Which Animals Like Cities Most? Slugs And Snails Top The List.

8:27 minutes

Small Roman snail or Burgundy snail (Helix pomatia) with light brownish shell crawling and crossing a brick pathway in a city in summer
Credit: Shutterstock

If you live in an urban environment, it might seem like the animals you see every day—birds, bugs, squirrels—have adapted perfectly fine to city life.

But according to a new study in PLOS ONE, that isn’t always the case. Urbanization is directly linked to biodiversity loss, but researchers at UCLA, including Joey Curti and Dr. Morgan Tingley, wanted to find out specifically which animals thrive and which struggle in urban environments. So they turned to iNaturalist, a crowd-sourcing app where users upload photos of flora and fauna they see, along with information like location and date.

The team combed through years of iNaturalist data in the Los Angeles metro area and developed an “urban tolerance score” for 511 animal species. This score, which incorporated data such as light and noise pollution from different sections of the city, was a factor tied to those species’ level of tolerance to the local environment.

They found that snails and slugs love urban environments, likely thanks to increased moisture from local landscaping. But most other animals, including native species, and especially bugs like butterflies and moths, were not as tolerant to the region.

Joey Curti, a PhD candidate in ecology and evolutionary biology at UCLA and a co-author on that study, sits down with guest host Arielle Duhaime-Ross to discuss the results of the study and what cities can learn from this kind of research to encourage healthy biodiversity.

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Segment Guests

Joey Curtis

Joey Curtis is a PhD candidate in Ecology and Evolutionary Biology at UCLA in Los Angeles, California.

Segment Transcript

ARIELLE DUHAIME-ROSS: If you live in a city, you might assume that the animals you see every day, like birds, bugs or squirrels, have adapted perfectly fine to living in an urban environment. Unfortunately, that’s not always the case. Urbanization is directly linked to biodiversity loss, but researchers at UCLA wanted to find out specifically which animals thrive and struggle in an urban environment.

So they turned to a large source of readily available data, iNaturalist, the app where people log the plants and animals they see along with their location and the date. And the scientists use this information to determine which animals were the most tolerant of urban environments over a period of time. And you might be surprised by some of the results.

Here to tell us about them as well as what city managers can learn from the data to increase biodiversity in urban environments is Joey Curti, PhD candidate in Ecology and Evolutionary Biology at UCLA. They were a co-author on that study published in PLOS One. Joey, welcome to Science Friday.

JOEY CURTI: Hi, Thank you for having me.

ARIELLE DUHAIME-ROSS: Thank you so much for being here. So if I’m an animal living in a city, what kinds of factors are going to be stacked against me in terms of my ability to survive and thrive?

JOEY CURTI: Yeah, that’s a great question. It’s going to depend a lot about your natural history. So you can imagine if you’re a flighted animal, say, a bird, there’s going to be a lot of things that you can encounter on the landscape, for example, buildings that have reflective surfaces that might pose a challenge to you as you’re navigating across the landscape. Similarly, you can imagine if you’re a nocturnal flighted animal, say, a bat, so you use echolocation to find prey. And maybe road noises are really impactful to your ability to do so.

Similarly, the way that we light cities is really impactful for a lot of animals. And this can impact your ability to find your home, your ability to find prey, for example. So there are a lot of factors in an urban landscape that can be really challenging for our animal species here.

ARIELLE DUHAIME-ROSS: And if you don’t mind me asking, why is it important for a city to have healthy biodiversity?

JOEY CURTI: Yeah, so I’m a conservation biologist, and I’m absolutely in the camp of biodiversity having an intrinsic value. And I think that it enriches my life every day to go outside into my yard and to see all the native animals flourishing. But I also know that a lot of people– it’s a more convincing argument to say biodiversity protects your health.

And so there are a lot of studies out there, especially with birds, that show that your exposure to animal-generated noises, for example, birdsong, really do a lot to reduce your stress levels and your levels of anxiety and depression. And so biodiversity inherently can have a major impact on our human health and our health span as individuals.

ARIELLE DUHAIME-ROSS: I want to ask you, though, I hear that you’re a big fan of the app iNaturalist. How did you use it in this study?

JOEY CURTI: Yeah, so you can’t go out and measure and monitor every different species as a group of scientists unless you want to spend a lifetime doing so. And so we were excited to be able to leverage this amazing crowdsourced data set that tons of people use across the city on their daily lives. So it’s an incredibly vast data set. So it’s about 189 million observations.

