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Keywords:

  • disturbance-dependent species;
  • intermediate disturbance hypothesis;
  • multipatch scale;
  • Natura 2000;
  • threatened and endangered species

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Nature and Effect of Military Disturbance
  5. A Heterogeneous Disturbance Hypothesis
  6. Conclusions
  7. Implications for Practice
  8. Acknowledgments
  9. LITERATURE CITED

Disproportionately large numbers of threatened and endangered species and unusually high biodiversity occur on active and former military training areas. Although this may seem paradoxical given the apparently destructive nature of military training, an evaluation of the nature and extent of the disturbances is enlightening. Military training frequently produces heterogeneous landscapes. Large portions of military training areas remain virtually untouched, favoring disturbance-averse species; other portions are heavily disturbed, favoring disturbance-dependent species. The rich habitat mosaics include the two extremes as well as the continua of disturbance and succession between them, thus providing suitable habitat for a very large number of species with widely varying habitat requirements. To explain the phenomenon, a heterogeneous disturbance hypothesis is proposed which suggests that biodiversity is maximized where multiple kinds, frequencies, severities, periodicities, sizes, shapes, and/or durations of disturbance occur concomitantly on a landscape in a spatially and temporally distributed fashion. The enhanced biodiversity occurring on active and former military training areas illustrates the need for restoration ecologists to restore or maintain an appropriate heterogeneous disturbance regime when attempting to restore ecosystem function and biodiversity.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Nature and Effect of Military Disturbance
  5. A Heterogeneous Disturbance Hypothesis
  6. Conclusions
  7. Implications for Practice
  8. Acknowledgments
  9. LITERATURE CITED

Military training areas are increasingly recognized as sites of high biodiversity and large numbers of threatened and endangered species. For example, to comply with the European Union’s Habitat Directive, all nations within the European Union are required to designate a certain percentage of their landmass as “special areas of conservation” to be included in the Natura 2000 network (EUROPA 2005). Nominations to the Natura 2000 network are to be based on the presence of high biodiversity, the occurrence of red list species (the European equivalent to the threatened and endangered designation), and priority habitats. Noting these characteristics on their military training lands, Denmark has nominated 45% of its military estate, the Netherlands has included 50% of its military training areas, and Belgium has proposed 70% of its training areas (Gazenbeek 2005). Germany has nominated 50% of its training areas (Naturstiftung David 2007). The U.S. Army maintains numerous training areas in Europe. As of this writing, 77% of those training areas have been nominated by the various nations where they occur.

Grafenwöhr and Hohenfels Training Areas, the two largest U.S. Army training areas in Europe, comprise 22,855 and 16,175 ha, or about 0.34 and 0.24%, respectively, of the land area in the German state of Bavaria where they occur. Approximately 688 species of plants are known to occur on the Grafenwöhr Training Area (IVL 2004), and 675 plant species have been recorded at the Hohenfels Training Area (Douglas et al. 2001). Schönfelder et al. (1990) list a total of 2,496 plant species in Bavaria. Thus, despite their small size and the intensive military training that occurs there, over 27% of the plant species in the entire state occur on each of the training areas. Similarly, in the Netherlands, 53% of all vascular plant species and 61% of all bird species known to occur in the nation are found on their military training areas which comprise less than 1% of the land area (Gazenbeek 2005).

Equally impressive numbers exist when only red list species are considered. Approximately 84 and 92% of Grafenwöhr and Hohenfels Training Areas, respectively, were nominated for inclusion in the Natura 2000 network, making them the sixth and seventh largest nominated sites in Bavaria (Fig. 1). Yet, based on numbers provided by the Bavarian government, the training areas provide habitat for a larger number and a greater density of red list animal species (the only life-form for which data are available) than the larger sites in the state, including national parks (Fig. 2). This is particularly impressive given that four of the other sites are located in the Alps which form the southern border of Bavaria and are said to represent a biodiversity hot spot in Europe (Davis et al. 1994).

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Figure 1. The seven largest sites nominated by the German state of Bavaria for inclusion in the Natura 2000 network include (in descending order by size) the Garmisch-Partenkirchen region of the Alps (G-P), Nationalpark Bayerischer Wald (NBW), the Obertsdorf region of the Alps (O), Nationalpark Berchtesgaden in the Alps (NB), the Mittenwald region of the Alps (M), Grafenwöhr Training Area (GTA), and Hohenfels Training Area (HTA).

