The souls reproduce by a female splitting into about a million "children". Since Wanda reacts very strongly by seeing a few dead souls and hates the idea of dead soul children, we must assume all of these 1 000 000 survive. But then:

1. Since these children all survive, and a soul cannot survive for long without a host body, there must be a million host bodies available at the time for reproduction, and souls along to carry on a million implantations, or possibly putting into cryotanks. How is this achieved?

2. Even if only 1/10000 of all the new "children" are females (the other being sterile "workers), this gives about 100 new females. This means that the soul population increases by at least a factor 100 for each generation. But then, it would just take about 30 generations for the souls to outnumber the elementary particles in the known Universe. How can that be?

Do the souls have access to faster than light travel and communication? I don't think so, since it takes many decades or centuries for the souls to travel from one planet to another. When discussing where to send the seeker, Wanda dissuades against sending her to the seaweed planet, which is located in the closest stellar system, and it takes only a decade to go there and back. This fits our closest star, Alpha Centauri, which is 4.3 light years away, assuming that they travel near, but not faster than light. Also, since Wanda talks about going there and back, it seems that they don't have faster than light communication either, because then, it would take less than a decade for the souls on the Earth to receive the information that the seeker was extracted from her host and sent away.

On the other hand, it seems that Wanda has information about the current status of the other planets, in particular the recently colonized dolphin planet, not the status decades or centuries ago, and likewise for souls on other planets. Otherwise, since the colonization of the Earth begun just about a decade ago or so, the souls on most of the other planets can't know that the Earth has been colonized. 

In particular, Wanda, while still on the seaweed planet, heard rumors about souls which had difficulties to control their hosts on the Earth. If the souls have no faster than light communication and travel, this must have happened in the very beginning of the colonization of the Earth, since otherwise, there was not time enough for this information to reach the seaweed planet and then for Wanda to go to the Earth. But it seems unlikely that this should have happened and the information about it being spread in the beginning of the colonization.

So, it doesn't seem to add up...

I have made it sure that to edit, that you must be a regristeded user on wikia to edit on here. Due to security issues that others wikia's are experiencing. So I am sorry about this, but I was doing what is deemed the safer option.

Can we talk about how ridiculous it is that the Souls took over the Earth without major bloodshed?

I realize the whole Invasion of the Bodysnatchers thing is outlandish and this is sci-fi yadda yadda, but all of that aside, I find it really hard to believe not a single world leader with a big red nuclear button (US, Russia, UK, France, Israel, China, India, Pakistan, North Korea, plus NATO nuke sharing states Belgium, Germany, Netherlands, Italy, and Turkey) caught wind of the invasion before the Souls had taken over enough of the military personel manning the silos, subs, and satellite command centers of all of those nations to stop the use of nuclear armageddon to prevent alien armageddon. I mean, it's implied that the Souls either swooped in an took humanity all in one big gulp or did it slowly over time, infecting more and more humans as time went on, right? They MUST have either waged a very bloody war against resistance fighters in the vast, unoccupiable wildernesses like the western United States, Siberia, and Amazon Rainforest, OR killed enough humans (in the hundreds of millions or possibly billions range) quickly enough to shock-and-awe Earth into surrender (a la the Combine from Half-Life).

they have a very strong relationship in the end, but at first they hated each other. Mealanie had a very good reason to hate Wanda through the whole movie and book, but she accepted Wanda and in the end really wanted Wanda to stay, just like Ian.