<I believe the Nartec are self-destructing. They are profoundly inbred. As I believe humans know, insufficient variety in the gene pool can lead to deterioration over time. The Nartec population is dropping. Fertility is dropping. Infant mortality from birth defects is rising. Life spans are shorter.>
"You mean they’re on the brink of extinction?"
<Yes. The high levels of radioactivity have allowed them to undergo accelerated rates of mutation. But now the destructive mutations are beginning to pile up. And they have insufficient sources of new genetic material.>
"Fewer ships sinking. They must have been breeding with limited numbers of surface humans, survivors of sinking ships."
<It would at best be a short-term and dangerous fix. The new breeding stock would have no doubt resulted in a relatively large number of Nartec born without their unique adaptations: gills and webbed feet.>
"So their dreams of conquering Earth are-"
<The desperate act of a race that knows itself to be doomed.>
--Animorphs 36: The Mutation
I think it's kind of funny how, with only a bit of tweaking, this passage could easily be applied to Voldemoron and his merry band of idiots. :D