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Figure 2. The land area of the seven largest sites in the German state of Bavaria nominated for inclusion in the Natura 2000 network, the number of threatened and endangered (red list) animal species reported to occur in each, and the density of threatened and endangered animal species in each (G-P, Garmisch-Partenkirchen region; NBW, Nationalpark Bayerischer Wald; O, Obertsdorf region; NB, Nationalpark Berchtesgaden; M, Mittenwald region; GTA, Grafenwöhr Training Area; HTA, Hohenfels Training Area). Data are provided by the Bavarian government.

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In 1992, 728 plant and animal species were listed as federally threatened and endangered in the United States (Flather et al. 1994). At that time, U.S. Department of Defense (DoD) lands harbored more threatened and endangered species than any of the other five major U.S. federal land management agencies, including the National Park Service (Fig. 3). As in Europe, the importance of DoD lands is even better illustrated when the density of threatened and endangered species is computed. DoD lands provide habitat for 3–18 times more threatened and endangered species per unit area than lands managed by the other major federal land management agencies. Indeed, DoD lands harbor approximately 21% of all federally listed species on just 3% of the federal land base (Groves et al. 2000). Utilizing a dataset of 1,184 species considered endangered, threatened, proposed, or candidates (Groves et al. 2000), the value of DoD lands for biodiversity is further illustrated (Fig. 4). NatureServe (2004) identified 476 species at risk (not yet federally listed) occurring on DoD lands of which 24 occur nowhere else.

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Figure 3. The land area administered by the six largest U.S. federal land management agencies, the number of threatened and endangered species found on those lands in 1992, and the density of threatened and endangered species known to occur on the lands managed by the agencies (BLM, Bureau of Land Management; FS, Forest Service; FWS, Fish and Wildlife Service; NPS, National Park Service; BIA, Bureau of Indian Affairs; DOD, Department of Defense). Data source: Flather et al. (1994).

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Figure 4. The land area administered by five U.S. federal land management agencies, the number of threatened, endangered, proposed, or candidate species found on those lands in 1996, and the density of those species (BLM, Bureau of Land Management; FS, Forest Service; FWS, Fish and Wildlife Service; NPS, National Park Service; DOD, Department of Defense). Data source: Groves et al. (2000).

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Many military training areas have been abandoned due to political developments around the world. They have often been transformed into legally protected nature conservation areas (IUCN 1996). Restoration practitioners are confronted with the major challenge of restoring disturbance processes that sustain the dynamic regime (Jentsch 2007). They need to understand the nature of the former military disturbance regime and find mechanisms to mimic the most crucial aspects of this driver of extraordinarily high biodiversity.

The Nature and Effect of Military Disturbance

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Nature and Effect of Military Disturbance
  5. A Heterogeneous Disturbance Hypothesis
  6. Conclusions
  7. Implications for Practice
  8. Acknowledgments
  9. LITERATURE CITED

In addition to natural disturbances, military training lands are subjected to a variety of significant anthropogenic disturbances. Land-based military training incorporates a variety of vehicles ranging from wheeled support vehicles to heavy tracked vehicles. All vehicles are prone to disrupting and compacting the soil and crushing vegetation when driven off-road, but the nature and intensity of disturbance vary by vehicle type (Hirst et al. 2003), whether the vehicles are turning or traveling in a straight line (Ayers 1994), soil type (Braunack 1986), and soil moisture content (Thurow et al. 1993). Bivouacking and even foot traffic from dismounted training can compact the soil and reduce vegetative cover (Trumbull et al. 1994; Whitecotton et al. 2000). Excavation of antitank ditches, weapon system emplacements, and foxholes causes inversion and mixing of soil layers and destruction of vegetation, while exploding munitions cause cratering, concomitant soil displacement, and wildfires (Demarais et al. 1999).

