The Earth may already have begun its descent into its sixth “mass extinction” event. Where the previous five were caused by external influences like meteor strikes or huge volcanic eruptions, this one is different.
This time, say scientists, the finger of blame points undeniably at us.
A collection of studies published this month in Science Magazine review the evidence that points to this sobering finding. If we don’t change our ways, they say, the Earth may well see the end of the Anthropecene period – the Age of Humans.
A mass extinction event means a rapid, widespread loss of life on Earth. From an evolutionary perspective, it means more species go extinct than new ones that flourish. Depending on the cause, between 70 to 90 percent of all species just disappear.
Taking all previous mass extinctions into account, scientists say that nine out of 10 of all types of life ever to have lived on this planet have gone extinct.
Mass extinction occurs due to a distinctive domino effect that’s triggered by “defaunation.” To regular Joes like you and me, this means a decline in animal population. Scientists believe we’re entering a period of “Anthropocene defaunation” – loss of animal population caused by human activity.
How do we cause that loss? We destroy critically important habitat and we overhunt large animals.
Beware Rodents Bearing Suitcases… and Diseases
Loss of habitat and human predation cause the loss of large animal species. Think elephants, zebras, lions, tigers, rhinos and giraffes. Once those major animal players disappear from a land-based ecosystem, the rodents gleefully move in and become king of the manor. There’s no animal bigger and badder to keep them in check anymore.
Rodents carry with them all manner of disease and pestilence. All of a sudden those diseases take hold, sweeping the world and cutting down all creatures in their path.
This isn’t guesswork. Scientists have watched it happen. According to the study team’s press release:
[P]revious 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. 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.
Consequently, the number of rodents doubles – and so does the abundance of the disease-carrying ectoparasites that they harbor.
“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 lead study author Rodolfo Dirzo, Stanford University professor of biology. “Who would have thought that just defaunation would have all these dramatic consequences? But it can be a vicious circle.”
Between 16 and 33 percent of all vertebrate species are considered globally threatened or endangered, according to the study. It’s not just the big animals we need to worry about, however.
Invertebrates – particularly insects like spiders, butterflies, worms and beetles – are in decline as well. The study notes that while the number of humans has doubled in the last 35 years, the number of invertebrates has fallen by 45 percent. This, too, happens because of “habitat loss and global climate disruption,” said the Stanford press release.
Nearly 100 percent of Orthoptera species such as grasshoppers, crickets and katydids are in decline. Butterflies and other Lepidoptera face a 35 percent drop. These are only two examples in a world filled with similar unfortunate statistics.
We need invertebrates if we are to survive. Insects, for example, pollinate three quarters of the world’s crops. Without them, processes like nutrient recycling and decomposition become problematic. Remember the problems we’re having because of the decline in bees? Expand those issues to include all invertebrates and the ramifications become deeply worrisome.
“We were shocked to find similar losses in invertebrates as with larger animals, as we previously thought invertebrates to be more resilient,” Ben Collen of the U.K.’s University College London, told USA Today.
It‘s Not Too Late, But We‘re Perilously Close
“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,” Prof. Dirzo said.
Estimates say we still have perhaps a century or so to adjust how we do things and reverse this mass extinction trend. The types of change we need to embrace require commitment and resolve.
Reversing the early stages of our plant’s sixth mass extinction event will require a wholesale course correction in how we’re dealing with climate change, say some experts. We will need to employ what Scientific American calls “aggressive conservation” to preserve species that are on the edge of extinction.
Can we do it? Can we get it together in time, or is the Age of Humans about to be consigned to history?
Photo credit: Thinkstock