Learn Italian: È importante che questa specie reagisca al clima. - It is important that this species reacts to the climate.

The phrase È importante che (It is important that) expresses an opinion or necessity, which always triggers the subjunctive mood. Since reagire (to react) is an -ire verb that takes the -isc- pattern, its singular present subjunctive forms end in -isca.

Does reagisca here match io, tu, or lui/lei?

It matches lei (she/it), referring to questa specie (this species). In the present subjunctive, regular -ere and -ire verbs use the exact same -a ending for the io, tu, and lui/lei forms. You have to rely on the context or the subject pronoun to know who or what is reacting!

The word specie ends in -e. How do we know it's singular here?

Specie is an invariable noun, meaning the singular and plural forms are identical (la specie and le specie). You can tell it is singular here because it is paired with the singular demonstrative adjective questa (this).

Why does it say al clima? Shouldn't a word ending in -a be feminine, like alla clima?