- Email: [email protected]
Biodiversity requirements for self-sustaining space colonies
Futures 110 (2019) 24–27
Contents lists available at ScienceDirect
Futures journal homepage: www.elsevier.com/locate/futures
Biodiversity requirements for self-sustaining space colonies Alan R. Johnson
T
Department of Forestry and Environmental Conservation, Clemson University, Clemson, SC 29634-0317 USA
ABS TRA CT
Human colonization of space or other planets is often justified as a bet-hedging strategy, an effort to avert human extinction in the event of a catastrophe on Earth. To fulfill this bet-hedging role, space colonies would need to be self-sustaining in the sense of providing long-term support for human life in the absence of material or technological inputs from Earth. Existing life support systems and experiments with closed ecological systems have demonstrated the difficulty of attaining such self-sufficiency. Natural ecological systems persist in the long-term by incorporating a high level of biodiversity, providing functional redundancy and adaptability in the face of changing conditions. Humans will invariably bring a diverse assemblage of microbial species with them into space, and macroscopic species can serve a variety of resource-provisioning or ecosystemregulating roles. I suggest that a high degree of biodiversity will be required to assemble truly self-sustaining ecological systems capable of providing for long-term persistence of human populations. The spread of Earth's biodiversity carries ethical implications which deserve careful consideration.
1. Introduction What if the end of the world were imminent? Of the many justifications given for human colonization of space or other planets, one recurring theme is the value of off-Earth colonies as a bet-hedging strategy to avert human extinction. The pioneering Russian rocket scientist and theorist Konstantin Tsiolokovsky thought it imperative for mankind to colonize the Solar System: All kinds of danger lie in wait for him on the Earth. We do not mean the difficulties we all daily experience: mankind will soon do away with these. We are talking of disasters that can destroy the whole of mankind or a large part of it. (Tsiolkovsky, [1914]2004, p.38Tsiolkovsky, 2004Tsiolkovsky, [1914]2004, p.38). Astronomer and science communicator Carl Sagan (1997) echoed this theme in his book Pale Blue Dot: A Vision of the Human Future in Space. More recently, in the 2017 BBC television program The Search for a New Earth, theoretical physicist Stephen Hawking declared his belief that humans must populate a new planet within 100 years if the species is to survive. Elon Musk, founder and CEO of SpaceX, similarly believes that humans must become a multi-planetary species if they are to survive, although he does not think that extinction is necessarily so imminent (Musk, 2017). Many catastrophic events can be envisioned, including an asteroid impact, thermonuclear war, a global pandemic disease, environmental degradation and/or climate change resulting from greenhouse gases and other pollutants, or unexpected consequences of advances in biotechnology, nanotechnology or artificial intelligence (Bostrom & Ćirković, 2008). Such catastrophes might lead to the collapse of human civilization, the extinction of humans as a species, or the complete annihilation of life on Earth. For humans to survive in space or on other planets, they must have a life support system which provides the necessities of life (oxygen, water, food, etc.) and which processes waste materials. Provision of these life support services by purely abiotic, physicochemical means is not feasible with current technology, and will probably never be the most desirable option. Humans will rely on a variety of other species, microbial and macroscopic, in order to survive. Indeed, humans themselves have a diverse assemblage of organisms associated with their bodies. It is estimated that humans carry 500 to 1000 bacterial species at any one time, as well as a diverse array of microarthropods, protozoa, fungi, viruses and phages (Gilbert et al., 2018). Human-associated microbiota play an
E-mail address: [email protected]. https://doi.org/10.1016/j.futures.2019.02.017 Received 15 August 2018; Received in revised form 16 November 2018; Accepted 18 February 2019 Available online 22 February 2019 0016-3287/ © 2019 Published by Elsevier Ltd.
