OIKOS 94: 533 – 547. Copenhagen 2001 Adding artificial feedback to a simple aquatic ecosystem: the cybernetic nature of ecosystems revisited John E. Petersen Petersen, J. E. 2001. Adding artificial feedback to a simple aquatic ecosystem: the cybernetic nature of ecosystems revisited. – Oikos 94: 533 – 547. A cybernetic system can be defined as one controlled by feedback, that is, a system in which input is partially determined by output. I explored the cybernetic properties of a simple planktonic ecosystem by introducing an artificial feedback loop; light energy delivered to the system was linked to the ecosystem’s productivity and respiration. Specifically, I programmed a computer to turn lights on and off when dissolved oxygen reached low and high setpoints, respectively. Three treatments were applied that differed in light intensity and in range between high and low setpoints. Experiments were repeated under high and low nutrient conditions. The added feedback did not substantially alter responses to limiting factors from those expected under fixed duration lighting. However, several novel features were observed, includ- ing poor coupling between productivity and respiration, similar patterns in energy demand among treatments, and oscillations in primary productivity. These observa- tions can be viewed as support for a holistic, cybernetic view of ecological systems. This view complements the dominant mechanistic-reductionist perspective on causal- ity in ecosystems. The experimental addition of new feedback is apparently a useful means of investigating the self-organizational properties of ecosystems and may also improve our understanding of the consequences of anthropogenically induced feed- back in natural and managed systems. J. E. Petersen, Oberlin College, Center for Enironmental Studies, 122 Elm St., Oberlin, OH 44074, USA ( john.petersen@oberlin.edu). ‘‘Neither a purely reductionist approach nor a merely holis- tic perspective is sufficient to encompass the intrinsic nature of the system’s behavior.’’ (Pahl-Wostl 1993) The mechanistic-reductionist approach to science is characterized by the search for cause at the next lower level in a hierarchy of causality (Ulanowicz 1990). This bottom-up approach to scientific inquiry is perhaps epitomized by the ‘‘central dogma’’ of biology (DNA codes for RNA which codes for proteins). Although this reductionist approach has contributed to the revo- lutionary advances that we have recently witnessed in molecular genetics, it has not led to similar advances in ecosystem science (Jørgensen et al. 1992, Ulanowicz 1997). Indeed, what might be termed the central mantra of systems ecology – ‘‘the whole is greater than the sum of its parts’’ – can be viewed as an explicit acknowledg- ment that there is a broad range of phenomena that can only be observed and interpreted at the whole-system level, and that these phenomena are therefore not amenable to the power of a purely reductionist approach. At the heart of the holistic versus reductionist schism is the issue of what controls the ecological behavior that we observe. During the early 1980s, ‘‘cybernetics’’, the study of control systems (Wiener 1948, Ashby 1963), provided the context for a lively debate of this issue (Engelberg and Boyarsky 1979, Patten and Odum 1981, Straskraba 1983). Patten and Odum (1981) of- fered a minimalist definition that distinguished cyber- netic systems from non-cybernetic systems by the presence of feedback control; in cybernetic systems, ‘‘input is determined, at least in part, by output’’. Accepted 2 April 2001 Copyright © OIKOS 2001 ISSN 0030-1299 Printed in Ireland – all rights reserved OIKOS 94:3 (2001) 533