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 Unit1126PLL wrote:
The Tyranids make no sense.

They'd be better off eating the CHNOPS from a nebula and never touching a planet ever.

They just don't make sense.

But what if they like the taste?

Or, given that they're animals, it's entirely possible they are simply not aware of such possibilities. Or they are and just happen to *also* eat planets. Or they are programmed to do so, by a creator. Or there is some odd agenda of theirs, perhaps trying to make extinct all sentient species so that they never have a predator.

Or the actual answer, which is just that the Nids are a metaphor for Mother Nature (and how she's a bitch), and we're overthinking this.

Tyranids does leave few planet alone!
Look at planet called Solemnace.

http://wh40k.lexicanum.com/wiki/Solemnace

It seem very rare to see Tyranids avoid Necron. And in Necron codex there are lore about Tyranids unable to pinpoint the bio rich-planet, Tyranids might be to near-miss the planet instead of ‘avoid’.

 Selym wrote:

 Unit1126PLL wrote:
The Tyranids make no sense.

They'd be better off eating the CHNOPS from a nebula and never touching a planet ever.

They just don't make sense.

But what if they like the taste?

Or, given that they're animals, it's entirely possible they are simply not aware of such possibilities. Or they are and just happen to *also* eat planets. Or they are programmed to do so, by a creator. Or there is some odd agenda of theirs, perhaps trying to make extinct all sentient species so that they never have a predator.

Or the actual answer, which is just that the Nids are a metaphor for Mother Nature (and how she's a bitch), and we're overthinking this.

Don't think of the nids as concious, thinking beings like we are. Think of them as an invasive insect on a galactic scale. There are countless ways the nids could not only 'win' and mow through the galaxy, but do so easily. Those don't happen because the nids don't have sciences, objective reasoning, or the like.

 NinthMusketeer wrote:

 Selym wrote:

 Unit1126PLL wrote:
The Tyranids make no sense.

They'd be better off eating the CHNOPS from a nebula and never touching a planet ever.

They just don't make sense.

But what if they like the taste?

Or, given that they're animals, it's entirely possible they are simply not aware of such possibilities. Or they are and just happen to *also* eat planets. Or they are programmed to do so, by a creator. Or there is some odd agenda of theirs, perhaps trying to make extinct all sentient species so that they never have a predator.

Or the actual answer, which is just that the Nids are a metaphor for Mother Nature (and how she's a bitch), and we're overthinking this.

Don't think of the nids as concious, thinking beings like we are. Think of them as an invasive insect on a galactic scale. There are countless ways the nids could not only 'win' and mow through the galaxy, but do so easily. Those don't happen because the nids don't have sciences, objective reasoning, or the like.

I still maintain that the Nids are a metaphor for nature. Their invasions are way more like a natural disaster than an actual invasion, like getting into a punchup with a tornado. Sure, there are factors that a tornado could "use" to become more powerful, but it's a tornado so it either does or does not, and you'll be thankful it's at least not actively malicious.

Only one who have witness void space intergalactic are The Silent King... He saw something horror out there and return to milkway galaxy.. Only if he would tell us what going on in intergalactic out there.

Codex:Necron

When a Necron is destroyed beyond repair it just phases out to be repaired on It's designated Tomb World. While it would be a problem if a Tomb World itself was being attacked, Tomb Worlds have what are known as "Null Fields" protecting them so strong that no Warp energy can manifest inside of them. As such any Tyranids would be cut off from the Hive Mind upon entering such a field.

The fact mentionned above that the pylons disrup psychic energies and that necrons are immune to genestealer cults is actually something to be reckoned with: the genstealer link to the hive or the psychic signature of a word is what allows the hive mind to notice it. If you think of it like a gargantuan scanner then there simply is no little red light for some necron tomb worlds and the tyranids miss it.

Plus how powerful the necrons are against tyranids and the fact that they can't be scavenged is something to take in account for the space bugs. So it makes sense that the tyranids should avoid the necrons since they are such a hard nut to crack.

