Are the krill sequestering more iron than they need? Assuming they aren't, then in this model processing by whales can lead to fatter krill (more biomass per unit of iron), but not more krill.

In most of biology, abundance of food would lead to more individuals as well.

You're talking about a fixed amount of iron. It can be in algae or it can be in krill.

But it can only be in one thing at a time. More algae can't lead to more krill if the existence of the larger krill population precludes the existence of the larger algae population.

Suppose I told you that by draining the iron from a bunch of humans we could promote the growth of iron-bearing plants that would feed a bunch of new humans. How much surplus iron could we drain from a living human? How many new humans could sustain themselves on that amount of iron?

But it's not necessarily true that both populations are limited by iron availability. These iron-hungry plants could be producing more, say, zinc than they themselves need, which could allow the human population to expand even with less iron available.

Give it a long time for coevolution to take place, then have something suddenly happen to the mosquito population that was extracting the iron from the people, and we could see the same dynamic.

> These iron-hungry plants could be producing more, say, zinc than they themselves need, which could allow the human population to expand even with less iron available.

There are a couple of ways this could work.

The human population might have access to a source of iron which is not available to the plants. In this case, we can model the system as iron going from a mine somewhere (say) to the plant population, mediated by human remains. The iron isn't being drawn from the human population.

Or, this human subspecies might store a lot more iron than it actually needs to survive. Maybe they grow their own metallic armor or something. In this case, the remains of one human contain enough iron to support several other humans, so returning human remains to the soil in order to grow more plants could cause in increase in the human population, though the new humans would have impaired armor or whatever.

The issue I see with the iron/algae/krill model is that it's explicit that the additional iron for the algae is coming out of the krill. So we have two questions:

(1) Can krill get their iron through some other means than eating it?

(2) Do krill sequester more iron than they need to survive?

I'm assuming that krill obtain their iron through their diet. So unless an adult krill sequesters more iron than it needs, it doesn't really matter how much algae the recycled iron can produce -- less iron in the krill population will necessarily mean a smaller krill population. If the krill population isn't limited by iron, it may be happy to grow to accommodate the new larger supply of algae. But in order to do that, it will need to keep a certain amount of iron. If that iron has to come out of the algae the new krill supposedly eat, then how much larger is the new supply of algae really? Once the krill population gets back up to its original size, all of the "additional" iron is gone.

Putting this another way, if we think that algae are limited by the availability of iron, and we also think that krill are limited by the availability of algae, then we aren't saying that krill are not limited by the availability of iron. They definitely are! But that limitation goes through the algae they depend on.