In February, the city of Denver  a segment of a restaurant-lined street in its town center to car traffic. The move ended a five-year experiment that created new space for fresh-air strolling and socializing during the COVID-19 pandemic.

Denver’s Larimer Street was one of hundreds of U.S. streets that opened to bikers and pedestrians during the pandemic, limiting car traffic. These street-focused emergency response measures, billed as “open streets,” “slow streets” or “shared streets,” allowed restaurants to expand their outdoor dining spaces and provide safe spaces for walking or rolling on bikes. Kevin J. Krizek, a professor of environmental design, has researched these measures.

“That was a valuable experiment, allowing cities and residents to see their public spaces in different ways,” Krizek said. “They showed the power of spurring vibrant spaces when people eat outside and brought different forms of society together in these areas.”

But five years later, many of these trial programs have ended as funding, permits and support ran out.

In 2022, Boulder, Colorado, reverted a stretch of its . In New York City, the number of open street project miles  between 2020 and 2022.

Krizek shared his take about how COVID-19 temporarily changed cities across the United States, what cities learned and why many of these forward leaning programs didn’t last.

How did COVID-19 reshape our cities?

During COVID-19, experimentation with slow or open streets was a big breakthrough. Neighbors saw their public space in new light and leaned on their nearby streets to provide space for walking, riding bikes and meeting people in safe spaces.

Americans often travel to Europe to experience the vibrant outdoor cafes and active street life. COVID allowed them to see the possibility of doing that here and the power of changing streets overnight. That's one of the most valuable lessons COVID taught us: Street space could quickly change and be used for more than moving and storing cars.

Are American cities designed to support pedestrians, bikes and safe public spaces?

No. American cities are mostly designed to maintain the swift movement of cars. If you take an aerial view of the country, roughly a third of our cities are devoted to space owned by the public, and most of that space is devoted to either storing or moving cars.

Coming out of the second industrial revolution in the 1930s, city designers began writing a whole labyrinth of legislative codes to prescribe the width of travel lanes, the turning radii and the length of a green light. All these rules were designed to swiftly move cars.

These outdated codes have become the architectural blueprint for most American cities and continue to be followed.

What are the consequences of that design?

In the past decades, cars in the U.S. have become larger, higher, more intimidating and heavier. They are now more dangerous, especially at high speeds.

Cities continue to allow these vehicles to dominate our streets, all in the interest of convenience and economic vitality. But swiftly moving traffic and safe walking are mostly mutually exclusive, particularly around intersections.

Traffic violence has killed more than 40,000 people each year over the past years in this country. That number equals a Boeing 737 plane going down every other day. When a plane crashes, we spend months investigating the cause. But when a car crash happens, we sweep up the carnage as quickly as we can, and we revert the system back to ‘normal’ conditions as quickly as we can. We fail to properly investigate and address the underlying factors that lead to these crashes. We've all become complacent with the risk and the need for speed, assuming there’s little to be done.

How did the public respond to these COVID-era experiments?

Many communities used these street changes as novel experiments. Studies found that, despite the isolation of COVID, people leaned into public spaces, which helped the local community integrate in ways that were previously unfathomable.

Why were they discontinued?

A lot has to do with how we frame the access to key spaces in our communities. Because American cities are designed for cars, many businesses are only accessible by driving there. Such businesses claim that if you diminish that access, you're compromising economic vitality.

That’s mostly an unsupported fear. There’s a slew of studies showing that if a town provides for multiple ways to get there, over time, more people access these businesses using varied modes.

What can individuals do to push for more pedestrian-friendly cities?

We, as a society, collectively accept an enormous amount of risk every time we get in and out of our cars. That risk comes with personal conveniences. We can ask ourselves to better understand the costs of these personal conveniences and to accept better street planning protocol that allows lower speeds and heightens safety on our transportation system.

People who want their cities to become safer and more pedestrian-friendly can approach their city council person and say, ‘We'd like to express our desire to move beyond the existing codes that lock us in.’ Changing existing codes, while challenging, is possible when citizens empower their city councils to think differently and create safer public streets. 

CU Boulder Today regularly publishes Q&As with our faculty members weighing in on news topics through the lens of their scholarly expertise and research/creative work. The responses here reflect the knowledge and interpretations of the expert and should not be considered the university position on the issue. All publication content is subject to edits for clarity, brevity and university style guidelines.

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Our sustainability impact by the numbers:

• First student-run campus environmental center in the U.S.
• No. 11 university for environmental and social impact in the U.S.
• First zero-waste major sports stadium in the U.S.

