Olivia: The original keystone refers to a wedge-shaped slab at the peak of a stone arch. If you remove the keystone, you risk the whole thing tumbling down. And some ecosystems work the same way, relying on one keystone species.

In biology, keystone species have a disproportionately large impact on their habitat, given their size or numbers. Their daily activities affect all kinds of other species, directly or indirectly. And a change in their number can cause a trophic cascade, where effects ripple throughout the ecosystem, often in surprising ways.

Take the grey wolves of Yellowstone National Park in Wyoming. Their numbers dropped to zero around the early 1930s, mostly because of hunting. But in 1995, wolves were reintroduced to the park, and the wolves’ impact has been monitored ever since.

Turns out, that impact is seriously important. The wolves’ absence and reintroduction changed Yellowstone in large and often surprising ways, right down to how the rivers flow.

For one thing, wolves prey on elk — and the elk know it! So their behavior changes when wolves are in the area. Elk are generally nomadic, staying on the move as they munch on plants. But without any wolves around, the elk became much less cautious, preferring to settle down and eat a lot in one place at a time.

The elk even ate all the way down to the riverbank – somewhere they wouldn’t normally dare linger if there were wolves around. Instead of eating a little bit from lots of different plants, they ate so much at a time that aspen and cottonwood trees declined, and so did the riverside willows that beavers and some songbirds depend on.

And beavers are also keystone species, because of their special engineering talents. Their dams slow the flow of rivers, preventing floods and offering a wide range of habitats for all kinds of species.

Ever since wolves were reintroduced to Yellowstone, willows have grown better, beaver colonies have increased from just one to nine, and songbirds are flourishing too. Elk numbers declined at first, but now seem to have stabilized. And bison numbers are up, probably because of a decrease in competition from the elk.

Research is ongoing to assess the wolves’ long-term impact, considering the many subtle interconnections in this complex ecosystem. But as top predators, it’s clear that the presence, absence, and then reintroduction of wolves has resonated throughout Yellowstone. And research into other wolf habitats will help work out what’s unique to Yellowstone and what can be applied elsewhere.

Elephants also do a lot to manage their environment — from what goes on in their mouths, to what comes out the other end. They’re sometimes known as the “mega-gardeners of the forest.”

For example, without African Forest elephants, a single species of acacia tree tends to dominate in African forests. It grows fast and shuts out the light from other plants. And the elephants help counter that. They sometimes knock down acacia in their search for food, opening up space and a critical shaft for light.

Other plant species seize their chance, adding to the forest’s biodiversity. The smaller branches they knock down provide hidey-holes for lizards, which are more diverse in places where elephants roam. When reaching for leaves or fruit, they’ll often knock off a bunch more. And these windfalls get picked up by smaller ground-dwellers like warthogs, as well as kudu, which is a type of antelope.

All this eating results in plenty of dung – around a metric ton every week. And all that poop becomes a mini-ecosystem all by itself, since it’s rich in nutrients the elephants couldn’t process. Fungi live in it, as do insects like beetle larvae, crickets and spiders. In 2009, three species of frogs were found happily living in Asian elephant dung. And of course, it’s excellent fertilizer.

Many plant species grow better in elephant dung than in poop from any other animal. Their wide roaming and variation in movements make them especially good at spreading seeds to new places. But you don’t have to be as big as an elephant to have a jumbo-sized impact on your ecosystem.

Parrotfish are named for their tough, almost beak-like mouthparts. These hardened lips are well adapted for plucking algae off coral, their main food source in their warm reef environment. It might not be glamorous, but it turns out to be vital to keeping the reef healthy. Without parrotfish, macroalgae can smother the coral to the point of killing it off. And no coral means no reef, which would be big trouble for the species that call coral reefs home.

A major report covering four decades of research in the Caribbean identified parrotfish as a group to keep a special eye on. Coral reefs are generally having a tough time of it: with climate change, ocean acidification and pollution steadily degrading the ecosystem. The parrotfish’s maintenance work helps keep reefs resilient, so they can recover from blows like sudden heating or a hurricane.

Caribbean reefs where parrotfish are heavily fished are suffering the most. So the report recommends more countries adopt policies to save the parrotfish, and hopefully save the reefs along with it. So, Like all keystone species, they really hold their ecosystems together.

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