The Wildlife Guide states about the Bottom Dwellers:

"This is the most difficult of the Pond challenges, so you should complete all previous ones before attempting any of the wildlife in this category!"

But do we really need to complete ALL the previous Tabs/Challenges 1st?
I understand how "Pond Myths", "Rainbow of Fish" & "Frog Breeding" are needed or play a part.
But must "Pong & Garden" , "Fishy Namesakes" & "Pondweed Combos" really be completed beforehand? Why, do they unlock something needed for BD? 

No, you have the right of it.  I think the point is just that you "need to know what you're doing" underwater before you approach the bottom dwellers.