The observations on Galapagos during El Niño show that one has to be aware of previous disturbances to an ecosystem when beginning an experiment. Many of the effects observed in the Galapagos were so long-term that only a study covering several years before the onset of an experiment would have allowed valid interpretation of the experimental period. To assume that a system under investigation is in equilibrium when an experiment is started seems quite unwarranted. This is clearly borne out by the examples in this chapter: 17 year seed dormancy, long-term effects of selection on finches and long-term after-effects on the age of structure of plant, reptile, and mammal populations are difficult to detect at the beginning of a study, but potentially have a great influence on later interpretation of the data. Furthermore, an experimental change in an ecosystem may change the conditions for immigration/emigration of organisms, and in this way change the composition of the system. Last but not least, El Niño demonstrated quite convincingly that variability in an ecosystem is an important component of the system. If such variance creates occasional bottlenecks for populations it may produce conditions for (more rapid) evolution of the component species. Experimentally altering conditions in an ecosystem and observing the response of the system and its components short term, may thus lead to very short-term predictions only if it is overlooked that important components of the ecosystem under study may change over a few generations due to directional selection under the altered conditions. 

ACKNOWLEDGEMENTS 

I would like to thank the Galapagos National Park Service under its Intendentes M.Cifuentes, F. Cepeda, and U. Ochoa for permission to work in the Galapagos, and to thank them and the directors of the Charles Darwin Station, C. MacFarland, H. Hoeck, F. Koster and G. Reck, for their continuous support of my own work in the Galapagos, as well as for many interesting discussions. Ole Hamann, Henning Adsersen, and Jonas Lawesson discussed botanical questions with me and kindly provided published and unpublished material. I greatly appreciate their help and patience. A. Laurie and H. and H. Snell filled me in with exciting information on iguana life during El Niño and allowed me to use their unpublished data. Max-Planck Gesellschaft, through W. Wickler's permanent help, encouragement and support, made my work in Galapagos possible. This is contribution No.433 of the Charles Darwin Research Station.