Speaking of large things in deserts, the movies Tremors and Dune both prepped me to believe that titanic wurm beasts could survive just fine in a desert landscape.

They could evolve to have a super ultra slow metabolism and feed on minerals deep underground.

This I will accept though if that is the case it would only need to come up to breathe and drink.

Well I still havent read dune but in the movie dont they use sound waves to bring them up? So they probably dont come up except for air and occasionally water or alien water analogue.

Once you start applying real-world science to Magic: the Gathering, it all starts to break down. You happen to have an interest in ecology, but using pretty much any science will show severe flaws in the worlbuilding. Try applying economics to the Jeskai, for example, or sociology to Ravnica, or geology to Jund, they will show flaws as severe as the Abzan's behemoths.

And none of them matter, just like the material physics of the Wall in Game of Thrones doesn't matter.

The Jeskai are said to be traders and artificers and ravnican guilds = political ideologies, so it makes sense

If one wants to talk about Jund's geology one first has to talk about speculative dimensional physics which would I'm sure would provide a good explanation without breaking everything done.

I actually can see someone building the wall in Game of Thrones.

Well, since economics was mentioned one thing that also always bothered me - elven economies. You see elves (like Tolkein's) with metal everything for their warriors. Where is that metal coming from? They're not strip mining for it and if they trade what exactly are they trading? And yes, I am aware the NonTimber Forest Products (NTFPs) can be worth 2.5 times as much as the timber, but what do the elves produce for trade? They don't really have agricultural capabilities because they don't have fields. Unless they're able to buy raw resources and skillfully turn them into something dramatically more valuable, they lack trade capabilities.

Probably elven wine and jewelry or something. If we were talking Tolkien style elves I'd assume they steal everything they use to trade.

Kaladesh does answer the question in that elves can grow metal from trees, but I have a feeling this is not an universal worldbuilding statement

Now that you mention it, how does other non-human economies work? What do the few goblin kingdoms trade in? What about Merfolk economies or Vedalken?

The Jeskai have no-one to meaningfully trade with and everyone spend one-third of their waking time practicing martial arts.

Ravnica's Guild's are somehow equally matched (sans the Gruul), but one group is feeding most of the populace and two groups have widely superior technology.

Like, I don't particularly care about any of that, but once you start applying real-world science, pretty much all worldbuilding breaks down. I just don't get why someone would spend a lot of time "disproving" huge beasts in a fantasy setting, unless you're creating your own fantasy setting in which you carefully mapped out the ecology or something.

If you want to talk about flaws in Magic's worldbuilding, there are much more fruitful discussions to be had than "Moldgraf Monstrosity doesn't have an ecological niche."

I assume that the goblins craft weapons or clothing and armour from hides and can offer those as trades to other goblins. Even things like bead jewelry can be valuable. I imagine that they mostly trade amongst themselves and loot for better things (and then trade what they looted). I imagine merfolk are similar in that regard. Perhaps they're more inclined to loot sunken ships as well. Merfolk may also be inclined to trade shells or other exotic tools or use coins they've salvaged to buy things from other races, depending on the world you're in.

Vedalken are artificers. They make machines. I imagine, if they were interested, that they could make worker drones for other groups. But I imagine them to be more like Mirrodin's vedalken and kind of just take what they want because of superior technology.

I'm not disproving anything. I'm merely pointing out that some creatures on some planes are not matched with suitable ecologies. That is the whole point of this discussion. Just me bringing up an issue that I thought was relevant.

Oh an Moldgraf Monstrosity is fine in my eyes.

I don't think goblins have much of an economy. The biggest guy gives the orders. THeir society runs on fear and force. They produce little.

Elves can do whatever they want because they are just better than us. I think most elves perform little economic activity. For trade they apply hundreds of tears of experience to crafting goods of the highest possible quality. And their position is usually secured by the fact that they have the longest-standing society and were able to craft the political situation to their liking as a result.

Merfolk are desperate for metal, and/or dry land to build foundries on. You ever try lighting a fire underwater? If they're lucky, they'll have some volcanic phenomena for smelting. I doubt it though.
What do they produce? Pirated goods and fish. Without metal tools it's hard to do anything else efficiently.

Also where do the Giants get their armor or even the clothes they were from? I'm gonna assume that if they are giants then they are also megafauna so they could take their fur/hides for them to wear but putting armor on any significant giant units would be an economical nightmare for anyone involved. Giants would have to stripmine every available deposit in the region depending on the rich it is to just armor one Giant.

I'm assuming the average height of a giant to be around 40 to 70 feet.

Most giants haven't moved into even bronze age technology. They're stone age for the most part.

True but some giants on some planes wear armor which bears the question: What sentient kingdom used a lot of resources and money armoring giants?

They have the Abzan.

And the Sultai