It may seem paradoxical that lands subjected to military training could provide habitat for disproportionately large numbers of threatened and endangered species and high overall biodiversity. However, one must recall that landscape disturbance is not a new ecological phenomenon; it has always existed. Within North America, vast herds of bison once roamed the Great Plains region, consuming vegetation, trampling the soil, creating wallows, and producing a much more heterogeneous landscape than is currently present (Knapp et al. 1999; Fuhlendorf & Engle 2001). In Europe, large herbivores such as the tarpan, auroch, European bison, and elk may have historically created a much more open forest than that exists today (Vera 2000). In Africa, wild ungulates continue to migrate across the savannahs, creating patchiness in soil and vegetation (Rietkerk et al. 2000). Wildfires consume millions of hectares of forest and grassland annually (Bond et al. 2005). Geologic phenomena such as earthquakes, volcanoes, and landslides also disturb large areas. On a landscape scale, all of the aforementioned forms of natural disturbance tend to be heterogeneous in size, shape, spatial and temporal distribution, periodicity, severity, and duration.

Because disturbances to ecosystems are generally associated with injury or death of plants, animals, and even humans, and because disturbed areas are often considered aesthetically unpleasing, the prevailing human perception of disturbance has been and continues to be largely negative. Where possible, humans often intervene and suppress natural disturbances. Examples include the suppression of forest fires, construction of flood control structures, and large-scale spraying for insect outbreaks.

In place of natural disturbances, humans have substituted their own forms of disturbance. However, in contrast to natural disturbances, most modern anthropogenic disturbances tend to be considerably more uniform in terms of spatial and temporal distribution, severity, and duration. In urban areas, vast tracts of land have been covered with buildings and highways. Where patches of green remain in the oceans of cement and asphalt, lawn mowers, herbicides, and fertilizers ensure aesthetically pleasing uniformity of plant height, species composition, and color. On agricultural landscapes, one finds vast monocultures of crops planted in carefully manipulated fields. Lands not suitable for cultivation are often grazed by domestic livestock where a basic premise of rangeland management is to maximize the uniformity of livestock distribution (i.e., disturbance) by optimizing the placement of water sources and dietary supplements and by fencing, herding, and/or implementing rotational grazing systems (Holechek et al. 2004). Forested areas are often clear-cut, leaving behind a relatively uniform landscape that does not mimic the landscape mosaic created by natural disturbance (Hansen et al. 1991). The net result of human intervention in natural processes is often the replacement of naturally occurring heterogeneously disturbed landscapes with more homogeneously manipulated landscapes.

Military impacts are dissimilar to the majority of anthropogenic disturbances inasmuch as they occur in a heterogeneous fashion. Mechanized training is naturally restricted to areas with relatively open terrain and slopes less than the operable range of the vehicles being used. Within any given landscape, some locations are more tactically beneficial than others from a military doctrinal standpoint and are therefore used disproportionately (Herl et al. 2005; Warren & Herl 2005). The use of live munitions is restricted to designated ranges that are usually surrounded by large buffer (safety) zones where human impacts are minimal to nonexistent. Even within the ranges, impacts are concentrated in areas immediately surrounding designated targets. The net result is that only a small fraction of the ranges are impacted (Houston & Doe 2005) and tends to diminish rapidly with distance from established targets. Overall, within the United States, less than 20% of the land area available for Army training and testing shows visible signs of physical disturbance (Shaw & Kowalski 1996). Thus, military training areas provide vast expanses of habitat for species that are disturbance-averse as well as for species that are disturbance-dependent.

