Human impact has a different effect on each species. In the case of the Peregrine Falcon, it's often said that it wasn't so much habitat destruction or a change in habitat; it was more the impact of the products, the pesticides that were used in the 40s, 50s and 60s, notably DDT. It's a killer, an unbelievable pesticide. It solved a lot of problems but the problem with
DDT is that its effect is cumulative, it's not biodegradable. We call that bioaccumulation and predators are the first to be affected by bioaccumulation. Predators are at the top of the food chain. They eat small rodents which of course eat crops, so they accumulate pesticides. A number of small rodents become infected with DDT, and a predator
can eat several rodents a day. The pesticide accumulates in the predator's body, and the impact that was noted in Peregrine falcons was the fragility of the shells of the eggs they laid. That's a true indicator of the state of environmental health. The Peregrine Falcon is only one example but it applies to all predators. If we want to determine if a forest, a
habitat or an ecosystem is healthy, we immediately check the health of the predators to find out what's happening in a particular habitat. If the situation is bad at the bottom of the food chain, the first to die are at the top of the food chain; they're the ones that will suffer the most direct impact.