I read the following sentence from an article online:
We are a species trapped by a series of brightly lit screens.
Is it grammatically correct? Or should it be "We are the species..."? Are both correct?
The verb are is irrelevant to the choice of article. What matters is what the article is actually referring to - the noun or noun phrase that follows it. The question is, should you describe humanity as "a species trapped by a series of brightly lit screens", or "the species trapped by a series of brightly lit screens". Now, it's probably true that we are the only species that could be described that way, but you would only use the definite article there if you were attempting to define or identify humanity by that characteristic. As it is only describing humanity, you want the indefinite article, a.