Wilderness is dead: Whither critical zone studies and geomorphology in the Anthropocene?

Anthropocene 2 (2013) 4–15

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Wilderness is dead: Whither critical zone studies and geomorphology in the Anthropocene? Ellen Wohl * Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, USA

A R T I C L E I N F O

A B S T R A C T

Article history: Received 14 December 2012 Received in revised form 18 March 2013 Accepted 19 March 2013

Numerous studies document the extent and intensity of human appropriation of ecosystem services and the manipulation of Earth’s surface and fluxes of water, sediment and nutrients within the critical zone of surface and near-surface environments. These studies make it increasingly clear that wilderness is effectively gone. This paper explores the implications for critical zone studies and management from a geomorphic perspective. Geomorphologists possess knowledge of the long history of human alteration of the critical zone. This knowledge can be applied to characterizing: historical range of variability and reference conditions; fluxes of matter and energy; and integrity and sustainability of critical zone environments. Conceptual frameworks centered on connectivity, inequality, and thresholds or tipping points are particularly useful for such characterizations, as illustrated by a case study of beaver meadows in the Front Range of Colorado, USA. Specifically, for connectivity, inequality, and thresholds, geomorphologists can identify the existence and characteristics of these phenomena, quantify and predict changes resulting from past or future human manipulations, and recommend actions to restore desirable conditions or prevent development of undesirable conditions. I argue that we should by default assume that any particular landscape has had greater rather than lesser human manipulation through time. This history of manipulation continues to influence critical zone process and form, and geomorphologists can use knowledge of historical context in a forward-looking approach that emphasizes prediction and management. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: Critical zone Connectivity Thresholds Sustainability Tipping points

1. Introduction Wilderness is defined in the U.S. 1964 Wilderness Act legislation ‘‘as an area where the earth and the community of life are untrammeled by man, where man himself is a visitor who does not remain.’’ This is a slightly more poetic rendering than the usual dictionary definitions of ‘‘a tract or region uncultivated by human beings’’ or ‘‘an area essentially undisturbed by human activity together with its naturally developed life community.’’ The common thread in diverse definitions of wilderness is the absence of humans and their influences. Opinions diverge on how strictly to interpret influences, or even on whether wilderness is anything but a social construct or a romantic myth (Lowenthal, 1964). Assuming wilderness is a useful designation for a landscape, can a region qualify as wilderness only if people have never influenced the landscape and ecosystem, or can it qualify if people are not influencing the landscape and ecosystem at present? To paraphrase Justice Potter Stewart, wilderness may be one of those entities that is

* Tel.: +1 9704915298. E-mail address: [email protected] 2213-3054/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ancene.2013.03.001

hard to define, although everybody knows it when they see it. Or do they? In this paper, I argue that in fact many of us mistake landscapes altered by humans in the past for wilderness that has never experienced substantial human influences, and that this misperception hampers our ability to understand the intensity and extent of human manipulation of Earth surfaces. By more fully comprehending the global implications of human manipulations during the Anthropocene, we can more effectively design management to protect and restore desired landscape and ecosystem qualities. This is a perspective paper rather than a presentation of new research results. I write from the perspective of a geomorphologist, but much of what I describe below applies to anyone who studies the critical zone – Earth’s near-surface layer from the tops of the trees down to the deepest groundwater – and who wishes to use knowledge of critical zone processes and history to manage landscapes and ecosystems. I use landscape to refer to the physical configuration of the surface and near-surface – topographic relief, arrangement of river networks, and so forth – and the fluxes that maintain physical configuration. I use ecosystem to refer to the biotic and non-biotic components and processes of a region. In practice, the two entities are closely intertwined because the landscape creates habitat and resources for the biota and biotic

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activities shape the landscape. I distinguish the two entities only because the time scales over which each changes can differ and the changes may not be synchronous. The title of this paper alludes to the now well-known paper, ‘‘Stationarity is dead: whither water management?’’ (Milly et al., 2008). I use the phrase ‘‘wilderness is dead’’ because I interpret wilderness in the strictest sense, as a region that people have never influenced. Given warming climate and rapidly melting glaciers and sea ice, even the most sparsely populated polar regions no longer qualify as wilderness under this interpretation. Just as stationarity in hydrologic parameters has ceased to exist in an era of changing climate and land use, so has wilderness. I use this realization to explore the implications of the loss of wilderness for critical zone studies and management from the perspective of a geomorphologist. I start by briefly reviewing the evidence for extensive human alteration of the critical zone. I explore the implications for geomorphology of a long history of widespread human alteration of the critical zone in the context of three factors of interest to geomorphologists (historical range of variability, fluxes of matter and energy, and integrity and sustainability of critical zone environments). I then explore how concepts of connectivity, inequality, and thresholds can be used to characterize critical zone integrity and sustainability in specific settings. A detailed case study of beaver meadows along headwater streams in the Colorado Front Range, USA illustrates how geomorphologists can uniquely contribute to managing the critical zone. The paper concludes with a discussion of my perspective on how geomorphologists can respond to the understanding that wilderness effectively no longer exists and that humans continually and ubiquitously manipulate the distribution and allocation of matter and energy. 2. Humans, humans everywhere, nor any land left wild Water, water everywhere, nor any drop to drink. – Samuel Taylor Coleridge. Numerous papers published during the past few years synthesize the extent and magnitude of human effects on landscapes and ecosystems. By nearly any measure, humans now dominate critical zone processes. Measures of human manipulation of the critical zone tend to focus on a few categories. (1) Movement of sediment and reconfiguration of topography. Humans have increased sediment transport by rivers globally through soil erosion (by 2.3  109 metric tons/y), yet reduced sediment flux to the oceans (by 1.4  109 metric tons/y) because of sediment storage in reservoirs. Reservoirs around the world now store > 100 billion metric tons of sediment (Syvitski et al., 2005). By the start of the 21st century, humans had become the premier geomorphic agent sculpting landscapes, with exponentially increasing rates of earth-moving (Hooke, 2000). The latest estimates suggest that >50% of Earth’s ice-free land area has been directly modified by human actions involving moving earth or changing sediment fluxes (Hooke et al., 2012). (2) Appropriation of ecosystem services. Human activities appropriate one-third to one-half of global ecosystem production, and croplands and pastures now cover about 40% of Earth’s land surface (Foley et al., 2005). Most measures of global human consumption have accelerated dramatically since 1950, including number of motor vehicles, fertilizer consumption, amount of domesticated land, and loss of forested land (Syvitski, 2012). The U.N. Food and Agriculture Organization estimates that 87% of the world’s commercially important marine fisheries are fully fished, overexploited, or depleted (FAO, 2012).

