Ecologies of Rotating Habitats: Basic Design Considerations and Modeling Possibilities

Hello, and welcome to a bizarre post for Beyond NERVA, which involves no nuclear reactions at all! I’m deep in the bowels of the next blog post on Topaz International Program, which has split into two (for long-following fans of the blog, this isn’t weird), and has taken me down many different research paths that I haven’t expected. As such, I want to devote more time and care to the next posts, but also want to keep bringing you content, so I’m bringing up an older (about 9 month-1 year) old post that went through editing, but never seemed to fit anywhere until now.

This is a field that I hope to be expanding into over the coming year: the ecological possibilities offered by rotating habitats.

This is an extension of my interests, and something that I discuss quite frequently with my ornithologist (and multispecies occupancy modeling) wife. It won’t be as pretty as usual, and nuclear reactions won’t be mentioned once, but I think that it’s an interesting topic that I haven’t seen covered in depth ANYWHERE. The sources are basically nonexistent, thanks to a failure of the system to save most of them and my personal arcane filing system which has epically failed me, but if you’re interested in this topic, PLEASE comment below, and I’ll do my best to both expand this line of research with better referencing in the future.

For those that aren’t familiar, I’m going to defer to my good friend, and frequent collaborator (I contribute to his work on a regular basis as part of his Production Team, the research and script-writing side, and we seem to have a future project we’re working on together in the works… but it’s a long term one, and not for this page), Isaac Arthur. For those that aren’t familiar with his work, the YouTube channel Science and Futurism with Isaac Arthur is an incredible introduction to the vast diversity of futurist possibilities, explorations of the minutiae of the Fermi Paradox, and many other concepts, and a font of novel concepts in futurism, from the settlement and development of the Solar system to stellar engines to (in the short future) the relocation of galaxies. I can’t recommend the channel enough, and I can’t point to any particular thing (other than Isaac’s brilliance) that is really the strength of the channel: the writing, editing, and custom visuals are literally world-class.

Here’s his video on O’Neill Cylinders, the concept that was popularized by Dr Gerard K. O’Neill in the 1970s. A strong recommendation is to read Dr O’Neill’s “The High Frontier,” currently in its third edition, but it’s not required.

Additionally, there’s a video on the environment of rotating habitats, which I consider to be relevant and educational, even if it’s far more futuristic than this post will cover, which will be a survey of what seems to be within the last five years of the field.

An ecosystem, on the other hand, is a system of interactions between biological organisms and their environment, and ecology is the study of these ecosystems. There’s a number of different ways to study an ecosystem, but probably the most useful for us would to be consider it as a structured set of systems that interact with each other. Each system in the ecology has its own subsystems, made up of different components, with some components having different roles to play in several of these subsystems. The more subsystems that a species is important within, the more important that species is for the ecosystem as a whole. These interactions between the systems allow for things like nutrient processing and transfer, energy transfer, population limitation, and habitat management, and many interactions are far from well understood. On top of that, often there are different species that perform these same functions between systems, but in different ways. This makes the ecosystem more robust, because it means that if something happens to one species, another is able to fulfill at least some of the roles that the species in trouble used to, keeping the whole system going.

heir environment, and ecology is the study of these ecosystems. There’s a number of different ways to study an ecosystem, but probably the most useful for us would to be consider it as a structured set of systems that interact with each other. Each system in the ecology has its own subsystems, made up of different components, with some components having different roles to play in several of these subsystems. The more subsystems that a species is important within, the more important that species is for the ecosystem as a whole. These interactions between the systems allow for things like nutrient processing and transfer, energy transfer, population limitation, and habitat management, and many interactions are far from well understood. On top of that, often there are different species that perform these same functions between systems, but in different ways. This makes the ecosystem more robust, because it means that if something happens to one species, another is able to fulfill at least some of the roles that the species in trouble used to, keeping the whole system going.

A good example of this is birds that distribute seeds for plants by eating fruit or berries, and defecating elsewhere: there are many different birds that eat different fruits at different times of the year, but just because a bird will eat a particular fruit doesn’t mean that it will eat ALL fruit – there are even cases where a berry that’s toxic to one bird isn’t toxic to another, maybe because the digestive tract of the first bird would destroy the seed, or for some other reason. If you’ve got the bush, but put in birds that can’t or won’t eat those berries, then that bush won’t spread very far, if at all. If you were counting on this bush for something else in the system, say for its leaves to be food for another animal, then you’re going to have a problem.

