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.