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(3) Alteration of biogeochemical fluxes. Irrigated agriculture has expanded globally by 174% since the 1950s (Scanlon et al., 2007), and this has been accompanied by substantially increased riverine fluxes of pesticides and nitrogen from fertilizers (Boyer et al., 2006). Although reservoirs store some of this increased flux (e.g., reservoirs store an estimated 1–3 billion tons of carbon; Syvitski et al., 2005), eutrophication of nearshore areas is now common around industrialized countries (Mitsch et al., 2001). (4) Total extent of alteration. In the first estimate of this type, McCloskey and Spalding (1989) suggested that one-third of the global land surface remained wilderness, although 41% of this wilderness was in the Arctic or Antarctica. More recent estimates indicate that >75–83% of Earth’s ice-free land area is directly influenced by human beings (Sanderson et al., 2002; Ellis and Ramankutty, 2008), and the remaining 25% is indirectly influenced by climate change and atmospheric deposition of human-derived contaminants. (5) River alteration. Over half of the world’s large river systems are affected by dams (Nilsson et al., 2005), and nearly all rivers are at least partly affected by dams, levees, channelization, flow diversion, and altered water, sediment and solute yields from the adjacent uplands (Wohl, 2004, 2011a). In the United States, only 2% of river kilometers are unaffected by dams (Graf, 2001). This equates to 1 dam per every 48 km of river among 3rd through 7th order rivers (Poff et al., 2007). Extensive flow regulation has resulted, among other things, in homogenization of flow regimes and reduced diversity of riverine biota (Poff et al., 2007). An important point to recognize in the context of geomorphology is that, with the exception of Hooke’s work, most of these studies focus on contemporary conditions, and thus do not explicitly include historical human manipulations of the critical zone. Numerous geomorphic studies, however, indicate that historical manipulations and the resulting sedimentary, biogeochemical, and topographic signatures – commonly referred to as legacy effects – are in fact widespread, even where not readily apparent (e.g., Wohl, 2001; Liang et al., 2006; Walter and Merritts, 2008). Initial clearing of native vegetation for agriculture, for example, shows up in alluvial records as a change in river geometry in settings as diverse as prehistoric Asia and Europe (Limbrey, 1983; Mei-e and Xianmo, 1994; Hooke, 2006) and 18th- and 19thcentury North America and Australia (Kearney and Stevenson, 1991; Knox, 2006). The concept of wilderness has been particularly important in regions settled after the 15th century by Europeans, such as the Americas, because of the assumption that earlier peoples had little influence on the landscape. Archeologists and geomorphologists, in particular, have initiated lively debates about the accuracy of this assumption (Denevan, 1992; Vale, 1998, 2002; Mann, 2005; James, 2011), and there is consensus that at least some regions with indigenous agricultural societies experienced substantial landscape and ecosystem changes prior to European contact. Many of the overview studies cited above also quantify the current magnitude and distribution of human alteration of natural fluxes, rather than explicitly considering interactions between humans and landscapes or ecosystems. Geomorphologists increasingly focus on such interactions in the form of feedback loops between resource use, landscape stability, ecosystem processes, resource availability, and natural hazards (Chin et al., in press). An example comes from the sediment budget developed for the Colorado River in Grand Canyon (Wiele et al., 2007; Melis, 2011). Much of the river sand within Grand Canyon comes from upstream and is now trapped by the dam, but sand also enters Grand Canyon via tributaries downstream from the dam. Sand present along the

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main river corridor at the time of dam closure can also be redistributed between channel-bed and channel-margin storage sites. Alteration of water and sediment fluxes by Glen Canyon Dam has led to beach erosion and loss of fish habitat in Grand Canyon, affecting recreational river runners and endemic native fish populations. Resource managers respond to these landscape and ecosystem alterations by experimenting with different ways of operating the dam. The availability and distribution of sand-sized sediment drives decisions as to when managers will create experimental floods by releasing larger-than-average volumes of water from the dam. 3. Geomorphology in the Anthropocene Given the documented extent and intensity of human alteration of the critical zone, a vital question now is how can geomorphologists most effectively respond to this state of affairs? More than one recently published paper notes the absence of a geomorphic perspective in discussions of global change and sustainability (e.g., Grimm and van der Pluijm, 2012; Knight and Harrison, 2012; Lane, 2013). Geomorphologists certainly have important contributions to make to scholarly efforts to understand and predict diverse aspects of global change and sustainability, but thus far the community as a whole has not been very effective in communicating this to scholars in other disciplines or to society in general. Scientists as a group are quite aware of existing and accelerating global change, but there may be less perception of the long history of human manipulation of surface and near-surface environments, or of the feedbacks through time between human actions and landscape configuration and process. Geomorphologists can particularly contribute to increasing awareness of human effects on the critical zone during past centuries. Geomorphologists can also identify how human-induced alterations in the critical zone propagate through ecosystems and human communities – that is, geomorphologists can contribute the recognition that landscapes are not static entities with simple or easily predictable responses to human manipulation, but are rather complex, nonlinear systems that commonly display unexpected responses to human alteration. Awareness of the ubiquity and long history of human manipulation of the critical zone, and of the complexity of critical zone processes, has implications for at least three factors with which geomorphologists are concerned and to which we can contribute specialized knowledge and methods of inquiry: historical range of variability and reference conditions; fluxes of matter and energy within the critical zone; and the integrity and sustainability of critical zone environments and biotic communities. 3.1. Historical range of variability Historical range of variability (HRV), like wilderness, has varying definitions. HRV is most commonly used to refer to the temporal and spatial range of variability in a specified parameter or environment prior to intensive human alteration (Morgan et al., 1994; Nonaka and Spies, 2005; Wohl, 2011b), but the phrase sometimes refers to variability during the period of intensive human alteration (Wohl and Rathburn, in press). I use the phrase here in the former sense. Ability to characterize HRV in a highly altered landscape inevitably relies on indirect indicators that range from historical (human-created archives of maps, text, or photographs), through biotic (tree rings, pollen in sediments, invertebrate fossils), to sedimentary and geochemical records. Geomorphologists are specifically trained to interpret past landscape process and form using physical records contained in sedimentary and geochemical data. We can thus make vital contributions to the collective effort to understand how a given

