They are sprayed on crops to kill any insects trying to eat them.
Well, it is either using fertiliesers/weed killers/pesticieds on the fruits they eat or deforestaion, they destroying of their habitat
Free-Range for - Space to roam freely, organic, no pesticieds used, higher quality produce. Against- More expensive, and hard to find open space for it
As far as ecological webs go, if you reduce the number of a species, other animals which that species fed on will thrive. Other animals which fed on the animal killed by the pesticide will not, quite the opposite in fact. Other than that, pesticieds aren't good. If toxins remain in an animal it will generally pass onto another which eats it. An ironic way of nature getting back at humans, if it goes all the way to us... It might also be a selective pressure for some fast reproducing species like bacteria, causing them to evolve.