Learn Polish: Ze względu na tropikalne gatunki musimy zamknąć drzwi. - Considering the tropical species, we must close the door.

The phrase ze względu na (due to / considering / with regard to) always takes the accusative case. Here, the phrase tropikalne gatunki (tropical species) is the accusative object.

Why does tropikalne gatunki look like it's in the nominative (subject) case if it's supposed to be accusative?

Because the singular noun gatunek (species) is masculine inanimate. In the plural, the accusative case for masculine inanimate nouns (as well as feminine and neuter nouns) is identical to the nominative case. So it looks the same, but it is grammatically accusative here.

Could I use z powodu instead of ze względu na?

You could, but it changes both the grammar and the nuance. Ze względu na implies consideration or care ("for the sake of"), whereas z powodu just states a cause ("because of"). Grammatically, z powodu requires the genitive case, so the sentence would have to change to: Z powodu tropikalnych gatunków musimy zamknąć drzwi.

Why is the perfective verb zamknąć used instead of the imperfective zamykać?

The perfective zamknąć is used because the speaker is talking about completing a specific action right now: getting the door closed. If they used the imperfective zamykać, the sentence musimy zamykać drzwi would mean "we must (regularly/always) close the door" as a general rule.