Biogeography of the Anthropocene

I Introduction

The prevalence of human influences on the biosphere requires rethinking the scope and goals of biogeography. If many ecological, chemical, geological, and evolutionary processes have been altered by people, then the evaluation of the implications of those changes should be a primary goal and organizing paradigm of the study of the distribution of living organisms on Earth. Recognizing the current time period as the ‘Anthropocene’ helps to elevate the recognition of human influences (Crutzen, 2002; Steffen et al., 2011). Here I present the first of three progress reports that will attempt to predict the scope of a new biogeography of the Anthropocene.

Homo sapiens is a primate species that converts the Earth’s land surface into cover types that provide food, fiber, and housing for itself and for commensal species domesticated and utilized for companionship, products, and services. Many other species have had their distributions and abundances changed by direct and indirect actions of this species, affecting global biodiversity patterns (Butchart et al., 2010). Given the associated alterations in global nitrogen, carbon, and water cycles, the planet’s biogeochemistry differs from what it would have been had this particular species not evolved (Rockström et al., 2009). In fact, H. sapiens may be unique with regard to the degree to which one particular species has altered global conditions. Current projections of altered climatic conditions because of human-caused increases of greenhouse gases, combined with the legacy and likely future trajectories of land-cover change, suggest a resulting directional trend in global conditions at least for the next several centuries (Diffenbaugh and Field, 2013). Not only will equilibrium biogeographical and ecological theories be inadequate for making predictions under novel conditions (Barnosky et al., 2012), but many assumptions of those theories will need to be recast and applied considerations reformulated.

As one example, consider that the goals of nature conservation in the Anthropocene would be quite different from goals developed for maintaining the species and ecosystems of the late Holocene, a realization that has implications for conservation biogeography and other applied uses of biogeographical theory. This progress report begins with an overview of how the Anthropocene can be recognized and delimited in the context of Earth history. Then the evidence for the likely presence of novel species assemblages in the future is evaluated, including an overview of the tools available for their assessment. Future progress reports will examine more closely the implications for other species of human influences on the biosphere.

II Recognizing the Anthropocene

The geological evidence needed to justify the recognition of the Anthropocene by Earth scientists includes changes in deposition and fossilization processes that would presumably be apparent in the geological record of the future. Zalasiewicz (2008) offers an entertaining perspective set 100 million years from now, as the role of humans in affecting the Earth is assessed using these criteria from the perspective of the distant future. Steffen et al. (2011) formally proposed that current biophysical conditions are no longer those of the Holocene, and that we should be considered to be living within a new geological epoch. Their in-depth assessment suggests that the Holocene-to-Anthropocene transition occurred in the 1800s, as they regard the rise of greenhouse gases, particularly carbon dioxide and especially as resulting from the burning of fossil fuels, to be a key diagnostic threshold (Steffen et al., 2007). There are many coincident or causally related changes affecting species extinctions, land-cover change, and the introduction of exotic species.

Ruddiman (2013) provides an alternative timing to the onset of the Anthropocene, assembling diverse data sets to suggest an earlier rise in greenhouse gases in human history, associated especially with carbon dioxide resulting from ancient human-caused deforestation, and also from methane resulting from paddy agriculture during the development of agriculture in Asia and due to the domestication of livestock in Europe and Asia. As he acknowledges, an earlier start of the Anthropocene complicates its recognition in the geological record given the heterogeneity of extent and type of agriculture around the world in the early and middle Holocene; he suggests that it may be best to utilize the ‘Anthropocene’ as an informal concept rather than as a strictly defined and formal geological time period.

Nevertheless, Wolfe et al. (2013) give specific criteria that they found useful for demarcating different depositional regimes affecting lake sediments in remote areas with minimal direct human impact. As they note, the switch from late Pleistocene to early Holocene sediments can be clearly detected by the presence of proglacial inorganic material overlain by later non-glacial organic materials. Similarly, they find a more recent switch to depleted stable nitrogen isotopic composition (N¹⁵) in the sediments from the 1950s to 1980s, and associated global nitrogen cycle shifts and local changes in types of diatoms and chrysophytes in the lakes. Brown et al. (2012) and Zalasiewicz et al. (2011) provide other criteria that can be used for delimitation of the Anthropocene as informed by geologic and geomorphic data and methodologies.

