If it´s not a guest race resource it´s there to stay
But how much of a problem it is to acquire enough changes all the time, usually those new resources are a challenge in the beginning, but after a chapter or 2 they become a much smaller problem. Until it becomes too easy, then Inno invents something new to make it a challenge again.
Orc are the first newly introduced resource that is not guest race only. However requirements are fairly low until chapter 15, when you suddenly need orders of magnitude more than previously. And it gets worse in chapter 16.
For mana chapters 9 and 10 are the most difficult ones, then you get the Dragon Abbey which makes it easier, after chapter 13 mana becomes a non-issue for a while until I think in chapter 18 they suddenly ramp up the need for mana again.
Seeds can be trouble in the beginning (chapter 11), if you have neglected AWs until then, since most of your production depends on your number of AW levels, but if you have the AW levels necessary, it´s an easy task until you reach chapter 15, when you suddenly need silly amount of sentient goods, which is the main consumer of your seeds by then. And once done with the chapter sentient needs (and consequently seed needs) decrease again.
those are just examples, all the resources introduced before and after go up and down in their demands as well, the interesting part is to figure how you can use that to your advantage