ECOSYSTEMS
Ecosystems (1998) 1: 546–557
r 1998 Springer-Verlag
Ecosystem Management in the Context of Large, Infrequent Disturbances Virginia H. Dale,1* Ariel E. Lugo,2 James A. MacMahon,3 and Steward T. A. Pickett4 1Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831–6036; 2USDA Forest Service, International Institute of Tropical Forestry, Rı´o Piedras, Puerto Rico 00928–5000; 3Department of Biology, Utah State University, Logan, Utah 84322; and 4Institute of Ecosystem Studies, Millbrook, New York 12545–0129, USA
ABSTRACT Large, infrequent disturbances (LIDs) can have significant impacts yet seldom are included in management plans. Although this neglect may stem from relative unfamiliarity with a kind of event that rarely occurs in the experience or jurisdiction of individual managers, it may also reflect the assumption that LIDs are so large and powerful as to be beyond the ability of managers to affect. However, some LIDs can be affected by management, and for many of those that cannot be affected, the resilience or recovery of the system disrupted by the disturbance can be influenced to meet management goals. Such results can be achieved through advanced planning that allows for LIDs, whether caused by natural events, human activities, or a combination of the two. Management plans for LIDs may adopt a variety of goals, depending on the nature of the system and the nature of the anticipated disturbance regime. Managers can choose to influence (a) the system prior to the disturbance, (b) the disturbance itself, (c) the system after the disturbance, or (d) the recovery process. Prior to the disturbance,
the system can be managed in ways that alter its vulnerability or change how it will respond to a disturbance. The disturbance can be managed through no action, preventive measures, or manipulations that can affect the intensity or frequency of the disturbance. Recovery efforts can focus on either managing the state of the system immediately after the disturbance or managing the ongoing process of recovery. This review of the management implications of LIDs suggests that management actions should be tailored to particular disturbance characteristics and management goals. Management actions should foster survival of residuals and spatial heterogeneity that promote the desired recovery pattern and process. Most importantly, however, management plans need to recognize LIDs and include the potential for such disturbances to occur.
INTRODUCTION
ecological studies (Tilman 1989) or management activities, and the intensity of LIDs is often of a magnitude that sets the system back to an early stage in its developmental sequence or initiates a different seral pathway. What distinguishes LIDs from other disturbances is their rarity and their amplitude. As is the case for small, frequent disturbances, ecologists now recognize that LIDs, rather than being catastrophic agents of destruction, are
Key words: disturbance; ecosystem management; land use; recovery; spatial heterogeneity; succession.
Large, infrequent disturbances (LIDs) are a part of many ecological systems. The spatial and temporal regimes of LIDs are larger than the scale of typical
Received 14 July 1998; accepted 16 September 1998 *Corresponding author; e-mail:
[email protected]
546
Ecosystem Management and Disturbances normal, perhaps even integral, parts of long-term system dynamics (Weatherhead 1986). In fact, some ecological systems contain species (for example, Pinus contorta) that have evolved in response to LIDs (Christensen and others 1989). The composition, structure, organization, and developmental and trophic dynamics of these systems are the products of LIDs (Denslow 1980; Leach and Givnish 1996; Lugo and Scatena 1996; Wootton and others 1996). This realization alters the way we consider management of disturbance-prone systems. As might be expected, small, frequent disturbances are easier to study, understand, and incorporate into planning and management than are LIDs. For example, aspects of silviculture are designed to replicate small disturbances in forests, yet large disturbances are seldom a part of the management plan. Therefore, LIDs have been less well accounted for in management (Baker 1992). This report presents a framework to encourage and facilitate incorporating LIDs into ecosystem management planning.
