Climate Change

Joe Craine is a research assistant professor at Kansas State University.

When the world gets warmer, what happens to bison?

And what does that imply for other grazing animals, like the 100 million cattle that graze on U.S. grasslands?

Due to efforts to restore bison in North America, The Nature Conservancy has been instrumental in helping predict the future of bison.

Grazers like American bison and domestic cattle are important components of grasslands worldwide. Approximately 500,000 bison and 100 million cattle graze grasslands in North America. Around the world, there are 1 billion cattle. Every time one of these grazers takes a bite to eat, they help regulate biodiversity. Every time grazers are harvested for food, it contributes to the economic sustainability of grasslands, making it less likely this habitat will be converted to other uses.

Climate change is likely to impact grasslands worldwide. Future grasslands will experience hotter temperatures, longer growing seasons, and more variable precipitation.

But how will climate change impact grazers like bison?

Scientifically, it can be a hard question to answer. There are many experiments that simulate the climates of the future for grasslands, but none of these experiments have grazers in them. Grazed and ungrazed grasslands are starkly different so the consequences of warming for ungrazed grasslands are unlikely to apply to grazed grasslands. If experiments are of limited utility, then we need to look at geographic patterns of bison that might give us a glimpse of bison in the future.

That’s where the Conservancy’s work to restore bison across North America has been critical.

The Nature Conservancy has now restored bison across eight different states and Mexico. At most sites, the bison are weighed every year. These weights have been an invaluable record of understanding how bison respond to hotter weather, but also hotter climates. The climate of Tallgrass Prairie Preserve in Oklahoma is 17°F hotter than at Cross Ranch in North Dakota.

To understand how hotter climates are likely to affect bison, I recently led a study just published in the journal PLoS ONE that synthesized weights of bison from Nature Conservancy herds as well as a number of other federal, state and private herds.

The results were pretty clear. Hot years do not necessarily affect bison much, but hot climates do.

For example, a 7-year old male bison at Ordway Prairie in South Dakota weighs, on average, 1,900 lbs. The same animal at the US Fish and Wildlife Service’s Wichita Mountains Wildlife Refuge in Oklahoma weighed 1,300 lbs.

Across all the bison herds in the study, the hotter the climate, the less bison weighed. For every 1°F warmer the climate was, these adult males weighed on average 20 pounds less. Younger males and females are also lighter in warmer climates, though not as much since they are lighter animals to begin with. As a note, by the end of this century, temperatures may rise by 10°F.

Whether temperatures rise by a few degrees or ten degrees, there are a lot of changes that are likely to happen to grasslands. But the effects on weight gain in grazers are likely to be the most important. Bison will be smaller, but cattle are also likely to gain less weight. It’s estimated that every 1.5°F increase in temperature could cost ranchers in the United States $1 billion dollars in lost income due to cattle gaining less weight.

Why warm climates would shrink grazers is still being worked out, but it appears that the soils of warm-climate grasslands supply less nitrogen to grasses and the grasses produce less protein in their leaves. Most grazers in the United States are limited by protein, which is expensive to supplement.

In all, a hot year doesn’t have much effect on bison. But many hot years in succession cause grasses to produce less protein and grazers to put on less weight.

The Nature Conservancy’s bison restoration program is increasingly providing important scientific answers – affecting the future of these iconic animals, grassland habitats and the sustainable economic utilization of ecosystems.

Opinions expressed on Cool Green Science and in any corresponding comments are the personal opinions of the original authors and do not necessarily reflect the views of The Nature Conservancy.