Futures 110 (2019) 24–27
A.R. Johnson
important role in both human and ecosystem health (Flandroy et al., 2018). So, the problem of establishing a self-sustaining human colony in space or on another planet is really one of maintaining a functioning ecosystem with humans as one of the components. Solving this problem has both practical and ethical dimensions. 2. Biodiversity and stability The role of biodiversity in determining the functioning and stability of ecological systems is one of the central questions of ecology. Despite intensive research efforts, there is still much we do not understand. The “problem” is, in fact, a cluster of many related problems, since both “biodiversity” and “stability” can connote a variety of different concepts. Biodiversity refers to the variety of life at all levels, from genetic, biochemical, physiological, morphological, taxonomic and behavioral diversity of individuals to multispecies communities and ecosystem processes. Species diversity is often the primary focus in conservation biology, but clearly biodiversity encompasses multiple other dimensions. Similarly, stability may indicate constancy of state or function, resistance to change, resilience or rate of recovery from perturbations, or persistence of species or ecosystem function over time. So there are multiple diversity-stability hypotheses that can be tested. Theoretical and empirical investigations have provided sometimes conflicting results, but the general consensus seems to be that diversity does, on average, contribute to stability in ecological systems, although much depends upon the type of diversity and stability under consideration and the details of interactions between species or system components (Ives & Carpenter, 2007; McCann, 2000; Pimm, 1991). 3. Closed ecological systems and life support Natural ecological systems on Earth are open to the exchange of organisms and abiotic materials across their boundaries up to the global scale. Research has been conducted on closed ecological systems (CES), which prevent movement of organisms in or out, and contain atmospheric gases and other materials within a bounded volume (Escobar & Nabity, 2017; Nelson, Pechurkin, Allen, Somova, & Gitelson, 2009). Once enclosed, ecological systems diverge from their initial state and from the trajectory that a comparable materially-open system would display. Maguire (1980) documented post-closure failure, as indicated by the loss of macroscopic organisms, in small freshwater ecosystems, generally finding longer persistence in larger systems. Clair Folsome and colleagues managed to create small enclosed ecosystems with marine species which have persisted for years (Shaffer, 1991), and which serve as inspiration for commercially-available “EcoSpheres” (www.eco-sphere.com). On a much larger scale, the Biosphere 2 research project in Arizona enclosed over 4000 macroscopic species (plus unquantified microbial diversity) with eight humans for a period of two years (Allen, Nelson, & Alling, 2003). Some species loss occurred, as was anticipated, and the dominance of successful species shifted in sometimes unexpected ways. Most problematic from a life-support perspective, biogeochemical cycles in the enclosed environment were disrupted, leading to critically low oxygen and high carbon dioxide levels in the atmosphere, such that material-closure had to be breached (Nelson et al., 2009). The take-home message seems to be that maintaining a persistent, functioning ecological system in an enclosure, particularly one that includes humans, is not an easy task. Numerous studies have investigated the maintenance requirements and reliability of Environmental Control and Life Support Systems (ECLSS), such as that aboard the International Space Station (ISS), or proposed systems for a lunar or Mars missions. Russell and Klaus (2007) found that the actual time devoted to ECLSS maintenance on Skylab and ISS was substantially greater than projected by system designers. Detrell, Griful I Ponsati et al. (2016), Detrell, Messerschmid et al. (2016) estimated the reliability as a function of time for critical components and whole life support systems. The systems under consideration, based on current technology, have high reliability (e.g., > 98%) only for a period of 3 years or less, and achieving this reliability requires substantial redundancy in the form of spare components or parts (e.g., 16 spare electrochemical depolarized concentrators for CO2 removal from the atmosphere). As the length of time increases, the probability of system failure increases. For a space colony to be self-sustaining, it would need life support that substantially transcends the capabilities of current technology. A self-sustaining colony could not long depend upon its initial stock of replacement parts, and would need to have the capacity to manufacture new electrical and mechanical