Reintroducing a species which we are familiar with, say after over hunting by poachers, is more likely to produce the expected results without complications because we have prior knowledge of that species role in nature.
I think of it like publishing an old book, or seeing an old movie. It is being a good steward of a legacy in a sense.
And asteroids. The dinosaurs got screwed because they hadn't yet developed a space program.
The African versions of big species were able to co-evolve along with humans as we developed spears, fire, etc. The American ones did not. This is why the American versions of everything from horses to cheetahs disappeared.
Bring them back I say.
Second, We are restoring the coevolution and making the system stronger in fact. A well managed predator benefit to thousands, maybe hundreds of species. Entire communities of insects and fungi need the temporary ecosystems created by predators. Preys benefit also by mean of disease control.
Attempts to introduce species that we thought we understood well have turned decidedly sour in the past, introducing species we know relatively nothing about is unlikely to go well.
Attempts to introduce species have turned decidedly sour in the past... if introducted in ecosystems where they do not belong. Reintroducting a locally extinct species is a different case.
Big felines are very well known animals. Present in all zoos of the world and reproducted in captivity since many years ago. There are entire books published about big feline surgery for example.
It is a conception because it is not the previous state you're returning to.
This is a false problem. The goal is not to achieve a theorical ideal previous state, is to restaurate a missing node to be able to reach a range of valid stable states. Is the same difference as "to reconnect a route bombed in 1940 to drive from a to b" or "to try to frozen the area in 1940". Nobody is talking about to living in caves again and working three hours each day.
If there's an insect that affected a plant... causing an imbalance that has not been allowed for.
All imbalances are "allowed" in fact, there is not a nature police, but your imbalance will never occur. Hervibores will just start eating other species of plants or migrate and quit the area followed by predators until the plant species recover. Mature ecosystems are buffered and can deal with known changes.
I could see bringing back a few specimens to life via cloning and whatnot - if only for being able to experience something from the past that we did not get to see.
And in the case of the woolly mammoth, woolly rhinoceros and cave bear, likely humans.
Could be- Or they could have been a good investment for the ratio of effort to food gained. In the case of the cave bear it may have been a case of killing of creatures that could be predators when humans were not in a pack and competed for the same shelter sources.
So, as one example, the earth has substantially less oxygen than it did millions of years ago. This means giant dragonflies cannot breathe in our current atmosphere. You would need to put them in a special chamber that recreates their niche, possibly without knowing what parameters are critical to their survival.
Anything that lived millions of years ago we're very unlikely to find enough intact genetic material to string a genome together anyway.
You would of course need some kind of zoo, reintroducing a new apex predator into the wild would probably wind up bad for somebody.