Many of the threatened and endangered species living on military training lands thrive there because of the disturbance, not despite it. For example, the endangered grey hairgrass (Corynephorus canescens), a poor competitor which needs frequent ground disturbance for seedling establishment, thrives on active and recently abandoned military training areas in Germany (Jentsch & Beyschlag 2003). In North America, the endangered Sonoran pronghorn antelope (Antilocapra americana sonoriensis) exhibits a significant preference for high-explosive target areas, presumably in response to increased productivity of grasses and forbs in those areas (Hervert et al. 1997; Krausman et al. 2005). In Hawaii, the Marine Corps has successfully doubled the population of the endangered black-necked stilt (Himantopus mexicanus knudseni), in part by instituting annual maneuvers utilizing amphibious assault vehicles on mudflats (Drigot 2001; Rauzon & Drigot 2002). The disturbance breaks open thick mats of invasive plants and creates a series of islands and moats that provide protected habitat for reproduction. Larvae of the endangered Karner blue butterfly (Lycaeides melissa samuelis) feed exclusively on the Wild lupine (Lupinus perennis) which occurs most frequently in oak and pine barrens, a disturbance-dependent plant community of North America. Within their native habitat, the occurrence of the lupine and the butterfly is positively correlated with military training activities, including vehicular traffic, bivouacking, and fires caused by munitions (Smith et al. 2002). In the United Kingdom, larvae of the endangered marsh fritillary butterfly (Euphydryas aurinia) depend on Devil’s bit scabious (Succisia pratensis) as a food source. The plant requires early successional conditions for survival and thrives in areas that are burned periodically. The Salisbury Plain Training Area is northern Europe’s most important site for the butterfly and contains 35% of the total English population due in large part to the fires that occur in the firing ranges (Gazenbeek 2005). The stone curlew (Burhinus oedicnemus), one of the United Kingdom’s most endangered birds, nests on bare or sparsely vegetated patches of ground (Green et al. 2000). One of the two largest population clusters occurs at the Salisbury Plain Training Area where tank maneuvers and artillery shells create approximately 26 ha of bare ground annually (Gazenbeek 2005).

Because many species are disturbance-dependent, the cessation of military training and loss of concomitant disturbance can have negative effects on their survival. Following the end of the Cold War, many former Eastern Bloc military training areas were abandoned. The military training areas represent some of the last remaining larger remnants of formerly widespread grassland communities in central Europe and are characterized by large numbers of threatened and endangered species and overall high biodiversity (Beutler 1993; IUCN 1996; Wanner et al. 2001). With the loss of military disturbance, many populations of disturbance-dependent species are threatened with extinction (IUCN 1996). Because military training areas have closed, scientists and environmentalists have explored alternative forms of disturbance to maintain the open areas and the species that thrive there, including the use of megaherbivores, mowing, tillage, and prescribed burning (e.g., Tschöpe et al. 2002; Jentsch et al. 2007).

A Heterogeneous Disturbance Hypothesis

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Nature and Effect of Military Disturbance
  5. A Heterogeneous Disturbance Hypothesis
  6. Conclusions
  7. Implications for Practice
  8. Acknowledgments
  9. LITERATURE CITED

The relationship between disturbance and habitat heterogeneity has been recognized for many years (e.g., Whittaker & Levin 1977; White 1979). Numerous hypotheses have been developed to explain the role of disturbance in the creation and maintenance of biodiversity (White & Jentsch 2001). One of the more popular contemporary hypotheses is the intermediate disturbance hypothesis which suggests that biodiversity is maximized where the frequency and/or intensity of disturbance occurs at an intermediate level (Connell 1978). The basic premise of the hypothesis is that mid-successional communities created by intermediate levels of disturbance will contain, in additional to typical mid-successional species, some early successional species that have not yet been competitively excluded and some late-successional species that are starting to become established. However, Mackey and Currie (2001) reviewed the contemporary literature and found that biodiversity peaked at intermediate levels of disturbance only 19% of the time while looking at a wide range of communities. Accordingly, Huston et al. (1999) predict that intermediate disturbance produces maximum diversity only where the population growth rate is moderate. At high population growth rates, diversity peaks at high disturbance levels; at low population growth rates, diversity is maximized at low disturbance levels.

A full review of the intermediate disturbance hypothesis and the role of disturbance in habitat heterogeneity would be quite lengthy and outside the scope of this paper. Suffice it to say that although the logic of the intermediate disturbance hypothesis is sound, and has been demonstrated to have validity in many cases (e.g., Sousa 1979; Aronson & Precht 1995; Molino & Sabatier 2001), it fails to consider the diversity of disturbance. This has been noted by numerous ecologists who have suggested that heterogeneous disturbance creates spatial heterogeneity or patchiness of habitats that, in turn, contributes to increased biodiversity at the landscape or multipatch scale (e.g., Collins 1987; White & Jentsch 2005). The intermediate disturbance hypothesis is limited to the patch scale. Indeed, Connell (1978) excluded the landscape scale, stating that the hypothesis would not hold for geographical gradients. In a landscape with a heterogeneous disturbance regime, different stages of succession are present simultaneously. Hence, the probability that a larger number of species can find suitable patches in a heterogeneously disturbed landscape is greater than in a landscape with uniformly distributed intermediate disturbance. To describe the role of heterogeneous disturbance on landscape scale biodiversity, we propose a heterogeneous disturbance hypothesis. The hypothesis suggests that biodiversity is maximized where multiple kinds, frequencies, severities, periodicities, sizes, shapes, and/or durations of disturbance occur concomitantly in a spatially and temporally distributed fashion. Where a heterogeneous disturbance regime exists, species can partition the varied conditions so that a multitude of species can coexist along gradients of disturbance and succession.

