A fellow ecologist has declared that it is nitrogen deposition, rather than climate change, that poses the largest threat to the world's ecosystems. Is he correct? Perhaps, but his extrapolations assume that tomorrow's human behaviors will be similar to today's – that people will continue with detrimental activity, either oblivious to the environmental consequences, or unwilling to act to avoid them. But what if people are more observant and adaptable than many ecologists would like to believe? Then, understanding the biggest threats – and the actions needed to minimize them – would require knowledge and assessment of human behavior.Any ecologist interested in ecosystem integrity and management – either today's or tomorrow's – should read Olsson and Folke's recent paper 1xLocal ecological knowledge and institutional dynamics for ecosystem management: A study of Lake Racken watershed, Sweden. Olsson, P. and Folke, C. Ecosystems. 2001; 4: 85–104Crossref | Scopus (219)See all References1, which describes a fishing association in Arvika, west Sweden, charged with management of the common-pool crayfish resource. Local citizens have perceived threats to the crayfish population as they have arisen, including water acidification, predation by mink, a fungal crayfish disease and overexploitation of the stock. They have not only acted to alleviate these threats but, more importantly, have done so in a manner that demonstrates a sophisticated understanding of resource and ecological dynamics, from the individual crayfish to the whole watershed level. For example, people are liming the lake, trapping mink, exchanging crayfish among different parts of the lake to prevent inbreeding, and have imposed temporary fishing bans. These are local and voluntary actions, and are not compelled through national guidelines or regulations.Olsson and Folke delve further in the natural-resource management literature, and demonstrate that the characteristics of the Arvika approach (flexibility, and intervention and monitoring at a variety of scales) are precisely those that are needed to sustain ecological systems. They discuss a current problem (too slow a recovery of crayfish populations) and its possible solutions, one involving restocking, the other managing key processes and functions in the crayfish habitat. Ecologists would choose from those alternatives by using models of crayfish dynamics, or experimental manipulations across a series of lakes. In doing so, these scientists would miss a key to the future success or failure of policies – the maintenance or erosion of the local institutions already devoted to sustaining lake functioning. The first solution requires only money, and could lead to greater initial increases in crayfish populations. The second requires an increase in local knowledge and coordination, keeping the stewards involved with their lake systems. This would lead not only to greater long-term success in addressing the current problem, but also to a greater capacity to respond to future problems.Ecologists tend to evaluate ecological dynamics or formulate ecological forecasts in the absence of sophisticated human behaviors and responses. They team too rarely with social scientists, policymakers and managers to understand how ecological systems that contain humans as an essential component might really work. A forecast or intervention that is unimpeachable in a scientific journal can fail dramatically in the real world. We must come to a more integrated and interdisciplinary knowledge of our ecological systems if we are to simultaneously sustain them and improve the human condition. The Olsson and Folke paper is a wonderful step in that direction.