Why are green plants called producers?

In a food chain comprising frogs, insects, birds and grass, which one of the organisms is likely to have maximum concentration of harmful non-biodegradable chemicals in the body.

In a certain study conducted on the occurrence of DDT along food chains in an ecosystem, the concentration of DDT in the grass was found to be 0.5 ppm. ln sheep, it was 2 ppm and in man, it was 10 ppm. Why was the concentration of DDT maximum in the case of man?