components. The problem of redundancy in the biological components of the ECLSS is more difficult. If a species that performs a critical function is lost (i.e., becomes extinct), a replacement cannot simply be manufactured. Perhaps another species will fill its ecological niche, or the lost function may be recovered by artificial selection or genetic engineering, but that assumes that sufficient species and genetic diversity is present to begin with. Holubnyak and Rygalov (2007) consider the role of physical buffers, such as oceans and soils, on the stabilization of biogeochemical cycles in the Earth’s biosphere, and develop a theoretical model to investigate the role of buffers in Closed Ecological Life Support Systems (CELSS). Their conclusion is that increasing the number or volume of buffers enhances stability in the sense of maintaining environmental variables within acceptable limits. If the buffering capacity is reduced, the system naturally displays greater variability, and maintaining environmental variables within specified limits can only be accomplished with increasingly accurate control algorithms and precise monitoring. This, typically, would require more human time and effort. While physical buffers enhance stability in life support systems, biotic diversity is also plays a crucial role. Morgan Irons recently obtained a patent for a design of a self-sustaining and resilient human habitation that could be deployed on the moon or Mars (Irons, 2018). The company Deep Space Ecology LLC is devoted to developing this technology (www.deepspaceecology.com). The system is envisioned as having three zones: a habitation area, an agricultural zone, and an ecological buffer. Expedited primary succession is employed to establish functioning biogeochemical cycles in the agricultural and ecological buffer zones. The resilience of the system is provided by a high biodiversity, providing competitive redundancy and ecological service reservoirs. As described in the patent, “[t]he ultimate problem with habitation systems that do not provide a full ecosystem component for the humans is that they do not 25
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provide for adaptive resilience…” The patent does not specify exactly the degree of biodiversity thought to be necessary for this resilience, but it is evidently quite high. The patent does specify the need for a wide range of taxa fulfilling a wide range of ecological functions, including food plants and non-food plants, herbivores, predators, pollinators, decomposers, and nitrogen-fixing bacteria. 4. Biodiversity requirements for long-term, self-sustaining life support Based on what is known from current life support systems, and experiments in CES ranging from small microcosms to large human habitations like Biosphere 2, we can try to infer the biodiversity requirements for a successful bet-hedging strategy in the event of a catastrophe on Earth. First, consider the temporal scale of the problem. If a catastrophe eliminated all life on Earth, the space colony would be the only repository of biodiversity remaining in the solar system (barring discovery of indigenous extraterrestrial life, the utility of which in a human life-supporting ecosystem is entirely speculative). Thus, we would need a self-sustaining ecosystem that could persist (with adaptation) for the remaining duration of the human species. Prothero (2014) estimates the average species longevity for large mammals as 3.21 million years, and Homo sapiens is thought to have arisen approximately 300,000 years ago. Thus, we can assume that humans could well endure for hundreds of thousands or millions of years into the future (although alternatives, such as replacement by genetically-engineered or artificial descendants much sooner, are also possible). Alternatively, a catastrophe which eliminates humans or causes civilization to collapse, but which does not destroy all Earth-based life, offers the possibility that a space colony could re-visit Earth to supplement its biodiversity. Still, the technological challenge of, say, launching a mission from Mars to Earth with only Mars-based resources and without an Earth-based mission control could be considerable. And, depending upon the nature of Earth-based catastrophe (e.g., large-scale thermonuclear war), the environment of Earth could be quite hostile to humans for a long time. So, even in this scenario, it seems we would need a self-sustaining ecosystem that could function reliably for hundreds or thousands of years. And, although some life (particularly microbial) may have persisted on Earth, the inventory of macroscopic species may be severely depleted. Next, consider the spatial scale required. A single colony, whether on Mars or in space, would be vulnerable itself to a catastrophic failure (e.g., a large impact, loss of containment, human-caused accident or