A single species can have tremendous diversity. Take dogs for example. Look at the variety of hair length, straightness and colour, the variety in size, in body form, in temperament. Like your neighbourhood, these places provide different species with homes, food and access to others of their own species. This is called an ecosystem and can refer to many areas such as a meadow, or a rainforest, or the entire world.
Biodiversity is a source of strength in the ecosystem.
A forest that contains many different types of trees will be stronger and more stable than a forest that has only one or two species. In a wheat field, there is only one species. If a disease attacks part of the wheat field, it can quickly spread to the whole crop. Genetic diversity within species makes it easier for species to adapt to new or changed environments. By having a large gene pool, a species is more able to adapt.