So it’s a ton of data to work with. And so we downloaded observations from across Los Angeles as well as 150 surrounding the city. And that left us with well over a million observations to work with across a bunch of different taxonomic groupings that we thought were important to monitor, including mammals, birds, amphibians, reptiles, and several different groups of invertebrates. And after pretty strict filtering, we were left with 511 different native species that we felt we had good data in order to move forward with our analysis.

We were really interested in understanding how these species on an individual level relate to measures of urban intensity. And for that, we mean things like light pollution, so artificial light at night, sound pollution, and measures of impervious surfaces, so our concrete or asphalt, things like that.

ARIELLE DUHAIME-ROSS: So what did you find? What animals did better than expected? Which ones did worse? And how did you come to those conclusions?

JOEY CURTI: Yeah, absolutely. So species can take on a negative value, meaning they have an intolerance to our measure of urban intensity, or a positive value, which means they’re positively associated with urban intensity. And we found that, on average, species had a negative association with urban intensity, meaning that they were more likely to be seen and observed in wild spaces across the city. So you can think your Santa Monica Mountains or your Griffith Park, for example.

We also found, though, that some birds, for example, did have positive associations with urban intensity. Most had negative interactions. And then, on average, the one group that did well seemed to be snails and slugs.

ARIELLE DUHAIME-ROSS: Was that something you expected to see?

JOEY CURTI: I would say that all of this was pretty surprising to me. I’m not a malacologist, but when we dig into the literature a bit, potentially ornamental landscaping, for example, lawns, different plants that we plant in our yards might have an impact on our native snails and slugs. Snails and slugs need moisture to survive, and so if we introduce a lot of moisture to irrigate our non native plants that we introduced into our lawns, potentially that’s actually creating good habitat if you’re a snail or a slug.

ARIELLE DUHAIME-ROSS: Hmm, OK. So what about the animals that did worse, the ones that were the least tolerant of urban environments?

JOEY CURTI: Yeah, so animals that tended to have negative relationships with urbanization tend to be our habitat specialist animals. So, for example, California quail, and these birds really rely on shrub cover. And of course, we don’t see those in downtown Los Angeles because that habitat no longer exists. So things like acorn woodpeckers or wrentit species– these are all birds– they tend to just really stick to these core habitat fragments that still exist within cities so, for example, Griffith Park.

ARIELLE DUHAIME-ROSS: So there’s a component of human behavior in this study as well. You were dependent on people logging sightings of various animals. And I would imagine that could introduce certain biases, maybe in the types of animals logged or in the times of day when people log them. How did you control for that?

JOEY CURTI: Yeah, that’s a fabulous question. We’re really lucky to be working directly with Morgan Tingley and his lab, which do a lot to work with these big data sets and be able to control for these different sort of confounding variables. And proximity to roads and trail networks is a major source of bias for these data sources.

Similarly, different areas of the city have different amounts of effort, for example. But essentially, if you’re in a grid cell in Los Angeles and any bird is observed, for example, we take that as an indication that that grid cell has had survey effort. And therefore, if we’re looking for, for example, a wrentit in that area and we don’t see it but we see some birds have been observed, we take that as a sign that at least some effort has been taken to try to find birds in that area. So it’s probably a true absence of that individual species.

ARIELLE DUHAIME-ROSS: Not to be too pessimistic here, but is it even possible for a huge urban landscape like LA to have healthy urban biodiversity?

JOEY CURTI: I think I have to be optimistic as a conservation biologist. There are a lot of initiatives here within the city to try to address this “no net loss of biodiversity by 2050” goal that we have. We actually have a wildlife ordinance that’s going through city council right now to make sure that the footprint of the new development is biophilic in nature and is not hindering our native species from moving across the landscape, for example. And so I’m really optimistic for our city to really do absolutely everything that it can to try to improve and maintain our biodiversity.

ARIELLE DUHAIME-ROSS: Did this research make you look at Los Angeles any differently?

JOEY CURTI: I’ve always been a big proponent of iNaturalist. I can actually be kind of an insufferable hiking partner here in Los Angeles, stopping every five seconds to take pictures of beetles, or bees, or your western fence lizard.


JOEY CURTI: But I think, really, I didn’t really understand until having worked with these data sets how valuable these crowdsourced data can be and how they can really advance science. And so now that I’ve worked with these data, I’m even more insufferable when I go outside. I document everything that I possibly can because I know that this is really feeding directly into tools that the city is using to evaluate its nature.

ARIELLE DUHAIME-ROSS: All right. Thanks for taking the time to explain all of this, Joey. I really appreciate it.

JOEY CURTI: It’s been an absolute pleasure. Thank you so much for having me.

ARIELLE DUHAIME-ROSS: Joey Curti is a PhD candidate in Ecology and Evolutionary Biology at UCLA.

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