We are currently in the midst of the sixth mass extinction event in the history of life on Earth, warns an international research team in a new scientific research review published in the journal Science.
The study was conducted by researchers from Stanford University, the University of California-Santa Barbara, Sao Paulo State University in Brazil, the National Autonomous University of Mexico, the Natural Environment Research Council Centre for Ecology and Hydrology in England and University College London.
Throughout the 3.5-billion-year history of life, our planet has gone through several mass extinction events, characterized by dramatic drops in species diversity. Unlike prior mass extinctions, which were caused by factors such as natural climate change or asteroid collisions, the
current mass extinction is caused exclusively by the activity of a single species: human beings.
In the new paper, the researchers refer focus on the severe drop in animal diversity that has occurred as a part of the mass extinction. They have dubbed this dramatic loss of animal life the
“anthropocene defaunation” — that is, the destruction of animal life associated with the era of human domination of the planet.
Cascade of effects
In reviewing the research, the study authors found that more than 320 species of terrestrial vertebrates have gone extinct since the year 1500, and the remaining species have had their populations decline by an average of 25 percent. Among vertebrates as a whole, 16 to 33 percent of all species are considered either threatened or endangered. The rates of decline are highest among larger animals (megafauna), which tend to grow and reproduce more slowly than smaller animals, and need larger areas of habitat to sustain them.
The loss of large vertebrates may have a cascade of ecological effects, the researchers warned. For example, studies found that areas of Kenya where large animals such as zebras, giraffes and elephants have been extirpated quickly became overgrown and experienced a boom in the population of rodents. This led directly to an increase in the rate of certain human diseases.
“Where human density is high, you get high rates of defaunation, high incidence of rodents, and thus high levels of pathogens, which increases the risks of disease transmission,” lead author Rodolfo Dirzo said. “Who would have thought that just defaunation would have all these dramatic consequences? But it can be a vicious circle.”
The mass extinction is not limited to vertebrates. The researchers found that, over the past 35 years (during which the human population doubled), populations of most invertebrate species decreased by 45 percent.
Invertebrates are animals without backbones, a vast category that includes everything from insects to spiders and from worms to a wide variety of ocean life.
As with the loss of megafauna, the loss of invertebrates can severely disrupt an ecosystem and thereby have serious effects on human health. For example, 75 percent of the world’s food crops are pollinated by insects. Invertebrates also provide key ecosystem functions such as decomposition and nutrient cycling.
“We tend to think about extinction as loss of a species from the face of Earth, and that’s very important, but there’s a loss of critical ecosystem functioning in which animals play a central role that we need to pay attention to as well,” Dirzo said. “Ironically, we have long considered that defaunation is a cryptic phenomenon, but I think we will end up with a situation that is non-cryptic because of the increasingly obvious consequences to the planet and to human wellbeing.”
The extinctions of both vertebrates and invertebrates are primarily caused by habitat destruction and climate disruption, the researchers found.
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