Walking through the forest in Guam is unsettling. Unlike tropical forests pretty much anywhere else in the world, it’s almost dead silent.
It’s quiet because there are virtually no birds. Brown tree snakes, an invasive species that was thought to have come to the island in the 1950s as a stowaway on a ship, feasted on the eggs of the island’s birds. Within just a few decades, nearly all of the 12 native species were pushed to the brink of extinction.
The birds played a critical role in the small island’s ecosystem. In their absence, Guam’s ecology has transformed in ways both dramatic and subtle. When integral species disappear, ecosystems become more unstable. That’s bad news for Guam, but it could also be a cautionary tale for animals (and the people that rely on them) elsewhere in the world. Huge populations of animals are already dying off around the planet, and just how that loss plays out, or what kinds of interventions should be taken to counteract that die-off, will depend on how well scientists understand its effects.
“Islands in general are good foretelling the future of mainlands because things that take a really long time on the mainland happen really fast here,” says Haldre Rogers, an ecology professor at Iowa State University who studies the Marianas.
Ecologists long ago coined the term “keystone species” to describe the species without which an ecosystem could cease to function. Rogers, who let me stay in her field house on a recent visit to Guam, calls the island’s birds a “keystone functional group.” They, along with the threatened Mariana fruit bat, are essential because they move seeds around.
Birds and bats eat the fruit from trees, but don’t fully digest the seeds. Instead, the seeds pass through their digestive systems, which removes seeds’ tough outer coats and pathogens. Then, the seeds are dropped, in a nutrient-rich pile of excrement, wherever the animals happen to be at the time. That means the seeds are more likely to germinate than if they’d just fallen off a tree. In addition, being farther away from their parents gives the seedlings a better chance of success (PDF) because they’re not competing with their tall, mature parents for resources like space, light, and water. Much of that wouldn’t have been possible without the flying animals’ help. Without any birds and very few bats, you get a forest full of gaps—fewer seeds are there to replace trees that fall.
“[Seed dispersal] is the engine that keeps the forest going,” Rogers says. “Without that process happening, without seeds being moved around, you get the slow degradation of the forest.”
The forest has changed in other ways since the birds started dying out. Fewer plant species now grow in Guam’s forest (this is also in part because invasive pig and deer eat some native species). And a person walking through the forest would find themselves constantly swatting spider webs from their face, because the (unsettlingly large) creators of those webs aren’t getting eaten by their natural predators: birds.
A less robust forest in Guam is at risk for profound disruption. Trees are more likely to fall during typhoons, which hit the island every few years. Since birds aren’t dispersing the trees’ seeds as efficiently, there are fewer trees left of reproductive age, so the forest will take much longer to recover from a damaging event like a typhoon. In short, the absence of keystone species leaves the forest more vulnerable to irrevocable damage, changing in ways that kill off or force out other animals that still live there. And because Guam is so small and isolated, animals that lose their habitat really have nowhere else to go.
🎧 For more intel on resilient ecosystems, listen to the Quartz Obsession podcast episode on kudzu. Or subscribe via: Apple Podcasts | Spotify | Google | Stitcher.
These disastrous scenarios aren’t guaranteed to happen. In fact, Rogers has been impressed with how resilient ecosystems can be—even on Guam, which she says is an “extreme” example of how profoundly ecosystems can be disrupted and still more or less function. Even when a particular species or type of organism disappears, the ecosystem often can often carry on. Forests may continue to provide a habitat for the organisms that are left—even if it’s a much less ideal habitat than existed before.
Sometimes, in ecosystems that lose ecologically important species, other species—new ones, or ones already present— fill the gaps. But in Guam’s forest, no other species has stepped in to take the birds’ place.
We’ve known for a while that top predators like wolves and tigers are disappearing from landscapes; only more recently have researchers realized that animals that had once been abundant, such as insects, frogs, and bats (all of which can be keystone species), are disappearing, too. Species don’t have to go extinct for them to no longer be able to fill their ecological niche, Rogers points out. Losing a big enough proportion of the individuals in the population might be enough to reduce its ability to fulfill its ecological role.
Laws intended to protect plants and animals, like the US Endangered Species Act, aren’t designed to take their ecological functions into account. As more of the world’s ecosystems start to look more like Guam’s forests, scientists may push to change that.
There are ways to make a difference without waiting for legal restructuring. On Guam, to get rid of the brown tree snake, researchers and government employees are experimenting with baited traps and have launched rapid response teams when locals spot them (they’re encouraged to call 1-671-777-HISS). Scientists are working with the US military to section off an area of forest on the Air Force base and ensure that it becomes and stays completely devoid of the invasive snakes. There’s talk of bringing the birds back to Guam; Rogers is optimistic. If it’s possible to resuscitate Guam’s forests, maybe we could do the same for other disrupted ecosystems elsewhere in the world.