2

Is it

These occur naturally in the Balkan Peninsula, Iberian Peninsula, Pannonian basin, and northern sides of the Black Sea.

or

These occur naturally in the Iberian Peninsula, the Balkan Peninsula, the Pannonian basin, and the northern sides of the Black Sea.

There seems to be occurences of both in places I checked.

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1

I agree with @WS2 that leaving out subsequent articles for 2nd and 3rd reads better. But unless you intend to say "... in the northern sides of the Black Sea.", I would reword for the 4th.

I.e. These occur naturally in the Balkan Peninsula, Iberian Peninsula, Pannonian basin, and on the northern sides of the Black Sea.

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  • 1
    You are right, in so far as I should have included an "on the" - now edited. But +1 – WS2 May 29 '17 at 18:27
  • OK tks for the +1 @WS2, I will reciprocate. – MikeRoger May 30 '17 at 9:22
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You can safely leave out all subsequent articles. indeed to shorten it further you could say - "These occur naturally in the Balkan and Iberian peninsulas, Pannonian basin, and on the northern sides of the Black Sea."

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