This is a sobering story often told to biology classes to illustrate the importance of a food chain and its ecological interrelations; The Royal Air Force and the World Health Organization years ago parachuted domestic cats into remote villages in Borneo in which all the local cats had died, allowing a population explosion of rats (which are potential carriers of all sorts of nasty diseases such as typhus, leprosy, and plague).
Why did all the native cats die?
Here is how it happened . . .
The insecticide DDT, had been sprayed to eradicate malaria-carrying mosquitoes (as many as 90 percent of the people suffered from malaria). The mosquitoes in the story were controlled by spraying the insides of village huts with DDT.
Malaria was indeed eradicated. All seemed well until the thatched roofs of the huts began to collapse on their occupants, and the village was over run by rats. It seems that the larvae of a moth that was normally present in the hut roofs - but never before in such numbers were eating the thatch. Apparently they'd undergone a population explosion. The DDT had also killed off the moth's predator, a parasitic wasp, but the moth larvae had the sense not to eat DDT.
Still, what is the loss of a few thatched roofs compared to eradicating malaria? There were yet further consequences - the DDT was eaten by cockroaches, though not in great enough quantities, alas, to kill them. Since DDT isn't broken down or excreted very well, it just builds up in the body. Next, the friendly neighborhood geckos, those lizards that walk across ceilings with their suction cup feet, ate the DDT-laden cockroaches. One gecko has to eat a lot of cockroaches to make a living, and the DDT from the cockroaches accumulated in the bodies of the geckos until it reached concentrations an order of magnitude higher than in the cockroaches. But, still not enough to kill the geckos.
The trouble was, the village cats while keeping down the rat population, also ate lots of geckos. Hundreds of cats were therefore accumulating the DDT ingested by millions of cockroaches. And while the DDT concentrations were never high enough to kill very many cockroaches or geckos, they finally did become concentrated one order of magnitude too high - and killed the cats. And saved the rats. And helped spread the other nasty diseases.
"Operation Cat Drop" eventually restored the cat population and eased the threat of plague, proving that ignorance is expensive. Just knowing that mosquitoes spread malaria and that DDT kills mosquitoes isn't sufficient - you've got to understand what else will eat DDT, even in sublethal doses, and so on up the food chain.
The whole system's the thing. It is called ecology. Our agricultural/medical/industrial society is dumping all sorts of new chemicals into the environment [or our bodies], with little knowledge of what they'll do. The remedy - if any - isn't usually as simple as parachuting cats.
Does this remind you of anything? Like perhaps, the Pandora's box we open with the overuse of antibiotics or other medications?