Defining plant species is even more difficult and subjective because through grafting and cross-pollination, plants with very dissimilar characteristics may be crossbred. Perhaps, deciphering the genetic code of different animals and plants will eventually allow biologists to more objectively define a species. About 2 million animal species have been defined, most of which are insects. Several million more are believed to exist. Again, most of them are insects that live in tropical areas.
Biodiversity is defined in different ways.
In most cases, biodiversity refers to the number of plant and animal species in a given area. The area of interest may range from an entire continent or ocean to someone's backyard. In general, biodiversity is usually greater in tropical regions than in dry or cold climates. Also, all else being equal, biodiversity usually increases with the size of the area of interest. Because of this fact, a park of ten square miles will have greater biodiversity and a healthier ecosystem than 10 parks of one square mile each.