ANALYSIS Nonlinear behavior of the socio-economic dynamics for lake eutrophication control Yoh Iwasa ⁎ , Tomoe Uchida, Hiroyuki Yokomizo Department of Biology, Faculty of Sciences, Kyushu University, Fukuoka 812–8581, Japan ARTICLE INFO ABSTRACT Article history: Received 28 May 2006 Received in revised form 2 November 2006 Accepted 3 November 2006 Available online 15 December 2006 To succeed in combating lake eutrophication, cooperation of local inhabitants, small factories, and farmers in reducing phosphorus discharge is very important. But the willingness of each player to cooperate would depend on the cooperation of other players and on the level of environmental concern of the society in general. Here we study the integrated dynamics of people's choice of behavior and the magnitude of eutrophication. Assumptions are: there are a number of players who choose between alternative options: a cooperative and environment-oriented option is more costly than the other. The decision of each player is affected by “social pressure” as well as by economical cost of the options. The lake pollution increases with the total phosphorus released, and a high pollution level in the lake would enhance the social pressure. The model includes a positive and a negative feedback loops which create diverse dynamical behavior. The model often shows bistability — having an equilibrium with a high level of cooperation among people and clean water, and the other equilibrium with low cooperation and polluted water, which are simultaneously stable. The model also shows fluctuation between a high and a low levels of cooperation in alternating years, cycle with a longer periodicity, or chaotic fluctuation. Conservatism of people stabilizes the system and sometimes helps maintaining cooperation. The system may show unexpected parameter dependence — the improved phosphorus removing efficiency might make water more polluted if it causes the decline in the environmental concern and cooperation among people. © 2006 Elsevier B.V. All rights reserved. Keywords: Socio-economic choice Ecosystem dynamics Social pressure Lake management Bistability 1. Introduction Most ecosystems receive a heavy influence of human activity over hundreds, sometimes thousands of years. To achieve a sustainable use of an ecosystem, we need to consider the integrated dynamics of ecological processes and socio-eco- nomic choice by human beings. For example, the water quality of shallow and large lakes is affected most importantly by nutrient input from various sources (Carpenter et al., 1998; Havens et al., 2001). For the success of combating lake eutrophication, an essential element is cooperation of many people, such as local inhabitants who implement efficient but costly sewage disposal, small factories whose operation is accompanied by reduced phosphorus discharge, and farmers who choose agriculture method with reduced phosphorus release from the farmland (Tabuchi, 2005). In these examples, cooperative players adopt environmentally benign but eco- nomically costly option over the alternative, and how much people are willing to contribute would be affected by the state of lake ecosystem. ECOLOGICAL ECONOMICS 63 (2007) 219 – 229 ⁎ Corresponding author. Tel.: +81 92 642 2639; fax: +81 92 642 2645. E-mail address: yiwasscb@mbox.nc.kyushu-u.ac.jp (Y. Iwasa). 0921-8009/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolecon.2006.11.003 available at www.sciencedirect.com www.elsevier.com/locate/ecolecon