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. Like human censuses, counting the number of different species in a large region may involve very rough estimates. Population estimates are especially uncertain if migrating birds and other mobile animals are considered.
People have many reasons for supporting increased biodiversity.
Not only is it beautiful to have a variety of plants and animals in a region, but also the ecology and food webs are much more resilient if plants and animals depend on a variety of other species for survival. Diseases, such as mad cow, foot and mouth, and the Irish potato blight, demonstrate that it's unwise for humans to depend on a limited number of genetically constrained crops and livestock. That is, food monocultures are not desirable. When genetic diversity decreases, the limited number of plants and animals become more susceptible to pests and diseases. In some cases, genetically restricted crops and livestock may be threatened with extinction from outbreaks of diseases and plagues of pests.