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Impending: Entering the Anthropocene Mass Extinction


Introduction

	What has become apparent to some scientists is the fact that there are dramatic trends indicating biodiversity on Earth is under attack (Andermann et al., 2020; Barnosky et al., 2011; Dirzo et al., 2014; Pievani, 2020; Steffen, 2011).  Biodiversity is important to human survival (Pievani, 2020. Steffen, 2011).  However, human activities have decreased the probability that many species could survive in the biosphere (Pievani, 2020).  In the last 500 years, unprecedented human impacts have been particularly profound, affecting the viability of entire ecosystems and the health of the biosphere as a whole (Steffan et al., 2011; Smithsonian Institution, 2018).  From the global changes in climate and atmospheric composition to the fragmentation and destruction of habitats, from the introduction of agriculture and livestock to the urbanization and human explosion, human existence and activity have transformed the carrying capacity of Earth for extant animal species (Pievani, 2014).  

	As larger body animals are disproportionately vulnerable to extinction (Barnosky et al., 2011), this paper will examine the current situation of mammals and what extinction risks mammals are vulnerable to.  With the influence mammalian presence has on other species in an ecosystem, extinctions of mammal species could trigger transformation of ecosystems including many co-extinctions (Dirzo et al., 2014).  Trends in mammal species losses are matched with comparable trends in the losses of other animal groups (Andermann et al., 2020).  Some have proposed that the Anthropocene mass extinction, the sixth mass extinction in the history of the world, is on the verge of beginning or is already underway (Andermann et al., 2020; Barnosky et al., 2011).  This is extrapolated from the current rate of biodiversity loss in studies that examine current rates of extinction as compared to background rates, the magnitude of extinctions compared with the magnitude that characterized past mass extinctions, and the rate-magnitude models that project time-frames within which current losses would reach the magnitude of the previous mass extinctions (Barnosky et al., 2011)  

	This paper is a discussion of each of these points; hopefully, the reader will become aware of the crisis at hand that life on Earth is under siege, something that could very well threaten the viability of Earth as a suitable home to Homo sapiens (Pievani, 2020).  Carl Sagan wrote in the Pale Blue Dot: “To me, it underscores our responsibility… to preserve and cherish the pale blue dot, the only home we've ever known.” (Sagan, 1997)

Discussion

	The word “Anthropocene” has been circulating for the last two decades to describe a new epoch, as subsequent to the Holocene epoch, to highlight the dawn of a new era characterized by significant human impact upon the Earth (Steffen, 2011).  This epoch is characterized by unprecedented and large-scale anthropogenic changes to Earth’s climate, land, oceans, and biosphere (Steffen, 2011).  That human beings bring about significant changes to the living world is directly caused by the exponential increase of human population in the past century and in the consequent and ever increasing demand for resources (Steffen, 2011).  As had happened in the rise of previous epochs, in the Anthropocene epoch, global changes to key processes of Earth have occurred (Zalasiewicz, 2011).  For example, anthropogenic global warming is leading to phenomena such as rising sea levels, changes to land and ocean ecosystems, retreating glaciers, sea ice, and ice sheets, biodiversity losses, and forest fires (Pievani, 2020).  Another example is the accumulation of CO2 in the ocean, which results in the continuing acidification of ocean water, with implications for marine life (Pievani, 2020).  Such a change to the chemistry of the ocean is unprecedented in the last 65 million years (Pievani, 2020).  The new possibility of a sixth mass extinction, the topic of this paper, arising from the scale of anthropogenic changes to earth, has been proposed by scientists (Barnosky et al., 2011; Pievani, 2020; Steffen, 2011).  Has a mass extinction resulting from human existence and activities already arrived?  Many impacts leading to biodiversity losses originate in human activities on this planet.  These include: co-opting resources, fragmenting habitats, degrading habitats, destroying habitats, bringing invasive non-native species, spreading pathogens, directly killing species by overfishing and overhunting, polluting, and changing the global climate (Barnosky et al., 2011).  Such human activities have decreased the probability that a multitude of species could persist in their habitats.  Compared with the rate of anthropogenic changes, evolution occurs on a much longer time scale.  While anthropogenic changes are not evolution itself, the fact is that species living in the human-impacted world have not had and do not have enough time for evolution to respond: there has not been time for genetic mutations to occur nor for adaptations to be selected for (Pievani, 2020).  Life on Earth has not had a chance to develop adaptations to respond to the precipitous alterations caused by humans (Pievani, 2020).  

