(4) Because of their countries’

Because of their countries’ great need for expertise in computer programming, the students were sent for studying in the united states of America.
Estimates by scientists suggest that only one per cent of the world’s extinct animals and plants have been indentified.
Any thing wrong in the above sentences?

Because of their countries’ great need for expertise in computer programming, the students were sent to study in the United States of America.
(This assumes the students are from more than one country. If all the students are from one country, then it should read: Because of their country’s great need…)

‘Estimates by scientists suggest that only one per cent of the world’s extinct animals and plants have been indentified.’ is okay.

Estimates by scientists suggest that only one per cent of the world’s extinct animals and plants have been identified.
Is there anything to use in stead of “by” here, which will be more suitable than “by”?

I can’t think of any single word that would be more suitable. Why would you want to change it?

Because some one suggest me that by is not perfect. That’s why. But after your word I am clear about it.

You could add more words, but would still need ‘by’:
Estimates made by scientists…
Estimates researched by scientists…
Estimates found by scientists…