Life as a Habitability Requirement
Astrobiologists often speak of a planet’s requirements for life, but can we turn that around? Is life a requirement for a planet’s habitability? A team of geographers from the UK, with help from an ecologist at Montana State University, decided to calculate the energy output of animals. The resulting calculation is astonishing.
Animals, considered as a dynamic factor of the biosphere, contribute a huge amount of energy to landscape changes on the earth — more than some geological processes. The research paper in PNAS by Harvey et al. explains the significance of their results, with some surprising numbers. This paragraph needs a “wow” emoticon next to it:
Animals profoundly influence Earth surface processes and landforms, but their collective significance has not been quantified. Integrating data across freshwater and terrestrial ecosystems, we uncovered over 600 animals with reported geomorphic effects, including five livestock taxa. Many more are doubtless overlooked due to inherent geographical and taxonomic biases in published research. We conservatively estimate that wild animal species collectively contribute ≈76,000 GJ energy or more to geomorphic processes annually, equivalent to the energy expended by hundreds of thousands of extreme floods. Livestock acting as geomorphic agents are estimated to exceed this contribution by three orders of magnitude. Our results reveal that the energy of animal geomorphic agents is a significant and overlooked driver of landscape change at the global level.
Their results are most likely underestimated by orders of magnitude. The title, “Global diversity and energy of animals shaping the Earth’s surface,” leads us to think of the many ways that “ecosystem engineers” large and small are at work in this essential role.
Animals cause landform change both directly, by mixing soils and sediments (bioturbation) and via the displacement of Earth materials (bioerosion and bioconstruction), and indirectly, by conditioning rock, soil, and sediment particles to be more or less susceptible to erosion and transport by geophysical processes. For example, riverbed gravels can become less mobile when bound by caddisfly silk or more mobile when disturbed by benthivorous feeding fish.
Even a lowly ant mound, termite pillar, gopher hole, or tortoise burrow contribute to the sum total of gigajoules of energy expended by animals in shaping the world. Here are some of the “zoogeomorphic species” of animals mentioned: salmon, burrowing scorpions, crayfish, beaver, worms, spiders, reptiles, frogs, fish, crustaceans, nesting birds, bivalves, gastropods, shrimp, kangaroos, boar, aardvarks… In short, almost every living thing is at work shaping the globe: “It has been suggested that all ecosystems on Earth are engineered by organisms to some degree,” they say. Watching elephants transform African landscapes makes us wonder what ecosystem roles were played by the mighty sauropods in times past.
We humans, of course, play a dominant role in altering the environment. But even in pre-industrial times, the vast herds of livestock managed by people groups around the world have contributed a thousand times the energy of undomesticated species. And consider that the UK scientists did not even attempt to calculate all the ecosystem engineering that occurs in the oceans. (Read here about the roles of salps and plankton in their diel migration activities that transfer carbon to the ocean floor, and read here about the cable bacteria that transfer protons from the seafloor.) These authors only focused on land animals and freshwater creatures. So yes, their calculation of 76,000 gigajoules is likely very low.
Life as a Requirement for Habitability
As the authors of this fascinating paper say in their conclusion, “Our analysis has revealed that the energy of zoogeomorphic species represents a significant and overlooked driver of geomorphic change at the global level.”
But now let’s ask if a planet needs living things to function as a habitat for life. That’s a different question. It could extend the long list of requirements adduced by scientists such as Michael Denton who argue that a planet suitable for complex life requires the fine-tuning of multiple physical factors. What if a planet also needs an active, energy-expending biosphere to be habitable?
That question was put forth by four scientists from Colombia on the arXiv preprint server, led by Jorge I. Zuluaga. They submitted their paper for publication to the journal Biogeosciences in 2014, but I could not find out whether it was ever published. We should only consider it, therefore, an interesting speculation. Nevertheless, it was seriously considered by The Planetary Society, and Zuluaga et al. did offer several empirical evidences in support of constraining “The Habitable Zone of Inhabited Planets.”
In the Planetary Society article, Jaime Green thinks that Zuluaga et al. are only answering the question, “Does inhabitation affect habitability” rather than “Is inhabitation a requirement for habitability?” He can accept that life might enhance a planet’s habitability but leaves it open whether the presence of life is a requirement. Recognizing that we have no data for an answer till we “touch down on alien soil” and make observations on another planet, he states in his conclusion,
In looking for Earth-like planets that might be home to life, we should be careful to keep our minds open to all possibilities, including that a planet might be habitable because life is there.