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For young salmon, the journey along the San Joaquin River in Central California is no small feat. Every spring and fall, thousands of these fish—each as long as a pinky finger— embark on a 350-mile race, swimming day and night and dodging predators along the way to reach the Pacific Ocean.

But survive the journey, and in some years, hardly any make it. Elevated water temperatures, dams and poor water quality all endanger the animal, but human-introduced predators, including striped and largemouth bass, kill most of them.

In a new CU Boulder-led study, researchers reveal how these salmon learn to swim in different parts of the river at different times of day to avoid predators and conserve energy. The study was Feb 24 in the journal Ecology Letters. 

“The salmon fishery in the San Joaquin River delta area is on the verge of collapsing,” said Mike Gil, the paper’s first author and assistant professor in the Department of Ecology and Evolutionary Biology. “We know these juvenile salmon are getting wiped out on their migration to sea. We need to know why and how this is happening, and if there are opportunities to leverage conservation practices.” 

After spending their first year in the river where they hatched, juvenile salmon migrate to the ocean to access the nutrients they need to mature. Once they reach reproductive age, adult salmon return upstream to spawn.

the California Department of Fish and Wildlife, the population of Chinook salmon migrating in the fall in the Sacramento and San Joaquin river systems has dropped from 872,669 in 2002 to 79,985 in 2022—a 90% decline in just two decades.

Gil and his team placed trackers in 424 juvenile Chinook salmon, as well as 23 striped bass and 17 largemouth bass. Using detectors placed along the riverbanks, the team monitored the activities of salmon and their predators, including when and where predators attack the most, for two months as they traveled through the San Joaquin River.

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Our sustainability impact by the numbers:

• First student-run campus environmental center in the U.S.
• No. 11 university for environmental and social impact in the U.S.

• First zero-waste major sports stadium in the U.S.

They found that salmon migrate over a much longer distance at night, a behavior that scientists had previously observed without fully understanding why.

The team’s data showed that during the day, predatory bass tend to concentrate and attack more frequently mid-river, where salmon prefer to swim. There, the currents flowing toward the sea are stronger, so salmon can ride the water downstream, saving energy.

To avoid those bass, young salmon have adapted to migrate mid-river at night. Meanwhile, by day, they seek refuge near the riverbanks— even though that means expending more than double the energy to swim the same distance.

“Intuitively, one would think these fish should just be taxiing right down the middle of the river all the time, so they can get out to the ocean and get away from all these terrifying predators as fast as possible. But that’s not what we saw,” Gil said. “Our study suggests that bass activities are forcing these fish to adopt a different strategy.”

The researchers also found that during dawn and dusk, predator attacks spiked. Gil said this is likely because striped bass, with their bigger eyes, can see better in low-light conditions than juvenile salmon which have smaller eyes.

“These fish seem to really pick up on changes in ambient lighting,” Gil said.

He hopes that the findings could help direct efforts to save local salmon populations.

For example, limiting light pollution at night in towns near the river and its estuaries could help these fish survive.

“We as humans are quite limited in our understanding of how animals in the wild behave.  By better understanding this, we can make the most informed decisions about how to keep these species around,” Gil said. 

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In 2024, annual average global air temperatures surpassed 1.5 degrees Celsius above pre-industrial levels for the first time, triggering extreme weather events like record-breaking rainfall and flooding events in the Sahara Desert and extreme summer heat waves across the planet. However, global warming will not stop at this level. 

Based on the current pledges of countries for limiting their emissions of greenhouse gases, global temperatures are projected to reach 2.7 degrees Celsius beyond pre-industrial levels by the end of this century. This scenario would dramatically reshape the Arctic, the fastest-warming region of Earth.

A new review paper, published in Science on Feb. 7, highlights these changes and their far-reaching implications. The paper, “Disappearing landscapes: The Arctic at +2.7°C global warming,” was led by , senior research scientist at the (NSIDC) and professor at the Centre for Earth Observation Science at the University of Manitoba.

“The Arctic is warming at four times the rate of the rest of the planet,” said Stroeve. “At 2.7 degrees Celsius of global warming, we will see more extreme and cascading impacts in this region than elsewhere, including sea-ice-free Arctic summers, accelerated melting of the Greenland Ice Sheet, widespread permafrost loss and more extreme air temperatures. These changes will devastate infrastructure, ecosystems, vulnerable communities and wildlife.”

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Our sustainability impact by the numbers:

• First student-run campus environmental center in the U.S.
• No. 11 university for environmental and social impact in the U.S.
• First zero-waste major sports stadium in the U.S.

In the review paper, the authors used the  as a starting point. They updated knowledge from the report about three specific areas of the Arctic environment, including sea ice, the Greenland Ice Sheet and permafrost, focusing on existing studies that show consensus about the changes that will take place in the region.

