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I'm writing a story for a character who is a an old monster hunter. In his younger days, he used to make a living by hunting the most dangerous monsters which are basically the story's equivalent of apex predators like tigers or lions. At one time he was tasked to clear a small island of tiger-like monsters, which he did, and he was paid well for that.

Later in his life, he returned to that island, only to find that it's been deserted by the people who lived there because of various ecological disasters. With lack of predators, its prey animals bred out of control, which decimated the vegetation to the point where even trees are eaten by the wildlife until there's none left, which ruins the soil and therefore agriculture (not to mention a period where the inhabitants on the island spend more time fighting off herbivores eating their crops than actually farming). This caused him to realize the error of his ways and he decided to be more conservative in his work, only hunting when it's absolutely necessary and even protecting monsters that are important to the ecosystem.

My question is, is this sort of ecological disaster even possible in a person's lifetime (ideally maybe 20-30 years)? The world of the story is inspired by Monster Hunter where fire or thunder breathing dinosaurs are quite normal and part of the ecosystem, so the science may be able to stretch a little bit.

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    $\begingroup$ Most real-world examples I could think of are not because of the removal of a species from the ecosystem but rather due to the introduction of a new invasive species. $\endgroup$
    – Philipp
    Feb 24, 2023 at 10:07
  • $\begingroup$ On a local scale and in an isolated geography such as you're suggesting, absolutely. If you think about it, humanity collapsed the ecology of the Bikini Atol in a fraction of a second. It's somewhat of a dramatic example, but it exemplifies my point. But it will be up to you as the author to put all the links together to show that the collapse was believable. I think that can be done. $\endgroup$
    – JBH
    Feb 25, 2023 at 6:14

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Yes, it can. A very similar thing has in fact happened on St Matthew Island, a smallish island in the Bering Sea off the coast of Alaska, where the US Coast Guard brought 29 reindeer in 1944 to serve as an emergency food supply for a local outpost. The outpost closed a few years later, leaving the reindeer behind. With no predators, they shot up in number, reaching some 6,000 by 1963, by which time they have eaten up most of the island's lichen, a crucial winter fodder. At this point a particularly severe winter caused the reindeer population to crash, and they were completely gone by the 1980s.

But in your setting, humans (farmers) remain after the tiger-equivalents are gone, and would be capable of replacing them as an apex predator... unless their society has a mechanism which prevents this?

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    $\begingroup$ The way I imagine it, the farmers didn't even think about replacing the tigers-equivalents. They didn't think overpopulation of prey animal is bad and just deal with whatever animal that comes to their farm instead of actively hunting them. It's not until some time later that the lack of trees start to influence the soil quality, at which point farming becomes more difficult and people start to leave the island. $\endgroup$ Feb 24, 2023 at 10:45
  • $\begingroup$ +1 Good one - I was thinking of Easter Island, but the timeframe was a few hundred years, not as fast as the OP wanted. $\endgroup$ Feb 24, 2023 at 11:56
  • $\begingroup$ Look also at Isle Royale which has had a wolf population crash nps.gov/isro/learn/nature/wolf-moose-populations.htm $\endgroup$
    – David R
    Feb 24, 2023 at 15:38
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    $\begingroup$ You can accelerate this process if a lightning storm causes a fire that burns up part of the island. That will abruptly tip the balance between consumer and food supply, and the remaining animals could easily devour what's left faster than the dead side can regrow. $\endgroup$
    – bta
    Feb 25, 2023 at 1:17
  • $\begingroup$ I think the farmers' failure to notice the situation is the most unbelievable part. Sure it would start as being a bunch of pesky prey but when it gets to the point of being unlivable, the inhabitants are more likely to start killing the prey. I like @bta 's idea as being a good tipping point and would cause many inhabitants to leave and the remaining prey to eat whatever was left. $\endgroup$ Jun 1, 2023 at 14:40
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Yes, the reduction of an ecosystem's health can be seen within the lifetime of one person. On the bright side, the reverse can also be true.

One real world instance of the crash and recovery of an ecosystem within a person's lifetime was the extermination of wolves within Yellowstone in the US.

The wolves were killed off in the 1930's. While the local elk were still preyed on by bears and cougars (and maybe coyotes), the lack of wolves removed a huge amount of predatory pressure. This allowed the elk population to grow enough to push the limits on the carrying capacity of Yellowstone.

With the lessened predation pressure, the elk would leisurely browse on the young willow, aspen, and cottonwood, which was quite hard on the beaver population, who needed willows to survive the winter. Without the beavers and their dams, the streams became clouded with silt and their shorelines had little vegetation, as what little there was was cropped or trampled by grazers.

When the wolves were reintroduced to Yellowstone, the area was fairly ecologically pour, with low populations of fish, small mammals, and birds, and the landscape around streams were mostly cleared by the elk and deer, which resulted in quite a lot of soil being washed into the streams.

Soon after the reintroduction, even with a low starting population of wolves, the elk and deer no longer felt safe loitering near the streams, and so the bushes and trees near the streams were able to grow and thrive. The beavers were able to thrive with the availability of the young willows, and with the increase in beaver population, new dams and ponds were created.

The beaver dams evened out the seasonal pulses of runoff, slowed the water which allowed the water table to be recharged, and the beaver ponds provided cold, shaded water for fish.

The newly robust willow stands also provided good habitat for songbirds. The streamside vegetation was also a boon for small mammals, giving them food and shelter. Even with this shelter, the large increase in their population meant that the coyote population also had a large increase in available food.

The scavenger populations also benefited, as instead of elk carrion being primarily available as winter-killed, it was now available year round as wolf-killed.

The elk populations themselves also benefited. Currently elk populations are about 3 times what they were, as the ecological health of the area has increased so much that there is plenty of forage for the elk, though they do have to migrate around to find it. Their forced migration to avoid wolves gives them access to additional food sources, while preventing them from overgrazing areas, which allows previously grazed areas to recover much more quickly.

Wolf Reintroduction Changes Ecosystem in Yellowstone

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    $\begingroup$ You know, I've seen so much ChatGPT answers that have the "yes, (question restated)" format that I've had a mini heart attack when reading your answer. $\endgroup$ Feb 24, 2023 at 22:34
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Forget "lifetime". You can see it in a single year.

The Grand Canyon Game Preserve (a predecessor to the modern Kaibab National Forest) was originally managed to maximize deer and elk populations for hunting, primarily by killing off predators (cougars and coyotes). Initially, it was wildly successful: hunters reported finding game animals in unheard-of numbers. A few years after management started, though, a winter was slightly harsher than average. The deer and elk stripped the forest of edible vegetation, then starved to death.

It took half a century for animal populations to return to pre-management levels, and the trees are still recovering.

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