sabotage, etc.). For persistence over the longer time scales of centuries to millennia or more, there would need to be many colonies. Indeed, I think it is arguable that for truly long-term persistence, a self-sustaining ecosystem occupying a significant portion of a planetary surface (such as Mars) would be required, whether that ecosystem is contained in enclosures or unenclosed in a terraformed environment. What level of biodiversity would be required to create an ecosystem that can reliably sustain human life for centuries or more with no inputs from Earth, and if necessary, provide the resources for terraformation of Mars or other bodies in the solar system? Ecological science is not capable of giving a precise answer to that question. Current technology has not demonstrated an ability for truly self-sustaining life support for more than a year or so. Reliability must be improved and time-to-failure increased by at least 2 or 3 orders of magnitude. The causes for life-support failure in the long term could be many, including evolution of new pathogens, extinction of species due to the cumulative stresses of extraterrestrial environments, or perturbations such as climate fluctuations or human errors in maintenance. Also, humans could require different nutrients or biological resources over time as they adapt, biologically and culturally, to the extraterrestrial environment. For all of these eventualities, resilience can only be assured if there is a large reservoir of biodiversity to start with. Many of the species in the initial species pool will undoubtedly undergo extinction, and species which may have initially seemed unimportant may prove to be valuable future assets, just as we continually discover new uses for existing biodiversity on Earth. I would venture that the 4000 macroscopic species included in Biosphere 2 should be regarded as a lower bound for the diversity required for long-term self-sufficiency, and that the composition of the ecosystem should include a broad range of taxa (vertebrates, arthropods, molluscs, worms, plants, fungi, etc.) as well as variety in ecological functions. Long-term survival of humans requires long-term persistence of a largely self-regulating and adaptable ecosystem, which requires a high level of biodiversity. 5. Ethical considerations For those concerned with the imminent loss of much of Earth’s biodiversity, the possibility of establishing species in self-sustaining space colonies may seem a welcome option. Indeed, the bet-hedging argument can be applied not only to Homo sapiens, but to other species as well. Why not take species that face extinction on Earth and give them a chance to survive elsewhere in the solar system? Critics might argue that it is unwise to divert resources and effort from species preservation on Earth to pursue the speculative venture of propagating them in space. Wouldn’t it be better to restore and protect the functioning ecosystems on this planet before we try creating new ecosystems elsewhere? And what value should we attach to a species preserved in an artificially-created habitat, whether an enclosed life support system or a terraformed planet? It can be argued that part of the value of wild nature derives precisely from the fact that it is independent of us, in the sense of not being a human creation or artifact (Lee, 1999). However, in the long run, one might argue, even human-created ecosystems in space will display their wild nature as the species evolve, adapting in unforeseeable ways to their new environment. Finally, we need to take seriously the fact that if humans and their accompanying biodiversity do establish a presence on other planets, life – at least microbial species – will almost inevitably escape containment and colonize habitable environments (Fairén, Parro, Schulze-Makuch, & Whyte, 2017). If the extraterrestrial environment lacks indigenous life, this would still constitute the first instance of establishing life on a lifeless world. If the environment does contain indigenous life, the act of introducing terrestrial organisms carries even greater ethical implications. Current planetary protection protocols are designed to prevent “harmful 26
