A bit of an oldish thread, but if we're to take away the metaphorical metaphysical aspect of this story's events, I'd argue there's a pretty solid ecological basis for what we see in the film. It's already been more or less expressed in bits and pieces throughout this thread, so this is a combination of those and me interjecting my own knowledge of ecological systems, and the Serengeti in particular.
So for starters, we know the Pridelands itself isn't the entirety of the Serengeti - which a lot of people for some reason tend to think it is - it's pretty much just the territory that Simba's pride holds, and they even have territorial borders just as any other stable lion pride does. (A good-sized pride's territory covers about 100 square miles, which is plenty of land for us to think that it's much more expansive than it actually is compared to the rest of the Serengeti alone.) Based on film lore and what we can observe in the various canon movies themselves, it seems the normal local large predator ecology comprises of the local lions, at least two crocodile floats, cheetahs, and the occasional passing leopard. The large, seemingly hundred-strong hyena clan lives beyond the Pridelands' borders and doesn't seem to have a large impact on the prey population since they are limited to occasional small party raids. You basically have a system that would be set up if lions did become sapient; they're only keeping out species that are niche competitors (chiefly hyenas and leopards; don't know if painted dogs have been seen in any of these films, but I'd assume a few small groups would be permitted to at least pass through the territory) and it seems that this has gone on for at least a few generations, so the prey base has become used to these population dynamics.
Once Scar took power and brought in the hyenas, now you have added literally over a hundred additional mouths to feed, which puts a lot of stress on the local environment even if everyone is just hunting as much as they need to sustain themselves. Then you add another pride of lionesses (Zira and her Outsiders) and you've now added even more mouths to the local environment. Without proper partitioning (which I assume is the real central role of the king and queen) it's easy to send the local ecosystem into chaos with this kind of sudden setup. Based on my biology-based timeline of the canon cartoon story, this would have gone on for some three years or so before Simba returned, and during that time the additional pressure put on the local herbivores would've resulted in both overhunting and ultimately driving a lot of the prey base away into other lion territories. (Hence Sarabi's statement to Scar: "The herds have moved on.") None of this suffering is necessarily dependent upon the weather or the local plant ecology, which would likely stay pretty consistent in the three year reign of Scar; it could've happened even without the annual stresses caused by months of drought during the dry season. All the prey animals had to do was get fed up with the massive local influx of predators and just up and left, leaving all the predators (the only animals we see later on) to starve during the annual drought. Simba came back towards the end of said drought, and in the ensuing months the rainy season rolled in and the prey animals returned.
So yeah, long story short, Scar can easily be explained as being directly responsible for destroying the local food web by bringing in the hyena clan (and bringing in another pride of lions) and presumably not enforcing rules to limit overhunting (and at least de facto encouraging it once it became clear that the lioness' normal hunting rate wasn't going to be enough to feed the pride given the added competition of the hyenas), ultimately driving off the remaining prey base into other lands such that during the dry season when Simba came back, everyone was starving from lack of prey.