Extinctions of large animals — a fate that could soon befall elephants and rhinoceroses — have a cascade effect on local ecosystems, including Northern California, where many smaller animals and plants died off after mammoths were wiped out, a team of scientists has discovered.

The size of elephants, wildebeests and other big plant-eaters makes them not only impressive and fascinating but also vital to the many species that live with and depend on them, according to a joint report by UC Berkeley, Stanford University, California State University Sacramento and the University of Chile.

“Ecological studies have shown that if you pull out a top predator or a key herbivore today, you get dramatic change in the ecosystem,” said Anthony Barnosky, a UC Berkeley professor of integrative biology and the study leader. “Our study makes it clear that in the past, such changes have lasted for thousands of years. These extinctions really do permanently change the dynamics. You can’t go back.”

The study, which was released Monday and is to be published in the journal Proceedings of the National Academy of Sciences, looked at extinctions in North and South America since humans arrived about 15,000 years ago.

The scientists found that the number and diversity of small animals and vegetation decreased all along the Pacific Northwest, as well as the western and northeastern United States, after mammoth and mastodon extinctions. In Alaska and the Yukon, what was once a mix of forest and grassland became mostly tundra after the loss of mammoths, native horses and other large animals, according to the study.

Nowhere has the change been more dramatic than in the Bay Area. Columbian mammoths were among thousands of now-extinct animals that roamed the region as late as 12,000 years ago. The shoreline was 12 miles farther out at that time, and a vast plain stretched from the Golden Gate, where a fast-moving river flowed, to the Farallon Islands.

Herds of mammoths, mastodons, camels, horses, llamas, elk, tapirs, moose and bison would have darkened the Farallon Plain. Mingling with these great herbivores were predators like the short-faced bear, saber-tooth cat, wolf packs and prides of California lions.

Some 60 large mammals have died out in North America since that time.

The situation is relevant today because previous studies have concluded that Earth is in the midst of a sixth mass extinction. A report published in June in the journal Science Advances found that animals are going extinct at a rate 100 times faster than they should. It concluded that humans are polluting the ecosystem, destroying habitat and killing off species at a rate so rapid that the demise of animals like elephants, tigers, rhinoceroses and others could occur within three human lifetimes.

The study released Monday explains how large browsing animals like mammoths, mastodons and today’s elephants, bison and moose change the ecosystem by eating small trees and shrubs, uprooting trees and trampling and churning up soil. They also distribute seeds and change nutrients in the soil when they defecate and urinate, according to the study.

The result is more open grasslands, fewer overgrown forests and a different mix of animals and plants.

“You see the impact of defaunation today in Africa, where the removal of elephant populations has led to these shrubby, scraggly acacias filling the savanna landscape,” said co-author Charles Marshall, a professor of integrative biology and director of the UC Museum of Paleontology. “Africa today, with its elephant populations, seems to fit the model of North America with its mammoths and mastodons.”

Peter Fimrite is a San Francisco Chronicle staff writer. E-mail: pfimrite@sfchronicle.com Twitter: @pfimrite