Comment on Humans – The Species that Changed Earth
In his wide-ranging and provocative essay, Erle Ellis (2016) introduces his new evolutionary theory – sociocultural niche construction – and uses it to explain how we “modern” humans uniquely developed the capacity to transform Planet Earth, why we have done so and continue to do so, and what we might to do make progress towards a better, more sustainable future (see also Ellis 2015). In this commentary, I reflect on whether this new evolutionary theory and the ongoing global anthropogenic transformation of the biosphere can fit comfortably within our standard theories and models of ecological and evolutionary dynamics or whether the patterns and processes of sociocultural niche construction in the Anthropocene necessitate new ways of thinking about and practicing evolutionary ecology.
Much has been written about whether cultural evolution is fundamentally different from biological evolution, if patterns and processes of cultural evolution can be comfortably mapped onto patterns and processes of biological evolution, or if the extended evolutionary synthesis (henceforth, EES) (and, by extension, Ellis’s new evolutionary theory of sociocultural niche construction [henceforth SCNC]) can be accommodated easily within the Modern Synthesis (e.g., Mesoudi et al. 2006 and commentaries therein, Sterelny 2011, Laland et al. 2014, Wray et al. 2014). My own view is that the basic theoretical construct of the Modern Synthesis and its underlying mathematics are flexible enough to accommodate any set of units that exhibit variation and are subject to selection. These may be genes, individuals, groups, species, or ecosystems (so-called “biological” units) or memes, individuals, family groups, clans, villages, city-states, or nation-states (“cultural” or “human” units) (Wilson and Hessen 2014). Prodding evolutionary biology to incorporate not only plasticity, epistasis, and developmental bias, but also niche construction, memes, cultural evolution, and horizontal transmission has been, and will continue to be, crucial for advancing our understanding of evolution writ large. Erecting new theories that are distinguishable from the Modern Synthesis only by the units of selection or new terminology, however, seems less valuable (Wray et al. 2014).
A stronger case for an EES or the SCNC could be made if cultural traits really were separable from “biology”. Ellis argues, following Sterelny (2011) and many other proponents of EES, that human adaptive fitness is a function of culture, not of biology, and that the transformative behavior of human societies cannot be explained by biology alone. But if behavior and cultural traits are not, at some deep level, biologically based, from where do they emerge and how are they perceived? Rather than retreat into a mind-body (or culture-biology) dualism, Mesoudi et al. 2006 defined culture as “semantic information that is represented primarily in the brain,” although they hedged their bets by acknowledging that “cultural knowledge does not exist solely in human brains and does not rely exclusively on face-to-face communication for transmission” (emphasis added). It is hard to argue with the latter statement, but our brains, and the organisms that house them, are biological structures, and one would be hard-pressed to argue that the evolution of these biological structures has not been part and parcel of our ability to store cultural knowledge outside the brain or to transmit and receive cultural information over long-distances (see also Alberti 2015). Memes and other cultural units are not completely separable from the organisms that originated them. Rather, they are extensions of ourselves – as biological organisms – and interpreted by other biological structures, including our autonomous nervous system and our brains.
New research also suggests that at least one animal besides Homo sapiens has evolved multi-level (hierarchically nested) societies via horizontal transmission of cultural traits (Cantor et al. 2015). Yet sperm whales have a much smaller ecological footprint than humans (and had a smaller one even before we hunted them nearly to extinction) and appear to be uninterested in selfishly appropriating more than half of the terrestrial and marine productivity for its own single species at the expense of the 10 million or so other ones on the planet. That trait, whether the biological expression of a blindly and rapidly breeding species functioning only as a vehicle for its genetic code (Dawkins 1976) or the sum of a suite of behaviors that are culturally transmitted, is what really sets humans apart from all other species.
Humans indeed are a great force of nature. From hunting large animals to extinction and domesticating smaller ones on farms and in our homes, through developing agriculture and moving species across the globe deliberately and accidentally, and by cutting down forests and fragmenting habitats, our ancestors and ourselves have altered ecological patterns and processes worldwide (e.g., Martin 1973, Lyons et al. 2016, Malhi et al. 2016). Our industries have changed our planet’s atmosphere, rivers and oceans, and biogeochemistry, in some cases possibly irrevocably (e.g., Zhang et al. 2016). And the rapidly changing environmental conditions in our cities – now home to > 50% of the Earth’s human population – are driving evolutionary shifts at unprecedented rates, causing novel and unexpected feedbacks between ecological and evolutionary dynamics in near-real time (Alberti 2015).
