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I have had an idea for a possible story, where the protagonists are some kind of future, sapient hominid descendants (Not descended from humans; bonobos or gorillas or something).

The main premise of the setting is that there is a huge diversity (In terms of number of species) of large mammal carnivores, specifically big cats, hyenas, bears and canines. I want to justify this scenario with some kind of plausible explanation, rather than "there just is".

The removal of humans is obviously going to help, and I never saw humans as being part of this age. I have my own idea, but I won't put it here; as I wouldn't want to start a discussion, which this site is obviously not for.

So, my question is; What scenario would be a plausible way to justify the appearance of all these large predator species?

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    $\begingroup$ Explaining abundance of life would be easy. Explaining diversity is a bit harder. $\endgroup$ – Alexander Aug 14 '18 at 17:50
  • $\begingroup$ Just to be clear: You want this to happen on Earth as we know it, just half a dozen million years into the future, right? $\endgroup$ – a CVn Aug 14 '18 at 17:51
  • $\begingroup$ @MichaelKjörling Yes, one of the difficulties I see with this scenario is how to get humans out of the situation so quickly without harming all the fragile predator families. $\endgroup$ – SealBoi Aug 14 '18 at 17:52
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    $\begingroup$ I regret that you need to learn to deal with uncommented downvotes. Though they aren't supposed to be a measure of popularity, they are nevertheless regularly used as an expression of popularity. Even experienced users post questions that get downvotes. Even questions that have graduated from our Sandbox get downvotes. It's a fact of life, like having to occasionally drink commercial lemonade and use restaraunt toilets. (Disclaimer, I have cast no votes.) $\endgroup$ – JBH Aug 14 '18 at 18:08
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    $\begingroup$ @hzmv 90% of the biomass, not 90% of the species. $\endgroup$ – SealBoi Aug 14 '18 at 18:28
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I tend to agree with YElm and TheTimeVoyager, and I'm mostly repeating some of their points hoping to say them clearer.

Gause's Law says that no two species can fill the same ecological niche. This is kind of self-fulfilling -- if two species coexist we can look for some way their ecological niches differ and always find something. It's kind of related to the Law of Fives. But it's related to reality.

To have more predators you should have more ecological niches for them. You need enough herbivore numbers to support many predator species. You want at least a breeding population of 10,000 for each predator, right? So if all the herbivores put together can only lose enough predated individuals to feed 100,000 predators, you are limited to 10 species.

One way to support a lot of predator types is to have a lot of herbivore types. Then the predators can specialize. Of course it's no good for a predator to specialize on a rare form of prey, so you need a lot of common herbivore types.

To get a lot of herbivore types you might want a diverse climate with diverse plants living on it. Maybe fifty kinds of dominant forests, with not 2450 but still hundreds of mixtures at their boundaries. Fifty kinds of scrubland with hundreds of mixtures. Dry land and moist land maybe not real close together but both present. Steep hills and flat plateaus.

Different habitats, different plants, different herbivores, different carnivores. Some specialized for particular situations. Some that generalize and can't compete in any one situation but can move in, get some benefit, and move on to something else.

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You need several ingredients:

1. Get rid of humans

As you correctly stated in your OP, humans are currently the greatest predators on earth. Get rid of them and evolution can take its course.

2. Big prey

Big predators need big prey to survive. We can see this dependence in dinosaurs like Tyrannosauridae and Sauropoda and later again in Entelodonts (giant pigs) preying on camels and Phorusrhacidae (giant birds) hunting down horses. Evolution works like an arms race in these cases: herbivores get bigger to get too big for carnivores, so they become bigger as well.

3. Extremely diverse biotopes in the same location

The reason why there are so many different species in the Amazon rain forest is that there are thousands of islands separated by the Amazon river. Any species that cannot cross the river will eventually evolve in a different way than their neighbors.

Additionally there is the annual flood that swamps enormous areas for several months. Aquatic and land-based species circle through times of seemingly limitless space and times of very limited space. During the flood, land animals can only live in trees and during the dryer season aquatic animals have to cope with the risk of falling dry.

