The impact of a change in the ecological conditions, like the introduction of the pesticide DDT, can cascade through an ecosystem with wierd and unexpected consequences. Richard Fagerlund explains:
In the 1950s, the World Health Organization sent supplies of DDT to Borneo to fight mosquitoes that spread malaria among the people. The mosquitoes were quickly wiped out. But billions of roaches lived in the villages, and they simply stored the DDT in their bodies.
One kind of animal that fed on the roaches was a small lizard. When these lizards ate the roaches, they also ate a lot of DDT. Instead of killing them, DDT only slowed them down. This made it easier for cats to catch the lizards, one of their favorite foods.
About the same time, people also found that hordes of caterpillars had moved in to feed on the roofing materials of their homes. They realized that the lizards that previously had kept the caterpillar population under control had been eaten by the cats. And now, all over North Borneo, cats that ate the lizards died from DDT poisoning. Then rats moved in because there were no cats to control their population. With the rats came a new danger: plague. Officials sent out emergency calls for cats. Cats were sent in by airplane and dropped by parachute to help control the rats.
— Fagerlund (2013): Take a lesson from Borneo — go easy on the bug spray on SFGate.com.