According to this article gramma terms,
Names what or whom a sentence is about. A subject is always a noun (or
noun phrase), a pronoun, or a word or word group acting as a noun
(such as a gerund or a noun clause). For example, the pronoun I is the
subject of the sentence I handed in my application for the job. The
noun clause Whatever you want to do is the subject of the sentence
Whatever you want to do is fine with me.
Is a verb form ending in -ing that acts as a noun. For example, the
gerund smoking acts as a noun in the sentence Smoking can be
hazardous to your health.
So the sentence becomes "(Providing pools of water for frogs when other water is scarce) is just one of the ways (how) elephants engineer ecosystems."
Note that the "engineer" above is a verb.