If we want to build a rotating habitat with an ecosystem on board, we need to understand what parts of the ecosystem we want: food, for instance, or air processing, or species protection and maintenance for endangered species. After we know that, then we need to figure out what systems are needed to support the end result we want, the systems that support them, and the environment needed to support all of those systems needed for the desired end result. Unfortunately, many of these subsystems are far from obvious, and as we learn more and more about each of these systems, and more about the species in each system, we discover more and more interactions that we never even suspected were happening, much less were as important as we now know.

This leads us to the conclusion that the more complex a system is, the more robust it is, and also means that we can get more benefits out of it as well. On Earth, we see many benefits of natural processes, beyond just having oxygen to breathe and beautiful places to go hiking, boating, fishing, or camping: ocean fishing of wild populations provides the majority of protein a large percentage of the world’s consumption; wild pollinators are a major part of agricultural production ( having wilderness integrated into our agricultural lands has been shown to have significant economic benefits (; ecosystems like mangrove forests or water lily fields have a huge impact on processing liquid wastes of many different types (, and it has been shown to have a huge impact on mental health ( There are lots of other benefits we get from having a robust ecosystem surrounding our living areas, like recreation, timber for construction and artwork, and other more immediate benefits, but in a lot of cases the things we don’t normally think of first are the things that can have the bigger impact on our well-being as a population, and a people.

These are called “ecosystem services,” and they’re going to be perhaps the biggest driving force behind which ecosystems are selected for various habitats, after the general environment of a habitat is chosen. Water purification and conditioning is something that a desert is going to be pretty much useless for, but a mangrove forest or swamp will be very helpful, and would also provide the ability to have aquaculture, shrimp or crayfish farming, and other food resources in the same location. Recreational areas are another type of ecosystem service, as are the scientific advances that studying these artificial ecosystems will provide. These services, the practical considerations of having these services available, and the aesthetic desires of the occupants of the habitat will define which ecosystems, and in what proportions, will be on a particular habitat.

Another concept we’ve mentioned before, but haven’t ever really looked at as anything more than a passing comment, is that rotating habitats can be used as ecological preserves for endangered species of plants and animals, and for some of the smaller species they offer the ability to expand their range hundreds of times over in very little space. This is especially true for many amphibians; many species of newt or salamander live along one particular creek and nowhere else, and even a fairly small rotating habitat can have lots of creeks on it; if you make it small enough, and if you plan some of the other complications well enough, you could even have a small rotating habitat with a salamander being the apex predator of the habitat. Now, this means you’ve got to have some way of controlling the population of the salamanders, but that’s doable, and we’ll look at some of the ways to do that later in this episode. However, it also means more territory without the threat of habitat loss and poaching for many other, more iconic endangered species, like giraffes, cheetahs, polar bears, wolves, and others. We looked a little bit at how to do this in the Exporting Earth episode, when we discussed the concept of a keystone species, or species that changes its’ environment so much, or is so important to many different species, that it in many ways defines an ecosystem.

One nice thing about a rotating habitat, though, is that if we aren’t trying to preserve a certain type of wilderness, we can to a certain degree mix and match parts of different ecosystems, either because of particular requirements of our rotating habitat or for aesthetic reasons. This has happened on Earth throughout our history, for better or for worse. This is, after all, a variation of both the agricultural revolution and the domestication of various animals, which were then introduced to other environments for the benefits of humans. There was a fairly large movement in the 19th and 20th centuries called the “Acclimatization Society” ( movement that took this a step further, wanting to introduce species not only for the practical benefit of humanity but also for their enjoyment – something that often backfired, since many of these species ended up going from introduced, or finding a reasonably non-destructive niche in the new ecosystem, to invasive, which means that the impacts on the ecosystem are destructive and destabilizing, and often can lead to major declines in population of many species, if not outright extinctions. The European starling is a good example from that time period of this sort of introduction in the Western Hemisphere; a more modern example of the same idea would be honeysuckle, an ornamental plant that has become incredibly invasive across much of the US. Islands are another great example of this sort of thing happening. Many Hawaiian bird species were already in dramatic decline by the time Western explorers and settlers came, but for many species that was the death knell. Many later settlers found the lack of birds depressing, so exotic species were brought in from South America, SE Asia, and other parts of the world, to the point that the vast majority of birds seen on the Hawaiian Archipelago are either introduced or invasive.