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contamination.” We must decide whether or not the contamination which is likely to occur is harmful, and, if it is, whether there are sufficient benefits to justify the harm. References Allen, J. P., Nelson, M., & Alling, A. (2003). The legacy of Biosphere 2 for the study of biospherics and closed ecological systems. Advances in Space Research, 31, 1629–1639. Bostrom, N., & Ćirković, M. M. (2008). Global catastrophic risks. Oxford: Oxford University Press. Detrell, G., Griful I Ponsati, E., & Messerschmid, E. (2016). Reliability versus mass optimization of CO2 extraction technologies for long duration missions. Advances in Space Research, 57, 2337–2346. Detrell, G., Messerschmid, E., & Griful i Ponsati, E. (2016). ECLSS reliability analysis tool for long duration spaceflight. ICES-2016-294. July 46th International Conference on Environmental Systems. Escobar, C. M., & Nabity, J. M. (2017). Past, present, and future of closed human life support ecosystems – A review. ICES-2017-311. July 47th International Conference on Environmental Systems. Fairén, A. G., Parro, V., Schulze-Makuch, D., & Whyte, L. (2017). Searching for life on Mars before it is too late. Astrobiology, 17, 962–970. Flandroy, L., Poutahidis, T., Berg, G., Clarke, G., Dao, M.-C., Decaesteker, E., et al. (2018). The impact of human activities and lifestyles on the interlinked microbiota and health of humans and of ecosystems. The Science of the Total Environment, 627, 1018–1038. Gilbert, J. A., Blaser, M. J., Caporaso, J. G., Jansson, J. K., Lynch, S. V., & Knight, R. (2018). Current understanding of the human microbiome. Nature Medicine, 24, 392–400. Holubnyak, Y. I., & Rygalov, V. Y. (2007). Human population growth and excess consumption: Understanding limits of stable functioning and algorithms of control utilizing CELSS technologies and developments (integrated applied modeling). IEEE: 3rd International Conference on Recent Advances in Space Technologies. Irons, M. A. (2018). Ecological system model for a self-sustaining and resilient human habitation on the mood and Mars and for food security and climate change mitigation anywhere on Earth. U.S. Patent 9,970,208 B2, filed July 11, 2017, issued May 15, 2018. Ives, A. R., & Carpenter, S. R. (2007). Stability and diversity of ecosystems. Science, 317, 58–62. Lee, K. (1999). The natural and the artefactual: The implications of deep science and deep technology for environmental philosophy. Lanham: Lexington Books. Maguire, B., Jr. (1980). Some patterns in post-closure ecosystem dynamics (failure). In J. P. GiesyJr. (Ed.). Microcosms in ecological research (pp. 319–332). Springfield, VA: DOE Symposium Series 52, CONF-781101. Technical Information Center, U.S. Department of Energy. McCann, K. S. (2000). The diversity-stability debate. Nature, 405, 228–233. Musk, E. (2017). Making humans a multi-planetary species. New Space, 5, 46–61. Nelson, M., Pechurkin, N. S., Allen, J. P., Somova, L. A., Gitelson, J. I., et al. (2009). Closed ecological systems, space life support and biospherics. In L. K. Wang (Ed.). Handbook of environmental engineering, volume 10: Environmental biotechnology (pp. 517–565). New York: Humana Press. https://doi.org/10.1007/978-1-60327140-0_11. Pimm, S. L. (1991). The balance of nature? Ecological issues in the conservation of species and communities. Chicago: University of Chicago Press. Prothero, D. R. (2014). Species longevity in North American fossil mammals. Integrative Zoology, 9, 383–393. Russell, J. F., & Klaus, D. M. (2007). Maintenance, reliability and policies for orbital space station life support systems. Reliability Engineering & System Safety, 92, 808–820. Sagan, C. (1997). Pale blue dot: A vision of the human future in space. New York: Ballantine Books. Shaffer, J. (1991). Stability in closed ecological systems: An examination of material and energetic parameters. (Doctoral dissertation). Manoa: University of Hawaii. Tsiolkovsky, K. (2004). The aims of astronautics. [1914]Barcelona: Athena University Press.
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Contents lists available at ScienceDirect
Futures journal homepage: www.elsevier.com/locate/futures
Biodiversity requirements for self-sustaining space colonies Alan R. Johnson
T
Department of Forestry and Environmental Conservation, Clemson University, Clemson, SC 29634-0317 USA
ABS TRA CT
Human colonization of space or other planets is often justified as a bet-hedging strategy, an effort to avert human extinction in the event of a catastrophe on Earth. To fulfill this bet-hedging role, space colonies would need to be self-sustaining in the sense of providing long-term support for human life in the absence of material or technological inputs from Earth. Existing life support systems and experiments with closed ecological systems have demonstrated the difficulty of attaining such self-sufficiency. Natural ecological systems persist in the long-term by incorporating a high level of biodiversity, providing functional redundancy and adaptability in the face of changing conditions. Humans will invariably bring a diverse assemblage of microbial species with them into space, and macroscopic species can serve a variety of resource-provisioning or ecosystemregulating roles. I suggest that a high degree of biodiversity will be required to assemble truly self-sustaining ecological systems capable of providing for long-term persistence of human populations. The spread of Earth's biodiversity carries ethical implications which deserve careful consideration.
