Astrobiology Workshop, Macquarie University July 12-13 2001

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Ecosystems, from life to planets to galaxies

Michael Burton (School of Physics, University of New South Wales, Australia and School of Cosmic Physics, Dublin Institute for Advanced Studies, Ireland)

Ecosystems are traditionally associated with life on the Earth, communities of organisms interacting and evolving in a manner determined by the totality of the biological and physical environment in which they exist. Driven by an energy source, typically the Sun, material is re-cycled and the system is in a self-organised, non-equilibrium, self-regulating state. Ecosystems are not confined just to regions on the Earth, however. The Earth itself can be regarded as such a system, the Gaia hypothesis itself argues that natural selection, from the time of the earliest bacteria, and the role of organisms in the chemical cycles of the biosphere, serve to regulate the contents of the oceans and atmosphere. On a larger scale, our Galaxy can be viewed as an ecosystem, the exchanges of matter and energy between the stars and the interstellar medium driving the cycle of star formation across the spiral arms in an apparently stable, but non-equilibrium state. It also produces the elements, and possibly the organic molecules, that are a necessary precursor to life.

The Galaxy is the simplest ecosystem we know of and also, perhaps, the most complex naturally occurring phenomenon not involving living organisms. As such it may provide insight into the far more complex ecosystems that do involve life. The existence of complex structures over such vastly disparate scales as life to galaxies is also reminiscent of the behaviour of a critical system, where structure exists at every scale, and every part is influenced by what happens elsewhere. This talk will use the example of the galactic ecosystem to illustrate some of these points.