On the other hand now the necrons are very likely to knock out tyranids where and whenever they can, because ruling a galaxy of barren worlds is way less fun.

Except that we know of multiple Tomb Worlds that have been destroyed by the Tyranids. Null Fields are a nice advantage, but not a definitive one, they will not stop a Hive Fleet by themselves.

Necron worlds are hard targets, but not unbreakable ones.

Actually its better to think of them as a doomsday weapon- a bioengineered plague. They are programmed to target living biomass to support their biomass, but much of their energy and manufacturing requires nonbiological activities.

They are a sort of slow moving Exterminatus, targeting organic life, that may have evolved somewhat since their original release.

According to the theory that it was created by the ancients then? If so it would be more consistant if they did have a go at necrons since the ancients hated they're guts and engineered species to counter them.

The thesis of an unregulated evolution and the fact that it should fezst on biomasse can hold true but it would be strange that they avoid or miss the necrons then, wouldn't it?

They could be a bioenginereed plague but not ancient-made. Why ancient made ? And then it make more sense

I agree with the "space plague" line of thinking. It really is just good to think of them as intergalactic space locusts with a unifying gestalt consciousness that consume biological matter (the more complex the better) and use it to both sustain themselves and to reproduce. Kind of terrifying actually. There are real life analogues to genestealer cults, like the fungus that takes over ant brains, and wasps/spiders that lay their eggs inside a live victim, which then has it's brain chemistry altered to perform behaviors that increase the chances of the eggs surviving.

w1zard wrote:
There are real life analogues to genestealer cults

True, but terrestrial animals don't (yet) figure out how to accelerate to over 11km/s to get off the planet, and don't have the option of consuming astronomical bodies to supply vastly greater quantities of materiel. Even just floating through space with a huge permeable "sail" could slowly absorb vast quantities of materiel more effectively than warring with intelligent species.

I don't think logic of any sort really holds the Nids up, but they work perfectly as a fictional representation of how mother nature is a bitch, and are an excellent contribution to making the universe seem like it is actively malicious to mankind (and that's what Grimdark is all about after all). The 40k universe hates humans. It has it out for us. So much that it will defy all logic and probability and have an extra-galactic natural catastrophe, that could have happily spent eternity consuming *any other galaxy* (as it is probable that 40k is the exception for difficulty of conquest), that could have targetted the actual best choices for living biomass (Orks), and that could have just contented itself with eating actual astronomical bodies for hundreds of thousands of years without even encountering inhabited 40k worlds (space actually is that big), actually head straight for the heart of the Imperium.

That's some really bad luck right there.

400-800 billion stars in the galaxy, humans have only managed to hold under a million. If the Nids acted on logic, humans might never even notice they were there.

Pharos did it

 Maréchal des Logis Walter wrote:
According to the theory that it was created by the ancients then? If so it would be more consistant if they did have a go at necrons since the ancients hated they're guts and engineered species to counter them.

The thesis of an unregulated evolution and the fact that it should fezst on biomasse can hold true but it would be strange that they avoid or miss the necrons then, wouldn't it?

Not the Ancients- they are extra galactic. Think Star Trek "Doomsday Device" but in the form of a bioweapon seeking out destruction of other biological organisms.

Attrition wise, they’re something you go after when you’re desperately desperate.

Gauss Weapons are essentially disintegrators, so the Hive Mind will rarely recover it’s own biomass.

Their foes phase out, so can’t be consumed piecemeal - you only get your snackycakes when the Tomb Complex is knackered - and sometimes not even then, if it has functioning dolman gates.

Rippers, an important stage in pre-digestion are tied up with Scarabs, further postponing your dindins.

They don’t panic. Ever.

They don’t breathe, so your microspores are of little use, as there’s no infection to weaken the defenders.

And the Shadow In The Warp means nothing to them. No general foreboding. Still able to communicate effectively.

So why bother? It’s a poor use of your ultimately finite resources. Yes, you’ll likely wind up scoffing it all in the end - but very much risk a negative balance, having expended far more biomass than you harvested.