1. Introduction What if the end of the world were imminent? Of the many justifications given for human colonization of space or other planets, one recurring theme is the value of off-Earth colonies as a bet-hedging strategy to avert human extinction. The pioneering Russian rocket scientist and theorist Konstantin Tsiolokovsky thought it imperative for mankind to colonize the Solar System: All kinds of danger lie in wait for him on the Earth. We do not mean the difficulties we all daily experience: mankind will soon do away with these. We are talking of disasters that can destroy the whole of mankind or a large part of it. (Tsiolkovsky, [1914]2004, p.38Tsiolkovsky, 2004Tsiolkovsky, [1914]2004, p.38). Astronomer and science communicator Carl Sagan (1997) echoed this theme in his book Pale Blue Dot: A Vision of the Human Future in Space. More recently, in the 2017 BBC television program The Search for a New Earth, theoretical physicist Stephen Hawking declared his belief that humans must populate a new planet within 100 years if the species is to survive. Elon Musk, founder and CEO of SpaceX, similarly believes that humans must become a multi-planetary species if they are to survive, although he does not think that extinction is necessarily so imminent (Musk, 2017). Many catastrophic events can be envisioned, including an asteroid impact, thermonuclear war, a global pandemic disease, environmental degradation and/or climate change resulting from greenhouse gases and other pollutants, or unexpected consequences of advances in biotechnology, nanotechnology or artificial intelligence (Bostrom & Ćirković, 2008). Such catastrophes might lead to the collapse of human civilization, the extinction of humans as a species, or the complete annihilation of life on Earth. For humans to survive in space or on other planets, they must have a life support system which provides the necessities of life (oxygen, water, food, etc.) and which processes waste materials. Provision of these life support services by purely abiotic, physicochemical means is not feasible with current technology, and will probably never be the most desirable option. Humans will rely on a variety of other species, microbial and macroscopic, in order to survive. Indeed, humans themselves have a diverse assemblage of organisms associated with their bodies. It is estimated that humans carry 500 to 1000 bacterial species at any one time, as well as a diverse array of microarthropods, protozoa, fungi, viruses and phages (Gilbert et al., 2018). Human-associated microbiota play an
E-mail address: [email protected]. https://doi.org/10.1016/j.futures.2019.02.017 Received 15 August 2018; Received in revised form 16 November 2018; Accepted 18 February 2019 Available online 22 February 2019 0016-3287/ © 2019 Published by Elsevier Ltd.
Futures 110 (2019) 24–27
A.R. Johnson
important role in both human and ecosystem health (Flandroy et al., 2018). So, the problem of establishing a self-sustaining human colony in space or on another planet is really one of maintaining a functioning ecosystem with humans as one of the components. Solving this problem has both practical and ethical dimensions. 2. Biodiversity and stability The role of biodiversity in determining the functioning and stability of ecological systems is one of the central questions of ecology. Despite intensive research efforts, there is still much we do not understand. The “problem” is, in fact, a cluster of many related problems, since both “biodiversity” and “stability” can connote a variety of different concepts. Biodiversity refers to the variety of life at all levels, from genetic, biochemical, physiological, morphological, taxonomic and behavioral diversity of individuals to multispecies communities and ecosystem processes. Species diversity is often the primary focus in conservation biology, but clearly biodiversity encompasses multiple other dimensions. Similarly, stability may indicate constancy of state or function, resistance to change, resilience or rate of recovery from perturbations, or persistence of species or ecosystem function over time. So there are multiple diversity-stability hypotheses that can be tested. Theoretical and empirical investigations have provided sometimes conflicting results, but the general consensus seems to be that diversity does, on average, contribute to stability in ecological systems, although much depends upon the type of diversity and stability under consideration and the details of interactions between species or system components (Ives & Carpenter, 2007; McCann, 2000; Pimm, 1991). 3. Closed ecological systems and life support Natural ecological systems on Earth are open to the exchange of organisms and abiotic materials across their boundaries up to the global scale. Research has been conducted on closed ecological systems (CES), which prevent movement of organisms in or out, and contain atmospheric gases and other materials within a bounded volume (Escobar & Nabity, 2017; Nelson, Pechurkin, Allen, Somova, & Gitelson, 2009). Once enclosed, ecological systems diverge from their initial state and from the trajectory that a comparable materially-open system would display. Maguire (1980) documented post-closure failure, as indicated by the loss of macroscopic organisms, in small freshwater ecosystems, generally finding longer persistence in larger systems. Clair Folsome and colleagues managed to create small enclosed ecosystems with marine species which have persisted for years (Shaffer, 1991), and which serve as inspiration for commercially-available “EcoSpheres” (www.eco-sphere.com). On