Why are top-level predators an important aspect to a healthy ecosystem?

They keep other nuisance species to a minimum, such as feral cats or deer. They prey on the injured, sick, and weak animals, leaving a better chance of survival for those animals that are fit and healthy. Predators control the size of prey populations, which in turn helps to slow down the spread of disease.

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What would happen if top-level predators were removed?

The most obvious result from the removal of top-level predators is the explosion of prey species. When prey become more limited the predator population in turn declines. What does this mean? It means that predators and prey balance each other out. A predator population only increases if a prey population increases. When top-level predators are removed, prey numbers explode and this leads to an unbalances ecosystem.