Under 2.7 degrees Celsius of warming, the Arctic region is likely to experience the following effects:

• Virtually every day of the year will have air temperatures exceeding pre-industrial temperature extremes.
• The Arctic Ocean will be free of sea ice for several months each summer.
• The area of the Greenland Ice Sheet that experiences more than a month of surface temperatures above 0 degrees Celsius will quadruple compared with pre-industrial conditions, causing global sea levels to rise faster.
• Surface-level permafrost will decrease by 50% of pre-industrial levels.

“Our paper shows that, already today, mankind has the power to wipe out entire landscapes from the surface of our planet,” said , professor for polar research at the University of Hamburg and co-author of the study. “It’d be amazing if we could become more aware of this power and the responsibility that goes with it, as the future of the Arctic truly lies in our hands.”

Other co-authors on the paper included Jackie Dawson of the University of Ottawa, Edward A.G. Schuur of Northern Arizona University, Dorthe Dahl-Jensen of the University of Manitoba and University of Copenhagen, and Céline Giesse of the University of Hamburg. Funding came from several sources, with the largest piece of Stroeve’s funding from the , C150 grant 50296. Data and information from NSIDC’s and projects were used in the review. 

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Giant clams, some of the largest mollusks on Earth, have long fascinated scientists. These impressive creatures can grow up to 4.5 feet in length and weigh over 700 pounds, making them icons of tropical coral reefs.

But these animals don’t bulk up on a high-protein diet. Instead, they rely largely on energy produced by algae living inside them. In a new study led by CU Boulder, scientists sequenced the genome of the most widespread species of giant clam, Tridacna maxima, to reveal how these creatures adapted their genome to coexist with algae.

The findings, Jan. 4 in the journal Communications Biology, offer clues about how such evolution may have contributed to the giant clam’s size.

“Giant clams are keystone species in many marine habitats,” said Jingchun Li, the paper’s senior author and professor in the Department of Ecology and Evolutionary Biology. “Understanding their genetics and ecology helps us better understand the coral reef ecosystem.”

A symbiotic relationship

Unlike popular myths—like the one in Disney’s “Moana 2” where the giant clam eats humans—these vegetarian mollusks rely on algae living within their bodies for energy. If giant clams ingest the right algae species while swimming through the ocean as larvae, they develop a system of tube-like structures coated with these algae inside their body. These algae can turn sunlight into sugar through photosynthesis, providing nutrients for the clams.

“It’s like the algae are seeds, and a tree grows out of the clam’s stomach,” Li said.

At the same time, the clams shield the algae from the sun’s radiation and give them other essential nutrients. This mutually beneficial relationship is known as photosymbiosis.

“It’s interesting that many of giant clams’ cousin species don’t rely on symbiosis, so we want to know why giant clams are special,” said Li.

In collaboration with researchers at the University of Guam and the Western Australian Museum, the team compared the genes of T. maxima with closely related species — such as the common cockle—that lack symbiotic partners. The researchers found that T. maxima have evolved more genes coded for sensors to distinguish friendly algae from harmful bacteria and viruses. At the same time, T. maxima tuned down some of its immune genes in a way that likely helps the animal tolerate algae living in their body long term, according to Ruiqi Li, the paper’s first author and postdoctoral researcher at the CU Museum of Natural History.

As a result of the clam’s weakened immune system, its genome contains a large number of transposable elements, which are bits of genetic material left behind by ancient viruses.

“These aspects highlight the tradeoffs of symbiosis. The host has to accommodate a suppressed immune system and potentially more viral genome invasions,” said Ruiqi Li.

The study also discovered that giant clams have fewer genes related to body weight control, known as the CTRP genes. Having fewer CTRP genes might have allowed giant clams to grow larger.

Conservation concerns

Last year, a giant clam population assessment by Ruiqi Li, prompted the 

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Our sustainability impact by the numbers:

• First student-run campus environmental center in the U.S.
• No. 11 university for environmental and social impact in the U.S.
• First zero-waste major sports stadium in the U.S.

International Union for Conservation of Nature (IUCN) to update the conservation status of multiple giant clam species. Tridacna gigas, the largest and most well-known species, is now recognized as “critically endangered,” the highest level before a species becomes extinct in the wild.  

T. maxima, because of its wide distribution, is currently classified as “.” But Ruiqi Li said it’s possible that different species are lumped into one category simply because they look similar.

“If you think these giant clams are all the same species, you might underestimate the threat they face,” Ruiqi Li said. “Genetic studies like this can help us distinguish between species and assess their true conservation needs.”

The team hopes to sequence the genomes of all 12 known species of giant clams to better understand their diversity.

