The evolutionary adaptations of an ecosystem would largely depend on the evolutionary path that the dragons themselves took. Basically if the dragons became quite large before developing flight, you would see different adaptations then you would if dragons developed flight before becoming large. Likewise the stage at which fire-breathing became viable would change the development of the surrounding species.
I think it would be a safe assumption that flight would be one of the dragons' early adaptations. Developing wings after getting too big would be troublesome for dragons who sprouted wings before the wings became strong enough or large enough to provide lift, the precursor to useable flight wings would be a liability for a large dragon and probably wouldn't pass on very well.
So, your prehistoric dragon would likely start off as a small, canopy dwelling predator with rudimentary gliding capabilities, like a flying squirrel. At this stage the going adaptations for avoiding dragon attacks would be:
- Being too big to be on the menu.
- Quick reflexes, being able to jump out of the way. Think crickets/grasshoppers.
Over time the dragons who developed stronger more specialized flight wings would become more maneuverable in the air, thus increasing their chances of catching jumping crickets. As their hunting prowess increased they would have some spare food energy that they could use to grow a bit larger bringing more things on to the menu.
Now dragons begin to look more like dragons, just smaller. Of course as the dragons become more proficient hunters, prey animals begin to develop counter measures. At this stage the going adaptations for avoiding dragon attacks would be:
Here dragons may begin to develop the precursors to fire-breathing, perhaps spitting sticky bile or venom at fleeing prey. This allows them to take down larger prey animals. As the dragons gain the ability to take down larger prey, they again gain extra resources that they in turn put toward getting larger.
But as they get larger, their energy requirements grow forcing them to choose larger prey animals, hunting rodents now expends more energy than they take in for their efforts. At this stage the going adaptations for avoiding dragon attacks would be:
- Growing still larger
- Thicker hides
As their prey gets bigger and starts to form herds and fight back, the dragons further develop their fire-breathing ability, sticky bile gets ignited to form an effective napalm. At first the fire adaptation is probably used to redirect and break up herds. At this stage the going adaptations for avoiding dragon attacks would be:
- Retreating to bodies of water.
You probably noted that being poisonous was a strategy developed early on that kept certain species from being eaten, this will probably remain one of the most effective deterrents.