	It has been proposed that three criteria would constitute the “perfect storm” for a mass extinction: accelerated climate change, changes to the atmospheric composition, and unusually intense ecological stressors (Pievani, 2014).  All three factors have been realized in the modern age (Pievani, 2014).  How these factors feed back into each other in complex ways would further contribute to the perfect storm that spells doom for biodiversity (Pievani, 2014).  It is not overstating the case to note that a drastic situation is upon us.  We do not realize the footprint human beings have on the environment and the biosphere, at least not as comparable to the greater forces of nature.  In fact, the scale of human impact upon the biosphere is so severe that it has become the specific force bringing about a mass extinction, something previously only possible by the powers of nature, in natural geological forces or an asteroid hitting earth.  In the Anthropocene epoch, human enterprises in sum has become an evolutionary force that rivals and even exceeds the powers of nature (Pievani, 2014; Pievani, 2020; Steffen, 2011).  As a result, the Anthropocene mass extinction is probably impending, if not already here (Barnosky et al., 2011).

	In the event of a mass extinction, the large-bodied animals are usually the hardest hit: these would include many mammals (Barnosky et al., 2011).  While later discussion will examine the expected ecological impact in the event of losing mammalian species, I will first discuss the current state of mammals.  Mammals have existed in most of Earth’s habitats (Schipper et al, 2008).  Their key ecological roles include predation, grazing, and seed dispersal (Schipper et al, 2008).  Mammalian diversity within a habitat usually depends on the availability of energy and the complexity of the area’s topography (Schipper et al, 2008).  For example, small ranges with complex features, such as the Himalayas, can support greater diversity (Schipper et al, 2008).  In particularly, if a small area has several climate zones, each species would be adapted to its own climate zone and restricted to that range, resulting in greater diversity in the habitat as a whole (Schipper et al, 2008).  Smaller geographic range size is one extinction risk predictor that could correlate with declines in populations (Dirzo, 2014).  Other extinction risk predictors correlating with population declines are low reproductive rates and relatedly, very large home range size, as well as the forementioned large body size (Dirzo, 2014).  Normal range size can vary a great deal for both land mammals and marine mammals.  For land mammals, normal range could be just a few hundred square meters to 64.7 million km2; for marine animals, normal range could vary between 16,500 km2 and 350 million km2 (Schipper et al., 2008)  That extinctions of individual species are preceded by decreasing population sizes and species range is particularly relevant when examining changing biodiversity trends and projecting the horizon that is the possibility of extinction.  In South and Southeast Asia, there is high species richness, high endemism, and high human pressure; this is an example of a region where land mammals are threatened (Schipper et al., 2008)

	There are currently 6,495 extant mammal species (Burgin et al., 2018).  According to the data compiled from studies of mammals across the world by over 1700 experts, there are 1139 extant mammal species currently threatened with extinction (Schipper et al., 2008)  Further, 188 species are critically endangered, with 323 more species considered near threatened (Schipper et al., 2008).  In the conclusion of this study, an alarming trend appears – 1 in 4 mammalian species is now threatened with extinction (Schipper et al., 2008), along with the finding that 1 in 2 mammalian species is in decline (Schipper et al., 2008).  Simulations using the statistical analysis software iucn_sim and the PHYLACINE database, with the projected trajectory of human density as a predictor, show that the rate of mammalian extinction is increasing presently, and predicts that 558 currently extant mammal species will have gone extinct by 2100 (Andermann et al., 2020).  The trend of mammalian species extinction has comparable trends in other animal groups (Andermann et al., 2020): later parts of the paper will examine the extinction rates of animals in general.  The ecological impact of mammalian extinctions can be understood through the interconnectedness amongst animal species in an ecosystem, which is not to disregard the relationships between animal and plant species.