MANAGEMENT NEEDS AND PROBLEMS OF LARGE, INFREQUENT DISTURBANCES Management plans need to include the potential for LIDs, as shown by three examples of managers facing the repercussions of large blowdowns in old-growth forests. The Nature Conservancy had no plan for dealing with the management issues associated with the blowdown of Cathedral Pines in Connecticut, and the Brandywine Conservancy in Pennsylvania assumed its old-growth forest had lost its value after a large portion blew down. However, in the aftermath of a large blowdown in the Tionesta Scenic and Natural Areas in Pennsylvania in 1985, the Forest Service reevaluated and reaffirmed its long-term policy of not salvaging in the old-growth forest of those areas. A long-term perspective requires that owners of large acreage consider the possibility of a disturbance occurring and have a management plan in place to deal with such events. Infrequent disturbances may not be included in planning because people’s perceptions are most influenced by what has occurred during their lifetimes (Christensen and others 1989). The temporal and spatial scale of LIDs is so large that our impressions are dominated by the lack of such disturbances. Even the actuarial literature struggles with computational approaches to include catastrophes [for example, see Peters and Mangel (1990) and Ludwig (1995)]. It appears that institutional memories are often no better than individual memories in dealing with LIDs (Yaffee 1996). Thus, management
547
goals typically do not consider what aims are appropriate in the face of disturbance. Management efforts should be goal directed and should identify the components, time frame, and spatial resolution needed to achieve the goals (Rogers 1997). Measurable end points should be specified as well. The end points could target a variety of public or landowner values, including aesthetically pleasing vistas, economic gain from natural resources, reduction of health or social welfare risks, or environmental benefits, such as habitat maintenance for species of concern. When LIDs do occur, they may trigger large departures from average events. These changes can result in a media and public outcry for immediate and costly action to return the situation quickly to seemingly normal conditions (Schullery 1989). Yet the costs and benefits of management actions are assessed at relatively small temporal and spatial scales. For example, even in hurricane-prone coastal areas, flood insurance is generally available to homeowners except in the days immediately preceding an impending hurricane. Furthermore, planning for response to LIDs does not typically consider the length of time required for repair to occur naturally. In the same sense, planners do not always recognize that human developments in the vicinity of the disturbance may have broad-scale implications. For example, although it may be cost-effective for an individual locality to establish a levee to protect it against floods, the levee can increase the chance of downstream floods (Collier and others 1997). Failure to anticipate decadal increases in the costs of flood damages in the United States is partially caused by this small-scale perspective. Traditional economic analyses do not anticipate such broadscale costs that result from suppressing LIDs on a local scale. In times of budget constraints, it may be easy to ignore the cost of preparing for a one-in-a-hundredyear event. In a sense, such actions mean that managers gamble that the current generation will not have to pay for the consequences of an infrequent event. Such gambles do not always pay off. In Florida, for example, budget cuts eliminated the use of weather-prediction models that enabled farmers to take protective actions and to avoid crop damage associated with rare freezes. Without the weather models, farmers were unprepared for the severe freeze in 1997, and multimillion-dollar damages occurred as a result. Ecosystem management of systems that may experience LIDs thus presents a variety of challenges to resource managers and ecologists. Many definitions of ecosystem management have been pro-
548
V. H. Dale and others Figure 1. Depiction of agents of large, infrequent disturbances ranges from those that are completely natural to those that are anthropogenic in origin. Some disturbances, such as fire, run the entire gamut, whereas others fall completely within the range of natural causes (for example, volcanoes) or anthropogenic agents (for instance, nuclear accidents).
