(NaturalNews) Planet Earth is on the verge of another mass extinction event, say scientists, and this time the cause is humans. A new study by researchers from Stanford University claims that losses to animal and plant life are reaching a tipping point that could signal an approaching mass die-off event, the sixth ever to occur, they say, in the history of the world.
Published in the journal Science, the review explains that more than 320 terrestrial vertebrates -- that is, land animals with limbs and spines -- have gone extinct since 1500. And among those that remain, their overall population numbers have declined by about 25 percent. Similar declines have also occurred among invertebrates, a phenomenon that the study's lead author says is attributable to human activity.
As far as vertebrate populations, up to 33 percent of all currently living species are either threatened or endangered, claims the study. And like the dinosaurs before them, large vertebrates like elephants, rhinoceroses and polar bears are most vulnerable to extinction, a trend that matches the lead-up to previous extinction events.
As go the large animals, so go the small animals
Though we are not as accustomed to seeing them in places like the U.S. and Europe, large animals are critical to the survival of small animals. Without them, the trickle-down effect would disrupt the entire ecosystem, as has been observed in experiments conducted in Kenya where large animal losses led to infestation with small rodents.
"Previous experiments conducted in Kenya have isolated patches of land from megafauna such as zebras, giraffes and elephants, and observed how an ecosystem reacts to the removal of its largest species," reads a Stanford news release.
"Rather quickly, these areas become overwhelmed with rodents. Grass and shrubs increase and the rate of soil compaction decreases. Seeds and shelter become more easily available, and the risk of predation drops."
Human development stamping out animal, plant habitats and spreading disease
Once again, the importance of biodiversity for maintaining life is on full display, the antithesis of which has become the norm in industrialized societies. As the human population has grown, and human developments expanded, diverse plant and animal life has slowly disappeared, only to be replaced with pests and disease.
"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," said Rodolfo Dirzo, a professor of biology at Stanford and lead author of the study. Dirzo is also a senior fellow at the Stanford Woods Institute for the Environment.
"Who would have thought that just defaunation would have all these dramatic consequences? But it can be a vicious circle."
Insect diversity also important for growing food
As the dominoes fall with larger animals, leading to losses of smaller animals, so go the insects. And while this might sound like a good thing -- insects are pests, right? -- it is important to remember that, without insects, 75 percent of the world's food crops would not get pollinated, which means no food for humans. Not only do insects pollinate crops, but they also contribute greatly to the nutrient cycle, as well as to the decomposition of organic materials back into the earth.
"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," adds Dirzo.
"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."