portion of the critical zone has varied through time in response to natural and human-induced disturbances. HRV is also sometimes delineated for contemporary landscape process and form at sites exhibiting reference conditions. Reference conditions can be defined as the best available conditions that could be expected at a site (Norris and Thoms, 1999) and described using historical or environmental proxy records or comparison to otherwise similar sites with lesser human alteration (Morgan et al., 1994; Nonaka and Spies, 2005). Interpretation of contemporary, relatively unaltered landscape units as indicators of reference conditions is a form of the traditional ‘paired watershed’ approach, in which differences between treated and reference watersheds that are otherwise similar are used to infer the behavior and significance of a particular variable. A paired watershed study might test for differences in channel morphology, for example, between a population of reference watersheds and a population of treated watersheds in which peak flow has doubled as a result of land use (David et al., 2009). Whatever approach is taken, HRV is difficult to quantify. There is the challenge of defining when humans began to intensively alter critical zone process and form. Process and form are complexly interrelated and change substantially through time and space in the absence of human activities, as well as in response to human activities. Ability to quantify the range of variability in individual parameters or entire ecosystems strongly depends on the length and completeness of proxy records, as well as scientific understanding of the operation of the unaltered ecosystem. Reliance on reference conditions in a contemporary, relatively unaltered ecosystem can be misleading because contemporary conditions reflect only a single state or limited portion of the HRV (SER, 2002). In other words, we cannot metaphorically point to some time prior to the development of agriculture or other intensive human activity and use information regarding ecosystem conditions from this time as a precise target for managing and restoring an ecosystem. But, geomorphologists can help to inform understanding of HRV, particularly by emphasizing (i) the depth and breadth of records of the critical zone contained in landforms, (ii) the extent, intensity, variety and duration of past human alterations of the critical zone, and (iii) the dynamic nature of landscape processes. 3.2. Fluxes Fluxes of matter and energy within the critical zone influence landscape configuration and the processes that maintain or alter that configuration – in other words, geomorphology. Since its origin, geomorphology has been especially concerned with the movement of water and sediment at the surface and near-surface (in the atmosphere and below the ground surface), and this focus has broadened to include solutes and particulate organic matter. Geomorphologists have numerous qualitative and quantitative models of water and sediment transport and storage, and many of these models are, or can be, coupled to solute fluxes for hillslope, river, glacial and other environments. Our specialized insight into fluxes – exemplified by equations such as those developed for soil production (Heimsath et al., 1997), hillslope sediment diffusion (Roering et al., 2001), rainfall-infiltration-runoff (Refsgaard and Storm, 1995), flow routing through stream networks (Marks and Bates, 2000), or bedload transport within rivers (Meyer-Peter and Mueller, 1948) – and storage within diverse landforms (e.g., floodplains, terraces, deltas, alluvial fans) positions us uniquely to quantify how past human activities have affected fluxes and to numerically simulate and quantitatively predict the effects of proposed future human manipulations on fluxes. Quantifying magnitude and spatial and temporal dimensions of fluxes is at the

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heart of understanding interactions between human resource use, landscapes and ecosystems, as illustrated by the earlier example of sand fluxes in the Grand Canyon.

incorporate some or all of these basic concepts (e.g., Bull, 1991; Simon and Rinaldi, 2006; Wohl, 2010; Chin et al., in press): in this section, I focus on the basic concepts.

3.3. Integrity and sustainability

4.1. Connectivity

Ecological integrity can be defined as the ability of an ecosystem to support and maintain a community of organisms with species composition, diversity, and functional organization similar to those within natural habitats in the same region (Parrish et al., 2003). This definition focuses on biota, although the physical and chemical processes that sustain the biota are implicitly included. The analogous geomorphic concept is that of physical integrity, defined for rivers by Graf (2001) as a set of active fluvial processes and landforms such that the river maintains dynamic equilibrium, with adjustments not exceeding limits of change defined by societal values. A river has physical integrity when river process and form are actively connected under the current hydrologic and sediment regime. One component of ecological or physical integrity is sustainability. Sustainability is most effectively defined within a specified time interval, but implies the ability to maintain existing conditions during that time interval. Another component of integrity is resilience, which refers to the ability of a system to recover following disturbance. A resilient ecosystem recovers the abundance and diversity of organisms and species following a drought or a tropical cyclone, for example, and a resilient river recovers channel geometry and sediment fluxes following a large flood. Drawing on concepts of ecological and physical integrity, a composite definition for critical zone integrity and sustainability might be a region in which critical zone processes respond to fluxes of matter and energy in a manner that sustains a landscape and an ecosystem with at least minimum levels of diversity. The core concept of this definition is that biotic and non-biotic processes can respond to fluctuations in matter and energy through time and space, rather than being rigidly confined to a static condition. In other words, hillslopes have the ability to fail in landslides during intense precipitation, rather than being shored up by rock bolts and retaining walls, and fish populations have the ability to migrate to different portions of a river network in response to flooding or drought, rather than being partitioned into sub-populations by impassable barriers such as dams or culverts. Layers of vagueness are built into this definition, however. Over what time span must the landscape and ecosystem be sustained? What constitutes an acceptable minimum level of physical or biological diversity? These are not simple questions to answer, but in addressing these questions for specific situations, geomorphologists can make vital and needed contributions to ongoing dialogs about how to preserve vitally important ecosystem services and biodiversity. Focusing on these questions can also force geomorphologists to explicitly include biota in understanding surface processes and landforms. The stabilization of hillslopes or the partitioning of rivers does not really matter in a purely physical context. Although geomorphologists may be interested to know that hillslopes cannot adjust because of stabilization or rivers cannot continue to move sediment downstream because of dams, these issues become critically important only in the context of increased hazards for humans in the hillslope example, or loss of ecosystem services for biotic communities in the dam example.