posed (Goldstein 1992; Slocombe 1993; AFPA 1994; Bureau of Land Management 1994; Grumbine 1994; Wood 1994; Christensen and others 1996; Fitzsimmons 1996; Haeuber 1996; Leslie and others 1996; Agee and Johnson 1998), but few operationally include LIDs. We endorse the definition of ecosystem management as ‘‘a collaborative process that strives to reconcile the promotion of economic opportunities and livable communities with the conservation of ecological integrity and biodiversity’’ (Keystone Center 1996). This definition of ecosystem management is compatible with understanding the role of disturbances in natural systems and with our capacity to elucidate principles that form the basis for evaluating the benefits and costs of disturbances as a part of management action. The complete range of disturbance regimes needs to be integrated into an understanding of system dynamics (Arnold 1995; Baker 1992) while retaining system resilience within the range of natural variation (Holling and Meffe 1996). Our goal in this article is to place what ecologists know about LIDs into the context of management decisions. Elsewhere in this issue, Turner and Dale (1998) introduce the concept of LIDs; Turner and others (1998) describe what we have learned about succession from studies of LIDs; and Romme and others (1998) summarize the commonalties and differences of disturbances. Ecosystem management in systems experiencing disturbances requires that we understand both natural disturbances and the role of humans in creating or altering largedisturbance effects and in managing the postdisturbance landscape. An impediment to our success in developing management recommendations is that the defini-
tion of disturbance is sometimes considered arbitrary, and in many cases the term is value laden. Even ecologists occasionally use lay terminology that has negative connotations for the results of disturbance. To offset this problem, this report places LIDs in a general context of system processes that allows a discussion of the implications of LIDs for management plans. The article then develops a set of general strategies for managing ecological systems in the context of LIDs.
NATURAL AND ANTHROPOGENIC CAUSES OF DISTURBANCE If the origin of the disturbance is considered as a continuum from entirely natural to entirely anthropogenic, patterns emerge (Figure 1). Some disturbances are physically forced solely by natural processes. El Nin˜os, hurricanes, and volcanic eruptions are examples of such disturbances. An important feature of naturally induced disturbances is that they may have complex origins. Several different disturbances, each with its own characteristics, can interact synergistically to produce different postdisturbance conditions. For example, the 1980 eruption of Mount St. Helens produced a complex of disturbance agents (pyroclastic flow, debris avalanche, mudflows, ash deposits, and blowdown), and several agents interacted at specific sites to affect the degree of damage and the course of recovery (Franklin and others 1985; Adams and others 1987; del Moral 1993). Additionally, combinations of postdisturbance site characteristics influence the biota. For example, the 1985 tornado blowdown at Tionesta created a variety of physical habitat types, each of which affected survival, recruit-
Ecosystem Management and Disturbances ment, and interaction of organisms that could contribute to the forest recovery (Peterson and Pickett 1990; Peterson and others 1990). Disturbances that are considered totally anthropogenic constitute another group. These disturbances can be accidental or purposeful. Oil spills are examples of accidental, anthropogenic events that have no analogue in nature. Some anthropogenic events, however, are similar to natural disturbances. The floods created in the Grand Canyon are an example of intentional human-caused disturbance that was meant to approximate natural floods (Collier and others 1997). In some cases, unintentional human-caused disturbances mimic nature, such as fires that are set by humans but have gotten out of control and become large disturbances. In contrast with abrupt human-induced disturbances, land use can be considered a long-term, broad-scale, chronic disturbance by humans (Foster and others 1998b). The diversity of land uses ranges from actions that wholly transform the system into a unique ecological situation to actions that mimic natural disturbances. Land use can replicate the effects of natural disturbances, as is the case with military training at Fort McCoy in northern Wisconsin. The training activities produce effects similar to fires that typically burned these areas; both create oak savannas which are requisite for the obligate host plant of the threatened and endangered Karner blue butterfly (Lycaeides melissa samuelis Nabokov), which thrives on the military reservation (Bleser 1992). Many LIDs are the result of synergistic effects of natural and anthropogenic processes (Paine and others 1998). Sometimes the factors instigating a disturbance can be detected, but often the prime causal force cannot be identified as either natural or anthropogenic because it involves both types of disturbances. In these cases, the historical influences may be subtle, and their effects may be indirect. For example, insect disturbances to forests are more frequent now either because many of the pest insects have been introduced by human activity or because humans have altered the state of some ecological systems and thereby increased their vulnerability to attack (Nothnagle and Schultz 1987; Swetnam and Lynch 1993). In other cases, natural or anthropogenic LIDs may be altered by historical factors whose effects on ecological systems persist for centuries. For example, in the Caribbean, the strikingly smooth and even-sized forest canopy structure, with a conspicuous absence of emergent trees, led H. T. Odum (1970) to suggest that this canopy structure was a result of constant trade-wind action and periodic hurricane-force wind effects. However, long-term
