Primary consumers are those that are herbivorous - they only eat producers. Primary consumers are the link between the base of an ecosystem's chain food - where simple substances are transformed into organic compounds by photosynthesis or chemosynthesis - and the upper layers - where energy is obtained from, mostly, other animals. If primary consumers were not found in an ecosystem, all other layers of the chain food on that ecosystem would be nonexistent. The ecosystem would only be made by producers.
Ecosystem without primary consumers: say for ex there are bears and fish, right, so if all the bears disappear or just all die suddenly than the fish aka the prey would benefit. Why? well because without having to worry about bears eating them they can live without fear and they can reproduce more families of fish, and even live longer, and also eat more. More prey without predators is basically the dream for prey, prey don't want to be eaten, but that's the food chain so consumers really don't care, they just want to eat, so it would be good for prey to live without predators.
I think the correct answer from the choices listed above is option A. Directional selection can be distinguished from disruptive selection by the elimination of extreme variations of traits. Directional selection is a natural selection where an extreme phenotype is favored than others.
