There’s been some interesting points raised here. I’ve typed a long response, so let me break it into several parts:
Biome Soundtrack Design Philosophy
In my opinion, yes. Or rather, there should be a desert theme which plays in deserts, and perhaps a mountains theme, and both can play in desert mountains. As the player becomes more familiar with the soundtrack, this should create a greater tonal variety while still fitting the gameplay environment, if that makes sense.
If you’re in the desert mountains and hear the desert theme, it may invoke memories of time you spent wandering the dunes, and when you then hear the mountains theme, it may invoke memories of time you spent scaling snowy mountain cliffs, both of which provide different and rich context to the current gameplay experience.
I think themes should strike a balance between specific and broadly applicable, while specific locations can be given a sonic identity using a combination of multiple themes and ambient tracks.
Biome Parameters & Exobiomes
I never responded to this, but a biome map something like this is what we should develop. Holdridge has 3 axes, but I for the simplified simulation we’re going for in Thrive, two parameters (temperature and humidity) ought to suffice, modulated by altitude. I may take a shot at defining parameters on a map like this in a way that Hadron and other people can reference.
Also, I’ve thought more about how to represent non-Earth biomes, and my intuition is that not much change is really necessary. I see no reason an alien planet with plant-analogs wouldn’t have analogs to deserts, scrublands, grasslands, and/or forests. With our two parameters, temperature and humidity, what kind of biomes might inhabit the extremes that we don’t see on Earth? An even hotter desert? An even wetter wetland? I think that such environments can be represented as unique subtypes of biomes we already have.
I’d like to direct attention to a source that’s been very inspirational for me when thinking about this topic, and that’s the work done by Nikolai Hersfeldt for his Worldbuilding Pasta blog. It’s quite well-researched, and he’s developed his own climate classification system which can handle biomes beyond what we find on Earth. Allow me to quote some of his work here. I highly recommend giving the whole blog post a read if anyone’s interested, I won’t be able to do it justice.
Based on our previous sources and other work, here’s a range of potential tolerance limits for minimum temperature encountered for more than a brief period in a typical year, for a range of life forms—do note that some of these are very approximate:
18 °C: tolerance limit for shallow marine coral.
10 °C: tolerance limit for some particularly warm-adapted tropical plants.
0 °C: the frost point, when ice formation may damage exposed soft tissues; tolerance limit for tropical plants; minimum growth temperature for even cold-adapted plants.
-15 °C: tolerance limit for frost-tolerant broad-leaved evergreens
-20 °C: minimum growth temperature for extremophile photosynthetic microbes.
-40 °C: tolerance limit for many non-boreal plants.
-60 °C: tolerance limit for needle-leaved evergreens; approximate freezing point of water-H2O2 mix.
-80 °C: minimum temperature with observed metabolic activity; potential atmospheric freezeout in CO2-rich atmospheres.
-97 °C: freezing point of water-ammonia mix.
-180 °C: atmospheric freezeout in N2/O2-rich atmospheres; freezing point of Li-ammonia brines.
There is some literature exploring the absolute limits of temperature at which plants are irreversibly damaged even in moist conditions that we can go off of here:
35 °C: bleaching temperature of even heat-tolerant corals.
40 °C: tolerance limit for aquatic and undergrowth plants.
50 °C: tolerance limit for most plants with exposed leaves.
70 °C: tolerance limit for desert-adapted plants, and complex life generally.
100 °C: water boiling point at 1 atm; tolerance limit for any life on land.
120 °C: maximum observed growth temperature for microbes in high-pressure water.
150 °C: limit of stability for Earth-like biochemistry.
200 °C: maximum observed survivable temperature for microbes with brief exposure.
374 °C: boiling/critical point of water even at high pressure.
Conveniently, complex Earth life simply isn’t able to thrive (get it) at temperatures much higher or lower than we already have on Earth. Without delving into literature, I would wager that’s no coincidence, as Earth’s biosphere is a self-regulating system deeply connected with the planet’s climate. Given that life in Thrive is pretty Earth-like (even with LAWK off), I would argue we simply don’t have much to worry about here.