549
study shows that forest structural parameters and species composition continue to change for more than 50 years following hurricane disturbances that last as little as 4 h (Lugo and others forthcoming). In spite of the dramatic effect of continuous exposure to hurricane disturbances and consequential forest responses, species distributions in a 16-ha experimental plot still show the effects of land uses that occurred 50 years ago (Willig and others 1996). In this plot, the passage of natural LIDs has not erased the legacy left from human modification of treespecies dominance. The effect of the anthropogenic LID has influenced how hurricanes—natural LIDs— impact the forest and, in turn, affect the functional diversity of soil bacteria (Willig and others 1996). The constraints and context of a disturbance may be influenced by human manipulation of disturbance events. For example, flood control has created systems that are now susceptible to large-scale flooding disturbances in a way they never were before (Sparks 1996). Similarly, the buildup of fuel on the forest floor as a result of fire suppression means that a fire might be particularly large and devastating [although Mortiz (1997) found that previous fire suppression had no effect on distribution of very large fires in the Los Padres National Forest, California]. As another example, the historical patterns of land use in an east Tennessee watershed have influenced subsequent patterns of insect outbreaks (Dale and others 1990). Clearly, humans can both instigate LIDs and influence their impact. Because humans can alter the susceptibility of given systems to LIDs, they can also alter disturbance regimes. The frequency, intensity, and size of a disturbance are subject to human modification for some types of disturbances. Fires may be the best example of this phenomenon (Schullery 1989). For example, at Eglin Air Force Base in northern Florida, decades of fire control led to the establishment of fire-prone forests dominated by sand pines (Pinus clausa). These pines are now being replaced by the naturally occurring longleaf pine (P. palustris) forests as biologists set controlled fires to return the system gradually to chronically burned forests (US Air Force 1993). One of the implications of recent rapid changes in climate may be alterations in the frequency and intensity of natural disturbances (Franklin and others 1992). Human actions also affect the rate of spread and size of some disturbances, such as fire (Clark and others 1996a, 1996b). These changes in disturbance regimes can alter the landscape attributes of a system (Turner and others 1994). For example, human-managed systems tend to have more linear features than do systems dominated by natural patterns of distur-
550
V. H. Dale and others
bances (Krummel and others 1987; Knight and Wallace 1989). Disturbance regimes can affect plant distribution [for example, see Coffin and Lauenroth (1988), Glenn and Collins (1992), and Brown and Brown (1996)], and the size, shape, and distribution of patches are influenced by disturbance frequency and size (Foster 1980; Romme and Despain 1989; Swanson and others 1990; Clark 1991; Bradshaw 1992). For instance, the substitution of a widespread, relatively uniform anthropogenic disturbance regime for the patchy disturbances of the presettlement forests of Wisconsin has resulted in a more homogeneous forest (White and Mladenoff 1994). Similarly, the geographic change in system composition and structure of vegetation in the Great Smoky Mountains is due, at least in part, to changes in the disturbance regime (Harmon and others 1993).
MANAGEMENT IMPLICATIONS For management purposes, the conditions that lead to or result from LIDs need to be understood so that alternatives for manipulation can be developed, when possible. Our poor understanding of some LIDs emphasizes the need for simultaneous programs of system management and research. Ideally, we could focus active adaptive management efforts (Walters 1997) in systems where a large disturbance is either imminent or recent. Such a focus would enable an evaluation of the implications of management actions. In the absence of a history of such adaptive management efforts, this report summarizes the implications of managing LIDs in a variety of situations. Management plans may focus on the potential for managing (a) the system prior to the disturbance, (b) the disturbance itself, (c) the system immediately after the disturbance, or (d) the recovery process (Figure 2).