Automatically Appended Next Post:
Another thought....

Nids do well because they so rarely leave any survivors to pass on tactical hints and tips on how to tackle the squad.

That’s not something easily achieved against Necrons, because they self-repair and come back. So a battle on Day One can be lost by the Necrons, but learned from, and adapted to on Day Two.

If they learn quick enough, than after each engagement, just turn the Gauss on any corpses. Apply a total scorched earth policy. Nothing left for even the tiniest gribbly to get something down it’s neck.

That all helps the Hive Fleet spend its fleshy bounty faster and faster.

It’s even doubtful that fog clouds of spores would affect Necrons one jot - I’d imagine there’s plenty technological trickery in even the humble Warrior to be able to switch between different bands of vision.

 Mad Doc Grotsnik wrote:

So why bother?

Tomb Worlds are threats, better to destroy a Necron Tomb World than let it hinder and even threaten the Hive Fleet's advance.

The same logic drives Kronos to attack Warp Rifts, as even though it gets no material benefit from fighting Chaos, each Daemon World destroyed is one less headache in the future.

I guess I can give in to the thesis of Mankind's utter bad luck then? It is true that the whole 40k universe is about just every single things wants to get us, so tha's just for the grim dark.

However the term "biological weapon" is incorrect if we go that way, unless if we assume they've been created by somebody (maybe in their former galaxy?)

All hypothesis left aside, the fact that the necrons are a real threat is actually enough to justify two opposed behaviours: either try to take them out in the first place, or grow enough to be sure to attack with overwhelming number, which the necrons actually worry about as we mentionned. However, since the hive spirit is said to be extremely cunning, it can't have been left unnoticed to him that the Imperium would probably finish them off if they went for the first option and chased the necrons. Instead it would be logical that the go get the food that we are and eliminate the threat we could be beforehand, and only when they have grown enough turn to the necrons while being sure they won't get a triangle war. I think you could then make it logical that the necrons should be ignored in the frst place.

 Maréchal des Logis Walter wrote:
I guess I can give in to the thesis of Mankind's utter bad luck then? It is true that the whole 40k universe is about just every single things wants to get us, so tha's just for the grim dark.

However the term "biological weapon" is incorrect if we go that way, unless if we assume they've been created by somebody (maybe in their former galaxy?)

All hypothesis left aside, the fact that the necrons are a real threat is actually enough to justify two opposed behaviours: either try to take them out in the first place, or grow enough to be sure to attack with overwhelming number, which the necrons actually worry about as we mentionned. However, since the hive spirit is said to be extremely cunning, it can't have been left unnoticed to him that the Imperium would probably finish them off if they went for the first option and chased the necrons. Instead it would be logical that the go get the food that we are and eliminate the threat we could be beforehand, and only when they have grown enough turn to the necrons while being sure they won't get a triangle war. I think you could then make it logical that the necrons should be ignored in the frst place.

The problem is that the Necrons aren't ignoring the Tyranids. There are plenty of examples in which Necron forces have attacked the Tyranids to the point that there are memes of the Necron-BA bromance.

To the Hive Mind, the Necrons cannot be ignored. Sure small Hive Fleets and splinter fleets will avoid Necron worlds as they do not have the power to overwhelm those defenses. But large fleets like Leviathan? That's a hammer big enough to shatter Tomb Worlds.

Of course, the current mess with Chaos may change the calculus once again, as everyone is kinda tied with the immaterial forces of corruption.

Tyran wrote:

 Maréchal des Logis Walter wrote:
I guess I can give in to the thesis of Mankind's utter bad luck then? It is true that the whole 40k universe is about just every single things wants to get us, so tha's just for the grim dark.

However the term "biological weapon" is incorrect if we go that way, unless if we assume they've been created by somebody (maybe in their former galaxy?)

