Define the levels of biodiversity and describe how species richness and evenness are measured.

What this dot point is asking

The three levels of biodiversity

Biodiversity is more than just a count of species. It is measured at three connected levels:

• Genetic diversity: the variety of alleles and genotypes within a single species. High genetic diversity helps a population adapt to change and resist disease.
• Species diversity: the variety of different species in a community or area.
• Ecosystem diversity: the variety of habitats, communities, and ecological processes across a region or the whole planet.

All three matter for the health and resilience of life. A region might have many ecosystems but, if each is dominated by one species, its species diversity could still be low.

Species richness and evenness

Species diversity has two components that must be considered together:

• Species richness is simply the number of different species present.
• Species evenness describes how evenly the individuals are spread among those species.

A community is more diverse when it has both high richness and high evenness. Two communities can have the same richness but very different diversity if one is dominated by a single species.

Diversity indices

To capture richness and evenness in one figure, ecologists use a diversity index such as Simpson's index. The index returns a value that rises as both richness and evenness increase, so it is a fairer comparison than a raw species count. The exact formula is less important at this level than the idea: a single number that reflects both how many species are present and how balanced their abundances are. Comparing index values lets ecologists track change over time or compare two sites objectively.

Sampling to estimate biodiversity

It is rarely possible to count every organism, so ecologists sample. Common methods include:

• Quadrats: square frames placed (often randomly) to count or estimate the percentage cover of plants and slow-moving animals.
• Transects: a line across a habitat along which organisms are recorded, useful for showing how communities change along an environmental gradient.
• Capture-mark-recapture: for mobile animals, a sample is caught, marked, and released, then a second sample is taken to estimate total population size.

Random placement and adequate sample numbers reduce bias and make estimates more reliable.

Why biodiversity matters

High biodiversity tends to make ecosystems more stable and productive, and more able to recover from disturbance. Diverse ecosystems provide services such as pollination, clean water, and climate regulation. Loss of biodiversity, through habitat destruction, climate change, pollution, and introduced species, reduces this resilience. Measuring biodiversity gives scientists a baseline to detect decline and to evaluate conservation efforts.

Endemism and hotspots

Some species are endemic, meaning they are found naturally in only one place, such as many Tasmanian species. Areas with high numbers of endemic species and high overall diversity are called biodiversity hotspots and are conservation priorities, because losing habitat there causes a disproportionate loss of unique life.