Earth systems science is concerned with the relationships between the various components that comprise the Earth as a system, notably environmental and biosphere interactions. Over the years, a wide spectrum of views has been expressed by scholars. At one extreme, the environment is the dominant influence, driving evolution within the biosphere (which is interpreted as being largely moulded by environmental forces). As an example, many have considered that changes in seawater chemistry and atmospheric oxygen levels triggered the Cambrian Explosion. At the other extreme, the Earth's environment can be perceived as the product of the biosphere. This is the position of Nicholas Butterfield, who has written a paradigm-shifting essay saying:
"it is clear that animals figure disproportionately in the maintenance of the modern Earth System, not least because they invented it."
Understanding the complexities of the Earth System is at an early stage (Source here.)
Earth systems thinking has a bearing on our approach to the environment. Is the Earth unstable, easily nudged to 'tipping point' and environmental melt-down? Alternatively, is it resilient, with negative feedback mechanisms operating to restore ecosystems to equilibrium? An inherently unstable Earth appears to be the verdict of Lenton and Watson in their book Revolutions that made the Earth (recently reviewed in Nature). They express concern about the effects of human activity that could destabilise our current planetary state. The reviewer draws attention to three other contributions to Earth System thinking:
"Lenton and Watson's thought-provoking book is the latest in a distinguished line of works that have altered our perception of the planet. Russian-Ukrainian geochemist Vladimir Vernadsky first discussed the deep involvement of life in planetary chemistry in his 1926 book Biosfera (Biosphere). In Gaia (Oxford University Press, 1979), James Lovelock brought the self-stabilizing mechanisms of life into view by seeing the planet as a partially self-regulating, living whole. And Hans Joachim Schellnhuber's book-length chapter in Earth System Analysis (Springer, 1998) laid out a blueprint for a scientific discipline concerned with the interplay of social and environmental dynamics."
Central to Butterfield's analysis is the transition from past environments where animals were absent to past environments where animals were plentiful. Animals were absent in the Proterozoic, and their numbers were much reduced after some of the great mass extinction events documented by the rock and fossil record. This is Butterfield's comment:
"In the absence of relatively large-celled eukaryotic phytoplankton, a positive feedback loop of cyanobacteria-induced turbidity is likely to have excluded higher light-demand eukaryotic algae (both phytoplankton and benthic macrophytes) from all but the most oligotrophic or shallow-water marine settings, thereby inducing widespread oceanic stratification. Stratified, bacterially dominated oceans are a signature of the Proterozoic and have generally been viewed in terms of atmospheric oxygen availability. However, without the water-clearing abilities of suspension-feeding animals, there would have been no mechanism for tipping the system out of this condition, irrespective of ambient oxygen. It is simply the default structure of aquatic ecosystems in an exclusively microbial world."
By contrast, oceans with animals are totally different.
"Organism size lies at the core of aquatic ecology and, in the modern ocean, ranges over 20 orders of magnitude, mostly by virtue of the extended trophic tiering of animals. Without such activity, micrometer-sized phytoplankton are not converted to millimeter-sized zooplankton, centimeter-sized zooplanktivorous fish, decimeter-sized piscivores, and so on. [. . .] Significantly, the body-fossil record of large, ornamented and/or biomineralized phytoplankton is limited exclusively to the Phanerozoic, especially in the highly escalated post-Paleozoic oceans, and fossil biomarker molecules point to a fundamental shift in marine export production accompanying the early radiation of animals: from primarily bacterial during the Proterozoic to primarily algal during the Phanerozoic."
The transition took place in the Ediacaran: that window of time between the end of the Precambrian and the beginning of the Phanerozoic. This was the biggest ecological revolution that has been documented in the rock record. Until recently, we had not realised how dramatic the environmental changes were: Butterfield refers to "some of the most pronounced biogeochemical perturbations in Earth history" and to "unprecedented shifts in C and sulfur cycling, iron geochemistry, phosphate deposition and oceanic oxygenation". Even more significant is the question: what was driving these changes? Why did the stratified, anoxic waters of the Proterozoic not persist? Butterfiled rightly questions the consensus view that incremental changes just happened and they were conducive to animal evolution.
"What is missing from such hypotheses, however, is an appreciation of just how pervasive the role of animals has been in defining the modern Earth system. By facilitating and forcing the diversification of, for example, eukaryotic phytoplankton, large body size, bioturbation and biomineralization, early animals reinvented the chemical interchange between the biosphere and planet. In this light, the biogeochemical perturbations of the Ediacaran-Cambrian interval are more likely to be the top-down consequences of animal evolution than its bottom-up cause. Early animals did not simply fill up previously existing but unoccupied niche space; they created the space itself."
If this radical idea is correct, it transforms our approach to ecology. It provides a different perspective on mass extinction events and global recovery:
"One approach is to examine the effects of Phanerozoic mass extinctions, which preferentially eliminate the largest and most specialized animals (and their associated 'ecosystem services'). Mass extinctions differ in their intensity, causes and evolutionary context, but in the oceans they are commonly accompanied by widespread stratification, bottom-water anoxia and spikes in cyanobacterial export. These intermittent returns to pre-Cambrian conditions point strongly to the top-down control not only of phytoplankton diversity, but also Phanerozoic ocean structure in general."
The implications of this new approach for evolutionary theory are not discussed by Butterfield. However, it seems to me that there are many. Most evolutionary biologists are quick to sign up to the importance of environmental changes as a driver for evolution, but the new approach presents much of this environmental change as an effect, not a cause. Modern Darwinists love the adaptive landscape model of evolutionary transformation, but the new approach considers that the "biosphere 'as we know it' is a space designed by metazoans". Environmental changes are frequently suggested to 'trigger' evolutionary transformation, but the hypotheses never get to the stage where they can be tested. Regular readers of this blog will be aware that I have been promoting the idea that the fossil record is not so much a story of evolutionary transformation as a history of the Earth's colonisation (go here for a recent blog on this theme - with links to earlier blogs). This conceptual model interfaces well with Butterfield's proposed Earth System.
"All organisms alter their physical environment to some degree, but the unique attributes of animals makes them particularly powerful 'ecosystem engineers'. Simply as a consequence of their motility, metazoans mix, ventilate and chemically alter the media though which they move. Such bioturbation is all but ubiquitous in modern soils and soft sediments, and imparts a first-order control over everything from sediment composition to landscape topography and biogeochemical exchange."
Animals and the invention of the Phanerozoic Earth system
Nicholas J. Butterfield
Trends in Ecology & Evolution, 26(2), February 2011, 81-87 | doi:10.1016/j.tree.2010.11.012
Abstract: Animals do not just occupy the modern biosphere, they permeate its structure and define how it works. Their unique combination of organ-grade multicellularity, motility and heterotrophic habit makes them powerful geobiological agents, imposing myriad feedbacks on nutrient cycling, productivity and environment. Most significantly, animals have 'engineered' the biosphere over evolutionary time, forcing the diversification of, for example, phytoplankton, land plants, trophic structure, large body size, bioturbation, biomineralization and indeed the evolutionary process itself. This review surveys how animals contribute to the modern world and provides a basis for reconstructing ancient ecosystems. Earlier, less animal-influenced biospheres worked quite differently from the one currently occupied, with the Ediacaran-Cambrian radiation of organ-grade animals marking a fundamental shift in macroecological and macroevolutionary expression.
Lucht, W., Earth systems: Shaped by life, Nature, 470, 460-461 (24 February 2011) | doi:10.1038/470460a
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