Astrobiology is island biogeography writ large.

Saturday

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This Island Earth

Some time ago (on Twitter) I observed that astrobiology is island biogeography writ large. I return to this idea regularly, but have not yet adequately fleshed it out. I touched on this again in From an Astrobiological Point of View, but it would take considerable exposition to do justice to the idea. This post is an unsatisfactory response to my return to an idea that deserves to be studied in his own right and at some length.

Chart of the Galápagos Islands

Island biogeography has its origins in the origins of Darwin’s Origin of Species. As we all know, Darwin visited the Galápagos Islands during the voyage of the Beagle that Darwin recounted in The Voyage of the Beagle. Decades of thought and gestation followed, but it was in part the peculiar mix of species in the Galápagos that was crucial for Darwin’s breakthrough to the idea of natural selection. I have myself visited the Galápagos Islands (I wrote about this in Happy Birthday Charles Darwin!) and it is a spectacular lesson in natural history that I cannot recommend highly enough.

Although island biogeography begins with Darwin, it was brought to explicit formulation and theoretical maturity by E. O. Wilson and Robert H. MacArthur in The Theory of Island Biogeography. There the authors say in their opening remarks:

“By studying clusters of islands, biologists view a simpler microcosm of the seemingly infinite complexity of continental and oceanic biogeography. Islands offer an additional advantage in being more numerous than continents and oceans. By their very multiplicity, and variation in shape, size, degree of isolation, and ecology, islands provide the necessary replications in natural ‘experiments’ by which evolutionary hypotheses can be tested.”

Robert H. MacArthur and Edward O. Wilson, The Theory of Island Biogeography, Princeton: Princeton University Press, 1967, Chap. 1, p. 3

Much of this remains valid when translated, mutatis mutandis, into astrobiology. The key, however, is how one goes about arriving at the mutatis mutandis. How can all other things remain equal when we are translating from terrestrial ecosystems in miniature, thus a bit easier to understand than the whole of the terrestrial biosphere, or some major division such as a biome, into worlds entire isolated in the blackness of interplanetary and interstellar space? The analogy is not perfect, but it is suggestive of parallel avenues of approach.

How do you quantify the life of an entire world? Higher biological taxa. This graph shows families rather than species.

Scaling up biogeography

While the flora and fauna of islands are sufficiently restricted in scope to make it possible to do a detailed count not only of species present (already in The Voyage of the Beagle we see Darwin noting the number of genera and species present on various islands), but sometimes also of individuals. Obviously we are not going to be able to count species, much less individuals, for entire worlds. We must draw back, look at the big picture, and employ the kind of metrics we see in studies of mass extinctions. In detailing the loss of biodiversity of mass extinctions it is not merely species or even genera that go extinct; sometimes entire families, orders, and classes go extinct. These we can count; in fact, we could reasonably expect to count higher taxa for entire worlds.

The reformulation of island biogeographical ideas for astrobiology will be the labor of the production of a new science. The scaling up of our scope to higher biological taxa is only one among many scaling changes in our thought we must pursue in order to develop concepts adequate to the fate of life in the context of galactic ecology.

Flight and its Technological Equivalents

Geologically young islands — as with the well-known example of the Galápagos Islands, mentioned above — are primarily populated by birds and marine animals. Birds bring with them a variety of plant life; moreover, many plants can float, and are brought to islands by ocean currents. Least common to arrive and to survive are those terrestrial species that find themselves on islands due to sweepstakes dispersal routes, i.e., somewhat unusual circumstances in which a breeding pair of terrestrial animals are able to ride a floating log or mass of vegetation to an otherwise isolated island and can there reproduce, like the marine iguanas on the Galápagos, who have learned to feed by diving into the ocean and forage on inter- and subtidal algae. That is to say, the least common colonists are life forms that cannot swim or fly; being able to traverse planetary distances is a limiting factor in the distribution of a life form.

Darwin conducted a simple yet ingenious ecological experiment in island biogeography that he recounted in The Origin of Species:

“I have before mentioned that earth occasionally, though rarely, adheres in some quantity to the feet and beaks of birds. Wading birds, which frequent the muddy edges of ponds, if suddenly flushed, would be the most likely to have muddy feet. Birds of this order I can show are the greatest wanderers, and are occasionally found on the most remote and barren islands in the open ocean; they would not be likely to alight on the surface of the sea, so that the dirt would not be washed off their feet; when making land, they would be sure to fly to their natural fresh-water haunts. I do not believe that botanists are aware how charged the mud of ponds is with seeds: I have tried several little experiments, but will here give only the most striking case: I took in February three table-spoonfuls of mud from three different points, beneath water, on the edge of a little pond; this mud when dry weighed only 6¾ ounces; I kept it covered up in my study for six months, pulling up and counting each plant as it grew; the plants were of many kinds, and were altogether 537 in number; and yet the viscid mud was all contained in a breakfast cup! Considering these facts, I think it would be an inexplicable circumstance if water-birds did not transport the seeds of fresh-water plants to vast distances, and if consequently the range of these plants was not very great. The same agency may have come into play with the eggs of some of the smaller fresh-water animals.”

