Explain how a toxin in a food web will harm some while killing others why did the mosquito and cats die while other things lived

I think that a toxin that was introduced into a food web can kill some species but not others because some may have resistances to the toxin, while others, it may be lethal. Also that it did not affect everyone because as Bioaccumulation has stated if affected one organism but their might not be much biomagnification as in much increase in the correlation between the food chains in that area. i think the cat were like the top animal so the key stone animal of the food chain and also it had no predators. 

Cbecause it is the only one describing a habitat