I reckon we get better value for money with the scripting you do.
Yes, farming and animals, but practically only at a later stage of fortress development, and probably only if they are different from those accessible easily on other levels. (If you are a dedicated windy like myself there might also be access to wind unavailable at ground level, which is what drew me to Probe long before I mentioned it to Fleeting Frames.) The reward has to be worth the effort, after all.
And yes I'm sure there are exceptions even here but for me, practically, it will never happen since I tend abandon 90+% of my forts (for reasons) soon after the first caravan. And in the few which make it longer I'm usually pretty determined with pushing on with whatever project/feature drew me to that fort anyways.
Back to the script: Rivers. Okay I got bored part the way through but maybe someone has additional tools/patience that they can bring to bear, or I'll do some more in a couple of days time. I found this which shows the boundary as showbiomes currently displays it:
Unfortunately it does not
accurately capture the boundary - although it does get pretty close. There were only tropical (aquatic) creatures in that river according to the (u)nits list. Here's a screenshot of the embark location (from the last worldgen I posted):
Embarking 3 embark tiles south (i.,e, bottom of that embark zone) also results in tropical but no temperate aquatic life while the river is displayed as entirely temperate. (I'm leaving out the creatures which appear in both lists in all cases.) But going south one region tile and embarking at top of that embark zone gives temperate aquatic life. So my first take was that the boundary is 5 embark tiles too close to the tropics.
However I thought to check with the next river over. So I tried embarking 7 region tiles west and one south at the top of the embark zone with river in the centre tile. All tropical creatures: so things are not so simple. I then embarked 3 embark tiles south, where the river splits in two and got all temperate aqautic life. Then came a bunch of inconclusive tests where I embarked multiple times in between the two latter embarks trying to find the exact line, going a bit east or west while still including the river for variation. Then I tried to replicate the initial tests from this region tile by embarking at the top, then 3 tiles down and all tests were inconclusive generating only creatures which inhabit both biomes, such as carp. (The real frustrating part was being unable to verify, or alternatively contradict, the initial tests in case there is some random element involved.)
So my very provisional tentative understandings (aka needs more testing). Tropical rivers extend slightly more into the temperate zone than showbiomes currently portrays, approximately 1/3 to 1/2 an embark tile. Suggested but needs more data points to rule randomness or contribution of other factors such as temperature etc. However currently it seems as if this boundary is not a straight one but wanders somewhat, for whatever reason. Tentatively again, but no cases of temperate and tropical aquatic life sharing the same (on embark map) river and a decent amount of cases (presumably where overlap of biomes occur) where neither specific biome types are present suggests the inability to get both tropical specific and temperate specific creatures in the same embark contained river - furthermore that mixed rivers will only ever contain creatures common to both temperate and tropical versions. Unfortunately both latter hypothesises are ones that are easy to prove wrong but can only have weight added by (exhaustive...) further testing.
Futurenotes: my testing was with 3x3 embarks but different elements could be tested better with smaller (does 1x1 still need a mod?) embarks - finding the dividing line, or larger embarks (16x16 possible?) for investigating what happens with rivers that contain overlapping biomes.