Based upon this answer, J.K. Rowlings has apparently stated "You cannot destroy Dementors, though you can limit their numbers if you eradicate the conditions in which they multiply, ie, despair and degradation."
Does this mean that the number of Dementors is always either increasing or remaining the same? If they cannot be destroyed, does that mean they are immortal? Or do they die of "natural causes"?
I interpret what JK Rowling has said to mean that however many Dementors already exist cannot be destroyed by any means, Wizarding or human, meaning there is no spell, potion, curse, etc, that will eradicate them. I expect there is the possibility that Dementors eventually die a natural death. I suspect that the number of Dementors in the world ebbs and flows with their contact with a ready supply of despair, unhappiness, degradation, hopelessness, etc. The less of these emotions they have to feed off of, the less Dementors there will be.
I think it's analagous, as JKR herself says, to depression. Depression is a condition that is never cured, but its severity can possibly be controlled with therapy, medications, and by tweaking other external factors that make it less likely for the depression to be debilitating, if that makes sense.
Anyhow, this is my interpretation of what JKR has said about Dementors.
Harry thinks in GoF that Sirius went into a southern country and he couldn't imagine a dementor surving for long in sunshine.
I agree with that, I believe dementors can be destroyed (because I don't think they're alive either to be killed) if they are subjected to sunlight for too long and then dissolve into nothingness.
What is more likely is that Dementors recognize what will kill them, and invariably high-tail it when confronted with such things.
I mean, if not even an uber-Patronus can destroy a Dementor, then why do Dementors flee when confronted with one? They could just make a tactical retreat and then return when the Patronus that forced them away has gone.
We-Know-Who spent a lot of time searching for immortality. If dementors were immortal he surely would have harnessed that ability by some dark ways.
