Question

What would happen if rats was eliminated from the environment

Answer 1

Rats account for a decent amount of biomass globally.  They are intermediate predators, eating a lot but also being eaten.  If they went extinct, they would stop eating many insects and seeds / fruits -- the former would gain significantly in population, and the latter would lose some of their pollination.  Both factors would harm the viability of at least some species of plants, by adding to plant eaters (insects) and reducing plant reproduction.  Rats' predators would also suffer if they went extinct.  The primary predators are birds of prey, such as hawks and owls.  However, other animals such as skunks, foxes, weasels, coyotes, and even wolves eat rats.  For predators in marginal environments, the loss of such a food source could drive them out of a habitat area.