Managing the System Prior to the Disturbance Preparatory actions are designed to manage the system so that it responds to disturbances in ways that do not compromise management goals. The system can be manipulated to alter its vulnerability, its resistance, or its response to an LID. An example of changing a system’s vulnerability is the alteration of forest fuel loads so that fire is more or less intense when it occurs. Resistance can be increased at the location in the system where the agent first exerts the disturbing force (for example, through firebreaks, spraying for insects, or planting windbreaks). Then the disturbing force can still occur with its full intensity, but the system will not be
Figure 2. A schemata showing the state of a system prior to a disturbance, the influence of the initial state on the disturbance, the state of the system after the disturbance, and the recovery process. Management actions can focus on (a) the system prior to the disturbance, (b) the disturbance itself, (c) the system after the disturbance, or (d) the recovery process. Ongoing evaluation of the impacts of management actions is a critical part of overall scheme.
altered to the extent it would have been without the management intervention. For example, restoration of the Kissimmee River in Florida involves enhancing the connectivity between the river and the floodplain. Channels were restructured so that flood events improve dissolved oxygen conditions in the stream and enhance the establishment of wetland communities (Dahm and others 1995). Greater attention is needed on the potential for managing ecosystems for resistance to a disturbance. Tools for accomplishing this goal are intercropping, silvicultural techniques, species selection, genetic engineering, and experimenting with new species combinations. For example, novel combinations of native species, naturalized species, and noninvasive, nonnative species can regenerate under the conditions created by LIDs and create a system that is more resistant to future disturbances. Evidence that a system can be made more resistant is provided by the historical development of New England vegetation. As the effects of LIDs such as hurricanes, winter storms, and massive deforestation, agriculture, and land abandonment have taken their toll, the types and spatial patterns of prevailing forest communities on the New England landscape have changed dramatically, as has the area’s susceptibility to disturbances (Foster and others 1992; Foster 1995). In some cases, the landscape impacts of changing the system’s resistance may be severe.
Ecosystem Management and Disturbances For example, in the aftermath of the fires at Yellowstone National Park, it was recognized that the large scale of those fires was, in part, the result of previous fire-control actions that created connected patches of fire-prone forests (Schullery 1989).
Managing a Disturbance The disturbance itself can be managed in one of three ways: no action, prevention of the disturbance, or manipulation of its effects. Note that ‘‘no action’’ is not a default but a conscious management decision based on knowledge about the disturbance, the system, and the management goals. No action is taken when nothing can be done about the disturbance (as in the case of most volcanoes or hurricanes) or when one can accept the consequences of the event. For instance, the 1988 fires in Yellowstone National Park were initially ignored because it was felt that the effects would be acceptable (Schullery 1989). As another example, it is prohibitively expensive to try to prevent the wide-scale destruction that can result from meteors crashing into the earth by using a proposed management plan of launching nuclear bombs on rockets to intercept the meteors, explode, and thus change the meteors’ courses (Solem 1994). Prevention is usually motivated by the desire to satisfy or protect a human concern. Managers attempt to prevent disturbance when the costs of such efforts are perceived to be balanced or exceeded by the anticipated benefits. Thus, dams, levees, reservoirs, and coastal barriers are implemented to prevent disturbances caused by water action. The occurrence or spread of fires can be manipulated by controlling the amount of fuel on the forest floor or by strategically placing firebreaks in the path of the fire. Rather than merely including ways to offset disturbance effects, plans for management need to consider producing large disturbances artificially or mimicking their effects to restore processes in ecosystems that have been inappropriately ‘‘protected’’ from LIDs by previous management actions. Such disturbance manipulation requires human intervention with the causal forces. When it is possible to do, disturbance manipulation is always costly and difficult. The LIDs most amenable to manipulation may be those that result from anthropogenic disturbances, such as oil spills. However, attempts have been made to create LIDs that mimic natural disturbances. For example, in March 1996, the waters of the Colorado River were increased by releasing flood-level volumes from Glen Canyon Dam (Adler 1996). The goal of the anthropogenic flood was to instigate geomorphic and biotic conditions similar to
551
those of the natural flooding regime of the Colorado River, kept in check by retention structures for many years. The result of the flood was a reduction in the density of nonnative riparian species, such as the naturalized and highly invasive plant, tamarisk, and an increase in suitable habitat for a variety of endemic fish species (Collier and others 1997). The flood could have been effectively planned and executed only on the basis of knowledge about the ways that large floods affect natural systems and a record of the magnitude and frequency of past flood events.
