I guess I'll take that explanation. Still, it would be very hard to not imagine whole swaths of villages and cities disappearing because a megafauna couldnt find anything to eat in the cracks of civilization.

As for Tarkir, beyond what other people have mentioned so far; that dragons are mostly unsustainable anyway (there there are more than enough goblins and humans to eat for a while when the big food sources begin to vanish), Lets look at the actual mega-fauna that existed at any given time and see how they stack up.

Ashcloud Phoenix - While I consider phoenixes to be mega-fauna, they are so steeped in red mana and their reincarnations as well as their smaller numbers leads me to believe that they are more a force of nature than a functioning part of an ecosystem like a Roc would be. Speaking of Roc's Tarkir's Wingmate Roc doesn't seem very big (or there aren't enough scale things to judge it's true size). I would put the Rocs of Tarkir along the size of maybe a very very large condor? That's still big enough to prey on people and goblins if the need for food is strong enough.

Conifer Strider - Tarkir does have some pretty big elementals. The question is not what they would eat however, but what might eat them.

Highspire Mantis - These large bugs have been known (in-story) to prey on humans and probably the bird-folk as well....or anything else they can catch.

Summit Prowler - I assume that yeti's eat smaller mammals and larger, injured prey. They are about as big as a small dragon so ta group of them could probably bring down a giant lizard sized snack.

Temur Sabertooth - These existed alongside things like mammoths. I'm guessing they also fed on your basic Feral Krushok.

Ancient Carp - This fish is pretty big, but I highly doubt there are tons of these things running around. This strikes me as maybe a one-of every several generations kind of thing.

Battlefront Krushok - I mentioned these up above, and they are pretty big, but probably as easy to feed as an elephant or other similarly large animal.

Ivorytusk Fortress - Speaking of elephants....the Azban seem to enjoy using these as mobile homes and beasts of burden. While large tracts of the Azban homeland is pretty deserty, I'm pretty sure I remember seeing some art that depicts more savannah-looking landscapes that a normal elephant would live in similar to areas of northern Africa. Of course, they also employed quite a spread of mammoths and non-mammoth beasts.

Tusked Colossodon - This mega-fauna seems to be of the hibernating sort, and I'd not hesitate t put the likes of Meandering Towershell on that list as well.

Giant Segmented Krotiq - The Krotiq seem to be a problematic sticking point, but we do live in a game world that wholly accepts the occasional Giant Spider. I would imagine that, contrary to the intuitive nature of insects, these either don't have a lot of babies very often or their young is a major source of food for lots of the other mega-fauna and very few make it to titanic adulthood.

Leviathans and Serpents - I assume these find enough food to survive among the fish and other moderately sized things that stray too near the shore.

As an odd aside: Does anyone remember anything at all about the Loxodon of Tarkir? They only got two cards in Khans, got nary a mention when we went back to the past in Fate Reforged, and were not seen at all (presumably eaten?) in Dragons of Tarkir.

I'm pretty sure they were confirmed to be extinct in the new timeline, yes.

Right, but they seemed not to exist in the past of Fate Reforged as well. Only in the Khans timeline was there a small amount of Loxodon in the present. You'd think we'd have seen a few back in Fate Reforged...or read on some flavor text somewhere in Dragons how they died.

Maybe they just slipped through Creative's attention when they were making DoT, or maybe they didn't find the right card to show it. I mean, there were a ton of references to things from the old timeline in the cards, but they probably couldn't fit everything in there. But Fate Reforged was the shared past of both timelines, so if there were loxodon in KoT, logic dictates they existed in the Fate Reforged era as well.

Fate Reforged existed in the pre-Mending timeline. It is entirely possible that, between Ugin dying, and the Mending, some other 'walker could have left some Loxodon behind for some reason. That being said, Occam's Razor...yadda yadda, you're probably right and Creative just forgot about them or didn't have room.

I agree with you on many of those creatures but not on all of them. Summit Prowler do not seem to be the type that travels in packs. They seem like loners that come together only to mate unless forced to group together by the actions of sentients. Like the wolves in Siberia that form megapacks due to various reasons.

I will grant you I have also seen savannah like landscapes but what flora is there not prevalent nor massive enough to feed all these creatures. Unless...the plane of Tarkir and therefore the deserts and savannahs of Tarkir are much bigger than we imagine it to be. That would be an acceptable explanation if it was true.