Connectivity is used to describe multiple aspects of fluxes of matter, energy and organisms (Fig. 1). Hydrologic connectivity refers to the movement of water, such as down a hillslope in the surface and/or subsurface, from hillslopes into channels, or along a river network (Pringle, 2001; Bracken and Croke, 2007). Sediment connectivity describes the movement or storage of sediment down hillslopes, into channels, along river networks, and so forth (Fryirs et al., 2007). River connectivity refers to water-mediated fluxes within a river network (Ward, 1997). Biological connectivity describes the ability of organisms or plant propagules to disperse between suitable habitats or between isolated populations for breeding (Merriam, 1984). Landscape connectivity refers to the movement of water, sediment, or other materials between individual landforms (Brierley et al., 2006). Structural connectivity characterizes the extent to which landscape units, which can range in scale from <1 m for bunchgrasses dispersed across exposed soil to the configuration of hillslopes and valley bottoms across thousands of meters, are physically linked to one another (Wainwright et al., 2011). Functional connectivity describes process-specific interactions between multiple structural characteristics, such as runoff and sediment moving downslope between the bunchgrasses and exposed soil patches (Wainwright et al., 2011). Any of these forms of connectivity can be described in terms of spatial extent, which partly depends on temporal variability. River connectivity, for example, fluctuates through time as discharge fluctuates, just as functional connectivity along a hillslope fluctuates through time in response to precipitation (Wainwright et al., 2011). Connectivity can also be used to describe social components. The terms multidisciplinary, interdisciplinary, holistic, and integrative, as applied to research or management, all refer to disciplinary connectivity, or the ability to convey information originating in different scholarly disciplines, the incorporation of different disciplinary perspectives, and the recognition that critical zone processes transcend any particular scholarly discipline. Beyond the fact that the characteristics of connectivity critically influence process and form in the critical zone, the specifics of connectivity can be used to understand how past human manipulations have altered a particular landscape or ecosystem, and how future manipulations might be used to restore desired system traits. This approach is exemplified by the connectivity diagrams for rivers in Kondolf et al. (2006) (Fig. 2).

4. Conceptual frameworks for characterizing critical zone integrity and sustainability The issues raised above are complex and difficult to address. Three concepts – connectivity, inequality, and thresholds or tipping points – can be helpful in characterizing critical zone integrity and sustainability in specific settings. Numerous conceptual models

4.2. Inequality Connectivity does not imply that all aspects of a landscape or ecosystem are of equal importance to fluxes of energy, matter, and organisms. As scientists from diverse disciplines improve the ability to quantify rates and magnitudes of diverse fluxes, it becomes increasingly clear that the majority of landscape change occurs during relatively short periods of time and that some portions of the landscape are much more dynamic than other portions, as illustrated by several examples. Biogeochemists describe a short period of time with disproportionately high reaction rates relative to longer intervening time periods as a hot moment, and a small area with disproportionately high reaction rates relative to the surroundings as a hot spot (McClain et al., 2003). Numerous examples of inequalities in time and space exist in the geomorphic literature. More than 75% of the long-term sediment flux from mountain rivers in Taiwan occurs less than

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Fig. 1. Schematic illustration of the six degrees of connection between rivers and the greater landscape. The segment of channel (lighter gray) shown here is connected to: upstream and downstream portions of the river network; adjacent uplands; the floodplain; ground water; the hyporheic zone (darker gray); and the atmosphere. The photograph for upstream-downstream connection was taken during a flood on the Paria River, a tributary of the Colorado River that enters just downstream from Glen Canyon Dam in Arizona, USA. In this view, the Paria is turbid with suspended sediment whereas the Colorado, which is released from the base of the dam, is clear. The photograph for the hillslope-channel connection shows a large landslide entering the Dudh Khosi River in Nepal. The photograph for the floodplain-channel connection was taken along the Rio Jutai, a blackwater tributary of the Amazon River, during the annual flood in early June. In this view the ‘flooded forest’ is submerged by several meters of water. The photograph for hyporheic-channel connection shows a larval aquatic insect (macroinvertebrate) as an example of the organisms that can move between the channel and the hyporheic environment. The photograph for atmosphere-channel connection shows a mayfly emerging from the river prior to entering the atmosphere as a winged adult (image courtesy of Jeremy Monroe, Freshwaters Illustrated).

1% of the time, during typhoon-generated floods (Kao and Milliman, 2008). Approximately 50% of the suspended sediment discharged by rivers of the Western Transverse Ranges of California, USA comes from the 10% of the basin underlain by weakly consolidated bedrock (Warrick and Mertes, 2009). Somewhere between 17% and 35% of the total particulate organic carbon flux to the world’s oceans comes from high-standing islands in the southwest Pacific, which constitute only about 3% of Earth’s landmass (Lyons et al., 2002). One-third of the total amount of stream energy generated by the Tapi River of India during the monsoon season is expended on the day of the peak flood (Kale and Hire, 2007). Three-quarters of the carbon stored in dead wood and floodplain sediments along headwater mountain stream networks in the Colorado Front Range is stored in one-quarter of the total length of the stream network (Wohl et al., 2012). Because not all moments in time or spots on a landscape are of equal importance, effective understanding and management of critical zone environments requires knowledge of how, when, and where fluxes occur. Particularly dynamic portions of a landscape, such as riparian zones, may be disproportionately important in providing ecosystem services, for example, and relatively brief natural disturbances, such as floods, may be disproportionately important in ensuring reproductive success of fish populations. Recognition of inequalities also implies that concepts and processresponse models based on average conditions should not be uncritically applied to all landscapes and ecosystems. 4.3. Thresholds/tipping points Geomorphologists are used to thinking about thresholds. Use of the term grew rapidly following Schumm’s seminal 1973 paper