A rotating habitat, on the other hand, IS a blank slate when you begin, so as long as the needs of the ecosystem are filled, HOW they’re filled is up to the ecologists designing the ecosystem. This means that species that fill different niches in an ecosystem from different places on Earth can exist side by side, and this could be perfectly OK for the overall ecosystem, like having berry-eating birds from South America, and insect eating birds of various sizes from SE Asia, Europe, and North America all side by side. Habitats may even decide to have this as a tourist draw, since bird watching is already one of the fastest growing hobbies and most active citizen science fields in the world. The same is true for mammals, reptiles, plants, and even microorganisms: as long as the interactions between the different species fill the needs of the overall system, and there aren’t any major conflicts between the different parts of the system, then the exact species of the individual parts of the system is less important. This is true of genetic engineering as well: if you want all of your butterflies to have metallic gold and silver wings, and the color doesn’t mean that they’re eaten more (or less), or that the nutrient or metamorphosis requirements are too high, then why not?

That being said, especially early on it’s probably best to try and transplant already-existing ecosystems, because these are systems that are known to work well. Just because a particular species fills a particular niche in one ecosystem doesn’t mean that it won’t change to a new niche if given different opportunities; this is a common problem with introduced species, which are meant to fill a similar niche in a different ecosystem and end up causing major problems for the ecosystem in other ways while ignoring their intended niche. Even doing things this way is no guarantee, though. It’s pretty much guaranteed that some species just won’t be able to adapt for one reason or another, but a substitute from a different ecosystem may be a better option than genetically modifying the original species to be able to survive. Another thing to consider in the process is that if a species decides to do something other than what was planned, but it still works will with the overall ecosystem, then this isn’t necessarily a problem. Another species may need to be brought in to fill a particular role as far as nutrient recycling or other roles, but if there aren’t causing problems for the ecosystem, then why not leave it?

Once an ecosystem is established, invasive species DO become a concern, so import/export controls on things like animals, plants, insects, soil, and that sort of thing would need to be put in place, but this is no different than what happens between most countries here on Earth, and keeping the environment of a rotating habitat isolated is far easier if there’s hundreds of kilometer of empty space between your ecosystems, instead of sharing the same atmosphere, oceans, and often animals. Even migratory birds can, in some instances, carry fungi, seeds, and even mollusks long distances, after all, and this can spread invasive species to new areas. With human interconnectedness growing every year, the artificial means of spreading invasive species have grown as well. Zebra mussels are a major aquatic pest that were probably transported in the bilge water of cargo ships, fruit fly infestations have been caused by air travel, and escaped pet snakes have become a major ecological problem in Florida, just to name a few examples. Because of this, it’s likely that biological containment is going to be a big priority for rotating habitats – one more reason to try and grow as much food domestically as possible. For more on that, check out our Space Farming episode.

That leads us to look at something that we haven’t even mentioned yet, but is more important than most people realize: the plants in an ecosystem. Plants emerged far earlier than animals in our evolutionary history, and far from being dumb, static food for animals to eat, plants have a huge range of different needs, capabilities, and even communication capabilities. Stands of trees release chemical signals, either through the air or the soil, to alert other plants that they’re under attack by an insect or fungus, for instance, and so they need to start producing more of a chemical that’s normally wasted resources, but is critical as an insecticide or fungicide right now. Plants are, in many ways, far more complex and difficult to study than animals, because of their incredible genetic and structural complexity. You can’t slice off someone’s nose and put on an elephant’s trunk, apply a poultice, and expect a good result, but this process is done all the time with trees, and has been done for thousands of years. This is called grafting, and is particularly common with citrus trees. Some new types of fruit are actually parts of different trees grafted together, like apple branches on lemon trees, which change the flavor of the resulting fruit. Likewise, while the human body has a lot of different chemical immune responses, a plant has far more options. They’re chemical synthesis laboratories and factories that are far more flexible than anything but a chemistry lab for a major corporation or university in the range of possible compounds that they can synthesize.