All hypothesis left aside, the fact that the necrons are a real threat is actually enough to justify two opposed behaviours: either try to take them out in the first place, or grow enough to be sure to attack with overwhelming number, which the necrons actually worry about as we mentionned. However, since the hive spirit is said to be extremely cunning, it can't have been left unnoticed to him that the Imperium would probably finish them off if they went for the first option and chased the necrons. Instead it would be logical that the go get the food that we are and eliminate the threat we could be beforehand, and only when they have grown enough turn to the necrons while being sure they won't get a triangle war. I think you could then make it logical that the necrons should be ignored in the frst place.

The problem is that the Necrons aren't ignoring the Tyranids. There are plenty of examples in which Necron forces have attacked the Tyranids to the point that there are memes of the Necron-BA bromance.

To the Hive Mind, the Necrons cannot be ignored. Sure small Hive Fleets and splinter fleets will avoid Necron worlds as they do not have the power to overwhelm those defenses. But large fleets like Leviathan? That's a hammer big enough to shatter Tomb Worlds.

Of course, the current mess with Chaos may change the calculus once again, as everyone is kinda tied with the immaterial forces of corruption.

The thing is that even if they do need to fight back when they are attacked, hence why confrontations with the necrons are possible, they will probably try not to engage themselves into a fight by their free will. They do defend themselves, but ignores the crons if they aren't tied in by the robots.

 Selym wrote:

I don't think logic of any sort really holds the Nids up, but they work perfectly as a fictional representation of how mother nature is a bitch, and are an excellent contribution to making the universe seem like it is actively malicious to mankind (and that's what Grimdark is all about after all). The 40k universe hates humans. It has it out for us. So much that it will defy all logic and probability and have an extra-galactic natural catastrophe, that could have happily spent eternity consuming *any other galaxy* (as it is probable that 40k is the exception for difficulty of conquest), that could have targetted the actual best choices for living biomass (Orks), and that could have just contented itself with eating actual astronomical bodies for hundreds of thousands of years without even encountering inhabited 40k worlds (space actually is that big), actually head straight for the heart of the Imperium.

That's some really bad luck right there.

400-800 billion stars in the galaxy, humans have only managed to hold under a million. If the Nids acted on logic, humans might never even notice they were there.

They only have the power to manipulate biological matter through complex genetic processes. Inorganic matter means absolutely nothing to them. That is, they need matter of at least the complexity of a simple cell to do anything with it.

As for humans only being able to hold a million stars, source? There are a million worlds in the Imperium sure, but there are probably more than one world in some systems, and I bet there are plenty of "mining systems" that have no habitable worlds, but are still claimed in the name of mankind. Hell, I'll bet there are a lot of systems the Imperium doesn't even exploit that we still claim as "ours". Your numbers of stars in the galaxy is also incorrect, it is currently estimated at 250 billion with a maximum possible at 400 billion.

w1zard wrote:

 Selym wrote:

I don't think logic of any sort really holds the Nids up, but they work perfectly as a fictional representation of how mother nature is a bitch, and are an excellent contribution to making the universe seem like it is actively malicious to mankind (and that's what Grimdark is all about after all). The 40k universe hates humans. It has it out for us. So much that it will defy all logic and probability and have an extra-galactic natural catastrophe, that could have happily spent eternity consuming *any other galaxy* (as it is probable that 40k is the exception for difficulty of conquest), that could have targetted the actual best choices for living biomass (Orks), and that could have just contented itself with eating actual astronomical bodies for hundreds of thousands of years without even encountering inhabited 40k worlds (space actually is that big), actually head straight for the heart of the Imperium.

That's some really bad luck right there.

400-800 billion stars in the galaxy, humans have only managed to hold under a million. If the Nids acted on logic, humans might never even notice they were there.

They only have the power to manipulate biological matter through complex genetic processes. Inorganic matter means absolutely nothing to them. That is, they need matter of at least the complexity of a simple cell to do anything with it.

As for humans only being able to hold a million stars, source? There are a million worlds in the Imperium sure, but there are probably more than one world in some systems, and I bet there are plenty of "mining systems" that have no habitable worlds, but are still claimed in the name of mankind. Hell, I'll bet there are a lot of systems the Imperium doesn't even exploit that we still claim as "ours". Your numbers of stars in the galaxy is also incorrect, it is currently estimated at 250 billion with a maximum possible at 400 billion.