A Bolder Claim
The argument of Zuluaga et al. seems bolder than a mere claim that life affects habitability. In a PDF from the Universidad Nacional de Colombia, they asked, “Is it possible to neglect the effects of life when calculating the boundaries of habitability?” Interested readers may find their arguments and diagrams worth considering. If a sterile planet has all the abiotic factors in place, will it be habitable, or at least less habitable than an inhabited planet in the habitable zone? For example, “an inhabited planet maintains habitable temperatures under a wider range of insolation conditions” than a lifeless planet, even if it orbits within the continuously habitable zone where water can exist in liquid form. Would Mars sprout organisms if it were moved into our sun’s CHZ, the ice melted, and water became abundant?
In a statement that might be of interest to design advocates, Zuluaga et al. point out that
Orderliness in life (which is incomparably higher than that of the surrounding environment) is supported in a way unprecedented in the inanimate world: via competitive interaction.
Here are three “bottom line” considerations from their presentation:
Biota-environment feedbacks are likely to (substantially) alter the environment of an inhabited planet.
The equilibrium state of a complex system cannot be predicted while neglecting one of its (major) components.
Living phenomena have (unique) properties able to drive the environment to (otherwise) unstable physical states.
A Bridge Too Far for Materialists
To a materialist or a naturalist, such talk can bleed over into the Gaia hypothesis, which many scientists find bordering on vitalism. Hungarian scientist Eörs Szathmáry commented on this borderland in PNAS, worrying about the hypothesis that the “biosphere has a decisive role in keeping the Earth habitable — in other words, the biosphere looks after itself, somewhat similarly to organisms that also look after themselves.” This sounds mystical to a materialist. Pointing to a recent paper by Boyle et al. in PNAS, Szathmáry offered hope that natural selection can be incorporated as a mechanism that maintains biogeochemical cycles in a non-vitalistic way.
Yet materialists are often surprised, if not shocked, by the early appearance of life on the Earth. Michael Marshall wrote in New Scientist,
When did life begin on Earth? New evidence reveals a shocking story. Fossils and genetics are starting to point to life emerging surprisingly soon after Earth formed, when the planet was hellishly hot and seemingly uninhabitable.
“The Song, Not the Singer
Is it shocking because the Earth needed life to be habitable? Szathmáry and Boyle each considered the views of W. Ford Doolittle who extends natural selection to everything, even the biosphere. “It’s the song, not the singer,” Doolittle famously proposes; any replicator, biotic or not, can be acted on by natural selection. This may be a bridge too far for materialists. Szathmáry says,
But then, what is the status of biogeochemical cycles if we think in terms of replicators, vehicles/interactors, and the levels of selection? In other words, do biogeochemical cycles evolve by natural selection after all? This issue is hotly debated. Doolittle offers the solution that biogeochemical cycles are interactors: “This works as long as there are replicators…that cause the differential formation of the interactors that favor their differential replication, reproduction, or persistence.” (ref. 4, p. 173). According to this view, multispecies communities, ecosystems, and even Gaia can undergo evolution by natural selection for persistence. Effects of genes can percolate upward to a Gaian level, as kind of extended phenotype sensu Dawkins. Gaia can be seen as the most inclusive clade, and clade selection for persistence could contribute to the survival of the biota. Time will tellwhat such an extended concept of evolution by natural selection is worth.
To ID advocates, though, neither Gaia nor vitalism are necessary to argue for a biosphere as a requirement for habitability. An intelligent and wise designer would have the foresight to understand all the requirements and supply them simultaneously. We can debate the hypothesis of biology as a prerequisite for habitability, while agreeing on the wisdom of a designer engineering a world that can sustain and extend habitability through the presence of a dynamic biosphere. As Zuluaga et al. argue, “Life alters the environment by taking and excreting energy and waste products giving rise to (powerful) feedbacks on the environment.” These feedbacks, in turn, work to optimize the habitability of an inhabited planet, they argue. And think about that; optimization is a key concept in intelligent design.
