A 2015 study found that humans activities are driving species loss at a rate 100 times faster than historical baseline levels — which the researchers behind the study characterized as a conservative estimate. This finding fueled speculation that we’re currently witnessing a sixth global mass extinction event .
New research published in the Proceedings of the National Academy of Sciences (PNAS) provides further evidence that, even if we haven’t already entered a sixth era of mass species loss on a global scale, it may yet be imminent.
John Alroy, a professor of biological sciences at Australia’s Macquarie University, examined local-scale ecological data in order to forecast potential global extinction rates and found that hundreds of thousands of species are at risk if humans disturb all pristine forests remaining in the tropics. “Disturbance is no small matter, because roughly two-thirds to three-quarters of all the world’s species are found in tropical forests even though tropical forests only cover about 10 percent of the entire Earth’s continental area,” Alroy said in a statement .
Scientists have long believed that the rate at which we are destroying tropical forests, and the habitat those forests represent, could drive a global mass extinction event, but the extent of the potential losses has never been fully understood.
Mass extinction will occur primarily in tropical forests because Earth’s terrestrial biodiversity is so heavily concentrated in those ecosystems, Alroy notes in the study. In order to examine just how severe the impacts might be, he applied a highly accurate method of estimating species richness to data from 875 ecological samples of trees and 10 other groups of organisms “of keen ecological interest,” including bats, insects (ants, butterflies, mosquitoes, and scarabs), large and small mammals, and other vertebrates (birds, frogs, and lizards). The samples were collected in a variety of habitat types in tropical zones that were originally forested, from primary and fragmented forests to plantations and pasturelands.
“About 41% of the tree and animal species in this dataset are absent from disturbed habitats, even though most samples do still represent forests of some kind,” Alroy writes in the study.
Image Credit: Alroy, J. (2017). doi:10.1073/pnas.1611855114
Alroy projects that, if Earth’s remaining tropical forests are completely disturbed, more than 18 percent of species will be lost in every group studied except large mammals and mosquitoes. Seven of the groups he examined will lose greater than 28 percent of species. Trees, for instance, stand to lose as much as 30 percent of species, while ants could lose as much as 65 percent.
“The overall implication of this research is that any substantial loss of primary forests will result in numerous extinctions across many groups,” Alroy said. He added that there is good reason to regard his estimates as conservative, as well, and that the full impacts of human activities on species survival could be more severe than he has predicted: “Even if we preserve forests of some kind in many places, unless we protect them from ever being logged, those forests may end up being empty.”
Alroy’s findings also suggest that numerous rare tropical species may have already disappeared. “The most important point, however, is that many species may have already gone extinct because their ranges are now entirely deforested,” he writes in the study. “Furthermore, many species in otherwise pristine forests may have already gone extinct because of stressors not related to habitat destruction, such as hunting, interactions with invasive species, introduced epidemic diseases, pollution, and the direct effects of climate change.”
Given how rapidly deforestation has occurred throughout the tropics, Alroy adds, it’s “conceivable that an event on the scale of a true mass extinction has already taken place” and that these losses simply went unnoticed by mankind.
“A mass extinction could have happened right under our noses because we just don’t know much about the many rare species that are most vulnerable to extinction,” Alroy said. “To figure out whether this is true, a lot more field work needs to be done in the tropics. The time to do it is now.”
There is reason to hope that we can still prevent such large-scale loss of life. Previous research has shown that protecting 50 percent of the planet’s land area is a sort of baseline for ensuring the health of ecosystems and, in turn, the survival of life on Earth. This concept, often referred to as “Nature Needs Half,” is widely regarded as not just valid but feasible by conservationists. In fact, a study released last month suggests that even though many ecosystems functions have already been highly compromised by human activities, we can still achieve the 50 percent protection goal by scaling up current conservation efforts and targeting those habitat types that are most crucial for preserving biodiversity.
A Jaguar (Panthera onca). Large mammals stand to lose more than 10 percent of species if tropical forests are completely disturbed, new research finds. Photo by Rhett Butler.
Alroy, J. (2017). Effects of habitat disturbance on tropical forest biodiversity. PNAS. doi:10.1073/pnas.1611855114
Ceballos, G., Ehrlich, P. R., Barnosky, A. D., García, A., Pringle, R. M., & Palmer, T. M. (2015). Accelerated modern human–induced species losses: Entering the sixth mass extinction. Science advances, 1(5), e1400253. doi:10.1126/sciadv.1400253
Dinerstein, E., Olson, D., Joshi, A., Vynne, C., Burgess, N. D., Wikramanayake, E., … & Hansen, M. (2017). An ecoregion-based approach to protecting half the terrestrial realm. BioScience. doi:10.1093/biosci/bix014
Follow Mike Gaworecki on Twitter: @mikeg2001