Going with Wizard on this, I dont think one million worlds is a hard number, more a general "the imperium has a million worlds... or more... or less.... were not sure right now" also that number clearly doesnt include worlds with humans on them that have been lost to imperial records, warp storms etc.

w1zard wrote:

 Selym wrote:

I don't think logic of any sort really holds the Nids up, but they work perfectly as a fictional representation of how mother nature is a bitch, and are an excellent contribution to making the universe seem like it is actively malicious to mankind (and that's what Grimdark is all about after all). The 40k universe hates humans. It has it out for us. So much that it will defy all logic and probability and have an extra-galactic natural catastrophe, that could have happily spent eternity consuming *any other galaxy* (as it is probable that 40k is the exception for difficulty of conquest), that could have targetted the actual best choices for living biomass (Orks), and that could have just contented itself with eating actual astronomical bodies for hundreds of thousands of years without even encountering inhabited 40k worlds (space actually is that big), actually head straight for the heart of the Imperium.

That's some really bad luck right there.

400-800 billion stars in the galaxy, humans have only managed to hold under a million. If the Nids acted on logic, humans might never even notice they were there.

They only have the power to manipulate biological matter through complex genetic processes. Inorganic matter means absolutely nothing to them. That is, they need matter of at least the complexity of a simple cell to do anything with it.

As for humans only being able to hold a million stars, source? There are a million worlds in the Imperium sure, but there are probably more than one world in some systems, and I bet there are plenty of "mining systems" that have no habitable worlds, but are still claimed in the name of mankind. Hell, I'll bet there are a lot of systems the Imperium doesn't even exploit that we still claim as "ours". Your numbers of stars in the galaxy is also incorrect, it is currently estimated at 250 billion with a maximum possible at 400 billion.

To the planets you can add various outposts here and there that logically ultimatly raise the alarm.

However be careful of the necron compound: it is called organic metal. While it has to be extremely hard to make profit of it, it might not be completly unusable to the hive. This requires diggin into it and is a hypothesis, but still. Altought gauss weaponry does mean on the other hand that the fallen tyranids will not be fully recovered.

I'd like to again push forword the hypothesis that it might be a kind of strategic choice for the Hive spirit, that does not wish to engage in a fight where it would be so weakend that the Imperium could then maul them down. Instad they might want to secure their position beforehands. As for the time span descirbed in the fluff, the necrons' strength is not ignored but since they are awakening, it leaves enough time to get the job down against other foes. This could also be one reason why whereas the Chaos Deamons are dealt with by Kronos, as they are an already strong and ready to fight menace, no fleet has yet evolved to have a go at the necrons.

Again pure suggestion here, but it is the most logical reason why tyranids would ignore the necrons whereas they are dangerous (granted logic is applicable to 40k at all)

 Maréchal des Logis Walter wrote:
To the planets you can add various outposts here and there that logically ultimatly raise the alarm.

However be careful of the necron compound: it is called organic metal. While it has to be extremely hard to make profit of it, it might not be completly unusable to the hive. This requires diggin into it and is a hypothesis, but still. Altought gauss weaponry does mean on the other hand that the fallen tyranids will not be fully recovered.

I'd like to again push forword the hypothesis that it might be a kind of strategic choice for the Hive spirit, that does not wish to engage in a fight where it would be so weakend that the Imperium could then maul them down. Instad they might want to secure their position beforehands. As for the time span descirbed in the fluff, the necrons' strength is not ignored but since they are awakening, it leaves enough time to get the job down against other foes. This could also be one reason why whereas the Chaos Deamons are dealt with by Kronos, as they are an already strong and ready to fight menace, no fleet has yet evolved to have a go at the necrons.

