A multice-choice question

B is correct.

'Two species... dominated'
'There are two species... that dominated'

Yes, a colon or an m-dash is needed after 'prairies'; a comma would suffice, but would not be as effective.

You certainly cannot omit the 'who' in your sample, Jeff: 'There is an old man who wants to see you' needs its 'who'.

Object relatives can usually be dropped: 'There is an old man (who/m) I wish to see'.
In continuous forms, the relative + 'be' can be omitted: 'There is an old man (who is) sitting on a stump'.

This is just off the top of my head, so if you can find an example of the omission of the subject relative pronoun, please show me.

Thank you, moderators.
Honestly, I do know the two rules,
"1, Object relatives can usually be dropped;
2, In continuous forms, the relative + 'be' can be omitted. "

But I was kinda misled by some grammar-teaching article. You know, I tried to google something about attributive clause before and found one, and bookmarked it. You may think I made up the sentence,
"There is an old man (who) wants to see you."
Actually, I quoted it from that article. And I though this one was special, 'cause it's led by 'there be'.

But it's okey now. I believe you two. Anyway, there's no article that is flawless.

Thank you again.

Jeffu.

I sympathize with Jeff.

When I was learning English--that's decades ago--I read a grammar book delineating the same rule as Jeff brought up. As years passed, I realized that's a rather weird rule. I checked it with native speakers, they said in unison to the effect that I should forget the rule.
Ever since I haven't applied it. I don't know whether it is the rule still in effect in British English or in some dialects as CJ has pointed out. I believe that in American English, it's certainly an outdated rule.

As far as I understand, there is only one exceptional case where nominative relative pronouns (who/which/that) alone may be omitted: when there is an inserted clause within the relative clause. This was already discussed in this forum.

ex) Periodically we mail product and service information (which) [we feel] would interest you.
However, if you prefer not to receive such mail or phone calls, please let us know.