	In an ecosystem, different types of organisms occupy different roles (Dirzo et al., 2014).  Sometimes, mammals are described as part of the megafauna, with megafauna meaning large animals in a given area.  There are various ways in which megafauna can act as ecosystem engineers (Smith, 2015).  For instance, they can break large trees and affect their canopy (Smith, 2015). Subsequently, this changes the amount of light reaching the lower vegetation and affects the composition of forest vegetation.  They can also contribute to the cycling of nutrients and the redistribution of nutrients over large spaces (Smith, 2015).  The changing availability of nutrients like sodium and phosphorus alter the fertility of ecosystems (Smith, 2015).  In the case of the biodiversity crash in the Late Pleistocene, which occurred around 12,000 years ago, the changed cycling of phosphorus has consequences still observable today (Smith, 2015).  Another way that megafauna affect the ecosystem arises from megafauna eating fruits with large seeds and dispersing them over long distances (Smith, 2015).  Grazers can reduce grass competition and help reduce the fuel load at the bottom of forests, decreasing the likelihood of forest fires (Smith, 2015).  The presence of predators increases species richness in communities (Smith, 2015).  Predation pressures change prey behavior and cause them to roam to different parts of the landscape (Smith, 2015).  Losses of megafauna could decrease species richness (Smith, 2015).  In particularly, simpler communities are not as resistant to changes in the environment of this sort (Smith, 2015).  These are some of the effects that could result from the loss of megafauna.

	Disappearing megafauna leave behind ecological niches which cannot be filled by smaller organisms (Dirzo et al., 2014; Smith, 2015).  Patterns of defaunation lead to changes in community composition (Dirzo et al., 2014).  Changes to the composition of a community can immediately impact the ecosystem, including the ways listed above (Dirzo et al., 2014).  While extinction tends to occur relatively slowly, declines in individuals could escalate to functional extinction very quickly (Dirzo et al., 2014).  Because of the interconnectedness of species inside an ecosystem, the extinction of even a few species of megafauna could lead to a cascade of co-extinctions in the community (Smith, 2015).  Changes to ecosystems around the world which result from cascades of co-extinctions may be accelerating at present; in the future, a tipping point into a mass extinction could be reached from which return is no longer possible (Dirzo et al., 2014).  

	To consider the bigger picture question of the causes of the Anthropocene mass extinction, and whether and how quickly Earth could be entering this event, studies have examined key predictors for extinction rates, current and future extinction rates, and also the magnitude and rate-magnitude factors.  Researchers have taken different positions regarding whether species extinctions in the past had been caused by climatic changes or anthropogenic impacts (Andermann et al., 2020).  For their study about past events of significant extinctions, Andermann et al. used statistical software and databases to distinguish between human-related predictors and climate predictors.  This study shows that, in prehistory, when Homo sapiens migrated to new continents, their arrival time coincided with mammalian extinction.  It suggested that these mammals were behaviorally naïve to the behavior of humans and easily fell prey to human hunting: this is particularly true for larger mammals (Andermann et al., 2020).  Similar outcomes occurred when humans colonized islands (Andermann et al., 2020).  In the last few thousand years, habitat destruction has caused extinctions of smaller species as well.    Simulations tested whether human factors or climate factors could predict mammal extinctions in the past (Andermann et al., 2020).  For human factors, human population size is one predictor variable (Andermann et al., 2020).  Another predictor for human factors is human land occupation, which consists of changes to total area occupied by humans (Andermann et al., 2020).  Because data suggests that human beings colonized a land mass to the full extent almost immediately in geological time, instantaneous occupation is assumed (Andermann et al., 2020).  These simulations showed that human population density used as a predictor explained mammal extinction patterns with 96% accuracy and human land occupation as a predictor explained the patterns with 97.1% accuracy (Andermann et al., 2020).  Contrast this with climate predictors that had 63.6% accuracy and temperature change that had 60.2% accuracy, which were closer to the null model that has 59.4% accuracy (Andermann et al., 2020).  According to this study, the current extinction rates are 1700 times the rate in the Late Pleistocene, the period when human migration could first be correlated with megafaunal losses (Andermann et al., 2020).  All this points to human activities causing animal species extinctions (Andermann et al., 2020).