It seems logical to conclude that the high biodiversity and large numbers of threatened and endangered species on military training lands occur there as a result of the varying conditions that are simultaneously present. The disturbance mosaic provides ideal habitat for species that prefer almost any position along the gradients of disturbance and succession. For example, the largest documented breeding populations of the black-capped vireo (Vireo atricapillus) and the golden-cheeked warbler (Dendroica chrysoparia), two endangered birds with contrasting habitat requirements, occur within Fort Hood, Texas (Tazik et al. 1993; Dearborn & Sanchez 2001). Whereas the vireo prefers to nest in scrub oak and juniper communities where periodic disturbance maintains a sparse, early successional habitat (Grzybowski et al. 1994), the warbler is dependent on mature juniper trees for nest building and foraging (Pulich 1976). Despite their widely divergent habitat requirements, the birds coexist in relatively close proximity because of the irregular fire history patterns that exist there and produce ideal habitat for both species. The only other location known to the authors where these two species coexist in significant numbers is Camp Bullis, Texas, another military training area.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Nature and Effect of Military Disturbance
  5. A Heterogeneous Disturbance Hypothesis
  6. Conclusions
  7. Implications for Practice
  8. Acknowledgments
  9. LITERATURE CITED

The data presented herein document unusually high biodiversity and exceptionally large numbers of threatened and endangered species on military training lands on both the North American and the European continents. A heterogeneous disturbance hypothesis is presented to explain the phenomenon in terms of the heterogeneous disturbance regime that exists on military training lands. The hypothesis suggests that biodiversity is maximized where multiple kinds, frequencies, severities, periodicities, sizes, shapes, and/or durations of disturbance occur across the landscape in a spatially and temporally distributed fashion. It borrows from and builds on contemporary theories of disturbance ecology and suggests that the mosaic left by heterogeneous disturbance provides habitat suitable for disturbance-averse species, disturbance-dependent species, and many other species along the continua of disturbance and succession. We do not seek to imply that military training is an ideal disturbance vector. However, for those involved in restoration ecology, we can learn from the ecological phenomena that occur on military training areas and propose that any effort to restore and/or maintain biodiversity in the landscape must necessarily include restoration of a disturbance regime that creates and maintains habitat heterogeneity, for the two are inextricably linked.

Implications for Practice

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Nature and Effect of Military Disturbance
  5. A Heterogeneous Disturbance Hypothesis
  6. Conclusions
  7. Implications for Practice
  8. Acknowledgments
  9. LITERATURE CITED
  • • 
    The exceedingly high numbers of threatened and endangered species and exceptional overall biodiversity that characterize military training lands illustrate the benefit of heterogeneously distributed disturbance on a landscape.
  • • 
    As implied by the heterogeneous disturbance hypothesis, any landscape-scale restoration effort should consider the reestablishment of a disturbance regime that may include multiple kinds, frequencies, severities, periodicities, sizes, shapes, and/or durations of disturbance distributed heterogeneously in space and time.
  • • 
    A heterogeneous disturbance regime provides suitable habitat for species that are disturbance-averse and disturbance-dependent, as well as species along the continua of disturbance and succession.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Nature and Effect of Military Disturbance
  5. A Heterogeneous Disturbance Hypothesis
  6. Conclusions
  7. Implications for Practice
  8. Acknowledgments
  9. LITERATURE CITED

This work was funded by the Installation Management Command—Europe Region, Heidelberg, Germany. Valuable comments were provided by C. Buhk and numerous anonymous reviewers.

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Nature and Effect of Military Disturbance
  5. A Heterogeneous Disturbance Hypothesis
  6. Conclusions
  7. Implications for Practice
  8. Acknowledgments
  9. LITERATURE CITED
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