Equilibrium and nonequilibrium in Australian bird communities - the impact of natural and anthropogenic effects

The concept of nonequilibrium has been applied to populations that do not trend towards an equilibrium point, and in which the direction of changes in population size appear not to be dependent on density (Rohde, 2005). Furthermore, communities, such as those on islands or in nature reserves, would be considered to be in nonequilibrium if their species richness or diversity changes progressively over time. Many studies on the concept of equilibrium have been on birds. In this chapter I identify several examples from Australian birds that I believe support the concept of nonequilibrium at both population and community levels. First, the majority of Australia is arid or semi-arid, with low and unpredictable rainfall, meaning that conditions are usually difficult for birds, but occasionally there are times of relative plenty, after heavy rain, which allow population increases. Next, I examine the concept of species equilibrium on Australian islands, and extend this to the declining woodland birds of the fertile grassy woodlands of southern Australia, which have become fragmented and degraded by human activity. Finally, Australia is experiencing climate change, which is likely to intensify in the future, so I shall consider the probable impacts of this on Australia's birds.