	Another study quantifies the current rate of extinction and compared it with the background rates, which were computed for periods of time when extinction rates are not outside the norms for evolution (Barnosky et al., 2011).  Using paleontology databases containing lists of recently extinct species, the study computed the background rates and recent rates within specified interval time bins using the E/MSY metric (Barnosky et al., 2011).  The E/MSY metric consists of “extinctions per million species-years” – for example, if there were 1 million species in existence today and the rate is 1/million species per year, then the rate predicts 1 species will go extinct each year (Barnosky et al., 2011).  The study found the rates for the most recent period (Barnosky et al., 2011).  When using 1-year bins, the computed value of E/MSY is 693, compared to the background rate of E/MSY = 1.8 (Barnosky et al., 2011).  Calculations show that no previous 500-year intervals had seen a comparable extinction rate of mammals.  In fact, if today’s threatened species were to go extinct altogether, then the outcome would be beyond any reasonable projection from the background rates (Barnosky et al., 2011).  Today, 20%-43% of all mammals on Earth are considered threatened (Barnosky et al., 2011).  If these species thus categorized were to all go extinct, then the magnitude of species loss could approach 40% (Barnosky et al., 2011).  The loss of 40% of mammal species, which would trigger co-extinctions of many other species, is a significant quantity because it indicates movement toward 75%, which if accounting for all species, would be the tragic milestone in the scale of species losses to be characterized as a mass extinction (Barnosky et al., 2011).

	The study by Barnosky et al., considered in the previous paragraph, also examined rate-magnitude information.  Although the Big Five extinctions (the previous mass extinctions pre-dating the presence of humans) took a period longer than 500 years, it is possible to compute what their extinction rates would be if their species losses were compressed into 500 years.  This value is higher than the extinction rate of the past 500 years since 1500, but if all presently threatened species were to disappear, then the current rate would be comparable (Barnosky et al., 2011).  The study asked how many years it would take before the current magnitude of species losses matches the magnitude of the Big Five (Barnosky et al., 2011).  If all threatened species were to be lost within this century, the two magnitude values would be comparable in 240 to 540 years; in the specific case of mammal extinctions, it would take 334.4 years (Barnosky et al., 2011).  From this outcome, Barnosky et al. predicted that the Anthropocene mass extinction may occur soon within the next three centuries (Barnosky et al., 2011).

	Simulations projected large increases to the current rate of extinction between now and 2100 (Anderson et al., 2020).  With many species endangered already and continuing growth of the human population, there will be large biodiversity loss in this time frame (Anderson et al., 2020).  A startling statistic finds that of all terrestrial animal species, numbering between 5 and 9 million, there are somewhere between 11,000 and 58,000 species losses every year, including many which have not yet been identified by scientists (Dirzo, 2014).  What would happen were Earth to irrevocably enter the Anthropocene mass extinction?  As had happened after each of the Big Five mass extinction, biodiversity would rebound afterwards (Barnosky et al., 2011; Pievani, 2013).  However, the time scale for such a recovery would be hundreds of thousands of years to a hundred million years in the future (Barnosky et al., 2011; Pievani, 2013).  The recovery of species through targeted conservation efforts at present is theoretically possible (Schipper et al., 2008).  Yet, the ecological consequences of human impact have not been moderated at a global scale (Dirzo, 2014).  Although conservation efforts are mounted in different regions of Earth, they have not been able to significantly reverse the trend of biodiversity loss (Steffan et al., 2011).  As Pievani, philosopher of biology, pointed out, while humans need a healthy Earth, the Earth does not need us (Pievani, 2014).  If we push ourselves and the Earth to the point where we ourselves become extinct, the ultimate cessation of human activities will allow new species to blossom around the world so that evolution would enter a new cycle, as had happened after each of the previous mass extinctions (Pievani, 2014).  

Conclusion

	The Anthropocene epoch indicates a new geological epoch where the force of human activities becomes comparable to the forces of nature in the evolutionary context.  The interconnectedness of species inside ecosystems show that the disappearance of single species can trigger ecological changes beyond the loss of the one species (Dirzo et al., 2014).  Mammals as a group are under intermediate threat (Dirzo et al., 2014).  Yet, with 1 in 4 species threatened with extinction, the situation may be more serious than the category suggests (Dirzo et al., 2014).  Through statistical methods, we see that rate, magnitude, and rate-magnitude models are all pointing toward significant losses in biodiversity in the near future (Barnosky et al., 2011).  If all currently critically endangered species were lost soon, the Anthropocene mass extinction, if not already here, will begin within the next few centuries (Barnosky et al., 2011).  Human impact on Earth may even lower the probability of civilization’s continuation and threaten the existence of Homo sapiens (Steffan et al., 2011).  Considering the drastic consequences of accelerating biodiversity loss, information for the public about the crisis must become accessible.  Research into how science communication can reach a widespread audience and how the role of science communicators can increase public awareness that could enable conservationist action could be important.  Research into how conservation efforts may be coordinated and expanded will also contribute to significant reversals of the trend in biodiversity losses. 


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