Charles Darwin, On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life, London: John Murray, 1st edition, 1859, GEOGRAPHICAL DISTRIBUTION. CHAP. XII., pp. 386-387

Such is the power of flight to widely disperse species over the surface of Earth. Flight has a value beyond the differential survival and reproduction advantage that it confers upon those species so endowed; it also plays a co-evolutionary role at the largest scale of planetary ecology. That flight should develop within a biosphere is perhaps not inevitable, but we could say instead that a biosphere in which flight emerges is likely to achieve much higher levels of biodiversity, and hence prove a more robust ecosystem. A robust ecosystem, in turn, is more likely to survive existential threats (such as the mass extinctions that have repeatedly punctuated the evolution of life on Earth), so that planetary biospheres of a given longevity are more likely to have flight than not.

Natural selection found several different solutions to the problem of flight. Some small plant seeds, and some very small animals (e.g., spiders), are light enough to be carried by the wind. Some animals fly by gliding (flying squirrels), and some animals employ wings for flight. Wings have emerged separately among insects, dinosaurs, birds, and mammals. Flying fish might also be said to have wings. Given a biosphere not disrupted by the anthropocene, flying fish might eventually transition to a fully flying way of life; this may yet happen in the distant future.

Flight?

The problem of flight at the level that concerns astrobiology is potentially as diverse as the solutions to the problem of flight in a planetary biosphere. We are only just beginning to understand the complexity of the universe in which we live, and we are continually discovering capacities of nature and of life that previously would have strained our credulity. Just last week on the second episode of The Unseen Podcast, host Paul Carr noted that, with all the exchange of material between the inner planets of the solar system, we would not be surprised to find that all this life comes to the same root, while we probably would be surprised, if found like the oceans of the moons of Jupiter and Saturn, if it came from the same root. That far out in the solar system, we would expect a second genesis if there is any life at all.

If there is life in the subsurface ocean of Europa, we expect that life to be the result of a second genesis.

That perspective on the likelihood the relations of life within the confines of a single solar system may change as we learn more about astrobiology. But so far this discussion is primarily a matter of naturally occurring dispersal vectors for species. We must consider astrobiology both before and after technologically-driven dispersal vectors, as well as in regard to terrestrial and to extraterrestrial dispersal vectors. Just as technological dispersal vectors have began to play a major role in our planetary biosphere, especially in relation to the distribution and introduction of invasive species, we would expect a mixture of both natural and technical dispersal vectors in astrobiology.

Spaceflight is to astrobiology as flight is to biogeography.

Given the continuity of natural history and civilization, that spaceflight is to astrobiology as flight is to biogeography follows naturally in the strict sense of “naturally.” In other words, there is a continuity from flight as the result of biology and flight as the result of technology; there is idea diffusion (or idea flow) from nature to civilization: we observe the existence proof of powered, heavier-than-air flight in nature, and we seek to reverse engineer this development and to reproduce it with technology. Thus, in a sense, technology is the pursuit of biology by other means. Thus spaceflight, as the technological equivalent of biological flight, will play a co-evolutionary role at the largest scale of galactic ecology.

It may be worth noting in this context that the cluster of developments dependent upon human activity — intelligence, technology, language, and civilization among them — could be said to represent a solution to the problem of survival, but it is a “solution” that we find no where else in nature except in ourselves. Now, in referring to “nature” in the previous sentence I here mean “in the terrestrial biosphere.” This is significant, because a viable solution to the problem of survival (as we can see from the example of flight, or I might also use the example of vision) tends to be repeatedly emergent in nature, so that we find multiple instances of homology and convergent evolution. We do not find this in regard to the human solution to the problem of survival.

If this is a solution to the problem of survival as posed by the terrestrial environment, why did no other species exploit this strategy?

On a larger scale, a scale at which “nature” does not mean the terrestrial biosphere but rather means the whole of the universe, we may well yet see the cohort of complexities associated with human beings repeated elsewhere, though we have to scale up our perspective, just as with scaling up island biography until it coincides with astrobiology. Metrics appropriate to human activity in a terrestrial context will not be sufficient for human (or, more generally, intelligent) activity in an extraterrestrial context. Another way to understand this is that, confined to the surface of Earth, distinctions that would be significant to civilization are conflated by contingent circumstances; raised off the surface of the Earth, and given energy and resources almost without limit, previously conflated properties of civilization manifest themselves in an extraterrestrial context and eventually become obvious as spacefaring civilizations undergo rapid adaptive radiation and come to exemplify different civilizational properties.

Terrestrial civilizations from an extraterrestrial perspective appear homogenous, but this may be a function of their being subject in common to specific terrestrial selection pressures.

But to return to the idea that technology is the pursuit of biology by other means, as I observed in my Centauri Dreams post, How We Get There Matters, existential ends are not indifferent to technological means. In the particular case of the pursuit of biological ends by technological means, this provides a context for thinking about astrobiology in an age of spacefaring civilizations.

Many metrics have been proposed for spacefaring civilization. I mentioned some of these in my last post, Thinking about Civilization, including metrics that I have myself attempted to work out. In that post I did not mention the metric that I proposed in my Centuari Dreams post How We Get There Matters (and which I followed with SETI Under Conditions of Constraint for Spacefaring Civilization), which concerned classes of starships. This is a metric immediately relevant to the question of spaceflight understood as the development of a continuum that begins with the first wind-blown distribution of seeds and spores, and which might some day mean the greening of the galaxy.

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