Describe the structure of ecosystems and explain why biodiversity at gene, species and ecosystem levels matters for ecological function.

What this dot point is asking

This dot point asks you to describe how an ecosystem is put together, and to explain why the variety of life it holds, its biodiversity, is essential to keeping that ecosystem working. You should be able to define the levels at which biodiversity is measured and link them to ecological function using real Tasmanian and Australian examples.

The structure of an ecosystem

An ecosystem has two parts that constantly interact. The biotic component is every living thing, from bacteria and fungi to plants and animals. The abiotic component is the non-living environment: sunlight, temperature, water, soil, nutrients, salinity and pH. These parts are organised into a hierarchy. A single organism belongs to a population of its own species, populations of different species form a community, and the community together with its abiotic surroundings forms the ecosystem.

Each species occupies a niche, which is its particular role and position in the ecosystem, including what it eats, where it lives, when it is active and how it interacts with others. The habitat is the physical place a species lives, while the niche is the way it makes a living there. When two species compete for the same niche, one usually outcompetes the other, so coexisting species tend to divide resources.

Levels of biodiversity

Biodiversity is described at three connected levels.

• Genetic diversity is the variety of genes within a single species. A population with high genetic diversity has more variation to draw on when conditions change. The Tasmanian devil shows the danger of low genetic diversity, because its limited variation helped Devil Facial Tumour Disease spread so quickly.
• Species diversity is the number of different species in an area (richness) and how evenly individuals are spread among those species (evenness). A Tasmanian wet eucalypt forest has high species diversity.
• Ecosystem diversity is the variety of ecosystems within a region, such as the alpine moorlands, rainforests, buttongrass plains and giant kelp forests found across Tasmania.

Why biodiversity supports ecological function

High biodiversity supports the processes that keep ecosystems running. Different species capture energy, recycle nutrients, pollinate plants, control pests and build habitat. Because more than one species often performs a similar role, this functional redundancy means an ecosystem can keep working even if one species declines. This buffering effect makes biodiverse ecosystems more resilient to disturbances such as fire, drought or disease.

Biodiversity also supports productivity and stability. A varied community uses available resources such as light, water and nutrients more completely than a single species could, so total productivity tends to rise. Diverse food webs have many alternative pathways, so the loss of one link is less likely to collapse the whole system.

Tasmania's giant kelp (Macrocystis pyrifera) forests are a strong example. These underwater forests once fringed the east coast, providing habitat, food and shelter for fish, invertebrates and seals. Warming waters and the spread of the long-spined sea urchin have destroyed most of them, turning structured forests into bare urchin barrens with far lower biodiversity and far less ecological function. The collapse shows how the loss of one structuring species and rising temperatures can strip an ecosystem of the variety that kept it productive.

Bringing it together

To answer this dot point well, describe the biotic and abiotic structure of an ecosystem, define the three levels of biodiversity, and then explain, with a Tasmanian example, how that diversity underpins productivity, stability and resilience. Use the kelp forest collapse or the Tasmanian devil to show what happens when biodiversity is lost.