Change can occur in ecosystems naturally over time (bushfires, succession) or as a result of human activity (land clearing, climate change).

Human Impacts on Ecosystems.

Bushfires

Information about the effects of bushfires on the environment from NOVA - Australian Academy of Science.

Climate Change

Information from the Australian Academy of Science about Climate Change, including links to Impact of Global Warming on Biodiversity, Rising Sea Levels; Coral Bleaching  and Effect of Global Warming on Human Health.

Natural Change in Ecosystems

This site, from Physical Geography, describes Plant Succcesion in detail.
Although this is an example form the northern hemisphere, the same principles apply in Australia - annual grasses and herbs give way to perennials, which are replaced by shrubs and then trees. This will only happen when the soil type and climate is suitable - in the western volcanic plains, the "climax community" was grassland due to deep cracking soils, temperate climate and relatively low rainfall.
succession.gif
Another example of ecosystem change is how coral cays form in tropical marine environments over time. The following example is from Encyclopedia Britannica.
coral_reef_formation.gif