In the early 1950s, there was a serious outbreak of malaria amongst the Dayak people in Borneo. The World Health Organization (WHO) decided to help and massively sprayed the area with DDT. The mosquitoes died in large numbers and the Dayak were happy - for a while. However, in complex situations the likely systemic impacts of actions, even if well intended, need to be taken into acount. In this particular case the Dayak got a very unpleasant surprise. The DDT did not only kill mosquitoes but also wasps. But these wasps used to feed on caterpillars that feed on thatched roofs. Without the wasps the caterpillars rapidly increased in number with disastrous effect on the structure of thatched roofs, which began to collapse. But that was not all. Gecko lizards ate the poisoned insects, and  as cats often catch and eat geckoes they  began to die in large numbers. Soon the number of rats increased. Not only were the rats a nuisance but they carried sylvate plague and typhus. This simple Sign Graph illustrates that they can be very effective in identifying unintended negative consequences, and on many occasions they can help to identify potential side effects before they become a serious problem.


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