First order consumers are:

Producers are those living organisms which produce energy in the form of food for themselves and other living organisms. Plants are the main producers in the environment.

Consumers are the organisms which cannot produce their own food and depend on other organisms for energy. As herbivores are the animals which eat plants, they depend directly on the producers for their food. Therefore, they are the first order consumers.

Carnivores and omnivores feed on herbivores or other second order consumers for food. Decomposers feed on dated and decaying matter. Thus, they are not the first order consumers.