Another thing we’ve just begun to realize in the last few years is how connected everything in the macrobiological world is connected to everything in the microbiological one. We used to see microorganisms as something that really only affected the things that we see in two ways: at some point, they’re food for something, which is eaten by something bigger, and something bigger than that, and so on; and we saw them as ways that these bigger things can get sick. Within the last couple decades, though, we’ve discovered that their impact is far more prevalent, and far more complex, than even the most avid fan of the microscopic world would have guessed even 30 years ago. Not a week goes by that we don’t hear about a new, usually overblown, discovery about the effects of our gut microbes on our brains, or a new treatment for mitochondrial diseases – which, remember, are absolutely critical for most life, but are completely genetically and evolutionarily distinct from their host – or some other surprisingly beneficial impact an unknown, or formerly feared, maybe even actively killed, micro-critter living on or in us imparts to us. The tree’s equivalent of mitochondria is symbiotic fungi in the root system and surrounding soil, which makes the nutrients in the soil possible for the plant to absorb Every species has similar microfloral ecosystems living within them, and often they’re completely different from the ones in humans – with important exceptions, like mitochondria. These symbiotic, beneficial microorganisms in one species can be a major cause of disease in other species in the same ecosystem, and even within a species what’s symbiotic in one part of the body can be a life-threatening infection in another. This is why, no matter how hard we try, disease will always be a part of any ecosystem – and this isn’t necessarily a bad thing! More on that later, though.

These microorganisms can have in terms of inter-species interactions, another newer discovery that has greatly changed how we view these organisms’ role in an ecosystem. We’re all familiar with the classically understood role that bacteria and fungi play in an ecosystem: they break down anything that’s dead into food for the living organisms in the ecosystem. This is an absolutely critical role, and one that’s not exclusive to bacteria, but it’s also a very small part of the role that these organisms play. In the last decade, there’s been a lot of research that’s shown that fungi and bacteria have a far larger role that certain types of microbiota play: that of executive assistants, or even administrators, of resources in an ecology. Just like we’ve discovered that bacteria in your gut can regulate your mood, your energy level, and other things that were traditionally thought of as a function the brain and its’ neurochemistry played, soil fungi and to a lesser extent bacteria also regulate how nutrients are distributed in an ecosystem. This area is well-established at this point, but due to the fact that these organisms don’t generally accept the confines of a petri dish, they’re also not well understood. Their importance, though, has been long-suspected, even if it was pure speculation at that point. Legumes, or beans, take their names from the characteristic nodules on their roots, which aren’t actually biologically part of the plant – they’re symbiotic fungi, which are critical for the plant’s survival, but different species in their own right, they’re even in a different kingdom – fungi. These nodules provide many nutrients to the plant, but also alter the soil chemistry, because nitrogen is a waste product for both the plant and themselves. Because of this, crop rotation since the early Bronze age in certain cultures has included legumes as a necessary fertilizing mechanism. It turns out that this is just the tip of the iceberg, there are species of fungi that manage nutrient availability depending on their proximity to certain plants within every forest, and presumably most other ecosystems, on Earth, between different species depending on their chemical waste products, their mature heights, and the needs of the fungi to survive in the soil. We genuinely don’t understand all of the mechanisms behind these very simple organisms, but just like mitochondria regulate any species that relies on aerobic respiration, these types of fungi (and there’s quite a few of them) regulate botanical ecosystems. If we need a lever for customizing biomass within a rotating habitat, this may be one of the most powerful available to us, but we don’t understand anything but the absolute basics of what it is or how it functions.

