Diversity implies difference. A diverse forest has many different species, both plant and animal.
Often, the more diverse a forest, the less chance there is that losing a single species will result in the loss of an entire forest. Many other species still survive and life goes on. Forest monocultures, or those with a single species, are more at risk from a new disease or insect destroying the entire forest. If there is only one species and it becomes extinct, the forest does not survive.
Because of time, native forests are usually more diverse than non-native forests. Time favors diversification, as new species evolve to fill specific niches or tasks in a forest.
Gradually, species which co-evolve together create interdependence. A bird, for example, may evolve to feed on certain insects. These insects may in turn pollinate only certain plants, which create shade favorable to the generation of young tree seedlings. This tree might be the only tree in which the bird can nest, and therefore reproduce.
In a natural system, populations of all species ebb and flow dependently. Too many birds, for example, can result in fewer insects, which can result in fewer trees to nest in. In nature, there is a constant balancing among species and their numbers. Normally this is a slow process which occurs over several generations.
When a new species is introduced imbalances can quickly occur. New introductions often have no controlling predators. Some introduced species are very competitive, and are able to replace the native species. This can be disastrous for those species dependent on the original native species.
A few introductions can threaten entire ecosystems. Unfortunately, many of Hawaii’s forests are living laboratories of such events, as non-native introductions have quickly replaced once native forests.