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While for humanity roads are essential means of communication, for animals it is the opposite: a barrier that can be lethal. In fact, every year millions of animals die trying to cross roads that cut their habitats in half: we see the same thing in Iran with the Asiatic cheetah that in the India, where elephants are run over (although yes, with trains) is one of the big problems of its railway system. One of the solutions proposed by conservation biology are wildlife steps: an infrastructure, whether a bridge, tunnel or walkway, that allows animals to move safely through their domain by crossing roads or railway tracks. California just took this idea to another level with the largest structure of its kind ever built. The megastep for California pumas. It is about the Wallis Annenberg Wildlife Crossinga colossal plant bridge about 64 meters wide that crosses the US-101 highway as it passes through Agoura Hills, in Los Angeles. The structure has more than 11.8 million kilos of concrete, 82 bridge beams and more than 6,000 cubic meters of living soil to house more than 50 species of plants native to the region. The idea is to faithfully recreate the coastal sage scrub or coastal sage scrub, aromatic shrubs that are abundant in the area, but on one of the busiest highways. The project is a public-private collaboration that started formally on Earth Day 2022 and has cost $114 million. Its inauguration is scheduled for autumn of this year, thus becoming the largest wildlife crossing in the world. California Government Why is it important. The United States National Park Service has spent decades documenting that mountain lions in the Santa Monica Mountains are genetically isolated by roads and urbanization. And an isolated population is doomed: it does not have genetic exchange with other groups, there is endogamy, genetic variability is lost and the species loses its capacity to adapt. That is to say, the passage of US-101 not only kills animals by being run over, but it is also an evolutionary trap. That said, the bridge is not an infrastructure that will benefit exclusively the puma: it is also designed for red lynxes, foxes, coyotes, reptiles and a long chain of species whose mobility is essential for the ecosystem. Finally, the structure is framed within the California 30×30 objectivewhich aims to conserve 30% of the state’s coastal lands and waters before 2030, in this case connecting the protected spaces of the Santa Monica Mountains with the environment. Context. Wildlife passages are not something new: the first were built in the 50s of the 20th century, in France. In fact, Europe has been developing this technology for more than 70 years and has a long list of structures of this type. What is unique about this project is not so much the structure itself, but its scale and location: it is in a huge city and crosses a 10-lane highway where More than 300,000 vehicles pass through each daynot on a secondary road in deep America. However, there are already promising precedents such as the recently inaugurated Colorado’s Greenland Wildlife Overpass on I-25, connecting approximately 15,800 hectares (39,000 acres) of habitat for deer, elk, mountain lions and bears. Scientific literature also supports the bet: after analyzing 89 fauna passages in Europe, North America and Australia, this study published in Biological Conservation concluded that they are highly effective and that they reduce animal mortality due to roadkill by up to 90% compared to unprotected stretches. Other wildlife passes. Although the Wallis Annenberg Wildlife Crossing will be the largest in the world, there are other wildlife crossings (mainly in Europe) equally ambitious: Natuurbrug Zanderij Crailooin the Netherlands. At 800 meters long and 50 meters wide, it is the longest ecoduct in the world. Doñana National Park It has a network of wildlife crossings and ecoducts built specifically for the Iberian lynx, whose 80% of deaths occur due to being run over. On the A4 highway in the Lower Silesian Forest in Poland there are 15 ecoducts and wildlife crossings. It has been monitored for three years its use by wolves, ungulates and other carnivores. Veluwe ecoduct network in the Netherlands. Nine ecoducts in a natural area of ​​1,000 square kilometers, with almost 5,000 deer and wild boar crossings documented in a single year. Ecoduct on the Türkiye – Central Europe highway. It was built after multiple attacks by brown bears, achieving reduce collisions to zero. Yes, but. Despite its potential ecological value, the project has faced criticism for its high cost: It started with a budget of 92 million and in addition to being delayed, it will end up costing approximately $114 million. In addition, there are those who wonder whether a single structure is enough to save an entire population, suggesting that it is more necessary to implement smaller-scale but more numerous interventions rather than a single megaproject. From a strictly scientific point of view, the effectiveness of these steps is neither automatic nor guaranteed. A paper published in the Journal of Applied Ecology cautions that most available studies measure the number of crossings but not the actual impact on population viability, and that population-level effects remain difficult to quantify. Furthermore, design matters: those structures less than 20 meters wide are used noticeably less by animals. And if it has a bad location it can end up being useless. 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