How would decreasing habitat size affect populations living in the habitat?

The size of the habitat and its health is crucial for the number of species that can live in it, as well as the size of the populations of the species that live in it. By experiencing a habitat decrease, thus losing parts of it, the species will face a big challenge in order to be able to survive. The reason for that is that the populations have been established in accordance with the habitat size, the food sources in it, thus how much members it can support of a certain population, but part of it, small or large is gone. That will result in a decrease in the size of the populations because they will not have enough space for living, nor enough food to be supported, thus lot of members of the population will die out, and the chances are that only the fittest will be able to get to the food sources and survive.