The E1 Nino -Southern Oscillation (ENSO) phenomenon affects the interannual fluctuations of climate over much of the globe (e.g. Rasmusson and Carpenter, 1982, 1983;Rasmusson, 1985; Ropelewski and Halpert, 1986, 1987). Droughts in Australia, for instance, usually coincide with El Nino events (e.g. Ropelewski and Halpert, 1987). There has been, however, some concern about the stability of ENSO and its associations with climate fluctuations. These associations have been mapped using only about a hundred years of data. Can we be confident that ENSO will continue to operate in the future, especially as the global climate changes? If we knew that ENSO has been operating for much longer than the period of instrumental data we might be more confident that it will continue to operate in the near future, especially if it has survived periods of substantial climate change.
Quinn et al. (1986) and Hamilton and Garcia (1986) used documentary records of heavy rainfall in Peru to determine the dates of pre-instrumental E1 Nino events. Quinn et al. presented a chronology of events starting in the year 1541. Nicholls (1988a) produced a chronology of Australian droughts from the start of European settlement in 1788, compiled from documentary sources, and demonstrated that the Australian droughts and E1 Nino events tended to coincide, confirming studies which use later, instrumental data. Studies of ice cores (Thompson and Mosley-Thompson, 1986), alluvial flood deposits (Wells, 1986;Craig and Shimada, 1986), beach ridges (Sandweiss, 1986), shell middens (Rollins et al., 1986), and fossil mollusks (DeVries, 1986) suggest that El Nino has been affecting coastal Peru for at least several millenia.
A different approach is proposed here to examine whether ENSO has been operating for only a few hundred years or for thousands of years or even longer. The approach relies on the amplification of climate variability caused by ENSO and the evolutionary implications of this 'excess ' variability. Schreiber and Schreiber (1984), Diamond (1985), and Gibbs and Grant (1987) have suggested that aspects of ENSO might have evolutionary consequences.
Nicholls (1988b) demonstrated that the relative rainfall variability (mean of the absolute deviations of annual rainfall from the long term mean, divided by the long term mean) tended to be a third to a half greater in areas affected by ENSO than in other areas of similar mean rainfall. This was so even outside the immediate E1 Nino area of the eastern equatorial Pacific and the Pacific South American coast. ENSO causes 'excess' variability of annual rainfall in the areas where rainfall and ENSO are related. Marked adaptation of the biota in ENSO-affected areas to 'excess' precipitation variability might imply that ENSO has been operating long enough to allow these adaptations to evolve. Over most of Australia rainfall is affected by ENSO (Ropelewski and Halpert, 1987) so there should be considerable evidence of adaptations to this variability, if ENSO has been operating and affecting Australia for millenia. If the Australian biota show no clear adaptations to variable climate, relative to the biota elsewhere, this would imply that ENSO is not a very old phenomenon. Some adaptations of Australian wildlife to variable rainfall are described below.