FormerlyHemlock
Hero
Something that can help, but might be a pain to do, is find some of the old AD&D monster books. They listed number appearing in their description, which can give you an idea of their organization (2E was better for this IIRC). BECMI might have the same info, but I don't remember. You could also just figure it out on your own, but that's also a lot of work. Anyway, once you have this information, you can easily have players make INT checks to get some of this information, which will help them plan accordingly.
Figuring it out on your own can lead to good things too though. For example, did you know that 100 square miles of territory (26,000 hectares, or 10 miles x 10 miles) will support 13 lions? Cold-blooded creatures need less food, so you might expect to find 130 crocodiles in the same area, but they are also less social so instead of one pack of 13 lions you'd see 130 lone crocodiles. Jackals need less territory than lions (1000 hectares per jackal, about half as much as a lion--I'm not sure if they can overlap). Also, in bad terrain without much food, animal territories can requires 10x as much area as normal to feed themselves (so roughly 3x as much distance in all directions). Birds can require more territory (golden eagles require 9000 hectares each, or approximately a 3.3 mile radius territory), and some territories are measure not in areas but linearly (some gulls claim 10m to 120m of coastline as their territory).
You could stick with these same numbers in-game (e.g. call it one owlbear in a 5 mile x 5 mile valley), or you could introduce new assumptions like rodent-sized rock-eating tribbles as the foundation of your fantasy ecology to let you pack more predators into a smaller space.
Even a passing familiarity with the way real-life animal ecologies work has the potential to improve your game.