‘‘Geomorphic thresholds and complex response of drainage systems,’’ although thinking about landscape change in terms of thresholds was implicit prior to this paper, as Schumm acknowledged. Geomorphologists typically define a threshold as a significant change in surface process or form, and distinguish extrinsic thresholds that occur under the response of an external variable from intrinsic thresholds that occur without a change in an external variable. Other disciplines such as ecology use thresholds in a similar manner, but the public may be more familiar with the analogous phrase, tipping point, thanks to Malcolm Gladwell’s 2002 book ‘‘The Tipping Point.’’ Gladwell described a tipping point as the point in time when change in a parameter or system is no longer progressive or linear but instead becomes exponential. In the context of the critical zone and geomorphology, we can focus on thresholds that are relatively easy to identify, such as exceeding a regulatory level for a specified substance. Examples include mandated total maximum daily load for a river, permissible nitrate concentrations in drinking water, or standards for particulate matter in the atmosphere. Understanding and manipulating the factors that cause a substance to exceed a regulatory level, or predicting the consequences of that exceedance, are typically more difficult, but at least the exceedance is relatively easy to identify. Identification of thresholds that cause the critical zone to move between alternative stable states is more difficult. Ecologists define alternative stable states as different stable configurations that an ecological community can adopt and that persist through at least small perturbations (Beisner et al., 2003). A community can move from one stable state to another by a sufficiently large perturbation applied to state variables such as population density (in this scenario, different states can exist

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specified time interval, but implies the ability to maintain existing conditions during that time interval. Thresholds associated with exceeding sustainability limits unfortunately seem to be most commonly identified once they have been crossed and a species has gone locally or globally extinct, a biotic community has disappeared locally or globally, or a human community can no longer use a resource such as agricultural soils that have eroded or become saline, fisheries that have collapsed, or ground or surface waters that are no longer potable. Clearly, there is an intellectual challenge in identifying these more complex thresholds before they are crossed, and meeting this challenge has the added substantial benefit of contributing to sustaining critical zone integrity. In addition to a tradition of explicitly identifying thresholds, geomorphology has established conceptual frameworks for considering scenarios in which thresholds are not crossed, as well as the manner in which a system can respond once a threshold is crossed. Relevant geomorphic conceptual frameworks include static, steady-state and dynamic equilibrium (Chorley and Kennedy, 1971; Schumm, 1977), disequilibrium (Tooth, 2000), steady-state versus transient landscapes (Attal et al., 2008), complex response (Schumm and Parker, 1973), lag time (Howard, 1982; Wohl, 2010), and transient versus persistent landforms (Brunsden and Thornes, 1979).

Fig. 2. Schematic illustration of changes in connectivity. The example illustrated here is relevant to the case study from the Colorado Front Range. An ecosystem with a stable, persistent configuration (1) results from the presence of beaver dams that limit longitudinal connectivity along the channel by obstructing flow and creating backwater zones that store water, solutes, and sediment. The beaver dams enhance lateral connectivity between the channel and floodplain by increasing the frequency, duration, extent, and magnitude of overbank flows, as well as the deposition and storage on the floodplain of fine sediment and organic matter. In a scenario of linear response (A), when the beaver dams are removed or fall into disrepair, longitudinal connectivity increases moderately and lateral connectivity decreases substantially (2). The ecosystem assumes a stable, persistent configuration that is very different than configuration 1. Return of beaver colonies and reestablishment of beaver dams can potentially return the ecosystem to a stable, persistent configuration the same as that of configuration 1 (indicated here as configuration 3). In a scenario of non-linear response (B), gradual disintegration of beaver dams once beaver leave a stream results in more rapid increase in longitudinal connectivity than loss in lateral connectivity (configuration 2). Enhanced longitudinal connectivity facilitates channel incision that eventually limits lateral connectivity (configuration 3). Return of beaver and rebuilding of dams quickly limits longitudinal connectivity (configuration 4), but accumulation of sediment along the channel may be required before lateral connectivity is fully restored (configuration 5).

simultaneously), or via a change in the parameters that determine the behavior of state variables and the ways they interact with each other (Beisner et al., 2003). As with ecological integrity, the definition of ecological alternative stable states implicitly includes physical and chemical processes, and can easily be broadened to include geomorphic process and form. Wohl and Beckman (in press), for example, describe wood-rich and wood-poor states in forested mountain streams, and quantify thresholds of instream wood load that can cause a stream to move from one persistent, stable state to another. Arguably the most difficult thresholds to identify, but also the most important, are those that define the limits of sustainability for a species, a biotic community, or a specific resource use by humans. As noted earlier, sustainability is most effectively defined within a

I propose that geomorphologists can effectively contribute to quantifying, predicting, and manipulating critical zone integrity by focusing on connectivity, inequality and thresholds. Specifically, for connectivity, inequality and thresholds, we can provide three services. First, geomorphologists can identify the existence and characteristics of these phenomena. What forms of connectivity exist between a landform such as a river segment and the greater environment, for example? What are the spatial (magnitude, extent) and temporal (frequency, duration) qualities of this connectivity? Where and when do inequalities occur in the landscape – where does most sediment come from and when is most sediment transported? What are the thresholds in fluxes of water, sediment, or solutes that will cause the river to change in form or stability? Second, geomorphologists can quantify changes in connectivity, inequality or the crossing of thresholds that have resulted from past human manipulations and predict changes that are likely to result from future manipulations. How do human activities alter fluxes, and how do human societies respond to these altered fluxes? To continue the river example, how did construction of this dam alter longitudinal, lateral, and vertical connectivity on this river? How did altered connectivity change the distribution of hot spots for biogeochemical reactions in the riparian zone or around instream structures such as logjams? How did altered connectivity result in changed sediment supply and river metamorphosis from a braided to a single-thread river, as well as local extinction of fish species? Third, geomorphologists can recommend actions to restore desired levels of connectivity and inequality, as well as actions that can be taken to either prevent crossing of a negative threshold that results in undesirable conditions, or force crossing of a positive threshold that results in desirable conditions. What characteristics of water, sediment and solute fluxes must be restored downstream from the dam, for example, to re-create key components of critical zone integrity and restore fish populations? What environmental flows must be maintained as flow regulation accelerates on this river (e.g., Rathburn et al., 2009)? 5. A case study from the Colorado Front Range, USA of geomorphic perspectives relevant to managing the critical zone I use the existence of beaver meadows along headwater mountain streams in the Colorado Front Range to illustrate some of