Still, Hersfeldt has neatly classified some exoclimates. These are exclusively hotter than Earth, because Earth temperatures already get colder than the absolute limits for complex Earth life.
Sufficiently high temperatures may then inhibit growth even among heat-tolerant plants, creating a period of interrupted growth in summer similar to that in winter for cold climates, and so a parallel sequence of increasingly growth-restricted climates:
Supertropical (HT), with only brief growth interruptions, allowing for evergrowth plants, named as the converse to subtropical zones.
Swelter (HD), with longer growth interruptions potentially favoring a more deciduous lifestyle.
Parch (HF), with a brief growing season to short for large vegetation.
Hot Barren (HG), with a growing season too short for almost any vegetation.
These zones can then be subdivided based on additional thermal tolerances, in particular the 70 °C and 100 °C boundaries:
Hot Summer (Hxa) climates have summers within the range of survival for complex life on Earth.
Torrid (Hxb) climates have summers beyond Earth’s complex life tolerances, requiring extreme adaptations to survive.
Boiling (Hxc) climates reach temperatures above boiling, fatal for all exposed life incapable of surviving total desiccation.
Because we’re trying to go for a pretty simple system, I don’t see the need for specificity like “Torrid Parch”, but we can definitely borrow some of this terminology.
My decisions here are somewhat arbitrary, but here’s the exobiomes I’ve devised for Thrive:
Biome type is on the left and Subtype is on the right. (Exo) indicates it’s a biome not found on Earth.
’Supertropical’ environments have brief interruptions to the growing season due to the extreme heat, and this term would indicate that. I would think they othwerwise can still support any tropical biome from Desert to Rainforest, though organisms in any of these environments would require special adaptations.
‘Torrid’ and ‘Boiling’ environments have summer or year-round temperatures beyond the tolerances of complex life. As such, I think these terms can only be fitting for superhot deserts.
Torrid deserts are only survivable to extremophile organisms. Boiling deserts are for our purposes not survivable, and no gameplay can take place there.
‘Parch’ environments can support vegetation, but not large vegetation, so this term works well for superhot Scrubland.
‘Swelter’ environments can support large vegetation, but with interruptions to the growing season in the summer. This term works well for superhot Forests and Dry Forests.
You could simplify this by only using the ‘Supertropical’ subtype to indicate temperatures higher than Earth, though I think using these several terms provides useful context to the player as well as flavor.
I’ve also added a ‘Hot’ and ‘Cold’ Barren subtype here, indicating that there is no life here because of inhospitable temperatures, rather than a lack of primary succession. These two biomes could populate worlds where there is no life, or where life can exist in water or underground somewhere but not on the surface.
I didn’t dive into the suite of Extraseasonal biomes in Hersfeldt’s system and how they may apply to Thrive (think a biome with swings into boiling and freezing temperatures but brief habitability), as I’m not sure how we’re going to handle seasons in Thrive.
Grasslands
Finally, coming back to Earth, there’s been some discussion on the community forums about how grasslands fit into this system. For posterity, I’ll copy my takeaways from that discussion here.
Actual grasslands are shockingly recent. Depending on your source and how they’re defined, grasslands as we know them today didn’t appear until after the Cretaceous, maybe even as recent as 5 million years ago.
Around 5 million years ago during the Late Miocene in the New World and the Pliocene in the Old World, the first true grasslands occurred.
I think for educational purposes it would be nice to make it somewhat difficult for grasses to evolve in Thrive, making it take a while after trees. Once they evolve and the conditions are right, then like on Earth, they should be able to dominate much of the planet.
As for biomes, I was going to suggest Grassland should be a subtype of Scrubland biomes, I just realized I’ve already got Steppe, which is a type of grassland. Prairie and Savanna are the other types. So, I could replace Steppe with Grassland, which can replace Scrublands once grasses evolve, and have Steppe, Savanna and Prairie be subtypes.
Edit: Grasslands can replace forests, scrublands, or even deserts, depending on the right conditions. Grasslands replacing forests was a major reason humans evolved to walk on two legs.
Here’s the layout of all the terrestrial biomes. I’ve added a T4 for post-grass, which I said I wouldn’t do, but the evolution of grasslands is a pretty major milestone.
