What do you call the number of species or organisms per unit area found in different habitats?

Biodiversity comes from two words Bio meaning life and diversity meaning variability.

Biodiversity is the variety of all living things; the different plants, animals and micro organisms, the genetic information they contain and the ecosystems they form.

Biodiversity is usually explored at three levels - genetic diversity, species diversity and ecosystem diversity. These three levels work together to create the complexity of life on Earth.

Genetic diversity

Genetic diversity is the variety of genes within a species. Each species is made up of individuals that have their own particular genetic composition. This means a species may have different populations, each having different genetic compositions. To conserve genetic diversity, different populations of a species must be conserved.

Genes are the basic units of all life on Earth. They are responsible for both the similarities and the differences between organisms.

Not all groups of animals have the same degree of genetic diversity. Kangaroos, for example, come from recent evolutionary lines and are genetically very similar. Carnivorous marsupials, called dasyurids, come from more ancient lines and are genetically far more diverse. Some scientists believe that we should concentrate on saving more genetically diverse groups, such as dasyurids, which include the Tasmanian Devil, the Numbat and quolls.

If we lose one species of dasyurid, we lose a substantial genetic resource. Several species of dasyurids are endangered and at least one, the Tasmanian Tiger, has disappeared forever since Europeans arrived in Australia.

Australia's frogs need your help. Donate to the Australian Museum's FrogID project and you'll be supporting essential frog research and conservation.

Species diversity is the variety of species within a habitat or a region. Some habitats, such as rainforests and coral reefs, have many species. Others, such as salt flats or a polluted stream, have fewer.

In Australia, more than 80% of plant and animal species are endemic, which means that they only occur naturally in Australia.

Species are grouped together into families according to shared characteristics. In Australia, it is not just the individual species that are endemic - whole families of animals and plants are endemic. Seven families of mammals, four of birds and twelve of flowering plants are endemic to Australia. No other country has as many endemic flowering plant families as Australia.

Invertebrates - animals without backbones - make up about 99% of all animal species, and most of these are insects. Invertebrates include crabs, snails, worms, corals and seastars, as well as insects, such as beetles and flies. Insects fill many vital roles in ecosystems as pollinators, recyclers of nutrients, scavengers and food for others.

While we may mostly notice mammals, they actually make up less than 1% of all animal species.

Ecosystem diversity is the variety of ecosystems in a given place. An ecosystem is a community of organisms and their physical environment interacting together. An ecosystem can cover a large area, such as a whole forest, or a small area, such as a pond.

An ecosystem is a community of organisms and their physical environment interacting together. An ecosystem may be as large as the Great Barrier Reef or as small as the back of a spider crab's shell, which provides a home for plants and other animals, such as sponges, algae and worms.

Megadiversity describes countries with very high levels of biodiversity.

Twelve of the megadiverse countries, including Australia, contain about 75% of Earth's total biodiversity.

As a 'developed' nation, Australia has a special responsibility for biodiversity conservation and management. Other megadiverse countries include Brazil, Colombia, Ecuador, Peru, Mexico, Democratic Republic of the Congo, Madagascar, China, India, Indonesia and Malaysia. It is estimated that there are 13.6 million species of plants, animals and micro-organisms on earth. Australia has about one million of these, which represents more than 7% of the world's total and is more than twice the number of species in Europe and North America combined.

Australia was once part of the great southern supercontinent Gondwana, which also included South America, Africa, India and Antarctica.

Due to the geological process known as plate tectonics, Gondwana began to break up some 140 million years ago and about 50 million years ago, Australia eventually split from Antarctica.

Australia's unique biodiversity and our high number of endemic species (that is, they only occur naturally in Australia) is mostly explained by the isolation of our continent from other land masses.

The best way to conserve biodiversity is to save habitats and ecosystems rather than trying to save a single species.

Many high profile conservation campaigns focus on rescuing a single endangered species from extinction, such as blue whale, bilbies, koala. No organism, however, exists in isolation. If a species is at risk, then the habitat in which it lives is probably under threat too.