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the ideas proposed in the previous section. Beaver (Castor canadensis in North America and C. fiber in Eurasia) are considered ecosystem engineers that change, maintain, or create habitats by altering the availability of biotic and abiotic resources for themselves and other species (Rosell et al., 2005). The most important ecosystem engineering undertaken by beaver is the construction and maintenance of low dams of wood and sediment. Beaver build dams on even very steep (>7% gradient) and narrow rivers, but where stream gradient is less than 3% and the valley bottom is at least two or three times the active channel width, numerous closely spaced beaver dams can create beaver meadows (Fig. 3). Dams vary from 7 to 74 per km along low gradient streams, with a typical value of 10 dams per km (Pollock et al., 2003). Beaver meadows – large, wet meadows associated with overbank flooding caused by numerous beaver dams along a stream – were first described in Rocky Mountain National Park by Ives (1942), but the term is now more widely used. A beaver dam creates a channel obstruction and backwater that enhances the magnitude, duration and spatial extent of overbank flow (Westbrook et al., 2006). Shallow flows across topographically irregular floodplains concentrate in depressions and this, along with excavation of a network of small, winding ‘canals’ across the floodplain by beaver (Olson and Hubert, 1994), promotes an anabranching channel planform (John and Klein, 2004). Overbank flows enhance infiltration, hyporheic exchange, and a high riparian water table (Westbrook et al., 2006; Briggs et al., 2012). Attenuation of flood peaks through in-channel and floodplain storage promotes retention of finer sediment and organic matter (Pollock et al., 2007) and enhances the diversity of aquatic and riparian habitat (Pollock et al., 2003; Westbrook et al., 2011). By hydrologically altering biogeochemical pathways, beaver influence the distribution, standing stocks, and availability of nutrients (Naiman et al., 1994). Beaver ponds and meadows disproportionately retain carbon and other nutrients (Naiman et al., 1986; Correll et al., 2000; Wohl et al., 2012). As long as beaver maintain their dams, the associated high water table favors riparian deciduous species such as willow (Salix spp.), cottonwood (Populus spp.) and aspen (Populus spp.) that beaver prefer to eat, and limits the encroachment of coniferous trees and other more xeric upland plants. Beaver thus create (i) enhanced lateral connectivity between the channel and floodplain, enhanced vertical connectivity between surface and ground water, and limited longitudinal connectivity because of multiple dams (Burchsted et al., 2010), (ii) hot spots in the river corridor and the greater landscape with respect to habitat and species diversity (McDowell and Naiman, 1986; Snodgrass and Meffe, 1998; Wright et al., 2002, 2003; Wright, 2009; Bartel et al., 2010), nutrient processing and biogeochemical reactions (Correll et al., 2000; Rosell et al., 2005), and carbon storage over time scales of 101–103 years (Wohl et al., 2012), and (iii) a stable ecosystem state that can persist over periods of 102–103 years (Kramer et al., 2012; Polvi and Wohl, 2012). Removal of beaver, either directly as in trapping, or indirectly as in competition with grazing animals such as elk or climate change that causes small perennial streams to become intermittent, drives the beaver meadow across a threshold. Several case studies (e.g., Green and Westbrook, 2009; Polvi and Wohl, 2012) indicate that within one to two decades the beaver meadow becomes what has been called an elk grassland (Wolf et al., 2007) (Fig. 3). As beaver dams fall into disrepair or are removed, peak flows are more likely to be contained within a mainstem channel. Secondary channels become inactive and the riparian water table declines. Peak flows concentrated in a single channel are more erosive: the mainstem channel through the former beaver meadow incises and/or widens, and sediment yields to downstream portions of the river increase (Green and Westbrook, 2009). Nutrient retention and biological

processing decline, organic matter is no longer regularly added to floodplain and channel storage, and stored organic matter is more likely to be oxidized and eroded. As floodplain soils dry out, burrowing rodents can introduce through their feces the spores of ectomyccorhizal fungi, and the fungi facilitate encroachment by species of conifer such as Picea (spp.) that require the fungi to take up soil nutrients (Terwilliger and Pastor, 1999). Once a channel is incised into a dry meadow with limited deciduous riparian vegetation that supplies beaver food, reestablishment of beaver is difficult, and the elk meadow becomes an alternative stable state for that segment of the river. Beaver were largely trapped out of the Colorado Front Range during the first three decades of the 19th century (Fremont, 1845; Wohl, 2001), but beaver populations began to recover within a half century. Beaver population censuses for selected locales within the region of Rocky Mountain National Park date to 1926, shortly after establishment of the park in 1915. Censuses have continued up to the present, and these records indicate that beaver were moderately abundant in the park until circa 1976. As of 2012, almost no beaver remain in Rocky Mountain National Park. This contrasts strongly with other catchments in the Front Range, where beaver populations have remained stable or increased since 1940. The National Park Service has undertaken riparian restoration in the park for 50 years, with increasing activity during the past decade and an emphasis on erecting tall fences that create riparian grazing exclosures designed primarily to exclude elk. Elk (Cervus canadensis) are native to the park. Predation by wolves historically limited the density of elk and kept the animals moving, but wolves (Canis lupus) were hunted to extinction in Colorado by about 1940 (Armstrong, 1972). Elk were hunted to extinction in the vicinity of what later became Rocky Mountain National Park by 1900, but 49 elk were transplanted from the Yellowstone herd in Wyoming during 1913–14 (Hess, 1993). The elk population reached 350 by 1933, when the population was judged to have met or exceeded the carrying capacity of the park’s lower elevation valleys that provide elk winter range (Hess, 1993). Although elk hunting is permitted in the surrounding national forests, hunting is not permitted within the national park and elk have learned to remain within the park boundaries. Elk numbers increased dramatically during the period 1933–1943, decreased in response to controlled shooting during 1944–1961, and subsequently rose rapidly to 3500 by 1997 (Hess, 1993; Mitchell et al., 1999). Like many grazing animals, elk prefer to remain in riparian zones, and matched photos indicate substantial declines in riparian willow and aspen during periods when elk populations increased. Although other factors may have contributed to the recent decline in beaver numbers, increased riparian grazing by elk likely influences beaver food supply and population. Beaver reintroduction in connection with riparian restoration requires, first, that beaver have an adequate supply of woody riparian vegetation for food and for building dams. About 200 aspen trees are needed by each beaver each year (DeByle, 1985). Second, reintroduction requires that the region includes sufficient suitable habitat to permit dispersal and genetic exchange between colonies of beavers on a river and between rivers. Beaver colony size can vary widely, but averages 5–6 animals. Each colony has a minimum territory of 1 km along a stream (Olson and Hubert, 1994). Third, successful reintroduction requires that human communities sharing the landscape accept the presence of beaver. Although the latter point might not seem as important in a national park, beaver continue to be removed in many regions because of perceived negative consequences of their presence, including water impoundments and overbank flooding, felling of riparian trees, and pulses of coarse wood to downstream river segments if beaver dams fail during peak flows.

