Why would a race (like elves) evolve to be immortal, or like super long lived? That seems like a way to easily overpopulate and break suspension of disbelief. And if they don’t breed very much in comparison to other races they’re going to get overrun…
Long-lived species is a common thing in speculative fiction. Most of your readers aren’t going to have their suspension of disbelief broken by that simple fact. “Long Lived” is a trope, after all. And for those readers that would, there are several things you can do to help curb that issue:
- Research human lifespan, the science behind our longevity and things that have contributed to longer lifespans in recent years.
- Look into other fictional long-living species and see what the author’s or creator’s gives as the reason for that possibility (if any).
- Consider how and why aging occurs in humans, give your elves a biological resistance or repairing system that prevents that or at least puts it off.
- Could organs be self-repairing…or even self-replicating if the need arose?
- What might be the cost for such an efficient biological system?
- How common is sickness and how is it handled that gives your elves the one-up on humans?
As far as population goes, you just have to consider balance. It doesn’t have to be one extreme or the other: overpopulation or extinction. Perhaps they have a more challenging time conceiving or carrying babies to term. Perhaps they have a small window of their life in which they can conceive. Perhaps there are biological, logistical, or cultural reasons to only have a small number of children. Perhaps they pop babies out like crazy, but living conditions or warring times mean not many of them live very long. It’s all about striking a balance between birth rates and death rates.
And, as always, know your tropes.