Plant diversity explodes when bison return to the prairie
Reintroducing bison to the grasslands doubles plant diversity and boosts resilience to extreme weather, according to a new study.
Somewhere between 30 and 60 million bison probably roamed North America before the mid-1800s, mostly on the Great Plains. But the massive mammals were nearly hunted to extinction.
Today, there are only about 30,000 bison in public and private herds in North America. About 400,000 other bison are raised as cattle.
“The plains bison, once one of the most dominant and widespread megafauna species in North America, nearly became extinct in the late 19th century and currently occupies less than 1% of its pre-European range. “said study author Zak Ratajczak, an assistant professor of grassland biology at Kansas State University, tells Treehugger.
“Like other megafauna, bison are thought to have an outsized impact on ecological dynamics because they can exert strong control over vegetation and tend to form a vast web of interactions with other species.”
For their study, Ratajczak and his colleagues explored whether bison can still have a significant impact on tallgrass prairie and, if so, what does that say about the effect they likely had in the past ?
Three decades of data
The researchers looked at about three decades of data from the Flint Hills area of Kansas. They analyzed information that began in the late 1980s and early 1990s, but revisited after noticing droughts that occurred in 2011 and 2012.
“These droughts resemble the climatic extremes that are likely to become more frequent in the near future, due to climate change,” Ratajczak says. “Therefore, the ecosystem response to these droughts could give us a measure of whether we can expect the ecosystem to be resilient in the future.”
The area they were studying was divided into three sections. In one there were no mega-grazers, one had reintroduced year-round bison grazing and the other had cattle grazing from April to November, which is the growing season. .
They found that the area with bison had an 86% increase in native plant species richness compared to areas without bison. Cattle grazing also increased plant species, but by less than half the amount associated with bison.
The results were published in the journal Proceedings of the National Academy of Sciences (PNAS).
The researchers say this is one of the largest increases in diversity ever caused by a grazing animal, and the grassland growth gains have withstood the extreme drought, which is likely to become more widespread.
“The tallgrass prairie (where the study took place) is one of the most productive grasslands on the Great Plains. There are a handful of very dominant grasses that can take advantage of this, becoming incredibly abundant, leaving very little room or resources for other species,” says Ratajczak.
Bison prefer to eat these dominant grasses and can consume large amounts of them. This leaves more space for other plant species, which leads to an increase in diversity.
Bison also help the ecosystem by wallowing. This is when they repeatedly roll over the dry ground to shed their winter coat or ward off insects. The behavior creates a depression in the land – called a swamp – which can promote plant diversity.
Resilience is key
It was essential to conduct a long-term study because some environmental responses to a change in land use can take years or even decades to appear, according to the researchers. In this study, they found an increase in plant diversity over three decades. If they had ended the research sooner, they would have thought the bison had a much lower impact than long-term.
“Like many grasslands, tallgrass prairie exists in a region of variable climate. It is ‘natural’ for the ecosystem and many organisms in this ecosystem are adapted to a variable climate,” says Ratajczak. “However, we expect the ecosystem to face even more variable climate, so having a long data record is important to understand how these ecosystems function on ecological and evolutionary timescales.”
Megafauna, such as bison, are absent from many, if not most, ecosystems.
“I kind of thought maybe the species that evolved to do well alongside bison might be locally extinct by now. Some probably did, but clearly many did not. This means that reintroducing bison to some tallgrass prairies could really increase their plant biodiversity,” Ratajczak says.
“I think the resilience that we saw in this study is particularly important. These drought episodes can be truly disconcerting, and while we can’t take this resilience for granted, I was heartened to see that the plant community was able to rebound quite quickly after a very extreme drought.