In the first, yes -- limits.No.2
Nature holds the species within bounds. Can I replace 'bounds' with 'limit'?
One important natural check is a limit on the amount of suitable habitat for each species. Can I replace 'limit' with 'bound'?
In the second, no.
I would need context. "Importation" can mean "imported product" but it usually refers to the act of importing.No.3
I got confused by 'importation' and 'import' when both of them mean 'something that is brought into a country in this way'. In other words, can I replace 'import' in the expression 'cheap imports of grain' with 'importation'?
IMO, "adjust" is often a quick and temprary change.No.4
'adapt' can mean 'to gradually change your behaviour and attitudes so taht your get used to a new situation and can deal with it successfully. 'adjust' can also mean 'to gradually get used to a new situation by making small changes to the way you do things'. Does it mean when used in this meaning they are interchangeable? For example, can I use 'adapting' to replace 'adjusting' in the sentence?
Adjusting to the tropical heat was more difficult than they had expected.
Looking forward to hearing from you.
Thank you in advance.
"Adapt" is a slower and often more long-lasting change.
You could use "adapt" there, but it would have a different meaning for me.