are the beginning link in a food chain while
are at the end of the chain

Producers are the beginning link in a food chain, while decomposers are at the end of the chain.

Explanation:

The food chain, in simple terms, is a sequence in which nutrients and energy are passing from one organism to another through 'predation'.

Producers are the organisms that produce their own food, they are also known as autotrophs. An example of this group of organisms is plants.

Then, producers are eaten by primary consumers, e.g. herbivores. Then, these organisms will be eaten by secondary consumers, which are carnivores. These (usually small) carnivores will be consumed by terciary consumers, which could be larger but are not yet considered apex predators, for example an eagle. Then, quaternary consumers, or apex predators, will eat these carnivores.

We could say this is the end of the food chain. However, this is not the case. Decomposers are organisms that feed on decaying matter and break down dead organic material. Examples are some species of bacteria, fungi, worms.

It is important to highlight that some researchers locate these organisms in a separate 'individual' trophic level because they can consume residues of plants or residues of predators. However, generally, they are considered the end of the chain or the 'cleaners' of the chain.

the darker peppered moth species were favored due to the fact that the industrial revolution had made all the nearby trees a darker color instead of white.