a much larger scale, the Biosphere 2 research project in Arizona enclosed over 4000 macroscopic species (plus unquantified microbial diversity) with eight humans for a period of two years (Allen, Nelson, & Alling, 2003). Some species loss occurred, as was anticipated, and the dominance of successful species shifted in sometimes unexpected ways. Most problematic from a life-support perspective, biogeochemical cycles in the enclosed environment were disrupted, leading to critically low oxygen and high carbon dioxide levels in the atmosphere, such that material-closure had to be breached (Nelson et al., 2009). The take-home message seems to be that maintaining a persistent, functioning ecological system in an enclosure, particularly one that includes humans, is not an easy task. Numerous studies have investigated the maintenance requirements and reliability of Environmental Control and Life Support Systems (ECLSS), such as that aboard the International Space Station (ISS), or proposed systems for a lunar or Mars missions. Russell and Klaus (2007) found that the actual time devoted to ECLSS maintenance on Skylab and ISS was substantially greater than projected by system designers. Detrell, Griful I Ponsati et al. (2016), Detrell, Messerschmid et al. (2016) estimated the reliability as a function of time for critical components and whole life support systems. The systems under consideration, based on current technology, have high reliability (e.g., > 98%) only for a period of 3 years or less, and achieving this reliability requires substantial redundancy in the form of spare components or parts (e.g., 16 spare electrochemical depolarized concentrators for CO2 removal from the atmosphere). As the length of time increases, the probability of system failure increases. For a space colony to be self-sustaining, it would need life support that substantially transcends the capabilities of current technology. A self-sustaining colony could not long depend upon its initial stock of replacement parts, and would need to have the capacity to manufacture new electrical and mechanical components. The problem of redundancy in the biological components of the ECLSS is more difficult. If a species that performs a critical function is lost (i.e., becomes extinct), a replacement cannot simply be manufactured. Perhaps another species will fill its ecological niche, or the lost function may be recovered by artificial selection or genetic engineering, but that assumes that sufficient species and genetic diversity is present to begin with. Holubnyak and Rygalov (2007) consider the role of physical buffers, such as oceans and soils, on the stabilization of biogeochemical cycles in the Earth’s biosphere, and develop a theoretical model to investigate the role of buffers in Closed Ecological Life Support Systems (CELSS). Their conclusion is that increasing the number or volume of buffers enhances stability in the sense of maintaining environmental variables within acceptable limits. If the buffering capacity is reduced, the system naturally displays greater variability, and maintaining environmental variables within specified limits can only be accomplished with increasingly accurate control algorithms and precise monitoring. This, typically, would require more human time and effort. While physical buffers enhance stability in life support systems, biotic diversity is also plays a crucial role. Morgan Irons recently obtained a patent for a design of a self-sustaining and resilient human habitation that could be deployed on the moon or Mars (Irons, 2018). The company Deep Space Ecology LLC is devoted to developing this technology (www.deepspaceecology.com). The system is envisioned as having three zones: a habitation area, an agricultural zone, and an ecological buffer. Expedited primary succession is employed to establish functioning biogeochemical cycles in the agricultural and ecological buffer zones. The resilience of the system is provided by a high biodiversity, providing competitive redundancy and ecological service reservoirs. As described in the patent, “[t]he ultimate problem with habitation systems that do not provide a full ecosystem component for the humans is that they do not 25
Futures 110 (2019) 24–27
A.R. Johnson
provide for adaptive resilience…” The patent does not specify exactly the degree of biodiversity thought to be necessary for this resilience, but it is evidently quite high. The patent does specify the need for a wide range of taxa fulfilling a wide range of ecological functions, including food plants and non-food plants, herbivores, predators, pollinators, decomposers, and nitrogen-fixing bacteria. 4. Biodiversity requirements for long-term, self-sustaining life support Based on what is known from current life support systems, and experiments in CES ranging from small microcosms to large human habitations like Biosphere 2, we can try to infer the biodiversity requirements for a successful bet-hedging strategy in the event of a catastrophe on Earth. First, consider the temporal scale of the problem. If a catastrophe eliminated all life on Earth, the space colony would be the only repository of biodiversity remaining in the solar system (barring discovery of indigenous extraterrestrial life, the utility of which in a human life-supporting ecosystem is entirely speculative). Thus, we would need a self-sustaining ecosystem that could persist (with adaptation) for the remaining duration of the human species. Prothero (2014) estimates the average species longevity for large mammals as 3.21 million years, and Homo sapiens is thought to have