Similar to corals, giant clams are facing increasing threats from climate change. When the ocean water becomes too warm, the clams expel the symbiotic algae from their tissues. Without the algae, the giant clams can starve.

“The giant clams are very important for the stability of the marine ecosystem and support biodiversity,” Jingchun Li said. She added that many creatures living in the shallow waters rely on their shells for shelter, and giant clams also provide food for other organisms.

“Protecting them is essential for the health of coral reefs and the marine life that depends on them.” 

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Climate change is melting away glaciers around the world, but in the Andes Mountains, a wild relative of the llama is helping local ecosystems adapt to these changes by dropping big piles of dung.

This finding, Dec 30 in Scientific Reports, revealed that the activity of this animal could accelerate the time plants usually take to establish on new land by over a century, highlighting a surprising way organisms are adapting to climate change.

“It’s interesting to see how a social behavior of these animals can transfer nutrients to a new ecosystem that is very nutrient poor,” said , the paper’s co-first author and a research scientist in the Cooperative Institute for Research in Environmental Sciences at CU Boulder. But the current pace of climate change still outpaces the ability of species to find new habitats, he warned. 

Vicuña latrines

The changemakers here are the vicuñas. They are one of two wild South American camelids, a group of animals that includes alpaca and llama, which are domesticated species. They live in the high alpine areas of the Andes. 

Vicuñas may be less famous than their celebrated llama cousins, but they are no less remarkable, particularly because of where they choose to poop.

Much like how humans use bathrooms, these animals get rid of their solid waste using a designated spot shared by multiple members of a social group. Scientists refers to these communal dung piles as latrines.

Over the past two decades, Steven Schmidt, the paper’s senior author and professor in the Department of Ecology and Evolutionary Biology, has studied how microbial life and plants are responding to retreating glaciers in the high-altitude Peruvian Andes.

The deglaciated soils are extremely depleted of nutrients and water—a sea of rocks and gravel that can remain plant-free for over a century.

But during expeditions over the last ten years, Schmidt and his collaborators began noticing patches of plants, all of which seemed to have emerged from vicuña poop piles.

Working with animal ecologist Kelsey Reider at James Madison University, the team trekked to sites in the Peruvian Andes, up to 18,000 feet above sea level, that were previously covered by glaciers. They sampled vicuña latrine soils in these areas and found that, compared to barren soils just a few feet away, soils with vicuña poop contained significantly more moisture and key nutrients, like organic carbon, nitrogen and phosphorus.

For example, latrine soil was made of 62% organic matter. In contrast, deglaciated soil that has been exposed for 85 years at the same location but without latrines contained only 1.5% organic matter.

At high elevations, temperatures tend to fluctuate significantly throughout the day, dropping below freezing every night even during the summer. “It's really hard for things to live, but that organic matter made it so that temperatures and moisture levels didn't fluctuate nearly as much. The latrines created a different microclimate than the surrounding area,” Schmidt said.

The team also found high DNA concentrations and a wide diversity of microorganisms in latrine soil samples, suggesting that the latrines provided vital ground for microbes and plants to thrive.

Adapting to climate change

The team said vicuña dung likely accelerated the timeline for plants to colonize a barren, lifeless habitat by a century. These animals deposit nutrients and plant seeds from lower elevations in their poop onto deglaciated ground, and then the seeds germinate, attracting other organisms, including animals that feed on the plants.

Camera footage showed that the patches of plants have attracted all kinds of animals, including rare species never before seen at such high elevations and large carnivores like puma. Vicuñas also eat the vegetation growing in their own latrines.

It could take hundreds of years for the deglaciated area to transition into grassland, which might help mitigate the negative impacts that many species preferring colder climates face as their habitats shrink from climate change, Reider said.

But even with the vicuña’s help, the rate of species colonizing new ground is much slower than the rate at which the glaciers are retreating.

Glacier melt across the world has accelerated over the past two decades. Between 2000 and 2019, glaciers other than the Greenland and Antarctic ice sheets lost about . If warming continues, the Earth could lose , a prior study estimated.

In parts of the Andes and other mountain ranges, including the Rocky Mountains, many people depend on mountain snow and glacier runoff for water. It is that shrinking glaciers and snow cover could threaten the water supply for nearly a quarter of the world’s population.

“The vicuñas are probably helping some alpine organisms, but we can’t assume they’ll all be okay, because in Earth’s history, we’ve never seen climate change happen at this speed,” Bueno de Mesquita said. “Current anthropogenic climate change is probably the most severe crisis our planet and all living things have faced in the past 65 million years.”

Ruth Quispe Pilco, graduate student in the Department of Ecology and Evolutionary Biology,also contributed to the study.

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