A major part of designing an ecosystem is the interaction between the organisms on the habitat and their environment, something that we discussed in the Environment of Rotating Habitats video. This can be anything from “how much rain does an area receive, and when does it receive it” to “what kind of seasons are on the habitat” to “what temperature is the atmosphere or the soil at over the course of a year” to “what day length is being used.” Every place on Earth has seasons, from the winter, spring, summer, fall cycle that’s familiar to most people in temperate zones to the extremes in day length experienced at the poles to wet and dry seasons along the equator. The species that live in these areas have evolved to need these variations in a lot of cases, especially a lot of the plants that ended up being bred to become our food crops. Freezing weather is important for insect and bacterial population control, because even beneficial pollinators and symbiotic micro-organisms can become the cause of major ecological imbalances if they get out of hand, while others become predatory. Many plant species use day length, average temperature, or both as indicators on when to start flowering, and when to start producing or ripening their fruits. Animals use these indicators, as well as plant growth cycles and other environmental factors, for everything from breeding to which types of food they’re eating to how far the young disperse after reaching maturity.

A last variable that we should look at before we start looking at how to actually go about designing an ecosystem for a rotating habitat is migration. Many types of species, from birds to fish to insects to mammals, migrate, and when they do this they perform many different roles not only in the ecosystems that they winter and summer in, but also the ecosystems in between. While there are some species that generally do migrate, but a certain part of the population decides not to due to new resource availability thanks to human activity, like the Canada goose in North America, there are other species that have something called “migratory restlessness,” where even if they don’t have to migrate, or can’t because their migratory route is blocked, or they’re in a zoo, or whatever, they will still do whatever the species equivalent of pacing restlessly, drinking black coffee and puffing on a cigarette irritably until 4 in the morning. Some species even have a higher likelihood of having strokes or heart attacks if they can’t migrate. The triggers and methods of migration vary wildly, too: some species follow food availability as it moves toward the equator for the winter, some use day length, some use temperature, and some just breed, and once the breeding is successful they leave – male hummingbirds are notorious for doing this, and migrate weeks or months before the females do, sometimes to completely different parts of the world, like Caribbean islands compared to Argentina.

Even species that don’t migrate change their behavior and movement patterns a lot between breeding and non-breeding seasons. Species that are intensely territorial during breeding and offspring raising often gather together in fairly sizable packs or flocks once the offspring are old enough to be at least self-sufficient, if not completely independent. Some species are nomadic, following learned patterns that are passed on through family groups, waterways, available patches of different foods, or moving up and down mountains due to altitudinal differences in plant availability and growth cycles. This is actually a major driver for plant propagation in a lot of cases, so it’s something that’s important for certain species, and detrimental to others. All of these things could be very different on a rotating habitat than they are on Earth, but how they’re different will have a big impact on what behaviors these species would show. Whether they will remain nomadic, or whether we want them to be nomadic or not, is going to be a question for the people who are actually designing these ecosystems.

We described an ecosystem earlier as a set of systems that interact with each other. This is a really useful framework, but we also need a way to define how that interaction happens. When we talk about interplanetary colonization, we talk about two things: energy available, and the constituent elements available. This is usually through mining, but things like fusion and fission can provide those as well. This is definitely NOT the way that modern ecologists define things in most cases, but there are a couple of fields in ecology that do. These are the fields of “ecological energetics” and “ecological stoichiometry,” with a healthy leavening of “island biogeography,” “patch dynamics,” and other disciplines.

Some of these concepts have been around for a very long time. Hunter-gatherer cultures worldwide used controlled burns of forests to create meadows and edge habitat, which encourages game animals and also makes it easier to hunt them, for instance, but the scientific study of these conditions didn’t start until the 1960s, when Robert MacArthur and RJ O’Rourke wrote “The Theory of Island Biogeography,” examining the biodiversity of islands in relation to their size and their separation from other bodies of land, either other islands or the mainland. On land, Jared Diamond posed the SLOSS question in the 1970s: is it better for species diversity and populations to have a Single Large Or Several Small patches of habitat? It turns out that the answer is: it depends on the species, the ecosystem, and the shape of the patches, just as it does on islands. These fields have continued to expand and be refined since, in the study of spatial ecology. This can also incorporate understandings of nutrient transfer, energy exchange, biological population sensitivity, and other variables that change the number and types of species that can thrive in a particular area, if the only thing that’s changing is the rearrangement of the same types of species.

