Living things are part of a food web. Therefore, they rely on other organisms for nutrients. Even plants cannot live in isolation. They rely on humus added to the soil to gain nutrients. These come from other organisms. Worms turn the soil and aerate it allowing nutrients and water to permeate. No matter how isolated a species is, it must have other organisms nearby to provide for its needs. While sometimes a population may become separated from a larger population, it will survive as long as its traits allow it to live in that environment. However, a small group has a smaller portion of the gene pool. If the environment ever changes, if the small group doesn't have the variations that allow it to adapt, it could face extinction. Isolation sometimes leads to the evolution of new species--they may be isolated by physical barriers, mechanically if their genitals are not compatible, due to behaviors, or temporal reasons. A species that is able to survive in this isolated environment will evolve over time and form a new species or it may die out completely if it can't.