There is a wide range of additional criteria that could be used by biogeographers to examine the biodiversity and ecosystem consequences of change associated with the Anthropocene. For example, Galetti and Dirzo (2013) discuss a series of feedbacks resulting from the loss of medium to large native vertebrates, often due to hunting and to habitat changes. In these defaunated landscapes, without top predators, there are immediate consequences for plant physiology and the behavior of the remaining animal species, longer-term effects for vegetation dynamics and associated ecosystem processes, and eventual evolutionary changes likely to affect the remaining flora and fauna. Dudgeon (2011) gives an aquatic example, using the dramatic differences in the post-dam Yangtze River, to point to pervasive effects of human alterations of water and sediment flows on native species of Asia and the concomitant consequences for water quality and fisheries. Rogers and Chown (2013) analyzed the bird species associated with non-native Acacia thickets in South Africa; they found that although some bird feeding guilds, including mixed feeders and insectivores, were well represented, native nectavores were not abundant in the anthropogenic vegetation. In addition to changes in trophic relations, other ecosystem changes with a human dimension include dramatic increases in nitrogen, alterations in hydrologic cycles, and both increases and decreases in carbon sources and sinks (Diffenbaugh and Field, 2013; MacDougall et al., 2012).

Humans have controlled the use of fire for many millennia (Bowman et al., 2009), altering land cover and disturbance regimes as a result. Agriculture permitted the settlement of larger populations, and further altered land cover, replacing native vegetation with crops, fallowed lands, pastures, and cities (Ellis, 2011; Ruddiman, 2013). From the very start, hunting, fishing, and harvesting of plant products have applied selective pressures on wild species, with conspicuous and useful species often over-exploited, resulting in reduced densities or even local extinction. Dangerous and nuisance species might be eliminated when feasible, or otherwise avoided. Kareiva et al. (2007) refer to these processes and events as the domestication of nature. The loss of some species and the spread of human-designed land cover will make different places more similar in terms of their biological diversity. Baiser et al. (2012) rigorously compared paired sites in terms of species similarity, showing that biotic homogenization proceeds as native species are lost and introduced species become more common; they tracked these changes through time and space, and found evidence that it is affecting spatial turnover for many different plant and animal groups.

In summary, global climate change will have worldwide dimensions and local consequences for plants and animals, and also for the ecosystems they help constitute. For some purposes, these changes are sufficient to recognize a new point in Earth history, at least with regard to living organisms. Mahlstein et al. (2013) found that the pace of change of climate zones will increase at the rate of warming, following a near linear trend. This puts about 20% of all land area into a different climate zone by 2100, using an analysis based on Köppen-Geiger climate types. The capacity of species to move, adapt, or persist will be crucial, which in turn will depend not only on species traits but on the landscape mosaics available for movement through and colonization within (e.g. Young, 2009). This is a unique transformation in terms of Earth history, given rates of change orders of magnitude faster than, for example, climate change in the Eocene (Diffenbaugh and Field, 2013).

III No-analog biogeography

Williams et al. (2007) did seminal research on novel or no-analog species assemblages resulting from likely unique future biophysical conditions. They calculated change from current conditions as standardized against interannual variability. Doing so revealed large expanses of tropical and subtropical land areas that will change dramatically by being placed into novel climatic conditions, including the biodiverse Amazonian and Indonesian rainforests. They identify other places likely to have ‘disappearing’ climates, especially notable in tropical mountain regions, suggesting the inadequacy of current protected areas for providing critical habitat in the future. Many places will have novel mixes of species, living in future climatic zones not currently found on Earth. They and other researchers have expanded upon that approach by further assessing the paleorecord for examples and inspiration, and by searching for empirical evidence of present-day novel species assemblages. Jackson et al. (2009), for example, illustrate the importance of different combinations of seasonal rhythms of temperature and humidity that collectively provide multiple ways that novel climatic conditions can be produced. They welcome more robust analyses that go beyond simple niche-climate correlations as the findings would presumably be more realistic, more useful, and more able to include the historical and spatial contingencies that are behind many place-to-place differences in biotic composition.

