A white rhino takes a nap at the San Antonio Zoo in this 2009 photo. Last year, the zoo opened a new white rhino habitat as part of its conservation effort for the species. “Rhino populations are dwindling worldwide due to habitat destruction and poaching,” said Tim Morrow, the zoo’s president and CEO. In a paper published in June on “biological annihilation” around the world, scientists cited the Sumatran rhino as one of the terrestrial vertebrate species “on the brink of extinction because they have fewer 1,000 or individuals.” Amy the Nurse, CC BY-NC-ND 2.0, via Flickr

Almost 10 years ago, biologists suggested Earth was entering a sixth mass extinction – defined as a loss of more than three-quarters of its species in a geologically short interval. Such events have happened only five times in the past 540 million years or so.

Five years ago, additional research confirmed that this sixth mass extinction was, in fact, under way. The work further noted that the number of species that became extinct in the past century normally would have taken between 800 and 10,000 years to disappear.

Then, a June 2020 paper in the Proceedings of the National Academy of Sciences (PNAS) noted that the extinction rate of terrestrial vertebrate species is significantly higher than prior estimates. Some 500 species have populations of less than 1,000 individuals and face likely extinction within the next two decades if nothing changes. That would bring total losses to an equivalent of those that would have taken place naturally across 16,000 years. Birds have the most species at risk, followed by amphibians, mammals, and reptiles. Further, experts predict a sharp increase in the current rapid vertebrate extinction rate in the future.

The news is no better for plants. A report in September from The Royal Botanic Gardens, Kew, in the United Kingdom warns that destruction of natural landscapes has put two in five of the world’s plant species at risk of extinction. This news looks all the more alarming given that nearly 4,000 species of plants and fungi were scientifically named for the first time as recently as 2019.

The Kew report states: “The World Checklist of Vascular Plants, the most comprehensive and regularly updated species list of its kind, records around 350,000 accepted species, of which 325,000 are flowering plants. Ten years ago, scientists thought that the vast majority of flowering plants had been described and named. But the subsequent stream of species revealed to science suggests there are many more to find, as do the experiences of botanists undertaking fieldwork in the tropics today.”

The causes

Previous extinction events resulted from catastrophic alterations of the environment – massive volcanic eruptions, depletion of oceanic oxygen, an asteroid colliding with Earth. It took millions of years after each of these events for the numbers of species to recover to pre-event levels.

Today, humans are the main driver of extinctions. Most plant loss, for example, results from destruction of wild habitat to create farmland. Over-harvesting of wild plants, building of infrastructure, introduction of invasive species, pollution and, increasingly, climate change represent important drivers as well.

Many extinctions have occurred just since our ancestors developed agriculture, about 11,000 years ago.

For animals, drivers of species extinctions include habitat loss and fragmentation (most of the 177 species of large mammals lost more than 80 percent of their geographic range in the last century), illegal trade, overexploitation, introduced domestic and wild species, pollution, and climate change.

The authors of the PNAS paper say this sixth mass extinction may be the most serious environmental threat to civilization, because it is irreversible. Acceleration of the crisis is certain, they add, given continued rapid growth in human numbers and consumption rates.

“We’re never going to solve this problem as long as the human enterprise keeps expanding,” says co-author Paul Ehrlich of Stanford University’s Center for Conservation Biology.

“Humanity is increasing in terms of population size and per capita consumption, and we are already at a level where we’re killing off the biodiversity of the planet,” Ehrlich said in an interview with Texas Climate News. “It all ties back to human activity, directly and dramatically.”

The costs

Apart from the tragedy of a plant or animal permanently disappearing – including iconic species such as polar bears – extinctions have a suite of consequences for humans.

Some of those newly discovered plants and fungi, for example, could help feed us. Scientists currently record more than 7,000 edible plant species, but only some 400 are considered food crops. The effects of climate change on agricultural practices and standard crops make the search for alternative crops more urgent.

Many plants also have medical value. At least 720 plant species currently used in medicine face risk of extinction, and we may lose others before even discovering their potential. Those new species identified in 2019, for example, included a Texas sea holly species whose relatives can treat inflammation.

Plants and fungi also could serve as sources of bioenergy, contributing to reducing both carbon emissions and energy poverty. The potential of fungi in particular is largely unexplored, as scientists suspect that more than 90 percent of fungi species remain unknown.

The legal and illegal trade in wildlife decimating many species has been linked to the Covid-19 pandemic, the PNAS paper notes. Scientists caution that continued destruction of wild habitat and unchecked wildlife trade almost surely presage more pandemics in the future.

Perhaps most significant, overall decreased biodiversity lessens the natural world’s ability to provide valuable ecosystem services such as water filtration, control of pests, and pollination. Extinctions change entire ecosystems, with the loss of one species potentially having far-reaching effects.

For example, disappearance of keystone species can trigger extinction cascades, or a series of extinctions throughout an ecosystem. Analysis shows that the disappearance of the Steller’s sea cow from the Bering Sea’s Commander Islands in the mid-1700s likely resulted from overhunting of sea otters a decade earlier. By feeding on sea urchins, which eat kelp, sea otters played a key role in maintaining the kelp forests on which sea cows depended for shelter and food. Large apex predators such as wolves and sharks also are known to be key to maintaining the health of their associated ecosystems.

“The costs of extinction are basically breakdown of the systems that support us,” Ehrlich says.

The cures

In previous periods of climatic and environmental change, resilient landscapes helped maintain terrestrial biodiversity. Unfortunately, recent research shows landscape resilience in North America at its lowest since end-Pleistocene megafaunal extinctions, due to an expanding human footprint and changing climate.

Further, the 196-nation Convention of Biological Diversity reported in August, the world has failed to meet a single target set by the treaty of that name for stemming the destruction of wildlife and ecosystems.

A November 2020 study provides a glimmer of hope: The alarming estimates of decline in vertebrate species are driven by less than 3 percent of vertebrate populations. The researchers found extreme declines in 16 of the taxonomic-geographic systems in the Living Planet Index, including birds and freshwater mammals in the Indo-Pacific and reptiles in North, Central, and South America, with loss occurring disproportionately in larger animals. Remove the not quite 3 percent of vertebrate species in free-fall, they note, and the picture changes dramatically. The fact that losses are concentrated also provides specific targets for conservation efforts.

Ehrlich cautions that the problem requires a big-picture approach, though. “The only thing we can do quickly is change our consumption patterns, but if we don’t start humanely changing our population patterns, that isn’t going to help either,” Ehrlich says. “We’ve known for years that if we keep dumping greenhouse gasses into the atmosphere, we’re in trouble. There is no way to continue a reasonable way of living for people on the planet unless we change how we deal with the problems of climate, biodiversity, toxification, and agriculture. We don’t have that much time.”

The most important thing that individuals can do, he adds, is become politically active. “We’ve got to educate people and get them out there to take action, rather dramatic action. What do you do about the growing population? The data show that if you empower women, birth rates go down. So, the first place to start is to fight for women’s rights.”

Gerardo Ceballos of the Universidad Nacional Autónoma de México’s Ecology Institute, lead author on the PNAS paper, along with Ehrlich and other colleagues, established a global initiative to try and turn things around called Stop Extinction. It aims to raise awareness and give people a role in changing the current trajectory.

Humans are the primary cause of the sixth mass extinction, and we also can be part of the cure.

Melissa Gaskill is a contributing editor of Texas Climate News.