Managing the System After the Disturbance The lack of anticipation of LIDs is often paired with a lack of recognition of the potential for recovery. In the aftermath of a disturbance, managers may think the ecological system will not be able to reestablish naturally. Yet, recovery may be possible. It is often a slow, step-by-step process by which individual organisms become established and eventually trophic interactions and other ecosystem processes become reestablished. Because the potential for recovery is seldom recognized by the public or the political system and because public attention is heightened after the LID, managers are often pressured to ‘‘do something’’ (Elfring 1989). Sometimes resource managers recognize that recovery will occur, albeit in a slow and step-by-step process; but their agencies, the public, or political representatives feel it is necessary to take some action in the name of facilitating recovery. In more than one instance, such actions have not only been very costly but also detrimental to the natural recovery process. For example, leaving the system alone made more sense than some of the intensive recovery actions taken after Hurricane Hugo, the eruption of Mount St. Helens, and the Exxon Valdez oil spill. We examine examples from these situations in greater detail below. After Hurricane Hugo passed over Puerto Rico in 1989, stream channels in the Luquillo Experimental Forest were packed with logs from trees knocked down by the high winds. Some of the logjams near bridges or recreation areas were safety hazards and were removed. In other locations, however, logjams were allowed to remain in the streams because prior research showed that such structures were important for providing cover and food for stream organisms and for retaining valuable nutrients, sediments, and organic matter in situ (Covich and Crowl 1990; Covich and others 1991; Covich and McDowell 1996). Because many resources stay in place behind logjams and can be used and reused by the stream fauna, the recovery of the stream commu-
552
V. H. Dale and others
nity following a flood is faster with the logjams than without them. This realization enabled forest managers to use their limited resources for solving safety hazards and for other forest-management priorities after a LID rather than for actions that would have been counterproductive or neutral in terms of sustaining natural-system functions. In contrast, the distribution of nonnative seeds to reduce erosion in the aftermath of the 1980 eruption of Mount St. Helens is an example of money poorly spent in recovery actions. Although petitions from scientists and a resolution from the International Congress of Systematic and Evolutionary Biology curtailed the wide-scale distribution of nonnative seeds, nevertheless $50,000 was spent distributing seeds by helicopter onto 32,000 ha of the new volcanic material. Subsequent studies showed that the nonnative seeds had no effect on the erosion problem (Franklin and others 1985). In fact, areas with the nonnatives had a higher mortality of conifer seedlings and a lower diversity of native species (Dale 1991). A similar experience occurred in landslide areas in the Luquillo Experimental Forest after Hurricane Hugo. Nonnative species planted on the landslides failed to grow and were rapidly overwhelmed by native vegetation. Steam cleaning after the 1989 Exxon Valdez oil spill may also have been unnecessary. Although more than 30,000 oiled bird carcasses were retrieved following the spill, the population dynamics of birds in the spill areas are difficult to distinguish from those of birds in the nonspill sites (Wiens 1996). Four years after the massive spill, little evidence remained of the event even in oiled environments. The demonstrated resilience of these communities raises the possibility that the effectiveness of expensive cleaning activities immediately after the spill requires more analysis and study (Starfield and Chapin 1996).
