A group of nature enthusiasts gathered at San Francisco’s Presidio for a pair of noble causes: celebrating the region’s biodiversity and trying to beat Los Angeles in a competition.

Saturday marked the first day of the BioBlitz, part of the City Nature Challenge in which people from cities across the world record plant and animal species.

At first, the challenge was only between San Francisco and Los Angeles. But now, the event that inspires people to appreciate wildlife fosters a friendly rivalry among about 60 cities vying to be named the place with the most diverse flora and fauna.

The Presidio Trust hosted the event, bringing out dozens of naturalists, old and young. They met at the Presidio Officers’ Club before branching out across hundreds of acres of open space at the Tennessee Hollow Watershed, Crissy Marsh and Mountain Lake.

“This started as a competition between Los Angeles and San Francisco to get people like you engaged in the incredible biodiversity that still exists in our cities,” said Lew Stringer, associate director of natural resources for the trust. “Much of what we’re doing today is looking at the incredible bounty of life.”

Before heading out, he went over how participants would record their findings. This year, they took wildlife photos and uploaded them to iNaturalist, an app network of citizen scientists, naturalists and biologists that allows participants to catalog what they found.

Not everyone needed a phone. Those seeking a repose from technology could join a group that was going to sketch observations instead.

Volunteer coordinator Jason Lisenby sent them off.

“On your mark, get set, go beat L.A. Go!”

Soon small groups fanned out across the region. The sketch group knelt in MacArthur Meadow among California poppies, bright green vegetation and tiny purple flowers.

John “Zip” Lehnus, a volunteer leading the group, said naturalists are part of “a huge geek club.”

“We got a bunch of people standing in the dirt drawing pictures of plants,” said Lehnus, 51. “I think that’s great. I think that’s what we all should be spending time doing.”

And it’s more than that. He bent down and smelled the pink clover he was painting — it smelled sweet, he said. It’s all about observing nature with your senses and recording everything you take in.

“That’s the idea of being a citizen scientist,” Lehnus said. “The more I pay attention, the more I see.”

Farther up the meadow, Martine Glaros sketched a species of Avena grass, locked in a rhythm of looking down at her sketch pad and then over at the plant.

“San Francisco is just such a dense and intense city,” said Glaros, who lives in the Presidio as an intern with the trust. “It’s really nice to be able to come somewhere that’s calm and peaceful.”

A short walk away, Pat Boyd recorded the wildlife at Thompson Reach. She’d seen several plants, but hadn’t seen many animals, apart from tiny white spiders “that kind of freaked me out.”

Still, the sense of kinship with nature brought her to the Presidio, she said.

“It’s part of our home,” Boyd said. “It feels like it’s ours in a way, and we should be supporting it.”

She took a photo of a plant with tiny purple flowers and uploaded it to the iNaturalist app to figure out what it was.

“Wild radish,” she said. “Who knew?”

Jenna Lyons is a San Francisco Chronicle staff writer. Email: jlyons@sfchronicle.com Twitter: @JennaJourno