One of the biggest problems until recently was that there was no framework for analyzing an ecology in a complete enough way to be useful in actually designing an ecosystem. As ecology developed, food chains became food webs, and flows of energy and nutrients through a system, and interactions between biological organisms and their geological environment, developed, but none of it was well integrated. Land biogeography and patch dynamics were possibly the first big step toward being able to do this, but at least initially, they weren’t enough to form the whole picture. Ecological energetics, biogeochemistry, and elemental cycles like the P, N, and C cycles have all been studied, and to a greater or lesser extent incorporated into various models, but this still wasn’t integrated enough to actually design a full ecosystem, just enough to better manage parts of an already-existing ecosystem.

With the rise of supercomputing, improvements in statistical and dynamic modeling of ecosystems, and the increase in available, detailed ecological data, new methods of ecological analysis that incorporates not only local geography, populations of animals, plants, and microorganisms, energy flows and also nutrient availability and biogeochemical processes have begun to reach maturity. One of these theories is called “ecological stoichiometry,” which integrates most, if not all, of these different systems into an overarching framework. The development of this system has already led to new insights in everything from species abundance to the effects of fertilizer runoff on ecosystems to management of fishing stocks around the world. While much of this work has focused on aquatic ecosystems, so far, the fundamental concepts should be able to be adapted to terrestrial ecosystems, and the interactions between watersheds, lakes and other aquatic ecosystems with their terrestrial neighbors as well. This could very well be the first time that tools are being built that make it possible to analyze, and eventually design, ecosystems on a rotating habitat.

The relationships between different species, their distribution, and the shape and size of necessary components in an ecosystem provides a good framework for the biological, geological, and environmental considerations that define the limits of what ecosystems require, but they also provide a set of engineering limitations for the design of the rotating habitat itself. From literally the ground up, each different type of ecosystem has different requirements as far as soil composition and depth, water content, and mass of the various plants, animals, and micro-organisms. These factors will significantly change the size of the habitat, how much mass is required in each area of the habitat, hydrologic considerations for weather patterns, and many other factors. An early succesional habitat, or scrub land, masses far less than a rain forest for the same given area, for instance, and a tree with a deep tap root will require a greater soil depth than one with a shallow, wider-spreading root system. These systems will also change over time, and the extent to which this change is managed, and how it’s managed, will change the requirements from a management point of view. Looking at the tree example, soil depth is something that will affect the mass distribution of the habitat, lower deck configuration (if there is one, which is likely), and other factors, so deeper-rooting trees may require more maintenance to ensure that they don’t spread beyond the boundaries of their suitable areas. Having roots growing through the roof of lower decks, or getting into water pipes, air ducts, or other engineering structures, is something that is best avoided whenever possible, after all. Another mass consideration for more wilderness preserve type habitats is large animal migrations: a wildebeest, bison, or elephant herd undergoing annual migration will significantly change the mass distribution within a habitat, so some sort of ballast system would be needed to make sure that rotational balance is maintained in the cylinder.

Another consideration when it comes to changes in mass distribution over longer timescales is that ecosystems are not static things. In nature, the distribution of plants and animals in areas that are even marginally suitable habitat changes over time, and effects like wildfires, droughts, and other events are necessary to revitalize soil, create new habitat for edge-dwelling creatures, and other biogeochemical effects. Changes in ecosystem distribution are driven by other factors as well, including various micro-organisms, animals, plant seed distribution methods, and environmental factors. One of the best-known examples of this is beavers altering the local hydrology of an area. Beavers are important in creating habitat for other species of both terrestrial animals and fish, flood and erosion control, plant population and distribution management, nutrient recycling, and several other ecological functions within the ecosystems that they exist in. Because of this, managing beaver population, through either population control or by providing suitable habitat in multiple locations to account for migration from one suitable location to another, could be a good example of using one particular species in order to leverage the ecosystem itself to make the desired changes over time without resorting to brute force methods of mass landscaping of large tracts of land through mechanical means.

While it may be possible to prevent this ecosystem evolution through brute force methods like forest thinning, fertilization, planting, and other techniques, whether this is something that’s desirable or not is going to be very design-specific, both for the habitat and for the ecosystems in that habitat. It may be that it’s simply not worth maintaining the same geographic distribution, and instead ensuring that a certain balance of proportions, and certain shapes and sizes of constituent parts of interrelated biomes, are maintained, but the exact location of these patches changes over time. This is actually studied to a certain extent already, and is known as patch dynamics theory. Certain things are probably going to be less dynamic on a rotating habitat than on Earth, like river locations, ponds, and lakes, but other, less mass-intensive systems could very easily be left to change over time like they do on Earth, constrained only by practical considerations like root depth or water requirements. Which systems are geographically limited, and by how much, is going to be a question that’s best addressed when looking at particular designs for rotating habitat ecosystems.