Again pure suggestion here, but it is the most logical reason why tyranids would ignore the necrons whereas they are dangerous (granted logic is applicable to 40k at all)

I agree. The most likely theory behind the nids not attacking tomb worlds is that the hivemind realizes that it would lose more than it gains by doing so, and that it is much better served by going after easier to attack Imperial worlds. I think if all of the food sources nearby were consumed, the nids would turn and attack the necrons because they are ultimately a threat to the hivemind. Until that happens there are simply better targets.

w1zard wrote:

 Selym wrote:

I don't think logic of any sort really holds the Nids up, but they work perfectly as a fictional representation of how mother nature is a bitch, and are an excellent contribution to making the universe seem like it is actively malicious to mankind (and that's what Grimdark is all about after all). The 40k universe hates humans. It has it out for us. So much that it will defy all logic and probability and have an extra-galactic natural catastrophe, that could have happily spent eternity consuming *any other galaxy* (as it is probable that 40k is the exception for difficulty of conquest), that could have targetted the actual best choices for living biomass (Orks), and that could have just contented itself with eating actual astronomical bodies for hundreds of thousands of years without even encountering inhabited 40k worlds (space actually is that big), actually head straight for the heart of the Imperium.

That's some really bad luck right there.

400-800 billion stars in the galaxy, humans have only managed to hold under a million. If the Nids acted on logic, humans might never even notice they were there.

They only have the power to manipulate biological matter through complex genetic processes. Inorganic matter means absolutely nothing to them. That is, they need matter of at least the complexity of a simple cell to do anything with it.

As for humans only being able to hold a million stars, source? There are a million worlds in the Imperium sure, but there are probably more than one world in some systems, and I bet there are plenty of "mining systems" that have no habitable worlds, but are still claimed in the name of mankind. Hell, I'll bet there are a lot of systems the Imperium doesn't even exploit that we still claim as "ours". Your numbers of stars in the galaxy is also incorrect, it is currently estimated at 250 billion with a maximum possible at 400 billion.

To address the point of nids eating non-living things:

If you lick a chunk of iron, you actually consume a small quantity of iron. That then gets processed through your body in exactly the same way as if you had found some iron in a steak. If you had some serious need for a ton of iron, you could actually just straight up eat iron. It's not healthy for humans, but if you were to engineer processes in, say, a huge bioship that could distribute that iron as needed to various systems and inhabiting organisms (which we do anyway, on a smaller scale, with billions of microorganisms), then you could eat an asteroid to cure iron deficiency. Take that on a large scale, and you'll find a heck of a lot of raw materials that can be processed in such a fashion far more efficiently than by finding those materials in animals.

But what of organic compounds, I hear you ask. Organic compounds are not just permanently recycled through living organisms, (though that does happen a lot), they are produced by simple microorganisms taking in the raw materials surrounding them and processing them into more complex arrangements. These are then eaten by more complex organisms, who repeat the same sort of function. Bacteria > tiny creatures > plants > bigger creatures > plants > bigger creatures again > whatever topped the food chain. There is nothing to stop the nids creating a spacefaring biocycle that eats whatever the universe has to offer.


Then to address the million worlds thingy:

The IOM has supposedly "a million worlds". If they are counting an average of more than one planet per system, that'll give them less than 1,000,000 significant worlds. If they then placed outposts on ten million other systems, that gives them maybe 11,000,000 systems of note. Given 400 billion stars in the galaxy, maybe half of those have non-stellar bodies around them. We'll call it at 200 billion edible systems, for an estimate.

200,000,000,000 / 100 = 2,000,000,000
2,000,000,000 / 100 = 20,000,000

The IOM therefore would be touching less than 1 percent of 1 percent of all edible systems, or 0.01%. The odds of the nids picking our galaxy is already small, and the odds of them being a high-profile threat if they followed logic is ridiculously low, unless they grew to numbers that would be visible from Andromeda. At which point the entire galaxy is buggered anyway.

