Today (May 22) is World Biodiversity Day.In celebration, below is a look at the world's ten most biodiverse countries as measured by species richness.This list takes a simplified approach, created a weighted index using five groups of animals — amphibians, birds, fish, mammals, and reptiles — and one group of plants — vascular plants. Author’s note: after posting, I located a better data set on plant species diversity, which altered the original rankings dropping Peru from #4 to #6. May 22 has been designated as the “International Day for Biological Diversity” by the United Nations to “increase understanding and awareness of biodiversity issues”. In celebration of World Biodiversity Day, below is a look at the world’s ten most biodiverse countries as measured by species richness. A few caveats before we get to the list. There are many ways to measure biodiversity. This list takes a simplified approach, created a weighted index using five groups of animals — amphibians, birds, fish, mammals, and reptiles — and one group of plants — vascular plants. Each country is ranked by its percentage of species in each group relative to the total global number of species for each group. There are obviously major omissions: the system don’t account for insects and other invertebrates, fungi, microorganisms, and a number of other large groups of living creatures. Nor does it attempt to measure diversity of populations within species, levels of endemism, or intactness of ecosystems. Nonetheless we believe the list represents a fair proxy for the planet’s most biodiverse countries. Some of the countries in the ranking could move up or down a few places in the rankings depending on one’s data sources and methodology.