The main thing that influences biodiversity is the availability of environmental resources. The rainforest has high precipitation leading to very increased levels of vegetation. Because of all the vast amounts of shelter, food, and water, an wide variety of species can be sustained with reduced stresses and competiton from other species. In the arctic on the other hand, the weather is very extreme, and there is no vegetation. This leads to reduced shelter and food, as well as making it difficult to just live in that freezing weather. There are high amounts of stresses and intense competition for food. This only permits the strongest species to survive.