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Fig. 3. (A) Schematic illustration of feedbacks between beaver dam and valley geometry that can result in the formation of a beaver meadow. Where beaver dam steep, narrow valley segments, only small backwaters form and the narrow valley bottom limits the extent of floodplain storage of water, sediment, and nutrients (lower portion of diagram). Where beaver build dams in lower gradient, wider valley segments, overbank flows of greater spatial extent can result in formation of secondary channels, infiltration and rise of the riparian water table, and storage of fine sediment and nutrients. Establishment of woody riparian vegetation such as willow (Salix spp.) is facilitated by higher water tables, providing a food supply for beaver, and facilitating creation of more dams and a stable, persistent beaver meadow. (B) Schematic illustration of feedbacks that can result in the formation of a beaver meadow or an elk grassland. Feedbacks in upper half of diagram that lead to a beaver meadow are as in (A). In lower half of diagram, absence of beaver dams reduces overbank flow. Riparian water table drops, secondary channels become inactive, and primary channel incises. Xeric upland vegetation encroaches on the valley bottom. Elk grazing can limit regeneration of deciduous woody riparian vegetation and thus limit stability of stream banks. Incised channel can also widen slightly.

Options for riparian restoration in Rocky Mountain National Park include gradual and more abrupt measures. Gradual measures include grazing exclosures that include some lag time for woody riparian vegetation to regrow, self-reintroduction of

beaver from populations outside the park boundaries, and measures to limit elk populations to 600–800 animals within the park. Relatively abrupt approaches to riparian restoration include engineered logjams or other obstacles to downstream

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fluxes of water and sediment within the primary channel, and active reintroduction of beaver. Geomorphologists can contribute to management decisions in at least three ways. First, geomorphologists can identify the existence and characteristics of longitudinal, lateral, and vertical riverine connectivity in the presence and the absence of beaver (Fig. 2). Second, geomorphologists can identify and quantify the thresholds of water and sediment fluxes involved in changing between single- and multi-thread channel planform and between elk and beaver meadows. Third, geomorphologists can evaluate actions proposed to restore desired levels of connectivity and to force elk meadows across a threshold to become beaver meadows. Geomorphologists can bring a variety of tools to these tasks, including historical reconstruction of the extent and effects of past beaver meadows (Kramer et al., 2012; Polvi and Wohl, 2012), monitoring of contemporary fluxes of water, energy, and organic matter (Westbrook et al., 2006), and numerical modeling of potential responses to future human manipulations of riparian process and form. In this example, geomorphologists can play a fundamental role in understanding and managing critical zone integrity within river networks in the national park during the Anthropocene: i.e., during a period in which the landscapes and ecosystems under consideration have already responded in complex ways to past human manipulations. 6. Toward a new mindset My impression, partly based on my own experience and partly based on conversations with colleagues, is that the common default assumption among geomorphologists is that a landscape that does not have obvious, contemporary human alterations has experienced lesser rather than greater human manipulation. Based on the types of syntheses summarized earlier, and my experience in seemingly natural landscapes with low contemporary population density but persistent historical human impacts (e.g., Wohl, 2001), I argue that it is more appropriate to start with the default assumption that any particular landscape has had greater rather than lesser human manipulation through time, and that this history of manipulation continues to influence landscapes and ecosystems. To borrow a phrase from one of my favorite paper titles, we should by default assume that we are dealing with the ghosts of land use past (Harding et al., 1998). This assumption applies even to landscapes with very low population density and/ or limited duration of human occupation or resource use (e.g., Young et al., 1994; Wohl, 2006; Wohl and Merritts, 2007; Comiti, 2012). The default assumption of greater human impact means, among other things, that we must work to overcome our own changing baseline of perception. I use changing baseline of perception to refer to the assumption that whatever we are used to is normal or natural. A striking example comes from a survey administered to undergraduate science students in multiple U.S. states and several countries. In the survey, students were shown an identical series of photos of river segments and asked to rate each river segment on a numerical scale in terms of being natural, esthetically pleasing, dangerous, and needing improvement. With the exception of the U.S. state of Oregon, and the countries of Germany and Sweden, students consistently rated river segments containing instream wood negatively, viewing these river segments as unnatural, dangerous, and in need of rehabilitation (Chin et al., 2008). This completely contradicts the manner in which river scientists view instream wood, and ignores the logical assumption that, since a much greater proportion of the world was forested historically, most river segments in forested environments would naturally contain a great deal of instream wood (Montgomery et al., 2003). The students’ negative perception of instream wood at least partly