A final thing to consider is the impact that the biological components of an ecosystem have on the environment as a whole. Forests, wetlands, and other ecosystems increase the amount of rain locally because of evaporation and transpiration; the extreme version of this is the rain forest, which often generates its’ own rainfall because of the large volume of water vapor that’s rising above the trees. Temperature is another example of an environmental effect of the local plant life, with less ground cover causing more energy to be reflected and absorbed by the air; this, combined with the albedo of the ground, is what causes the urban heat island effect, making cities hotter than the immediately surrounding countryside. Wind patterns, local humidity, and many other environmental conditions are also modified by the plants, and to a certain extent animals, in the area. This could be used as a way to modify the environment of the habitat, but is at the same time something that has to be considered during the design of the initial ecosystems, as well.

The ecosystems of rotating habitats are a unique area when it comes to ecological design and management. Not only do the wide variety of sizes and internal structures provide unique geographic limitations on the structure of an ecosystem, but unlike on Earth, these structures truly are islands: their energy supply, elemental and geological composition, nutrient recycling systems, and geographic location are entirely isolated from each other, and inputs into the system have to be intentional and planned in most cases. They are also truly a blank slate from an ecological point of view: with the exception of accidentally introduced species, something that is likely to happen but is also something that strong controls to prevent would be put in place, everything that exists in these ecosystems is something that has to be intentionally placed, in relation to the other components of the ecosystem and the other structures of the habitat, in order to create a stable system. The challenges of constructing such an ecosystem are huge, and the understanding of these systems isn’t understood well enough to be successful today, but ecology is developing to the point that these concepts can be considered from a scientific point of view. The time scales needed to develop this understanding, both for data collection and model development and testing, is going to be long, but so is the time frame until these structures are being built. The challenges on the ecological side, though, are likely far higher than the engineering challenges, so beginning to consider and research these ecological systems now is not premature. Now is the time to begin to lay the groundwork.

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  1. My 2-¢: One of the biggest problems will be stabilization- preventing boom-bust cycles that would threaten to reduce food/oxygen supply below human survival levels. For example, see for one example of how certain growth rates if not regulated lead to chaotic fluctuations in population. I see the following considerations:

    •Relative scale is critical: a certain amount of habitable volume or biomass might stably support a population of tiny multitudinous organisms like algae, worms or insects, but be more problematic for, say, grass and mice; and even more tenuously stable for anything as large as humans and their crop plants. In particular it must always be remembered that plants only show a net uptake of carbon dioxide when they’re gaining mass: a mature forest has a net uptake of close to zero.

    •The largest ecologies will be the easiest to keep stable but also the most expensive to build, In particular the temptation will be to try to keep biomes as densely populated as possible; but this will exacerbate stability problems. Aquarium managers are familiar with this problem: one dead guppy in a 1000-gallon tank is trivial; one dead guppy in a fifty-gallon tank threatens to drop oxygen levels to the lethal level. People instinctively think of the biodensity of a rain forest while forgetting the meters of soil and kilometers of atmosphere and ocean supporting it. Relatively speaking, life on Earth is a thin film of lichen growing on a rock. On Earth, a large wildfire is of inconsequential impact on the atmosphere; in a space habitat it would probably make the atmosphere unbreathable (hence depictions of densely arboreal space habitats is probably unrealistic).

    •In consequence, all but megastructure-scale ecosystems will probably need artificial stabilization to support anything as large and complex as humans and megafauna. The chiefmost concern will be oxygen/carbon dioxide levels. For example, rapid-growing plants like bamboo might be harvested and then pyrolyzed to reduce their cellulose content to charcoal and water vapor, to remove excess carbon from the atmosphere as required.

    •Ecologically, humans are extremely expensive. They’re large, have a slow reproduction cycle, require high-quality food, and correcting population imbalances by die-offs or culling is not considered an acceptable solution. This last consideration drives all others; above all the supply of food, water and air must continue.

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