While we hear alot about the mountain peaks, I imagine that to complete the ecology we need a better picture of those 'fertile valleys' between them. I'd wager that abundant prey species are down there ripe for the picking, but Ogres themselves stick mostly to the high-country, eating the other predators that decend into the valley for meals themselves. Ogres eat a lot, but I picture them doing it in bursts; like Walrus or Seals with lots of time of 'lounging' inactivity between big feats that cut down on the food needs.
Plus, Ogre diet is probably supplemented with a great deal of Gnoblar, and Gnoblars are likely lichen-scrapers, rat hunters, grub diggers, nest-theives: all the ways of getting nutrients from the environment that is beneath an Ogre's notice and packaging it up in a crunchy green skin for them.
Dark Elves, though, are more problematic. Their chief source of food must either be magically derived (a little reckless use of Lore of Life and Lore of Beasts can keep people fed I imagine); or derived from humans (soylent maurauder); or (most likely) pulled from the sea: fish and kelp and whatnot; and 'big game hunting' in those monsterous waters: whale and kraken for dinner again!