Explain why toxic compounds usually have the greatest effect on top-level carnivores

This is because toxins are ingested by organisms and accumulate in the fatty tissues. Theses toxins then become more concentrated in successive trophic levels of a food web biological magnification. Magnification occurs because the biomass at any given trophic level is produced from a much larger biomass ingested from the level below and therefore, top-level carnivores tend to be the organisms most severely affected by toxic compounds.