Managing the Recovery Process Because managing recovery can be thought of as managing succession, knowledge of natural succession processes increases the likelihood that the management plan will be achievable [see Luken (1990)].The first step in managing recovery is to evaluate the site potential. This includes evaluation of the spatial pattern of the effects, the biological and physical residuals of the disturbance, the site environmental conditions, the potential for propagule rain to aid in the recovery, and the influence of structural heterogeneity on the reestablishment of plants and animals. Following a disturbance, the site may be changed in many ways, but one important set of characteris-
tics of the disturbance may be what is left afterward, that is, the residuals (MacMahon 1997; Foster and others 1998a). Residuals include organisms, their propagules, abiotic site characteristics, and organic matter. These are soon joined by migrants whose rate of arrival can be characterized by their distance from the site and their vagility. These organisms may interact in a myriad of ways by providing shade, moisture, nutrients, or future propagules. A common goal of recovery management is to shorten the process of succession or to maintain the process of succession at one particular state that is considered desirable for human purposes. Thus, following some LIDs, the recovery plans for the area may include altering the patterns of landscape connectivity or suppressing or removing undesirable residuals, under the assumption that such action is cost-effective. In some cases, after a significant disturbance, nonnative plant species may occupy the open site to the detriment of the native species that one is attempting to reestablish (Putz 1997). The negative influences of the nonnative cheat grass (Bromus tectorum) on sagebrush community recovery following wildfire are well documented (Mack 1981). A classic tropical example of this phenomenon is the invasion of grasses (for example, Imperata cylindrica L.) after a disturbance and subsequent inhibition of forest recovery—a situation that can be avoided by site preparation or planting of native trees (Otsamo and others 1995, 1996). Thus, humans can act as agents of migration for a variety of desirable species, thereby shortening the process of succession and perhaps causing the establishment of species that prevent invasion of undesirable forms. The establishment process can be hastened in a variety of ways: by fertilizing, providing shade, providing mulch, adding mycorrhizae, or other activities that enable the desirable species to establish and reproduce rapidly. Management of the interactions of species and of landscape attributes is a bit more difficult and costly. Often we know so little about these interactions that the only alternative is to set up a scenario in which normal processes are likely to occur and then let the community develop on its own (MacMahon 1997). Sometimes, the effects of a LID or the combined effects of a LID and human modification of sites (for example, past land uses) can alter the direction of succession to such an extent that it is impossible to exclude nonnative species. This situation may occur when sites are highly degraded after the LID. Aide and colleagues (1996) found that forest recovery in abandoned pastures in Puerto Rico involved nonnative species (10 of 112 species enumerated) and experienced a different rate of succession than
Ecosystem Management and Disturbances Table 1.
553
Relationship Between Management Interventions and Succession Processes
Successional Processes
Management Interventions Add
Kill or Remove
Alter Structure
Migration
Seeds; eggs; lure, feed, or attract dispersers
Flooding system (zebra mussels); frighten or terrify; remove dispersal agent
Bird perches (seed dispersal); landscape structures
Establishment (ecesis)
Saplings
Postemergent herbicides, weeding, thinning, sterilize adults
Nest boxes, landscape structures
Biotic interactions
Predators, herbivores, rhizobium, mycorrhizae
Competitor control, food-web cascade, predators, herbivores, rhizobium, mycorrhizae
Beehives (pollination), landscape structures
Reaction
Fertilizer, irrigation, mulch
Raking, thinning, edge manipulation
Alter dispersion patterns, landscape structures
sites recovering only from hurricanes. Under the former conditions, species that are usually considered ‘‘undesirable’’ because they are invasive or nonnative become ‘‘desirable’’ because they play a positive role in the repair of site conditions and the eventual restoration of native species. Similar trends appear in temperate successions (Krause and DeSteven 1996) and should be assessed in management scenarios. Many LIDs may be so dramatic that, financially, the most feasible approach to system management immediately after the disturbance is to simply allow the area to naturally regenerate. Some selective management to ensure that undesirable, outside forces do not influence the trajectory of recovery may be added. In other cases, extensive management activities may be necessary to attain the desired goal. The balance between cost and desirability of return to a given end point must be weighed in making these decisions. In developing recovery plans, managers should focus on critical stages of the successional process (MacMahon 1987; Luken 1990): migration, establishment, biotic interactions (such as competition, predation, or mutualism), and the reaction of the systems to the changes that occur over successional time. For each successional stage, a management prescription can be developed that offers three alternative interventions: add something to the system, kill or remove a biological component, or alter the structure of the system (Table 1). For example, during the migration process, a manager could add seeds, eggs, or attractants for propagule dispersers (McDonnell and Stiles 1983). Alternatively, one could physically remove existing
