On the ridge of Knowland Park overlooking five counties and the sparkling waters of the bay, the Oakland Zoo’s expensive and contentious expansion is beginning to take shape.

After 2½ decades of planning and years of wrangling with local opponents, construction has sketched the outline of the zoo’s ambitious California Trail exhibit. The first of 16 eight-person aerial gondolas imported from Switzerland to carry visitors over the exhibit was being mounted onto its wires Thursday.

The California Trail attraction, envisioned as a showcase of the state’s biodiversity, ironically drew the ire of local conservationist groups in its planning stages because of its potential impact on Knowland Park’s wildlife, particularly the federally protected Alameda whipsnake and a rare type of chaparral plant.

Zoo officials say the expansion will encourage stewardship by offering a unique look at California wildlife past and present, and that building on lower ground would have increased the project’s environmental impact, not lessened it.

“This project is really the only sort of place in California where we’re talking about California’s biodiversity, past present and future — looking at what we have in the state and how can we take care of it,” said Nik Dehejia, chief financial officer for the Oakland Zoo.

The $80 million expansion will bring the zoo’s footprint up to 100 acres, making it the same size as the San Diego Zoo, one of the country’s largest.

At the Kaiser Permanente Visitor Center, a wall made of windows looks out on the city of Oakland stretched out below and the Golden Gate Bridge in the distance.

Those spectacular views come from the expansion’s use of the ridgeline habitat in Knowland Park, an aspect of the plan that drew criticism in the planning phases from some opponents because the expansion reduced public trail access to one of the park’s most beautiful overlooks.

California Trail will eventually be populated with some of the most formidable predators to roam California. The exhibit will house grizzlies, black bears, jaguars, mountain lions, bald eagles and condors.

Bison will roam a large enclosure by April. Although the viewing area for their exhibit will not be accessible to the public until the expansion opening in summer 2018, they will be visible from the park’s existing gondola system.

The group of about 20 bison is being provided to the zoo by the Blackfeet Nation tribe of Montana, with the agreement that the offspring the bison might bear at the zoo will be returned to the herd in Montana.

A subspecies of grizzly bear, the California grizzly, once roamed the state by the thousands, but was hunted to extinction by the early 20th century.

In 2016, the Center for Biological Diversity — incidentally, one of the groups that voiced opposition to the zoo expansion in 2014 — unsuccessfully petitioned federal wildlife officials to reintroduce grizzly bears to the California wilderness.

Now grizzlies will return to Northern California, albeit within the confines of the Oakland Zoo’s enclosures.

Most of the California Trail exhibits will be populated with animals who cannot be released into the wild, like young orphaned bear cubs who are bottle-fed by rescuers and lose their natural aversion to humans, making it dangerous to release them once they’re grown.

“In the case of the bald eagle, for instance, they’ll receive an injury — typically a wing injury — and they won’t really have the full strong flight necessary for them to be able to hunt and feed themselves,” said Colleen Kinzley, the zoo’s director of animal care, conservation and research.

In many such cases, animals are euthanized if no facility is available to take them in.

Among the most touted aspects of the expansion is the Interpretive Center, which will encourage visitors to consider their place in California’s ecosystems.

“It’s very tactile and hopefully really leads kids and adults to think about what it’s like for these animals out in the wild,” said Kinzley. “They’ll go around all the areas that these animals occupy and at the end, the last area is the bay, as a way to say: This is where we live; what are the responsibilities that go with that?”

Filipa A. Ioannou is a San Francisco Chronicle staff writer. Email: fioannou@sfchronicle.com Twitter: @obioannoukenobi