Some of the clearest evidence for the likelihood of novel species assemblages in the future comes from the extrapolation of findings from paleoecological studies that reveal species living together despite their not sharing ranges today (or vice versa). Such data make untenable simplistic ecological assumptions of species being highly co-evolved and co-adapted. Instead, the simplest working premise is that species move through climatic and ecological space independently from each other, such that climate or other global or regional biophysical change results in new groupings of species in particular places. Corea-Metrio et al. (2012), as one example, were able to use pollen and charcoal records from lake sediments in the Petén of Guatemala to put together an 86,000-year record of biophysical change and associated biotic shifts. They found no-analog climate conditions and interpret associated rapid ecological changes seen in altered pollen frequencies as being associated with both global warming and cooling processes, as mediated through local climates, depositional processes, and forest dynamics. Because climate predictions for their study region are more extreme in terms of temperature increases than any they document over 86 millennia, they end by forecasting a future no-analog climate and local species extinctions. Mergeay et al. (2011) examined an 1800-year record of 10 Daphnia species in lake sediments of a site in Kenya, with important shifts in presence and abundance best explained through the effects of contingency interacting with species traits and lake levels, in turn affected by climate. Finally, Lorenzen et al. (2011) used species distribution modeling, along with fossil evidence and DNA analyses, to show how humans have affected differentially the demographic processes of large mammals that made up Pleistocene to Holocene megafauna in the Northern Hemisphere. Some species thrived and some went extinct, sometimes in ways that suggest that climate was mostly to blame, in others that the human role was much more important. Each species evaluated responded individualistically, with climate change the best explanation for extinctions of Eurasia musk ox and woolly rhinoceros, while human-climate effects likely most affected wild horse and steppe bison.

A rather different way of thinking about no-analog situations comes from researchers concerned with the prevalence of novel species assemblages formed, not by possible future climate combinations, but as the result of species introductions by people and land use. For example, Hobbs et al. (2006) find biogeographic novelty coming from the recovering ecosystems in places where intensive monoculture land use has been halted, and where the dominant colonizing species are invasives from other continents. They wonder if these species assemblages will persist on their own in the future. They suggest that ecosystem values come from such situations, although they may require new and proactive management goals and methods to be maximally useful for environmental services (Seastedt et al., 2008).

IV Tools for evaluation

Recent studies have included a variety of approaches for examining possible unique future biophysical conditions acting upon both species and ecosystems, including modeling, the comparison of rates of biophysical change contrasted with dispersal rates of different kinds of organisms, and experiments using relocations and transplants or physiological measurements of tolerance to impending conditions (Dawson et al., 2011; Jackson et al., 2009). Murray et al. (2012) reviewed the usefulness of carefully evaluating the predictions of dynamic global vegetation models. Elith and Leathwick (2009) and Svenning et al. (2011) provide reviews of ways that species distribution modeling may be used to assess both past and future conditions. Loarie et al. (2009) undertook a modeling study on how fast climate change may affect the natural environments of California, with especially rapid change predicted for areas with relatively little topographic relief. Soudzilovskaia et al. (2013) assembled large data sets of plant traits to predict vegetation responses to future warming. They hope to use such approaches to also evaluate the services provided by likely future ecosystems. Finally, Lorimer and Driese (2014) summarize experiments done to ‘recreate’ wild environments. They have done this in the Netherlands with findings that they think challenge how nature conservation has been carried out. As can be seen, these varied research strategies permit examination of novel conditions from multiple perspectives.

Veloz et al. (2012) worry that niche theory and the use of species distribution modeling may not be adequate for predicting these kinds of future biogeographic changes. They are able to draw upon records of past plant distributions as represented by fossil pollen to show that in some cases it would have been impossible to predict the actual species ranges given existing knowledge of climatic controls. Some species may have been associated in the past with climate types that do not exist today. Hence, their realized niches are not static over millennia, or their fundamental niches are broader than current distribution data would suggest. Serra-Diaz et al. (2013) quantified possible futures of eight endemic California tree species by working in a relatively small geographic region, by considering topographic relief, and by including possible migration routes; however, it was also notable that each tree species differed in bioclimatic velocity and predicted consequences of future range shifts.

One important limitation on this new approach to biogeography is a lack of ecological knowledge on how novel species assemblages will co-exist and interact – for example, if they will compete, facilitate, or even consume each other. De Frenne et al. (2013) look at the effect of future conditions favoring dominance by warm-climate species, which they call ‘thermophilization’. However, their empirical data from hundreds of permanent vegetation plots point to differences in response of canopy-forming compared to understory species. They interpret these dissimilarities as the consequence of forest canopies increasing in cover and hence shielding the smaller stature, understory plant species from the direct effects of climate change.

