Mass extinctions had shaped the global diversity of our planet several times during the geological ages. The fossil record indicates that more than 95% of all species that ever lived are now extinct. During times of normal background extinction, the taxa that suffer extinction most frequently are characterized by small geographic ranges and low population abundance. Occasionally extinction events reach a global scale, with many species of all ecological types dying out in a near geological instant. In a conservative palaeontological sense, a mass extinction occurs when extinction rates accelerate relative to origination rates such that over 75% of species disappear within a geologically short interval (typically less than 2 million years).
Over the past 500 years, humans have triggered a wave of extinction, threat, and local population declines that may be comparable with the five previous mass extinctions of Earth’s history. Although anthropogenic climate change is playing a growing role, the primary drivers of modern extinctions seem to be habitat loss, human predation, and introduced species. The term defaunation was created to designate the declining of top predators and herbivores triggered by human activity, that results in a lack of agents that control the components of the ecosystems vegetation.
The most recent Living Planet Index (LPI) has estimated that wildlife abundance on the planet decreased by as much as 58% between 1970 and 2012. Several species of mammals that were relatively safe one or two decades ago are now endangered. The highest percentage of decreasing species is concentrated in tropical regions, mostly in the Neotropics and Southeast Asia. In 2016, there were only 7,000 cheetahs in existence, and less than 5,000 Borneo and Sumatran orangutans. Populations of African lion has dropped 43% since 1993, and populations of giraffes dropped from around 115,000 individuals in 1985 to around 97,000 representing what is now recognized to be four species (Giraffa giraffa, G. tippelskirchi, G. reticulata, and G. camelopardalis) in 2015.
Amphibians offer an important signal to the health of biodiversity; when they are stressed and struggling, biodiversity may be under pressure. Decreasing amphibians are prominent in Mexico, Central America, the northern Andes, Brazil, West Africa, Madagascar, India, Indonesia and Philippine. In the case of reptiles, the proportional decline concentrates almost exclusively in Madagascar; and decreasing species of birds are found over large regions of all continents. Other studies document that invertebrates and plants are suffering massive losses of populations and species. Long-term distribution data on moths and four other insect orders in the UK show that a substantial proportion of species have experienced severe range declines in the past several decades. Therefore, the acceleration of extinctions over the past decades, in which humans have played an increasingly important role, has left a number of hard questions about how the Anthropocene should be defined and whether or not extinctions should contribute to this definition.
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Rodolfo Dirzo et al., Defaunation in the Anthropocene, Science 345, 401 (2014); DOI: 10.1126/science.1251817