 Selym wrote:

Then to address the million worlds thingy:

The IOM has supposedly "a million worlds". If they are counting an average of more than one planet per system, that'll give them less than 1,000,000 significant worlds. If they then placed outposts on ten million other systems, that gives them maybe 11,000,000 systems of note. Given 400 billion stars in the galaxy, maybe half of those have non-stellar bodies around them. We'll call it at 200 billion edible systems, for an estimate.

200,000,000,000 / 100 = 2,000,000,000
2,000,000,000 / 100 = 20,000,000

The IOM therefore would be touching less than 1 percent of 1 percent of all edible systems, or 0.01%. The odds of the nids picking our galaxy is already small, and the odds of them being a high-profile threat if they followed logic is ridiculously low, unless they grew to numbers that would be visible from Andromeda. At which point the entire galaxy is buggered anyway.

Again, please stop misrepresenting numbers. There is an estimated 250 Billion stars in the milky way, not 400 billion. 400 Billion is the maximum upper bound and is extremely unlikely to have that much.

I could easily say that for every system with a habitable planet, the Imperium still claims and defends 100 systems. This actually wouldn't be too far off the mark as habitable planets are relatively rare from what we actually know about the universe, and entire systems would probably be strip mined to supply the Imperial war machine. This means roughly 100,000,000 systems are inhabited/claimed by the Imperium out of 250 Billion total. Still not a lot, but that is (1/2,500). Every time the nids invade a system that is a (1/2,500) chance it is an imperial controlled system. With the amount of systems a splinter fleet invades, they would run across humans very quickly, and their finely tuned instincts for seeking out more biomass would take over.

The idea that Tyranids don't use any inorganic materials is wrong. Just because they don't have visible riveted plates of metal on them doesn't mean they don't use those inorganic elements in their biology:

Strangle-vines tightened their grip on the ruins of an Imperial outpost, rendering steel and plastic down to their constituent parts ready for absorption.

p. 42, 3rd edition Tyranid Codex

Curving tusks of adamantium-laced chitin sprout from the Tyranid's head, allowing it to effect a devastating charge.

p. 33, 4th edition Tyranid Codex

However the Tyranids are not purely about just quantity of materials. They are also after genetic material, which they will not get by chomping lifeless asteroids.

Asking why don't they just be harmless autotrophs instead of being predators towards whole planetary ecosystems, is like asking why lions and tigers on Earth don't just eat grass or photosynthesize and make food from sunlight, water, and air when there is so much of it around them. Predators exist because it is a valid strategy to survive and it provides energy in a concentrated form. Autotrophs like plants or herbivores like cattle spend all their time either rooted to the ground photosynthesizing or spending nearly every waking hour grazing.

The Tyranids are a mobile predatory ecosystem that is as much about seizing new genetic resources as quantity of biomass. That's why as a first preference they go consume worlds infected by their Genestealer Cults, because they don't just want lifeless places with organic compounds.

Asking the Hive Mind why it eats others instead of making its own food is like a bacterium asking the same of human. The question may well be meaningless or irrelevant to the Hive Mind. Just as you don't care about the concerns of the bacteria on the food you eat, the Hive Mind does not care about the protests of the individual creatures consumed by its fleets. If the original primordial Tyranids were predators, then being a predator may well be their fundamental nature and the question of "Why?" may have either never occurred to the Hive Mind or dismissed as irrelevant.

Iracundus wrote:
snip

I actually didn't know that Tyranids used inorganic material like that, that is actually really cool to learn. I had thought that all they were after was biological material (aka biomass).

Biological material is made of plenty of inorganic material. We are far more than only carbon. The iron and oxygen in our blood, the calcium in our bones, and of course, the majority of our body mass comes from water, which is an inorganic compound.

The amount of metals in the human body is relatively trace. Even one of the most common, iron, amounts to only about a nail's worth distributed across the entire body. You don't literally eat a nail, but you get the iron in all the other food consumed.

For a Tyranid hive fleet, what counts as a trace amount could still add up to many hundreds of thousands of tons, ultimately spread out among all the ships that constitute the fleet. A few tanks and bases dissolved down and absorbed is the hive fleet getting its trace elements.