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If an intelligent alien species landed on the small bit of galactic rock that we call home, they might get out of their spaceships, have a look around, and decide that we—that is, our species—are the master builders on our planet. There would be plenty of reasons to think so. We build bridges spanning enormous waterways, aptly named skyscrapers, and stadiums that seat tens of thousands. And that doesn’t even scratch the surface of the diversity of structures we build: schools, airports, apartment buildings, shopping malls, a Starbucks on every corner. 

But we’re not the only species that modifies the environment to suit its needs. Some animals simply set up shop in pre-existing spaces, like bears that spend the winter hibernating in natural caves. Some can build a home by simply moving around an impressive amount of dirt, like gophers and ants. Some animals, however, create more elaborate spaces, transforming their environment in the process. If a group of aliens did land on our planet, they would just have to look a bit harder to see it.

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One common way in which animals change their environments is by building their homes. Birds are the most well-known nestmakers, to be sure. While some are content to just pile up some leaves and twigs and clumps of mud, other species are more skilled in their architecture. Bowerbirds create their nests  bowers by gathering up as many colorful and shiny objects as they can. They will readily combine flowers, shells, and bits of plastic, and many work hard to coordinate colors. But while the choice of materials may seem haphazard, the arrangement certainly is not: The objects are carefully laid out so that, thanks to forced perspective, the male will seem physically larger to a passing female. The larger the male, the more likely the female will mate with him. Sociable weavers, a bird native to Southern Africa, also build elaborate nests that are like apartment complexes dominating the skyline upon the otherwise flat desert floor.

But not all nest-builders are birds, and some have a larger impact on their local ecology. Our closest primate cousins, the chimpanzees (as well as gorillas and orangutans) construct nests high up in the treetops of the forests they call home. Their large mattresses in the sky are made by weaving branches into large, round beds, anywhere from 15 to 80 feet above the ground: high enough to escape ground-dwelling predators.

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In a 1962 paper in Annals of the New York Academy of Sciences, Jane Goodall writes that chimps choose their trees according to food availability. While they don’t ever nest in fruit trees, they almost always nest in trees just adjacent to food patches. The chimps also have a defined architectural style, finding one main branch to serve as the “foundation,” and then bending 6–10 “crosspieces” over that to make the bed. Sometimes they line their nests with leaves, using the softest ones for makeshift pillows.

Each chimpanzee makes its own nest, with the exception of the infants, who sleep with their mothers for as long as two and a half years. Surprisingly, they construct new nests each night, though they may weave a new nest in the same tree two nights in a row. “As these nests often remain recognizable for several months,” Goodall notes, “they are conspicuous features of the countryside.” 

The real animal architects, though, are beavers. These semi-aquatic rodents, the second largest in the world after capybaras, are classified as “ecosystem engineers,” because of the effects of their dams on streams: mucking up water chemistry, altering biological diversity, and changing the population of streamside trees and plants. 

When a beaver builds a dam, the community of river-dwelling invertebrates that prefer running water is wholly replaced by a new community comprised of pond-dwelling critters. 

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Beaver dams trap an appreciable amount of water, creating large ponds that are key for the animals’ survival: They increase the amount of habitat available to beavers, who build their homes in the ponds; encourage the growth of plants that beavers like to eat, in places they can easily reach; and help them evade their main predators, wolves, who can’t reach beavers’ underwater lodges. In the northernmost reaches of North America, beavers can cut more than a metric ton of wood each year, all within 100  meters of their dam. Dam construction even alters the amount of carbon and nitrogen that gets recycled by the water. The organic carbon in a flowing stream gets replaced every 24 years; it takes 161 years to replace the carbon in a dammed pond.

But dam construction affects the environment even more profoundly. When a beaver builds a dam, the community of river-dwelling invertebrates that prefer running water, like blackflies, scraping mayflies, and spinning caddisflies, is wholly replaced by a new community comprised of pond-dwelling critters like dragonflies, worms, and filtering clams. More impressively, dams increase the total biomass of water-living organisms by two to five times, compared with the flowing patches of river between dams. Beaver dams actually create entire wetland ecosystems, which host great biodiversity, recharge aquifers, absorb pollutants, and buffer against both flood and drought; they are consequently some of the more important parts of the natural world.

Of course, our buildings are not the only way we change the planet: We cut or burn through acres of trees. We split mountains into pieces in search of minerals and gems. We put vast amounts of carbon dioxide into the atmosphere.

Some other species also change their environments as a side effect of their industriousness. Photosynthesis in plants, for example, produces the oxygen in our atmosphere, which all animals are dependent on. Indeed, oxygen in Earth’s atmosphere might be the most obvious sign of life for an alien to notice from afar, and is arguably a greater change than anything humans have done. But oxygen is a byproduct of plant life, not an “effort” by plants to make their own lives better. For this reason, processes like photosynthesis are fundamentally different from the sorts of construction that birds and chimps and beavers do. 

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Humans are undoubtedly the most influential engineers, at least among animals. The beavers will have to be satisfied with silver-medal status.

Jason G. Goldman received his Ph.D. in developmental psychology at the University of Southern California in Los Angeles and writes a blog called The Thoughtful Animal, hosted at Scientific American. His doctoral research focused on the evolution and architecture of the mind, and how different early experiences might affect innate knowledge systems.

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