Grammatically, passive voice (détruit as participle) and détruit just as an adjective are formally indistinguishable in this case. But the meanings are different. If you see it as passive voice, it has the connotation of an ongoing process, whereas as adjective it is more a statement of a fact.

To make you recognize the flavor of the passive voice, you could also say: ...are being destroyed... Here are three tenses in passive voice -- present, imparfait, and plusqueparfait. You can pick up on the progression between them and how the form of the verb être is modified from one to the next. From this is clear, that ...est détruit is perfectly good passive voice present tense.

Un hectare est détruit. -- One hectar is destroyed. (..is being destroyed)
Un hectare était détruit. -- One hectar was destroyed.
Un hectare avait été détruit. -- One hectar  had been destroyed.

To illustrate the difference in meaning between passive voice and a simple adverbial statement, look at these sentences:

La souris est mangée (par le chat). -- The mouse is eaten (by the cat).

This is uniquely passive voice (...par le chat ) and it has the feeling of a process. (What is happening to the mouse? -- It is being eaten.)

La souris est mangée. -- The mouse is eaten.

Here, you can look at it as an adverbial statement (How is the mouse? -- It is eaten.) and it is an expression of the state of the mouse. Not what is being done to the mouse.

Because something is happening chaque seconde , it has to be a process rather than a state, and therefore the passive voice rather than an adjective.