Herds of wild zebra, buffalo, deer, and other herbivores on six continents can't can keep up with all the extra plant life dominating grasslands around the world.

Certain grasses have been growing taller, thicker and faster because of the nutrients and fertilizers humans have introduced both directly and indirectly into otherwise wild places. Some biologists had theorized that hungry wild herbivores would adjust and chow them down.

But, as it turns out, there simply aren't enough grazers left in most places to forage or mow down the extra plant life, according to a study led by the University of Minnesota at more than 50 grasslands worldwide.

"We're seeing, yes, wild herbivores can counteract this to an extent, but the widespread issue is that numbers of wild herbivores around the world have been massively reduced," said Elizabeth Borer, lead researcher. "So it isn't just that there's more nitrogen in these ecosystems, there's also less herbivores to counteract it."

Extra plant life is a problem because each fall as the plants die, all that added weight and biomass becomes fuel for wildfires. In places less prone to fire, such as Minnesota, exotic nutrient-loving grasses are strangling out native plants and small flowers needed by bees and other pollinators.

The complex diversity of plant and animal life is being lost as one species takes over.

The grasslands study was replicated at 58 sites across more than a dozen countries. Not every location experienced the same results.

In some places, such as Tanzania, wild animals have been able to keeping up with the rapid plant growth. Tanzania still has large herds of zebra and other grazers, and the herbivores are eating almost everything the grasslands are providing them, Borer said.

But in most places, including Minnesota, that's not the case.

"Our big obvious herbivores — bison — are gone," she said. "We still have deer and rabbits and domestic grazers, but they are not keeping up."

In drier climates, wild animals were able to eat more of the excess grasses and plants. But as regions became wetter and more fertile, herbivores had a harder time keeping up.

Nutrient pollution has become so prevalent that humans are essentially fertilizing every grassland on Earth, Borer said. It's coming from the land, and what farmers and homeowners are spreading on fields and lawns, and from runoff into rivers and streams.

Studies are also finding that nutrients are being carried by the air, in dust particles, and building up in high enough quantities to change the makeup of life in lakes high in the Rocky Mountains. They may also be causing toxic algae blooms in some of the wildest and most remote lakes in northern Minnesota and Isle Royale National Park.

The study's findings in places like Tanzania show there is hope to manage grasslands with herbivores, Borer said.

"The solution in northern Europe is going to be very different from Minnesota or California," she said. "But this suggests that restoration of wild herbivores and even some form of domestic grazing, with cattle or possibly goats, seem promising to avoid things like fire risk or the loss of species and biodiversity."

Greg Stanley • 612-673-4882