Another constraint is a paucity of actual measurements of dispersal and few assessments of how and when successful colonization takes place. In both of these situations, there will also be continuing directional, perhaps stepped, changes in temperature, precipitation, soil moisture, and other biophysical controls on species distributions that will complicate prediction given that there is no foreseeable endpoint to ecological change. Biermann et al. (2012) suggest the need for new institutions to deal with the research and management challenges inherent to the Anthropocene, although they do not discuss the role of the discipline of biogeography. Biogeographers could organize research networks that support monitoring and experimentation.

In terms of biodiversity conservation, Richardson et al. (2009) propose ways to evaluate managed relocations of species affected by climate change, altering some of the recommendations of the Hoegh-Guldberg et al. (2008) scheme by including additional ecological data and social values for consideration in decision making. Dawson et al. (2011) review ways to assess vulnerability arising from the interaction of the degree of tolerance of the species, the likelihood of habitat shift, the capacity of the species to migrate, and its propensity for extinction. There are rich materials here for applied and experimental biogeographical studies.

Ackerley et al. (2010) demonstrate how landscape-scale issues affect the ways that climate change will alter biodiversity patterns. For their study area in California and Nevada, they show the probable loss of high-elevation climate types and associated specialized biota. Most of the protected areas they evaluated will be greatly affected by climate change, suggesting that these conservation areas will be foci of change, rather than refugia from it. Bradley et al. (2012) raise concerns associated with conflicts between food security and nature conservation under future climates. They note that protected areas were in many cases placed in areas judged to have little agronomic value, but that in future climates such areas may be desirable for wheat and maize cultivation, creating social dilemmas that will complicate conservation decisions. Both these studies reinforce the need for evaluating novel biogeographical contexts for biodiversity conservation efforts.

Conservation paleobiogeography looks for lessons from the past that are relevant to current or predicted future conservation challenges. Posadas et al. (2013) and Wen et al. (2013) have recently reviewed trends in the study of historical biogeography, pointing out how advances in methods and data management are permitting new insights into the phylogeography and evolution of lineages. Meadows (2012) illustrates the advantages of using a longer-term perspective to help inform future predictions of change. Williams et al. (2011) illustrate a variety of examples of abrupt ecological change in the paleorecord, some of which were driven by climate and show coherency over relatively large areas, but others of which were much more idiosyncratic, caused presumably by more local factors such as disturbances and feedbacks with topography. Finally, Vegas-Vilarrúbia et al. (2011) give examples using Quaternary paleoecology to inform nature conservation efforts. Like similar studies, their analysis is premised on species behaving individualistically. They found that species extinctions did not happen if refugia were present. They also identify an important fire-climate-people synergy that further highlights the importance of considering human roles in shaping biogeographic outcomes.

V Summary

Next steps for the study of the Anthropocene include further organization of the scientific and educational infrastructure needed to evaluate this time period from a multidisciplinary approach (e.g. Biermann et al., 2012; Steffen et al., 2011; Wen et al., 2013). Harden et al. (2014) outline ways that Earth surface studies of many kinds could be reoriented to examine the Anthropocene. Biogeographers could have a particularly important role in this endeavor, as demonstrated in this progress report. Much evidence of human impact is seen in the presence of exotic species and with divergences in species abundances, with some species eliminated and others favored. Many of the land-cover changes and associated shifts driven by altered global atmospheric and hydrologic processes will result in novel vegetation types and faunistic assemblages with no previous history on Earth.

There are challenges, in terms of research goals, methodologies available, and data sets that would need to be addressed in studying the Anthropocene. The assumption of individualistic species behavior will not work for pairs of obligatory mutualistic species, and there may be other situations in which such assumptions are untenable. Some scholars have raised almost existential concerns about views of nature, as recognition of the Anthropocene makes it obvious that humans are part of nature and nature is in part a reflection of the actions of people (Lorimer, 2012; Robbins and Moore, 2012; Sayre, 2012). Ogden et al. (2013) posit a set of social justice concerns that can and should be approached through global stewardship, while Caro et al. (2011) worry that assuming humans have altered everything will wrest away interest and passion for resolving some kinds of environmental problems.

Conservation biogeography has been viewed as the application of biogeographic research to environmental concerns as they affect biological diversity (Richardson and Whittaker, 2010; Whittaker et al., 2005). Reorganizing it to include a direct, yet critical, focus on the Anthropocene would appear to be a useful addition to this and other aspects of applied biogeography. It may also provide a means to continue to examine human-nature relationships in unexpected and multidimensional ways.