Global Warming Smaller Bison
June 21, 2013

Hotter Climate Means Shrinking Bison

Brett Smith for - Your Universe Online

As climate changes and global temperatures increase, the average size of the iconic American bison will decrease, according to a new study in the journal PLOS ONE.

"Bison are one of our most important conservation animals and hold a unique role in grasslands in North America," said study author Joseph Craine, a biology professor at Kansas State University. "In addition to their cultural and ecological significance, they're economically important both from a livestock perspective and from a tourism perspective. There are about half a million bison in the world."

In the study, Craine analyzed a data set from several nonprofit, federal and state agencies that included 290,000 weights, ages and sexes collected from 22 bison herds across different parts of the US. The data for each animal was matched to climate data for where the animal roamed.

Based on size differences, Craine concluded that future generations of bison over the next 50 years will be smaller and weigh less. The biologist said a reduction in the nutritional quality of grasses will cause the size drop.

"We know that temperatures are going to go up," Craine said. "We also know that warmer grasslands have grasses with less protein, and we now know that warmer grasslands have smaller grazers. It all lines up to suggest that climate change will cause grasses to have less protein and cause grazers to gain less weight in the future."

Craine said the effects increased temperatures can already be seen when comparing bison in cooler, wetter area to those in warmer, more arid regions. According to the study, the typical 7-year-old male bison in South Dakota weighed 1,900 pounds, while the average weight of a 7-year-old male bison in much warmer Oklahoma was about 1,300 pounds.

"The difference in temperature between those two states is around 20 degrees Fahrenheit, which is about three times the projected increase in temperatures over the next 75 years," Craine said.

"That's a pretty extreme difference and beyond the worst-case scenario,” he added. “But it is a clear indicator that long-term warming will affect bison and is something that will happen across the U.S. over the next 50-75 years."

Craine noted that lower quality grass will also affect cattle, resulting in a big economic hit for livestock farmers. Although a similar study on cattle has yet to be done, the KSU scientist estimated that over 90 million cattle in the US could be affected. Both animals share similar physiologies and diets.

If the bison-based study were to translate for cattle in roughly equivalent numbers, every 1.5 degree Fahrenheit temperature increase could cost American cattle producers about $1 billion in lost income, Craine said. Farmers could try to mitigate their herd’s protein deficit with supplements or simply accept the new cattle sizes, but both approaches will negatively impact their pocketbook. Some studies have predicted that temperatures in the US will increase between 6 and 8 degrees Fahrenheit over the coming 75 years.

This week President Barack Obama announced that his administration will pursue strategies for reducing the effects of climate change, including reducing the emissions of power plants.