destruction of habitat for animal severely affects all the plant species that depend on them. Mass extinction Biodiversity generally refers to the variety and variability of life on Earth, and is represented by the number of different species there are on the planet. Since its introduction, Homo sapiens (the human species) has been killing off entire species either directly (such as through hunting) or indirectly (such as by destroying habitats), causing the extinction of species at an alarming rate. Humans are the cause of the current mass extinction, called the Holocene extinction, driving extinctions to 100 to 1000 times the normal background rate. Though most experts agree that human beings have accelerated the rate of species extinction, some scholars have postulated without humans, the biodiversity of the Earth would grow at an exponential rate rather than decline. The Holocene extinction continues, with meat consumption, overfishing, ocean acidification and the amphibian crisis being a few broader examples of an almost universal, cosmopolitan decline in biodiversity. Human overpopulation (and continued population growth) along with overconsumption, especially by the super-affluent, are considered to be the primary drivers of this rapid decline. The 2017 World Scientists' Warning to Humanity stated that, among other things, this sixth extinction event unleashed by humanity could annihilate many current life forms and consign them to extinction by the end of this century. A 2022 scientific review published in Biological Reviews confirms that a biodiversity loss crisis caused by human activity, which the researchers describe as a sixth mass extinction event, is currently underway. A June 2020 study published in PNAS argues that the contemporary extinction crisis "may be the most serious environmental threat to the persistence of civilization, because it is irreversible" and that its acceleration "is certain because of the still fast growth in human numbers and consumption rates." Decline in biodiversity Defaunation is the loss of animals from ecological communities. It has been estimated that from 1970 to 2016, 68% of the world's wildlife has been destroyed due to human activity. In South America, there is believed to be a 70 percent loss. A May 2018 study published in PNAS found that 83% of wild mammals, 80% of marine mammals, 50% of plants and 15% of fish have been lost since the dawn of human civilization. Currently, livestock make up 60% of the biomass of all mammals on earth, followed by humans (36%) and wild mammals