Rat poison is ripping through the food chain, threatening predators at the very top

Rats thrive around humans, for good reason: They feed off crops and garbage and readily adapt to many settings, from farms to the world's largest cities. To control them, people often resort to poisons. But chemicals that kill rats can also harm other animals.

The most commonly used poisons are called anticoagulant rodenticides. They work by interfering with blood clotting in Animals that consume them. These enticingly flavored bait blocks are placed outside of buildings, in small black boxes that only rats and mice can enter. But the poison remains in the rodents' bodies, threatening larger Animals that prey on them.

My colleagues and I recently reviewed studies from around the world that sought to document wild mammal carnivores' exposure to anticoagulant rodenticides. Many animals tested in these studies were already dead; others were alive and a part of other studies.

Researchers detected rodenticides in about one-third of the Animals in these analyses, including bobcats, foxes and weasels. They directly linked the poisons to the deaths of one-third of the deceased Animals — typically, by finding the chemicals in the Animals' liver tissues.

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Most poisons that these studies detected were so-called second-generation anticoagulant rodenticides, developed since 1970. These products are used exclusively in residential and urban areas and can kill a rat or mouse after just one night's feeding. First-generation rodenticides, which typically are used only on farms, require several doses to kill.

These poisons are widely available, and their use is largely unregulated in most countries. Using rodenticides is projected to increase and may be contributing to declines in many carnivore species around the world.