التنوع الأحيائي مهم.

It is a nominal sentence in Arabic.

Its basic structure is:

• التنوع الأحيائي = the subject/topic
• مهم = the predicate/comment

In traditional grammar terms:

• المبتدأ: التنوع الأحيائي
• الخبر: مهم

So the sentence is built like:

• [topic] + [comment about it]

In Arabic, the verb to be is usually not expressed in the present tense.

So Arabic says:

• التنوع الأحيائي مهم
• literally: Biodiversity important

But the meaning is naturally:

• Biodiversity is important

If you wanted was or will be, Arabic would normally use a verb such as كان or another future expression.

Because in Arabic, adjectives usually come after the noun they describe.

So:

• التنوع = diversity
• الأحيائي = biological

Together:

• التنوع الأحيائي = biological diversity / biodiversity

This is the normal Arabic order:

• noun + adjective

not the English-style adjective + noun order.

Because الأحيائي is an adjective describing التنوع, and Arabic adjectives must agree with the noun they describe in definiteness.

Since the noun is definite:

• التنوع = the diversity

the adjective must also be definite:

• الأحيائي = the biological

So Arabic uses:

• التنوع الأحيائي

not:

• التنوع أحيائي

This is a very important rule: when an adjective directly describes a noun, it agrees with it in things like:

• definiteness
• gender
• number
• case

Because مهم is not an adjective attached to التنوع inside the noun phrase. It is the predicate of the whole sentence.

Compare the roles:

• الأحيائي describes التنوع
• مهم tells us something about التنوع الأحيائي

So:

• التنوع الأحيائي = the full subject
• مهم = what is being said about that subject

In Arabic, predicates like this are often indefinite, so مهم appears without الـ.

Because التنوع is a singular masculine noun in Arabic grammar, so the predicate matches it.

Even though English learners may think of diversity as an abstract idea with no gender, Arabic nouns still have grammatical gender.

Here:

• تنوع is grammatically masculine
• so the predicate is مهم and not مهمة

If the subject were grammatically feminine, you would usually expect a feminine predicate.

In التنوع, the ل of الـ is assimilated because the next letter is ت, which is a sun letter.

So although it is written:

• التنوع

it is pronounced roughly:

• at-tanawwuʿ

not:

• al-tanawwuʿ

Also, the final letter ع is a special Arabic consonant with no exact English equivalent. Beginners often just aim for a constricted throat sound at the end.

This is a tricky word for many learners.

A few useful things to notice:

• It begins with الـ
• then comes a clear hamza sound at the start of the word
• it contains ح, which is a deep, breathy h sound
• near the end there is another hamza-related break in the pronunciation

You do not need perfect pronunciation immediately. A good beginner goal is:

• say الـ clearly
• make ح distinct from ordinary ه
• do not smooth everything into one English-style syllable

If you are reading carefully, think of it as a formal adjective meaning biological.

In fully vowelled MSA, it would be:

• التنوّعُ الأحيائيُّ مهمٌّ

These endings show that the words are in the nominative:

• التنوّعُ
• الأحيائيُّ
• مهمٌّ

In normal unvowelled writing, these short endings are usually omitted, so you just see:

• التنوع الأحيائي مهم

And in pause, speakers often do not pronounce the final short vowels anyway.

Yes. Here it functions as an adjective meaning something like biological or biotic.

It is attached to the noun:

• التنوع = diversity
• الأحيائي = biological

So together they form one noun phrase:

• التنوع الأحيائي

That whole phrase then acts as the subject of the sentence.

No. Another very common expression is:

• التنوع الحيوي

Both refer to biodiversity.

So if you see either of these, that is normal:

• التنوع الأحيائي
• التنوع الحيوي

The sentence you were given is still a good example for learning grammar, especially adjective order and agreement.