arisen approximately 300,000 years ago. Thus, we can assume that humans could well endure for hundreds of thousands or millions of years into the future (although alternatives, such as replacement by genetically-engineered or artificial descendants much sooner, are also possible). Alternatively, a catastrophe which eliminates humans or causes civilization to collapse, but which does not destroy all Earth-based life, offers the possibility that a space colony could re-visit Earth to supplement its biodiversity. Still, the technological challenge of, say, launching a mission from Mars to Earth with only Mars-based resources and without an Earth-based mission control could be considerable. And, depending upon the nature of Earth-based catastrophe (e.g., large-scale thermonuclear war), the environment of Earth could be quite hostile to humans for a long time. So, even in this scenario, it seems we would need a self-sustaining ecosystem that could function reliably for hundreds or thousands of years. And, although some life (particularly microbial) may have persisted on Earth, the inventory of macroscopic species may be severely depleted. Next, consider the spatial scale required. A single colony, whether on Mars or in space, would be vulnerable itself to a catastrophic failure (e.g., a large impact, loss of containment, human-caused accident or sabotage, etc.). For persistence over the longer time scales of centuries to millennia or more, there would need to be many colonies. Indeed, I think it is arguable that for truly long-term persistence, a self-sustaining ecosystem occupying a significant portion of a planetary surface (such as Mars) would be required, whether that ecosystem is contained in enclosures or unenclosed in a terraformed environment. What level of biodiversity would be required to create an ecosystem that can reliably sustain human life for centuries or more with no inputs from Earth, and if necessary, provide the resources for terraformation of Mars or other bodies in the solar system? Ecological science is not capable of giving a precise answer to that question. Current technology has not demonstrated an ability for truly self-sustaining life support for more than a year or so. Reliability must be improved and time-to-failure increased by at least 2 or 3 orders of magnitude. The causes for life-support failure in the long term could be many, including evolution of new pathogens, extinction of species due to the cumulative stresses of extraterrestrial environments, or perturbations such as climate fluctuations or human errors in maintenance. Also, humans could require different nutrients or biological resources over time as they adapt, biologically and culturally, to the extraterrestrial environment. For all of these eventualities, resilience can only be assured if there is a large reservoir of biodiversity to start with. Many of the species in the initial species pool will undoubtedly undergo extinction, and species which may have initially seemed unimportant may prove to be valuable future assets, just as we continually discover new uses for existing biodiversity on Earth. I would venture that the 4000 macroscopic species included in Biosphere 2 should be regarded as a lower bound for the diversity required for long-term self-sufficiency, and that the composition of the ecosystem should include a broad range of taxa (vertebrates, arthropods, molluscs, worms, plants, fungi, etc.) as well as variety in ecological functions. Long-term survival of humans requires long-term persistence of a largely self-regulating and adaptable ecosystem, which requires a high level of biodiversity. 5. Ethical considerations For those concerned with the imminent loss of much of Earth’s biodiversity, the possibility of establishing species in self-sustaining space colonies may seem a welcome option. Indeed, the bet-hedging argument can be applied not only to Homo sapiens, but to other species as well. Why not take species that face extinction on Earth and give them a chance to survive elsewhere in the solar system? Critics might argue that it is unwise to divert resources and effort from species preservation on Earth to pursue the speculative venture of propagating them in space. Wouldn’t it be better to restore and protect the functioning ecosystems on this planet before we try creating new ecosystems elsewhere? And what value should we attach to a species preserved in an artificially-created habitat, whether an enclosed life support system or a terraformed planet? It can be argued that part of the value of wild nature derives precisely from the fact that it is independent of us, in the sense of not being a human creation or artifact (Lee, 1999). However, in the long run, one might argue, even human-created ecosystems in space will display their wild nature as the species evolve, adapting in unforeseeable ways to their new environment. Finally, we need to take seriously the fact that if humans and their accompanying biodiversity do establish a presence on other planets, life – at least microbial species – will almost inevitably escape containment and colonize habitable environments (Fairén, Parro, Schulze-Makuch, & Whyte, 2017). If the extraterrestrial environment lacks indigenous life, this would still constitute the first instance of establishing life on a lifeless world. If the environment does contain indigenous life, the act of introducing terrestrial organisms carries even greater ethical implications. Current planetary protection protocols are designed to prevent “harmful 26
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