A team of Stanford University biology professors suggested Earth may be on the brink of the sixth mass extinction.
The planet's current biodiversity is the highest in the known history of life, but new research suggests it may have reached a "tipping point," the university reported.
The study, published recently in the journal Science, looked at previous scientific literature and analysis. The team's findings suggest losses and declines in species' populations could be indicative of the world's next mass biological extinction.
Since the year 1500 over 320 terrestrial vertebrates have gone extinct; current species show a 25 percent average decline; similar numbers have been observed in invertebrates. Sixteen to 33 of all vertebrate species are believed to be endangered; some of these populations include elephants, polar bears, and rhinoceroses. Invertebrates such as "beetles, butterflies, spiders and worms" have declined by 45 percent in the past 35 years while human populations have doubled.
Past massive extinctions have been linked to "natural planetary transformations" or catastrophes, but this one may be largely caused by human activity.
Larger animals are more likely to be targeted by humans and have slower population growth rates. Experiments in Kenya showed when megafauna such as giraffes and elephants were removed, areas tended to become overwhelmed with rodents. Droppings from these smaller animals caused shrubbery to increase, allowing them to hide from predators more easily and providing an abundance of food.
"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."
Finding a solution to this problem is not an easy task. Immediately reducing behaviors that inflict habitat change would help, but these actions must be carefully personalized to each specific habitat.
"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."
