It started as a duel: Los Angeles or San Francisco? Which city’s citizens could document more plants, animals and insects in the course of a week? 

In its second year, the City Nature Challenge has expanded beyond the California rivalry (which Los Angeles won, barely). Now, the organizers, the California Academy of Sciences and the Natural History Museum of Los Angeles County, want to know which city in the United States has the most nature?

Between Friday and Tuesday the two museums are asking residents of 16 U.S. cities to take to the streets armed with their smartphones and a free app called iNaturalist. Participants are asked to take a picture of any wild plant or animal, describe what they see and upload it through the app. Locally, participants are also encouraged to post images to social media tag with #NatureinLA. Then, on April 22, the results of the competition will be revealed.

A couple of tips before you head out:

• When you sign up for iNaturalist, click the box that says you consent to sharing your observations with scientists.
• Make sure you include a photo, coordinates and the date on every photo you upload, plus a description of what you saw.
• Make sure your photos are in focus! Watch this video for tips on how to take better photos.
• It’s OK if you don’t know exactly what animal or plant you’re looking at. Just describe what it looks like. Scientists will try to identify your observation later.
• If you can’t get close to the animal or plant, tightly crop your photo before uploading. That will make it easier for others to identify
• Try to only take pictures of wild organisms, not plants or animals in zoos, gardens or museums. If do you take a picture of a captive or cultivated species, click the button on iNaturalist that indicates that. And no pets!
• More FAQs here. 

The goal of the City Nature Challenge is to collect a ton of new scientific information while building community around exploring and observing nature. The information gathered will provide scientists with data they can use to develop a baseline for the natural environment in each city. By conducting the competition each year, scientists and local governments can track changes to the environment. 

“A lot of people have this perception of L.A. as a concrete jungle with no nature, just smog and traffic and buildings. But we’re in a biodiversity hotspot. There are species here that are nowhere else,” said Lila Higgins, who organized the project for the Natural History Museum of Los Angeles County. “We need to understand how urbanization affects nature. You can’t understand that without collecting big data sets, and you can’t do that without citizen science.”

In a city like L.A., community involvement is key because there's "too much private property,” according to the competition website. “Citizen science is the best way to gain a better understanding of […] current wildlife community in urban, suburban, and rural areas.”

While it’s way too soon to call the winner, as of Friday Houston was in the lead in the total number of observations. Higgins said there would be multiple winning categories, including the total number of species observed. L.A. and San Francisco have an obvious advantage over many other U.S. cities. Coastal California is one of 36 international biodiversity hotspots, according to Conservation International. Southern California's Mediterranean climate exists in only four other places in the world, all of which are on the hotspot list, and it hosts more than 2,000 endemic plant species. 

Correction: An earlier version of this story incorrectly identified the winner of the first competition. Los Angeles won by recording more citizen observations. However, San Francisco documented more species.