Keystone species are fascinating and show how we really need more systemic thinking.
The one that tons of people have seen is how reintroducing wolves into Yellowstone changed the course of rivers, made them less eroding, and generally increased ecosystem diversity: https://www.youtube.com/watch?v=ysa5OBhXz-Q&t=1s
Beavers are another one: https://www.youtube.com/watch?v=5pk_VD1-8BM Remember how we learned a lot of westward expansion in US colonial days was led by mountainmen trapping beavers to use their pelts in fancy european hats and we kept going further west because we kept killing off the beavers while demand for fancy hats kept going up? Turns out when you remove all those beaver ponds that would slow down water flow, you end up with a lot more erosion, incision, and flooding -- the landscape we have now. Fisherman think beaver ponds decrease fish populations because they tend to have warmer water, but this doesn't quite pass the thousands-years-of-coevolution-before-beaver-extermination sniff test... some experiments have showed that beaver ponds actually increase fish stock because they create lots of fish nurseries, and trout and salmon can instinctively swim up through beaver dams.
Then when you consider how things like aquaponics incorporate fish because fish poop makes great fertilizer, it starts to become clear how important beaver ponds can be to nutrient cycling in these ecosystems. One of the most fascinating fish-based nutrient flows is salmon migrations... they're hatched upstream, put on most of their growth and biomass in the ocean, and then migrate back upstream to die. Well, when they die, their bodies bring all those nutrients that washed down out of the watershed back up into the watershed, where they're spread by the bears and hawks that eat them, creating a grand flow of energy through the landscape that gives ecosystem stability. And beaver ponds have historically created the habitat needed to support this critical piece of the machine.
A fascinating example of how removal of keystone species can permanently stunt ecosystems is this guy trying to reforest scotland: https://www.youtube.com/watch?v=nAGHUkby2Is We think of scotland as grassy rolling hills, but the scotland of lore was actually covered in forest. This guy is finding that the forests can't grow back because deer populations are too high and they just eat any small saplings that try to take hold. Without reintroducing keystone predators like wolves, he's found that small interventions like fencing off the edges of tree thickets for a few years until the saplings can take hold can start the healing process.
You're right, but it's also not a settled or well understood concept yet. This whole idea got running with ecological cascades, and it turns out that these are all complex systems. But just adding or removing a species doesn't have one effect, or even an effect at all in some cases. We have to be careful not to trumpet one or two successes of eco-forming, because lots of people will read that and repeat it and think that doing X is always going to have a positive effect.
Reintroducing the wolves was part of a much larger mis-management of the park where we basically destroyed 80-90% of Yellowstone over a half century, by doing things like screwing with the animal population, or the controlled burning by indigenous/native populations.
How much does a wolf cost? wouldn't it be more cost effective to buy a bunch of wolves and set them loose, rather than maintaining fences for years and then building a new set of fences?
The one that tons of people have seen is how reintroducing wolves into Yellowstone changed the course of rivers, made them less eroding, and generally increased ecosystem diversity: https://www.youtube.com/watch?v=ysa5OBhXz-Q&t=1s
Beavers are another one: https://www.youtube.com/watch?v=5pk_VD1-8BM Remember how we learned a lot of westward expansion in US colonial days was led by mountainmen trapping beavers to use their pelts in fancy european hats and we kept going further west because we kept killing off the beavers while demand for fancy hats kept going up? Turns out when you remove all those beaver ponds that would slow down water flow, you end up with a lot more erosion, incision, and flooding -- the landscape we have now. Fisherman think beaver ponds decrease fish populations because they tend to have warmer water, but this doesn't quite pass the thousands-years-of-coevolution-before-beaver-extermination sniff test... some experiments have showed that beaver ponds actually increase fish stock because they create lots of fish nurseries, and trout and salmon can instinctively swim up through beaver dams.
Then when you consider how things like aquaponics incorporate fish because fish poop makes great fertilizer, it starts to become clear how important beaver ponds can be to nutrient cycling in these ecosystems. One of the most fascinating fish-based nutrient flows is salmon migrations... they're hatched upstream, put on most of their growth and biomass in the ocean, and then migrate back upstream to die. Well, when they die, their bodies bring all those nutrients that washed down out of the watershed back up into the watershed, where they're spread by the bears and hawks that eat them, creating a grand flow of energy through the landscape that gives ecosystem stability. And beaver ponds have historically created the habitat needed to support this critical piece of the machine.
A fascinating example of how removal of keystone species can permanently stunt ecosystems is this guy trying to reforest scotland: https://www.youtube.com/watch?v=nAGHUkby2Is We think of scotland as grassy rolling hills, but the scotland of lore was actually covered in forest. This guy is finding that the forests can't grow back because deer populations are too high and they just eat any small saplings that try to take hold. Without reintroducing keystone predators like wolves, he's found that small interventions like fencing off the edges of tree thickets for a few years until the saplings can take hold can start the healing process.