Food chains may become poisoned when one organism that is consumed by another contains a toxic substance. The act of poisoning may begin at any level of the food chain. Depending on the kind of toxic substance, the poisoning can affect many levels of organisms.
Let's imagine a few scenarios.
First, let's consider a food chain which is poisoned from the producers onward. Imagine someone sprayed a pesticide on their lawn to kill some ants. A rabbit who eats some of this grass might ingest the pesticide and become ill. A larger consumer — say, a cat — who eats the rabbit may also become sick.
In another scenario, primary consumers may be the origin of poisoning in a food chain. Imagine a mouse has eaten some poison set by a human, and is later caught by an owl. The owl becomes sick from the poison in the mouse and dies, later to be eaten by a coyote. This scavenger may also become sick, depending on the strength of the poison.
In these previous two examples, the poisoning was directly or indirectly caused by humans. Though this is a major issue in our world as waste disposal has great effects on our environment, human intervention is not the only source of poison. Many plants, animals, and fungi naturally produce toxins of some kind. If a poisonous frog or mushroom is eaten by a consumer, the process of poisoning has not been initiated by a human but can affect a food chain all the same.
Bioaccumulation also presents a real problem in poisoning animals. The higher up you move in a food chain, the more food is necessary to survive. If a small animal eats just a little bit of poison, they might be okay. A large animal who eats many small — but poisoned — animals ultimately ingests quite a lot of poison. This is especially harmful in cases where organisms do not have a highly diversified diet — animals who rely on just one or two food sources become more susceptible to poisoning by bioaccumulation.