Just as a thought exercise, how would an ecosystem have to adapt in order to avoid destruction?
The ecosystem just has to deal with it. It has to accept the new strong monster into its ecosystem with all the pain that comes from it. The ecosystem just has to deal with keystone species dying due to hunters and undergo massive change, likely causing mass death. If there is not enough prey, the predators die until the prey-predator equilibrium is regained.
The ecosystem, like in real life, needs protection by a outside force, be it magic, intelligent guardian animals, or humans in the case of a fantasy setting from destabilizing forces. They are not built to withstand a lot of change.
It's literally the same concept, just quantified with numbers imo
When a group goes out to hunt wolves and bring back 6 pelts for a quest, is it an hours job, or a week of hunting in the wilds? Or can they grind out 200 wolves in a day?
Players can kill an amazing amount of stuff, especially if higher level than the area.
UO originally wanted to have animals breed in the wild and produce more animals, giving the game real world mechanics. It didn't work because players killed everything too fast.
I think it's going to be different in every story, depending on the rules of the world or the game. And that's part of the fun of reading the stories.
Intra-wildlife in an RPG-world would work about the same as IRL. The "higher-Leveled individual" would get a higher chance to breed, would be better at hunting (or grasing) and would become a "higher-Leveled individual". Exactly the same as IRL.
There are, however, two problems.
1, Enter monsters. Not mundane animals, but aggressive ones with a cheat-power gimmick. Slimes are just one example, and the lowest Level bottom-feeder. Dragons, Wyverns, Undead, Trolls, you name it, and the ecosystem suffers massively.
2, enter Humans or any other species, that realises, killing stuff give you EXP. We more or less succeded in killing off any large predator in Europe and the American Buffalo, but that took a couple of thousand years (or a hundred or so for the buffalo), and that was without a System giving out EXP. Why are still Wolves, Bears, Big Cats, other stuff around, when killing them is good for Leveling and every basic quest is "Kill 10 Wolves"?
Rpgs were created the way they were to basically model how a real person gets more skilled, stronger and older over time. Books and novels didn't need to use that system since you could write whatever you wanted. That's why old fantasy novels sometimes get called progression fantasy too since the people in those stories learn how to use their abilities better over time, just without it being the main driving factor of the story.
Now you've got books with an rpg system inside of them. You're basically putting the model into a format that doesn't really need it. So the only difference you'll really find is in things that are outwardly fantastical. Magic, enchanting, mana, monsters, stuff like that. What you should be wondering about is how these fantastical elements actually affect the story, since the system is just a way to interact with them.
Also animals can benefit from exercise but they don't. So that might also help a bit.
On the other hand, if both predators and prey species gain EXP at similar rates, then that balance would be much easier to find. Oh, there would still be wide spread extinctions but the general structure of ecosystems should be fairly similar to what we know today.
Another question to consider is how many species are included? I think the most balanced and interesting might be if everything is included. I think that in this scenario it would be life on steroids. Plants would grow faster and sturdier, but biomatter would decay quicker as well. The balance between prey and predator would be preserved. Life in general would be more than it is today, but the general shape would be similar.
It really all depends on what type of system you are using. What benefits and what doesn’t? How fast do they grow and what do they need to do in order to grow? Do animals hunt only for food or for EXP as well? Is there magic? Can they become immortal? Are they ruled by instinct or do they gain intelligence?
I don’t know. But one thing to keep in mind is that ecosystems are not static even today. You just need to change the timeframe from our short human lifespans to hundreds of thousands of years. Climate changes, continents move, comets fall, Earth wobbles, and more.
I guess this could all even out to an exaggerated yet still analogous reflection of real life by virtue of an “effective” level cap in the way of the most powerful beings simply running out of other beings powerful enough to provide enough EXP to level up. At that point I see one of two options: 1. The strongest beings around eventually find and kill each other until there is a single apex predator who can never realistically be challenged (and I’m talking one single being, not a species), or 2. The strongest beings become vaguely conscious of their pre-apex predator geopolitics and naturally decide to basically establish regional control, effectively creating a sort of system of animal feudal protectorates.
All of this is assuming that humans or some other equally intelligent species (i.e. capable of mass scale and complex ecological regulation) doesn’t become the dominant species as in our case, because if that happens I’d assume that we would basically become world equilibrium enforcement in an even more direct way than we do now. That would mean making sure to tamp down on invasive species and other ecological stewardship, but mostly I think it would be setting an arbitrary level cap for nonhuman creatures and culling any that go above that.
If XP is gained for other things, say, crafting, cleaning, or anything, really, the non-combattants would be still in the game. There would probably still be a level-gap between murderhobos and normal people, but less than in the alternative.
For the environment that would mean, herbivores would gain XP by doing herbivore things. Dunno, letting methane into the atmosphere, being very good at chewing plants, or... hey, wait! Even herbivores fight for dominance! Only the best plant-cheving-methane-leaker would have the right to get know the pretty, female part of the population intimately :)
The animal world would still be under the same rule as the Earth-ecosystem. If the carnivores are overly successful, they will decimate the herbivores and they (the carnivores) wouldn't find enough food. Self-regulating.
Earth managed to make it work for what? More, than a billion years? Only when those pesky hairless monkeys fell down from a tree and realised, killing stuff is easier when using tools, got the system f***ed. Even than, it took what, a million years?
As long as the animals aren't sapient to realise, killing stuff is what make them stronger (i.e. level up), the Earth-rules apply. There is only so much game a predator can kill and only so much grass a prey can convert into methane.
The really iffy part are two things:
1, sapient creatures, that KNOW, killing stuff is good for them
2, invasive fantasy monsters
A practical way around this for the OP is twofold. Diminishing returns and practical limits. While it is possible to wipe out all life on an island, say, it's impractical and boring after a while if you're doing it the slow way (one at a time). Diminishing returns means you stop getting the same amount of xp after a certain point. That point might be when you reach a certain level, making hunting rabbits nearly pointless from then on. You might require higher level meat and plants to survive, meaning you'd have to hunt/gather those higher level resources or starve. You might only be able to take in a certain amount of XP per day, and anything after that would be wasted.
Take xp and use it like food and you get things like satiation (limit on how much xp you can have at a time), but also things like food poisoning, starvation, and the like. XP bonuses might come from certain activities, nerfs from others. Might be an xp drain occurs at certain points- say, holding too much without using it to level up, applying it to skills or abilities, or the like. Use it like air or water and you get things like anoxia or drowning- too little or too much xp could harm you.
The take away is that the story happens first then you apply the rpg mechanics in nearly every instance. So when disaster occurs, as it does in real life, life itself finds a way. Even if major global disasters occur, the Earth hits the big red reset button and starts over with some bacteria and extremophiles and works its way up from there. An extinction even for the sentient beings does not necessarily mean the end for life itself.