While I give you that the Tusked Colossodon hibernates for an undetermined amount of time, Meandering Towershell cannot. Because it is a reptile. Reptiles do go through a process called brumation which is similar to hibernation. The difference is however, that reptiles do not go to "sleep". They enter a period of suspended animation as in their metabolism drops dramatically, moving only when temperature warms up for them to make movement possible and get some water. Since Meandering Towershell lives in the jungle part of Tarkir where I will assume it is always hot since these guys are running around it has no reason to go into brumation. Sure, I'll give you that they might sleep for a few years or so but seeing as the card does not have any trees growing on it's back like this guy, I don't think they sleep for decades.

I'm just gonna go out and I say that Giant Spiders are a thing because the ecology they live in is huge like the pictures. I will accept the fact though that they're many young and Krotiq's many young may feed other fauna and flora and few make it to adulthood but that has always been the way for insects and arachnids and yet they are still a whole bunch of adult spiders and insects running around.

And yes, I believe Loxodons where hunted to extinction.

We can definitely rule out the planeswalker explanation, because Creative stated that no planeswalkers visited Tarkir between Fate Reforged and the present (they said that to calm people down about the time travel). And when you think about it, if the loxodon died out because they got eaten by dragons, it makes sense that there are no physical remains left, like zombies or skulls etc. They just got swallowed whole and were never seen again.

In regards to phoenixes, dragons and hydras, these tend to be mana manifestations, so they likely do NOT operate on normal ecology

How so?

No, those are all biological creatures in most situations.
(okay, less certainty with phoenixes)

Dragons maybe, but what little exploration of hydra flavour we have seems to imply they're not natural creatures, at least on Naya

Hydra Broodmaster

And there's plenty of reason to suspect apocalypse hydra and feral hydra are natural too.
The only exception is Progenitus, but that's because he's an avatar.

Dragons have plenty of history showing natural reproduction. Dragon broodmother, broodmate dragon, dragon egg, and the entire thing tracing their lineage back to the elders, which had siblings and everything.

I think this thread starts by assuming anyone is interested in realistic ecosystems when it comes to MtG worldbuilding. Creatures are huge to tell you something about the world. Dragons are bad for the environment to drive home a particular point. The Abzan have monumental creatures in their employ in order to make the nomads but still give them access to castle-like structures.

There is a sort of fantasy worldbuilding that cares about things like ecological niches and evolution. Magic: the Gathering isn't that type of fantasy. Magic uses the fantastical to explore certain topics or dynamics. It doesn't particularly mind if a Worldspine Wurm doesn't make a ton of sense, in terms of food consumption.

It's been years, but I still get pissed off every time I see that stupid wurm...

Okay, that might be a bad example. That one just doesn't make sense on Ravnica at all and doesn't work with the themes of Ravnica either. It was just the first thing that came to mind as "impossibly large creature." Pretend I said Pelakka Wurm instead.

Which would be totally fine if they kept it in card form and did not expand on the lore. They did however and when one puts gigantic creatures in an unfantastical ecology then that's not exploring certain topics. That's just lazy writing especially since there are settings in MTG that can Sustain their Megafauna.

I specifically named the Abzan and their creatures. In the lore, it's a way to pair the concepts of nomads and castles and to have their soldiers contrast with the Mardu warriors. No-one at Wizards ever given it a second of thought as to how they sustain those beasts or where they get the material for their castles. No-one is being annoyed that we're not seeing sand-stone quarries.

In fantasy, it's often perfectly acceptable to have fantastical elements without a direct explanation. I wouldn't particularly call it lazy writing, but rather just not something important or crucial to the plot.

If you think about it too hard you start asking even stranger questions, like...what do the Abzan do when the beasts carrying their homes gets too weak, injured, old, or dead to continue carrying them? How do they transfer a stone fortress from one beast to the next? How do they even breed/train these beasts to carry all that weight? Especially so calmly into battle? What if one falls over? does everything in the fortress break and is the weight too much for the beast to get back up? Is the Salt Road littered with the remains of sideways stone homes that couldn't be lifted out with some mammoth wearing a crane and set on a new beast?

You wanna pair nomads with castles? Fine, but you have to justify their existence in the lore or at least in the art itself. The Rule Of Cool is not a justification.

Within reason. Creatures that big living in a environment like a desert is not within reason and needs a direct explanation. Yes I get that the story isn't a character-driven story but a more wider scope story and so does not require much in-universe explanation but that does not make it any less needed in some way.