This will CERTAINLY need dowels (some will say use SPS, I don't have any experience with it myself). Anything that big would crush itself if you don't support it.
You absolutely will need support. (A cake drum is the thick board for the bottom .) You'll also need extra support inside that super tall tier. Check out SPS! It's very easy to use, inexpensive and strong!
More info in my siggy.
O.k. ..... let's dissect this thing
The two bottom tiers are stacked - need SPS OR good doweling in the bottom one *&* in the top one..
Then that middle tier - is probably two 4" tiers stacked as one cake.
Under it is (probably) a styro dummy wrapped in ribbons. No need to dowel it, just put that center extra tall cake on it. Then there is another styro dummy between the middle tier and the top one. So not only will you need to put support/dowels/straws in the bottom 1/2 of the tall tier, you will also need to put support/dowels/straws in the upper 1/2 (to support the styro & top tier. Make sure you off-set those two - if the support in the bottom is + then put the support in the upper 1/2 in an X so they are not right over each other.
thanks very much everyone!