This data is very inaccurate but it gives roughly an idea of how the spawns changed in my city thanks to the event. I know it's inaccurate because my scanner got nerfed as the event started, so I don't have the full picture of the same area that was scanned: first day area of ~75km2 ; second day area of ~6km2 . This means not only that the sample is smaller, but I also left out some big nests (shown like this).
Besides this, my scanner today is only picking up a fraction of the spawns that showed yesterday.. as I haven't had the time to actually play (I'm in the office, don't judge..) I cannot know if the spawns actually went down, or if it's my tracker that isn't working properly.
Methodology: I scanned a big area yesterday with two hours difference (at 14 and 16 hrs CEST) and counted (not me, the scanner) the amount of each 'mon on my city. I was planning to do the same today, but as the scanner is not working properly I was only able to scan a fraction of the area, so I had to trunk the data of the first day to be able to compare it to today's data. So I took the average of the two scanns I had, divided it by the amount of spawns the scanner got today, and multiplied each average to this factor:
1st scan: 6391 'mons
2nd scan: 6447 'mons
average: 6379 'mons
3rd (today) scan: 831 'mons
After this I was able to have at least an idea of what 'mons got their spawns reduced (or completely disappeared), which ones appeared, and where there's no change.
Geodude: 1 -> 74
Graveler: 0 -> 1
Slugma: 1 -> 57
Magcargo: 0 -> 5
Rhyhorn: 1 -> 39
Omanyte: 1 -> 38
Kabuto: 1 -> 30
Onix: 1 -> 7
Schuckle: 1 -> 7
Sudowoodo: 2 -> 10
Big missing (not one found):
Aerodactyl, Golem, Rhydon, Kabutops, Omastar, Larvitar, Pupitar, Tyranitar
No significant changes:
Got their spawns slightly increased (+90% to +25%): Chinchou, Magikarp, Teddiursa, Poliwag, Psyduck, Abra, Octillery, Staryu, Goldeen (yes, "water" biome)
Got no apparent change (+5% to -10%): Weedle, Dratini, Dunsparce, Ekans, Tentacool, Venonat, Paras, Slowpoke
Got their spawns slightly decreased (-20% to -40%): Slowpoke, Ledyba, Hoothoot, Spinarak, Rattata, Pidgey, Furret, Pidgeotto, Hoppip, Mr. Mime, Nidoran♀, Murkrow, Sentret, Ledian
And finally the biggest reductions (or complete disappearances):
The following 'mons got their spawns reduced between a 50% and a 90%, ordered by the magnitude of the reduction from (from a 48% to a 88%): Oddish, Natu, Horsea, Wooper, Gastly, Nidoran♂, Aipom, Kakuna, Misdreavus, Raticate, Seel, Caterpie, Eevee, Shellder, Magnemite, Spearow, Marill, Zubat, Krabby.
The following 'mons completely disappeared (ordered from biggest to smalles amount pre-event): Swinub, Jynx, Bellsprout, Sneasel, Drowzee, Xatu, Jigglypuff, Voltorb, Skarmory.
Conclusions: I feel the data shows by itself how some common 'mons got their spawns reduced and others completely eliminated to make room for the event-spawns. Would have loved to have the scanner running properly to have a real picture, but I feel I actually got a nice approach.
Some 'mons got partially, other completely replaced by event 'mons.
Link to the spreadsheet - Unfortunately all my excel formulas disappeared when I copied the data to the google spreadsheet, but you can still use the rows "E"+"F" for the pre-spawns, and the "J" for event-spawns.
Bonus: Clearly the event is biome-related, as the desert biomes are getting Larvitars, but we don't :( ... why, Niantic, why....