Proteins have structural roles throughout the body. Carbohydrates are the major energy source for cells in the body. I am unsure if lipids or nucleic acids apply.
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.
A marked change in the environment favors some characteristics of plants, animals and microbes over others. These character differences are often genetically based so that favored characteristics may increase in the next generation. The shorter the generation time, the faster this evolutionary change can occur.