But these dynamics derive from aspects of our biology and the biology of the species with which we interact. Homo sapiens is a unique blend of r– and K-selected species (sensu MacArthur and Wilson 1967) and comes as close as possible to being a true Darwinian demon (sensu Law 1979). Despite our long lifespan and apparent (at least from our own perspective) late age of maturity, we have the potential to reproduce often and aseasonally, we invest a lot of care in our children and they have a high likelihood of survival to adulthood. Consequently, our population continues to grow at an exponential rate. Past projections that the human population would stabilize (e.g., Pearl 1925) have continually failed to be borne out by data, and current projections are that global population will increase to between 9.6 and 12.3 billion by 2100 (Gerland et al. 2014).
The remarkable ecological and evolutionary success of Homo sapiens, whether measured in our species’s total biomass, extraordinary biogeographic distribution, facility at ecological engineering and environmental transformation, or ultrasociality (Hill et al. 2009), is certainly an evolutionary anomaly (Boyd and Richerson). Although our biological and cultural traits make us unique among the diverse species with which we share the Earth, they do not place us outside the laws of evolution or the rules of ecology. Just as the Modern Synthesis needs to expand to accommodate the evolution of behaviors and cultures, ecological theory needs to expand to accommodate humanity.
In particular, human decision-making engenders feedbacks that are not yet well accounted for in contemporary ecological theories. For example, standard discussions of the effects of human actions (broadly construed as land-use change) on biodiversity and subsequent ecosystem processes, functions, or services portrays a linear (or at most direct + indirect) chain of cause-and-effect: humans à ecosystem processes or humans à biodiversity à ecosystem processes (e.g., Tilman et al. 2014, Newbold et al. 2015). Alberti (2015) goes several steps further and illustrates a number of feedbacks between ecological interactions, heritable traits, ecosystem functions, and evolutionary processes in urban environments, but still considers humans (and the habitats that they modify) only as drivers of the eco-evolutionary dynamics.
But both biodiversity and ecosystem functions also drive land-use changes and other human decisions (biodiversity + ecosystem function à human actions). For example, people choose to site agriculture in high-productivity systems (e.g. alluvial plains, rich mesic grasslands). “Improving” these sites may alter local biodiversity and ecosystem processes, often in ways that make them even more attractive to further human use. People choose to site parks and reserves in areas of high biodiversity or great scenic beauty. These amenities often lead to further land-use changes as tourists and developers aggregate around parks, resulting in new feedbacks not only on local biodiversity but also on landscape-level heterogeneity (beta diversity; see McGill et al. 2015), with further downstream effects on ecosystem functions that in turn feedback on human land use. These and other types of feedbacks between human and non-human eco-evolutionary processes rarely are analyzed or modeled but could be examined or accounted for within existing ecological theory and its mathematical frameworks.
Finally, Ellis implores us to move beyond the idea that a balance of nature will rescue humanity. Here, he takes on one of the most persistent myths that has determined not only human attitudes about nature but also ecologists’ models and analyses (Kricher 2009, Botkin 2012, Ellison 2013). This is perhaps one of the greatest challenges for contemporary ecological theory, which, either out of mathematical convenience or wishful thinking, remains dominated by equilibrium models and stable states. If the natural world ever was stable or balanced (in any ecological or evolutionary sense) – and the available data are at best agnostic on this point – the evolution of modern humans and their ongoing transformation of the Earth demonstrably illustrate that we live in a nonequilibrium present and future. We already have non-equilibrium theories and models. Now we need to start using them.
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 A disclosure. I was the Editor-in-Chief of Ecological Monographs who, in 2012, invited Ellis to explain why humans have reshaped more than 75% of the terrestrial biosphere from “natural” biomes to anthropogenically-determined “anthromes”. I also was the handling editor for the paper, which was published as a Centennial Paper in the journal, and is available as “green” open access online from the journal’s website: http://onlinelibrary.wiley.com/doi/10.1890/14-2274.1/full.
 This list of units of selection is not meant to be exhaustive or exclusive.
 As I write this, the now nearly pandemic Zika virus has encouraged even the Pope to support contraception. So perhaps the first horseman of the apocalypse may yet slow human population growth. But history is not a source of optimism on this count.
 Ironically, that part of ecological research that currently focuses the most on transient, non-equilibrium dynamics – the study of tipping points and regime shifts – implicitly assume that either before or after the regime shift that ecological conditions are broadly stable. Botkin (2012: xii) expressed this cognitive dissonance most succinctly: “[i]f you ask ecologists whether nature is constant, they will always say ‘No, of course not.’ But if you ask them to write down a policy for biological conservation or any other kind of environmental management, they will almost always write down a steady-state [i.e., ‘nature is stable’] solution.”