A mass extinction occurs when 75% of the planet’s biodiversity disappears during a relatively brief unit of geological time. Estimates suggest that 95% of past life is now gone and that five prior mass extinctions account for much of this loss. When evaluating the status of biodiversity it is useful to inventory what is present in order to recognize when it is gone. Clearly, this type of inventory is virtually impossible, and while molecular taxonomists are busy counting what is here today, such meticulous work falls into a pattern “one step forward and two steps back.” For example, nearly every careful inventory of a small reef patch, tract of rainforest or microbiota community reveals a new subspecies or species. Metaphorically, the act of counting every existing species is like counting the grains of sand on a wave terraced beach. Much like the waves that deposit and steal grains of sand with each incursion, natural selection alters the genetic composition of one population, while anthropogenic effects eliminate others.
Most who spend their lives studying biodiversity recognize that ours is the era of the sixth mass extinction. With a total of 1,371,500 described animal species, scientists can’t agree on an actual animal diversity count let alone the total number of all species on the planet today. What is known is the following: Of the cataloged vertebrate species on the planet, 26% of mammals, 13% of birds and 41% of amphibians are threatened with extinction. This adds up to short list of 4,529 terrestrial vertebrates. Bear in mind that this number does not include the threatened reptiles and fish nor does it include the countless number of invertebrate species that make up the bulk of animal biodiversity. When statistical estimates of total planetary biodiversity are considered, scientists estimate that 10 to 690 species go extinct every week. Now if this doesn’t make you say “holy shit,” I’m not sure what will.
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