. Coral species such as Acropora millepora show measurable variation in heat-shock protein alleles between northern and southern reefs, and individuals with these alleles bleach less readily.

Species level. The reef supports about 1500 fish species, 400 coral species, six of the world's seven marine turtle species and over 30 species of marine mammal.

Ecosystem level. Within the reef system there are coral reef, seagrass meadow, mangrove forest, lagoon and deep slope ecosystems, each with different physical conditions and species.

A bleaching event reduces species biodiversity directly (coral mortality) but also removes the habitat that fish species depend on (ecosystem effect) and selects against heat-sensitive alleles in surviving corals (genetic effect).

Common traps

Confusing species richness with species biodiversity. Richness is one component of species biodiversity; evenness is the other. Two communities with the same richness can have very different biodiversity.

Treating ecosystem biodiversity as a synonym for "ecosystem". Ecosystem biodiversity is the number and variety of ecosystem types in a region, not a single ecosystem.

Ignoring micro-organisms. Bacteria, archaea and fungi contribute heavily to all three levels. A handful of forest soil contains more bacterial genetic biodiversity than the entire vertebrate phylum.

Listing examples without linking back to a level. Markers want each example tagged: this example shows genetic biodiversity because, this example shows species biodiversity because.

In one sentence

Biodiversity is the variety of life at the genetic level (alleles within species), the species level (richness and evenness within communities) and the ecosystem level (the variety of habitats and ecological processes), and the three levels are nested so that loss at one cascades to the others.