What do the dog things eat?

You don't have to see other animals in order for them to exist. We saw Riddick carrying around a giant drumstick later in the movie. Maybe the dogs eat whatever that thing came from.

Maybe liike wild dogs, dingos and such they are scavengers, and eat what they can find.

OP made a good point, the film keeps trying to portray how really dangerous the planet Riddick stranded on is by showing savage carnivores all around yet there is a total absence of herbivores without which carnivores can't survive much less thrive so well to be that numerous in population.

But then again, this is not animal planet and maybe the film's budget limited what they could show on screen.

-----
wat are you lookin' at...

[deleted]

They probably didn't have the budget to add in enough animals to show a more believable ecosystem. Pitch Black had the same problem, according to the trivia page. Plus, the pacing could've been slowed down by focusing on creatures that didn't want to eat the human characters.

Thit and thpin!