Do I use 'is' or 'are' in this sentence?

You use “is” because the subject is “research”.

Research is the subject. It is singular. So is is correct. Oops, amaanderveen beat me to it. (Good going!) One thing I would add is that I find it helpful to say the sentence out loud both ways.

…is

IS.

I think the strange word here is “declines.” ‘Is’ is definitely correct for the word you’re iffy on, but the reason the sentence sounds funny is “species and their declines.”

I know you’re talking about each species individual decline, so “research of neotropical species and their decline” would be wrong, that has a conflict between the subject (plural species) and the verb (decline). Perhaps a different word altogether?

is because their is a collective pronoun
or something like that

Is.

Many writers are confused by the noun nearest the verb, which here is “species.” But “species” is not the subject. It’s in a prepositional phrase. “Research” is the subject of the verb:

“Research (of neotropical species and their declines) is a prominent topic.”

Neither biology nor the form of the noun “species” nor the meaning of “their” has anything to do with it; those are all distractions. When you lift out the prepositional phrase, it is simple and direct.

Edit: The nearest noun is actually “declines,” sorry. But the point is the same. Both “species” and “declines” are distractors inside a prepositional phrase.

@jeruba, I think the reason declines looks funny in my head is because in this case it is a noun, but often it is a verb.

is this case

of neotropical species and their declines is a prepositional phrase and isn’t part of the essential sentence

essential sentence is just Subject-verb-Predicate nominative or direct object

@Perchik, that is true with very many English words. You’re wise to be alert to it.

Unfortunately, the fact that something looks funny is not usually a reliable guide to grammar, especially when we are surrounded by poor models. In the present case, the word “of” (a preposition) should light up in neon to say something is happening in front of the verb—look out, let’s make sure we aren’t getting misled. The object of a preposition follows a preposition, and grammatically you can always lift out the entire prepositional phrase (making sure you gather up all the objects if there is more than one) to see what’s going on without it. Prepositions are very, very often found in the vicinity of a grammatical error.

Well I was gonna say are but apparently is are the correct answer.
(If you don’t get the joke by now, read the sentence again).

uh, is.

why not change it so it’s easier in general?
“the research into the decline of neotropical species, is a prominent subject.”

@cage, That comma certainly doesn’t belong there, at least.

If you want to sound like a pirate, throw an “Arrrr” at the end.
But otherwise, just use “is”

@Cage, I thought perhaps the questioner was not looking for editing help but trying to understand a homework problem. This was my inference from the first sentence of the question. Sentences written to illustrate or test a grammatical principle may not be the best writing but still serve a purpose.

whoot whoot jumping on the IS train…all aboard

Thank you all… FYI this is a graduate school application essay thingy, the sentence is actually much longer than the one posted :)

@steveprutz:, Just a suggestion, you might not want to use the word thingy on your application essay. ;) Good Luck!

Iz.