Yesterday, the World Wildlife Fund activist group published its Living Planet Index 2014 report that calculates that the Earth is home to about half the number of vertebrates (mammals, birds, reptiles, amphibians, fish) that it hosted in 1970. Let's be clear: The report is NOT saying that half of vertebrate species have gone extinct, but that the overall number of wild vertebrates have declined by half. The trend is calculated using a complicated system for weighting the declines in various vertebrate species populations. Interestingly, this report comes just two months after a study—"Defaunation in the Anthropocene," published in Science—reported:
Among terrestrial vertebrates, 322 species have become extinct since 1500, and populations of the remaining species show 25 percent average decline in abundance.
The comparable LPI terrestrial vertebrate figure is 39 percent since just 1970.
In any case, the LPI parses data from 10,380 populations of 3,038 species out of an estimated 62,839 vertebrate species that have been described globally. From the report:
The Living Planet Index (LPI), which measures trends in thousands of vertebrate species populations, shows a decline of 52 per cent between 1970 and 2010 (Figure 2). In other words, the number of mammals, birds, reptiles, amphibians and fish across the globe is, on average, about half the size it was 40 years ago. This is a much bigger decrease than has been reported previously, as a result of a new methodology which aims to be more representative of global biodiversity.
Biodiversity is declining in both temperate and tropical regions, but the decline is greater in the tropics. The 6,569 populations of 1,606 species in the temperate LPI declined by 36 per cent from 1970 to 2010. The tropical LPI shows a 56 per cent reduction in 3,811 populations of 1,638 species over the same period. Latin America shows the most dramatic decline – a fall of 83 per cent. Habitat loss and degradation, and exploitation through hunting and fishing, are the primary causes of decline. Climate change is the next most common primary threat, and is likely to put more pressure on populations in the future.
The report also notes some countervailing population trends:
Even though slightly more populations are increasing than declining, the magnitude of the population decline is much greater than that of the increase, resulting in an overall reduction since 1970.
The LPI finds that 37 percent of the population declines result from direct exploitation, e.g. overfishing; and 31.4 and 13.4 percent is from habitat degradation and destruction, e.g., cutting down tropical forests.
Steep declines in animal populations have happened before. A 2008 article in the Proceedings of the National Academy of Sciences calculated that at the end of the last Ice Age human hunters so decimated the populations of large tasty critters that the total biomass of the world's terrestrial animals did not recover to its previous level until the Industrial Revolution. But then most of the recovered biomass consisted of human beings and our domesticated animals. By one estimate the world's farms and ranches harbor today about 1.4 billion cattle, 1.9 billion sheep and goats, 980 million pigs, and 19.6 billion chickens.
Just two months ago, in my article "Predictions of a Man-Made Sixth Mass Extinction May Be Exaggerated," I argued that trends in population growth, reforestation, agricultural productivity,and urbanization all point in a more hopeful direction over the balance of this century with regard to protecting wild species.