reflects the fact that most of them are used to seeing rivers with very little instream wood, even in forested environments, because of historical and continuing wood removal. Wood-poor rivers now seem normal and natural to most people. Those of us who work in rivers and are familiar with the scientific literature on instream wood, as well as the idea of dramatic historical change in landscapes and ecosystems, can metaphorically step back and shake our heads at the students’ misperceptions, but identifying our own unexamined and misleading perceptions is much more challenging. The default assumption of greater human manipulation of the landscape appears to apply broadly to temperate and tropical zones, whether arid, semiarid or humid. Archeologists have developed convincing evidence that the seeming wilderness of the pre-Columbian Amazon basin hosted many more people than initially thought, although estimates range enormously from 500,000 to 10 million people (Mann, 2005; McMichael et al., 2012) and remain controversial. Certainly some of these people intensively managed the surrounding vegetation and soils, as reflected in the persistence of dark-colored, fertile terra preta (Liang et al., 2006) soils that were created by pre-Columbian Indians from 500 to 2500 years BP. Prehistoric agricultural societies in central Arizona, USA created an extensive network of irrigation canals that resulted in soil salinization that persists today (Andrews and Bostwick, 2000). Only very limited areas of high latitude (Antarctica, parts of the Arctic) and high altitude appear not to have been manipulated by humans at some point during the past few millennia (Sanderson et al., 2002; McCloskey and Spalding, 1989). Faced with the realization that most landscapes have been and continue to be manipulated by humans in ways subtle or obvious, geomorphologists can make at least three important contributions to sustaining critical zone integrity. First, we are explicitly trained to consider the historical context of landscapes, in the broadest sense of ‘historical’ – reaching back through the Quaternary and beyond. We can enhance our efforts to focus on the time period that includes human presence on the landscape, and to characterize how past human manipulations continue to influence the critical zone. Second, we can apply our knowledge of connectivity, inequality, and thresholds to landscape and ecosystem management. I use management here to refer to coordinated and directed actions, rooted in scientific understanding, that are designed to maintain or enhance the integrity and sustainability of a landscape or ecosystem. This form of management contrasts with individualistic, narrowly focused manipulation of landscapes and ecosystems designed for immediate survival or economic profit, which characterizes most of human history. On the one hand, I am uncomfortable with the notion of management and the underlying hubris, because I see so much evidence that we cannot or do not intelligently or sustainably manage highly complex landscapes and ecosystems. On the other hand, we have been manipulating landscapes and ecosystems for millennia, and our manipulations will only continue to accelerate as human populations and access to technology increase. So, we might as well attempt to improve our management. Among the ways to improve management are to emphasize adaptive management (Walters, 1986), which involves monitoring system response to specific human manipulation and, if necessary, altering manipulation to obtain desired outcomes. Another obvious improvement would be to practice integrated management that considers, for example, not only how a proposed dam will alter hydroelectric power generation and river navigation, but also river connectivity, biological connectivity, sustainability of riverine and nearshore ecosystems, and so forth. Adaptive and integrated management can be most effective if underpinned by a conceptual framework that includes fundamental geomorphic

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concepts such as feedbacks and thresholds (e.g., Florsheim et al., 2006; Shafroth et al., 2010; Chin et al., in press). Finally, geomorphologists can quantify thresholds, alternative stable states of a landscape, landscape resilience, and critical zone integrity. To return to the beaver meadow example, the input of ecologists is needed to specify parameters such as minimum water table elevation to sustain willows, minimum food supply to sustain each beaver, and minimum genetically sustainable populations of beaver. Geomorphologists can quantify the channel obstructions and channel-floodplain connectivity necessary to maintain an anabranching channel planform, or the differences in overbank deposition rates of fine sediment and organic matter under singlethread versus multi-thread channel planforms. Quantitative thresholds can provide targets that management actions are designed to achieve, as when environmental flow regimes are designed around exceeding thresholds such as mobilizing bed sediments or creating overbank flows (Rathburn et al., 2009). 7. Conclusions A wide variety of metrics – loss of soil fertility, proportion of ecosystem production appropriated by humans, availability of ecosystem services, changing climate – indicates that we are in a period of overshoot (Hooke et al., 2012). Overshoot occurs when a population exceeds the local carrying capacity. An environment’s carrying capacity for a given species is the number of individuals ‘‘living in a given manner, which the environment can support indefinitely’’ (Catton, 1980, p. 4). One reason we are in overshoot is that we have consistently ignored critical zone integrity and resilience, and particularly ignored how the cumulative history of human manipulation of the critical zone has reduced integrity and resilience. Geomorphologists are uniquely trained to explicitly consider past changes that have occurred over varying time scales, and we can bring this training to management of landscapes and ecosystems. We can use our knowledge of historical context in a forward-looking approach that emphasizes both quantifying and predicting responses to changing climate and resource use, and management actions to protect and restore desired landscape and ecosystem conditions. Management can be viewed as the ultimate test of scientific understanding: does the landscape or ecosystem respond to a particular human manipulation in the way that we predict it will? Management of the critical zone during the Anthropocene therefore provides an exciting opportunity for geomorphologists to use their knowledge of critical zone processes to enhance the sustainability of diverse landscapes and ecosystems. Acknowledgements I thank Anne Chin, Anne Jefferson, and Karl Wegmann for the invitation to speak at a Geological Society of America topical session on geomorphology in the Anthropocene, which led to this paper. Comments by L. Allan James and two anonymous reviewers helped to improve an earlier draft. References Andrews, J.P., Bostwick, T.W., 2000. Desert Farmers at the River’s Edge: The Hohokam and Pueblo Grande. City of Phoenix, Phoenix, Arizona. Armstrong, D.M., 1972. Distribution of Mammals in Colorado. Monograph of the Museum of Natural History, No. 3. University of Kansas, Lawrence, KS. Attal, M., Tucker, G.E., Whittaker, A.C., Cowie, P.A., Roberts, G.P., 2008. Modeling fluvial incision and transient landscape evolution: influence of dynamic channel adjustment. Journal of Geophysical Research 113, F03013, http://dx.doi.org/ 10.1029/2007JF000893. Bartel, R.A., Haddad, N.M., Wright, J.P., 2010. Ecosystem engineers maintain a rare species of butterfly and increase plant diversity. Oikos 119, 883–890.

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