Resource and Landscape-Scale Habitat Availability Shape Insectivorous Bat Assemblages in a High-Altitude Andean City

In the high, thin air of the Andes, where the sun beats down fiercely by day and the temperature plummets at night, life exists in a delicate balance. For animals that rely on the night, such as bats, the city is a new and often hostile landscape. Urbanization does not affect all wildlife in the same way; some species vanish while others adapt, but this response depends heavily on the specific environment. In dry, high-altitude regions, water is scarce and food is limited, creating a harsh setting where even small changes can have big consequences. Scientists have long known that bats need two main things to survive in a city: a place to hide or roost, and enough insects to eat. They also know that artificial lights at night can confuse or repel many species, while green spaces usually offer a refuge. However, most of what we know comes from studies in temperate forests or lowland tropics. The high, semi-arid valleys of the Andes, with their unique mix of steep slopes, fragmented nature, and rapid urban growth, remain a mystery. Understanding how bats navigate this specific, extreme environment is crucial, not just for the animals themselves, but because they act as a barometer for the health of the entire ecosystem.

To uncover these secrets, researchers turned their attention to La Paz, Bolivia, a city that sits at an average elevation of 3,600 meters above sea level. This is a place where the urban landscape is a chaotic patchwork of concrete and scattered greenery, built into a deep valley with little natural planning. The city is home to roughly 750,000 people, yet its natural remnants are small, isolated, and under constant threat from expansion. In this challenging setting, a team of scientists set out to see what was happening to the local insect-eating bats. They chose 29 different spots across the city, ranging from the dense, gray center of town to the greener, quieter edges where nature still clings to the hills. Over several months during the warmer, wetter season, they spent hours each night listening for the high-pitched calls of bats using sensitive recording equipment. They did not just listen; they measured the amount of vegetation, the intensity of streetlights, and the distance to the nearest patches of wild nature. Crucially, they also set up traps with ultraviolet lights to catch a sample of the insects available in the air, knowing that the bats' success depends entirely on the food supply.

The researchers were looking for patterns in how the bats responded to their surroundings. They wanted to know if the bats cared more about what was happening right next to them, like a single park or a bright streetlamp, or if they were influenced by the broader landscape, such as the availability of nature patches miles away. They also wanted to see if the amount of food available was the most important factor, or if the physical structure of the city mattered more. After recording thousands of bat passes and analyzing the data, a clear picture emerged that challenged some common assumptions. The most powerful force shaping the bat community was not the greenery or the lights, but the availability of insects. Where there were more insects, there were more bats and a greater variety of species. This suggests that in this high-altitude, dry city, the bottom line for survival is simply having enough to eat. The landscape also played a role, but it was secondary; the bats seemed to respond to the broader context of where nature existed within a kilometer of their location, rather than just the immediate neighborhood.

When the scientists looked at the specific species they found, the story became even more nuanced. They identified three main types of bats that dominated the recordings: Myotis oxyotus, Tadarida brasiliensis, and Histiotus montanus. Each of these species reacted to the city in a completely different way. The Myotis oxyotus, which tends to hunt near trees and bushes, seemed to care about the local environment, showing a preference for areas with more greenery and specific lighting conditions, though food was still a major factor. The Histiotus montanus was the most dependent on food availability; its activity levels rose and fell directly with the number of insects present. The third species, Tadarida brasiliensis, which is known for being very adaptable to cities, did not seem to care about the local vegetation or lights at all. Instead, its presence was best explained by the spatial arrangement of the city itself, suggesting that where it roosts or how it moves through the landscape is more important than the immediate resources around it.

One of the most surprising findings was that the amount of green vegetation and the brightness of artificial lights did not strongly predict the overall success of the bat community. In many other cities around the world, parks and dark skies are considered essential for bat survival. Here, however, the small, often degraded green spaces in La Paz did not seem to provide enough value to attract the bats on their own. The native patches of wild vegetation that remained in the city, though fragmented, appeared to offer a combination of shelter and resources that the manicured urban parks could not match. The artificial lights, which often drive bats away in other studies, did not show a clear negative effect on the group as a whole, though they may have influenced specific species differently. This indicates that in the unique context of the high Andes, the scarcity of food is a much stronger constraint than the presence of light or the lack of green space.

The study highlights that saving bats in cities like La Paz requires a shift in how we think about urban planning. Simply planting a few trees or creating a small park is not enough if the insects are not there to support them. The researchers suggest that protecting the existing natural remnants and ensuring they are connected across the landscape is vital. These patches of wild land provide the necessary structure and resources that the city's artificial green spaces often lack. By focusing on the availability of food and the broader landscape context, city planners can better support the wildlife that shares their space. The work in La Paz serves as a reminder that nature in extreme environments operates by different rules, and what works for bats in a temperate forest may not work for those struggling to survive in the thin air of the Andes. The bats are not just surviving; they are responding to a complex web of resources and space, proving that even in a rapidly growing city, the wild can persist if the right conditions are met.