Multiple Averages by Groups

Hi guys, I'm new to the forum so please bear with me. I am familiar with excel but I would by no means cal myself an expert. I am currently working on a spreadsheet with over 3000 data entries, so I need to create a macro as a huge time saver. What I need to do is extract averages from a set of values, but I have groups that vary by size. The data is first broken down into groups of 8 (1a_1_0...) and then by tree species. I need to crank out average BA for each tree species in each group. This table is an example of what I am looking to do

Transect_P Tree_Spec Basal Area Mean BA
1a_1_0 Pr se 44.17860938 47.22202469
1a_1_0 Pr se 50.26544
1a_1_0 Th oc 78.53975 159.4684174
1a_1_0 Th oc 132.7321775
1a_1_0 Th oc 153.93791
1a_1_0 Th oc 201.06176
1a_1_0 Th oc 176.7144375
1a_1_0 Th oc 213.8244694
1a_1_50 Th oc 78.53975 60.08290875
1a_1_50 Th oc 44.17860938
1a_1_50 Th oc 86.59007438
1a_1_50 Th oc 28.27431
1a_1_50 Th oc 95.0330975
1a_1_50 Th oc 86.59007438
1a_1_50 Th oc 33.18304438
1a_1_50 Th oc 28.27431
1a_2_0 Fr am 324.2925127 324.2925127
1a_2_0 Ma sy 993.1458202 993.1458202
1a_2_0 Th oc 214.0837291 328.9373273
1a_2_0 Th oc 506.7070511
1a_2_0 Th oc 45.6036346
1a_2_0 Th oc 153.278883
1a_2_0 Th oc 729.6581536
1a_2_0 Th oc 324.2925127

I am just looking for ideas to make this go much much faster. Thanks in advance guys. I uploaded these values in an attachment as well. Apologies for the bad formatting.

Put this in D2 and drag down.
=SUMPRODUCT(-($A$2:$A$25=$A2)*($B$2:$B$25=$B2),$C$2:$C$25) / SUMPRODUCT(--($A$2:$A$25=$A2)*($B$2:$B$25=$B2))

Just on an aside - have you considered using a Pivot Table for your analysis ?

With Transect_P & Tree_Spec as ROW Fields and Basal Area set as DATA Field (set to AVERAGE)

Pivot Tables will give you a lot of reporting flexibility for little / no effort.