Every year, humpback whales travel for thousands of miles from the polar regions down to the tropics for a variety of reasons, including to give birth to their young. When they live and feed in the high-latitude places, they build up blubber by consuming loads of krill, their typical food. As they migrate toward the equator, they live off that fat, pooping plumes of nutrients, fertilizing tropical waters and fueling the growth of microorganisms that other marine creatures feed on. In short, the iconic marine mammals act as giant, globe-trotting, nutrient-dispersing factories.
In a similar way, thousands of species, counting billions of animals, make migratory journeys each year on land, in the oceans and skies. They cross countries and continents, with some traveling thousands of miles to feed, breed or overwinter. They play important roles by transporting nutrients, preying on pests or pollinating flowers, including crops. “Migratory species are integral parts of the ecosystems where they’re found,” says Amy Fraenkel, executive secretary of the Convention on Migratory Species, an international body that aims to conserve and protect these animals. “And what makes them unique is the fact that they move, which means that they can deliver some specialized services.”
Now, a new State of the World’s Migratory Species Report released by the Convention on Migratory Species reveals the shocking and dire state of our planet’s animal pilgrims. Out of thousands of migratory creatures that inhabit earth, the body lists 1,189 as needing international protection. That includes 962 species of birds, 94 species of terrestrial mammals, 64 aquatic mammal species, 58 species of fish, 10 species of reptiles and one insect, the monarch butterfly. The report was prepared for the convention by conservation scientists at the United Nations Environment Program World Conservation Monitoring Center.
Overall, more than one in five species listed by the convention are considered threatened with extinction based on assessment by the International Union for Conservation of Nature (IUCN), a global organization that keeps a “red list” of species in trouble. The IUCN uses a slightly different definition of migratory species than does the Convention on Migratory Species. The IUCN defines migratory species as those that must travel from one habitat to another during different times of year to sustain themselves—think humpback whales, red knots or monarch butterflies. The Convention on Migratory Species’ framework is more human focused. “The definition that we use under the convention has to do with whether species cross national borders routinely and regularly,” explains Fraenkel.
Forty-four percent of species listed by the Convention on Migratory Species have populations in decline, and the organization points out that its list is not comprehensive. Almost 400 migratory species that are threatened or near-threatened with extinction according to the IUCN are not currently on the convention’s list.
The risks aren’t the same for different creatures. While birds have the highest number of species on the list, migratory fish are in most decline. Nearly 97 percent of the listed fish are at risk of disappearing.
Benjamin M. Van Doren, a biologist at University of Illinois Urbana-Champaign who wasn’t involved in the report but whose research focuses on migratory birds, was startled by this fact. “The area of the most concern to me, or most surprising, given that I focused on birds, was that fish are so threatened,” he says. North America alone has lost about a third of its bird population since 1970. “But compared to birds, the report showed that migratory fish are in even more dire straits. I’m sure this wouldn't come as a surprise to a fish biologist, but for me, it was really eye-opening.”
Both migratory fish and birds provide huge benefits to the different ecosystems they travel to. When salmon migrate upstream to spawn, they serve as food to land creatures, such as bears. Both salmon carcasses strewn near the waterways and bear poop go on to fertilize the forest. And various migratory birds that fly up and down the planet eat and excrete seeds on their journey, making plants grow where they wouldn’t otherwise.
One reason migratory creatures are withering is because traveling comes with many perils. Many migratory animals regularly cover thousands of miles, explains Fraenkel. On the way, they deal with unpredictable weather, changing climates, loss of habitat, shortage of food sources, predation, diseases and other dangers. They rely on at least two ecosystems to sustain their lifestyle—and in many cases on some transitory stopover sites. And if one of these spots can no longer sustain them, or if they can’t physically get there, their entire population may wither, sending ripples through entire ecosystems.
Van Doren shares one telling example. “The Delaware Bay in the eastern United States is a crucial stopover point for many migratory shorebirds, especially the red knot, which is a poster child of a threatened migratory species,” he says. “And if the horseshoe crab eggs they feed on aren’t available at the right time and in the right quantities, then they won’t make it to the Arctic in time to breed successfully, and their population may crash.”
Fraenkel says countries’ policies often present obstacles to animals surviving, thriving and sustaining their habitats. Fraenkel cites elephants, which are often described as “ecosystem engineers.” When they feed on young trees and bushes, they thin out the new shoots that compete for water, space and sun. The saplings that survive have a huge advantage and grow taller and stronger, and sequester more carbon in their trunks. If elephants cross from a country where they are safer to one where poachers kill them, the forest on both sides of the border will suffer.
“Most of the threats to the species are from human activities,” Fraenkel says of the report’s findings. “The reason why [the Convention on Migratory Species] exists is to bring countries together to agree on the key priorities for shared species.”
The report finds that humans are the primary cause of the decline of species on their list. Hunting and overfishing, whether intentional or as bycatch, can be harmful. Humans build roads, fences and train tracks, and cut down forests. They use pesticides, dump plastic waste, and create a lot of noise and light pollution.
A single road can disrupt a terrestrial animal’s migration path that had been used for centuries. A dam can render it impossible for fish to accomplish annual spawning journeys. A windmill farm in the path of migratory birds can decimate flocks over the years. Overall, the degradation and fragmentation of the creatures’ original habitats and paths serve as a huge disruptor to their movement. Three in every four listed species are affected by degrading and fragmented habitats.
Fraenkel points out that humans can solve many of these problems. When building roads and tracks, humans can create overpasses and underpasses that serve as so-called wildlife corridors. The passageways connect two sides of a forest or mountain range, allowing the species to move as they did for decades before. Windmill farms can be built away from the birds’ and bats’ flying paths. If a city is expanding, more parks and green spaces can be made so that birds retain their stopover spots. And some species may simply need stronger protections, consistent across neighboring countries. “These organisms are having important contributions to all of the places where they spend their time,” Van Doren says, so their decline will inevitably affect more than one ecosystem, including in entirely different parts of the world.
Fraenkel cites a few success stories from the recent past to demonstrate that recovery is possible. In the 1990s, the saiga antelope of Central Asia had suffered a 95 percent decline, from 1.5 million to barely 50,000. Saigas were traditionally hunted for food and their horns, but they also could no longer migrate as needed because human-built obstacles disrupted their paths. That caused overgrowth of grasses that saigas no longer grazed, changing the entire landscape. A joint international effort led to the revival of the steppe and their migration paths, including by removing fences that obstructed the routes. Local communities also reduced hunting—helping the animals rebound, at least in parts of their range, such as Kazakhstan. According to the IUCN, their population bounced back up to about 1.3 million in 2022.
Similarly, when whaling was made illegal, many of these marine mammals came back from the brink of extinction.
Migratory species benefit ecosystems that support human and planetary health. If humans want to have functioning forests, rivers, mountains, steppes and oceans to enjoy, they need to figure out how to keep these animals alive and well, Fraenkel notes. “A forest without species is not exactly a functioning forest,” she says.