When land is cultivated by mankind, small natural areas occasionally remain. These are islands of untouched nature in a sea of a completely altered habitat.
One example of this is the Brazilian rainforest. There were clearings, thus small tree islands often remained. The vegetation around these islands grew but mainly bushes and herbs, no forest.
Scientists investigated these forest clearings and revealed that on the edges of these areas more and more species were disappearing, some even after 20 years. Some species disappeared due to the changed conditions, others because the areas were simply too small to maintain a healthy population.
Similar effects can be observed in forests which have been divided by roads. An obvious example is a group of birds who live on ants. These ant-birds only hunt in the dark forest and never in open areas. However the ants that they feed on need large jungle areas, in order to find sufficient food for themselves. When one forest clearing does not offer enough they move on to the next one. As the ant-birds won't cross open areas such as roads, they stay behind in the original forest clearing and soon starve to death due to lack of food.
One species after another has become extinct as habitats are carved up. This obviously leads to a paring down of the eco-system. In the end we are left with a shadow of the former forest, which bears no likeness to the original thing.
Such isolation processes are occurring more and more frequently. Nature reserves are also islands in this way, the smaller they are, the more endangered their inhabitants become.