Scientists have long understood that climate change has an effect on the numbers and distribution of animal species, but exactly how big an impact and why has remained largely speculative, until now.
New research shows that the American pika, the mountain-dwelling relative of the rabbit roughly half as big as a football, began rapidly fleeing lower elevations and dying off in the Great Basin region of the U.S. West a decade ago.
The change coincides with the time frame when human-generated greenhouse gas emissions began to leave a massive imprint on climate and sea levels, scientists say, and the implications are considerable for a multitude of species.
"The rules of the extinction game have changed," said Erik Beever, a research ecologist at the U.S. Geologic Survey's Northern Rocky Mountain Science Center in Montana and lead author of the peer-reviewed paper, which will soon be published in the print edition of the journal Global Change Biology.
"Typically, species have been endangered by habitat loss, exploitation and hunting, and to a lesser degree invasive species," Beever told SolveClimate News. "In the case of the pikas, none of these are the issue."
Beever and his colleagues recorded more extreme temperatures and less precipitation at sites where the rabbit-like critters are going extinct than at those where they are thriving. Higher highs and lower lows consistent with global warming models could have made the pint-sized pikas more susceptible to disease, heat stress or predation, Beever said.
His team compared historical distribution records of pikas in the Great Basin from 1898 to 1956, compiled by various academic and government organizations and summarized by E.R. Hall, a well-known mammalogist, in the second half of the century. They carried out their own population surveys from 1994 to 1999 and again from 2003 to 2008.
The researchers then analyzed which factors were primarily driving extinction in the 20th century compared to the past decade. Last century, variables like the amount of suitable habitat in the mountain range affected pika survival more strongly than changing climate.
But in the last ten years, the researchers found that climate became the dominant factor.
Their findings show that colonies of pikas disappeared from entire sites once every 10.7 years during the 20th century. Since 1999, their extinction rate jumped to once every 2.2 years. Pikas also migrated nearly 475 feet up-slope in the past decade, an 11-fold leap over the roughly 40-feet-per-decade shift seen throughout the 20th century.
For Beever and his team, it was no surprise why.
Sites where pikas had gone extinct averaged 4.5 degrees Fahrenheit warmer in the summer than at those where they are alive and well, they found. Temperatures also frequently dipped below minus 7.5 degrees Fahrenheit in the winter due to a lack of insulating snow, and received 1.75 times less precipitation.
Previous studies have identified shifts to higher elevations in species due to climate change, including checkerspot butterflies (Camille Parmesan, University of Texas, Austin), amphibians (Alan Pounds, Golden Toad Laboratory for Conservation, University of Miami) and pygmy rabbits (Eveline S. Larrucea, University of Nevada, Reno).
But Beever's results represent the first time scientists have demonstrated a clear and direct connection between modern global warming impacts and increasing extinction rates.
Similar though largely unpublished field surveys conducted by researchers at the University of Colorado, Boulder, California Polytechnic State University, the Utah Division of Wildlife Resources and the University of California, Berkley, show pikas are also disappearing in the Rocky Mountains, Manti Range in Utah, Sierra Nevada Mountains and the Cascade Mountains — albeit not as rapidly as in the Great Basin.
The findings have crucial implications for many species across the continent.
American pikas are often dubbed "ecosystem engineers" because they serve as important prey for birds and mammals and greatly affect plant composition and soil nutrients in their surrounding areas. Their disappearance can be felt throughout the food chains of mountain ranges across North America.
The precise climate change-related impacts that have led to increased pika extinction can also serve as early warning indicators for other species, Beever said.
For instance, the decline in snowpack from warmer temperatures meant pikas suffered and possibly died from a physiological response to cold-related stress. This finding can be applied when analyzing climate change effects on other species that are reliant on snowpack such as the wolverine.
"We can no longer just keep looking at the conventional factors to explain extinction," said Beever. "We have to consider contemporary climate change-related causes. What we've learned from the pikas — understanding the mechanisms driving this extinction — can be applied for looking at both mountain- and valley-dwelling species spread across all body sizes."