These conditions offer many niches for many species to evolve.

4. Separation between species

There can only be one prime predator. Entelodonts and Phorusrhacidae wouldn't have evolved at the same time in the same area because they are direct competitors for the same resources (big herbivores). You can have one big predator and one big scavenger in the same area, but never two species that are too similar.

So let the continental drift rip Africa apart, push the Antarctic into habitable temperatures, remove land bridges and pile up mountains between habitats to separate species.

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  • $\begingroup$ I've been to Africa on numerous occasions, and in a typical day's drive, you can see lions, leopards, hyenas, crocodiles, cheetahs and sometimes even wild dogs, all in the same area; many of their "menus" overlap, and indeed they compete, but they persist, and all save cheetahs are doing well aside from anthropogenic threats. My point is: there can and will be competition, but it does not immediately obliterate all but one species. Otherwise, a fine answer. $\endgroup$ – SealBoi Aug 14 '18 at 19:24
  • $\begingroup$ @SealBoi To be honest, I didn't count species like wild dogs, leopards and cheetahs as "big predators" in the context of this question. I thought you were aiming for bigger in body size. Wandering ants are devastating predators as well, but they aren't big... $\endgroup$ – Elmy Aug 14 '18 at 19:36
  • $\begingroup$ Fair enough, I guess I just meant "megafaunal predator". Even still, lions and hyenas occupy, more or less, the exact same niche (Kruuk 1972), and yet both species are common in places where human damage is deliberately minimized. $\endgroup$ – SealBoi Aug 14 '18 at 19:52
  • $\begingroup$ Good to see an answer based on the mammalian megafauna. $\endgroup$ – a4android Aug 15 '18 at 1:52
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If you look at speciation charts for the time immediately after major population bottlenecks there are often explosions in particular families but most of the new species don't go anywhere. For example immediately after/above the K-T Boundary mammals diversified greatly but most of the new species have since died out. They were able to diversify because there were many empty niches in the life web. Given the way humans suppress (read kill) large predators their sudden decline would have a similar effect, suddenly opening many ecological niches for which a number of species are competing, this would explain a massive, but short-lived, expansion of species diversity among large predators.

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    $\begingroup$ A good answer, but are you sure this would work, considering the clades which the predators hail from? They are big cats, hyenas, canids etc., and I'm not sure if they can experience a diversity surge when the rough niche/guild I plan for them to occupy in the future is the same niche they occupy today. $\endgroup$ – SealBoi Aug 14 '18 at 19:00
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    $\begingroup$ @SealBoi If you posit that humans more-or-less wiped out those clades, leaving only a couple of species in each then as those remnants spread out into their clades' traditional ranges' you'll get major geographic specialisation and speciation. Having a fixed producer/hunter/scavenger niche doesn't mean that the geographic niches are fixed for a given clade. $\endgroup$ – Ash Aug 14 '18 at 19:07
  • $\begingroup$ Well said, I stand corrected. $\endgroup$ – SealBoi Aug 14 '18 at 19:25
  • $\begingroup$ @SealBoi All good it was a legitimate concern. $\endgroup$ – Ash Aug 14 '18 at 19:41
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Although it isn't incredibly common to have many large predators in one locality, it has actually happened before. One case of this occured in the Late Cretaceous of Northern Africa, preserved in places like the Kem Kem Beds. Here, we have many large theropod dinosaurs running around together in the same place, including Carcharodontosaurus, Sauroniops, Abelisaurid dinosaurs, Sigilmassasaurus, and Spinosaurus. One hypothesis regarding their coexistence is that each animal inhabited a separate niche, hunting a very particular food source or inhabiting a particular habitat. For instance, Spinosaurus and Carcharodontosaurus might not have needed to compete because one hunted primarily in and around the water while the other preferred dry land.

These types of specialist animals might be your best chance for a plethora of large carnivores.

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