propagules from the system by events, such as the intentional flooding to remove zebra mussels from rivers. On the other hand, a manager could frighten away or exclude animals that might eat the propagules or could remove the dispersal agent of undesirable seeds. Structural additions to the system might include bird perches and alterations in landscape structure so that seed input is enhanced. Successional trajectories after a LID event can follow a variety of directions (McCune and Allen 1985), but in many cases they appear to culminate in functional communities similar in structure and composition to the original ones. Alternatively, the successional trajectory can involve novel or unfamiliar seral communities because the magnitude and low frequency of the LID sets succession back to early or uncommon stages. For example, after Hurricane Hugo, stands in the Luquillo Experimental Forest in Puerto Rico were invaded by vines and herbaceous species not previously observed at that site in more than 50 years of research (Scatena and others 1996). Yet, this early stage in the recovery of the rain forest quickly gave way to more familiar successional trees such as Cecropia peltata. Seven years after the hurricane, vines and herbaceous vegetation had assumed their previous low abundance in specialized locations, such as in the canopy gaps of tall forests. However, their dominance after the hurricane was critical in the immobilization of nutrients released by the LID and in the reestablishment of forest conditions (Scatena and others 1996). In this case, nonarboreal vegetation did not inhibit forest regeneration. The traditional management practice of elimination of vines and weeds when they occur in tree plantations would have been
554
V. H. Dale and others
counterproductive in the context of managing a hurricane disturbance. Anyone who witnessed a damaged stand covered by vines might not have imagined that the trees covered by native members of this life form would eventually grow and replace them. This lack of foresight underscores the need to base LID management on ecological principles and not on headlines, personal memory, or ideas about what is attractive.
MANAGEMENT QUESTIONS There are few systems in which it is possible to have long-term or large-area management without allowing for disturbances [for example, see Pickett and Thompson (1978)]. In spite of the vintage of this recognition, it has not previously stimulated a general management strategy such as the one proposed here to alert managers to the ecologically based choices they have in dealing with the inevitable LIDs. Even small areas intended to be sustainable over the long term are likely to experience LIDs at some time. This analysis of the management implications of LIDs can be summarized by addressing three questions. Can we manage large-scale, infrequent disturbances? The answer is a qualified yes. A prescription for management must be tailored to particular disturbance characteristics and must have clear and achievable goals. Some disturbance agents and some systems are amenable to management because disturbances can be both endogenous and exogenous and because resistance of a system can be managed in the face of some physical forces. What have we learned from LIDs that is applicable to human-caused disturbances? First, although it may take time, recovery does occur. The observation of succession following a variety of LID events consistently shows that sites that appear to be destroyed develop into vigorous and diverse ecological systems just a few years or decades later. The rate of recovery depends largely on the intensity and extent of the initial disturbance, the environment of the particular system, and the presence and pattern of residuals left after the disturbance. Documented examples of the naturalness continuum (Figure 1) include the 1988 Yellowstone fires, the 1989 passage of Hurricane Hugo over Caribbean forests, and the 1980 eruption of Mount St. Helens (Walker and others 1991, 1996; Turner and others 1997). Uninformed attempts to speed recovery can have negative effects. Therefore, it is crucial to understand how different systems are likely to recover from LIDs. Residuals are often critical in the
recovery process. Managers should act to maximize survival of residuals and the spatial heterogeneity that fosters succession. One way to do this is to implement actions that mimic the key consequences of natural disturbances. The many potential functions of residuals should not be underestimated. Thus, overcleaning of disturbed sites should be avoided. Finally, the intensity and frequency of the event influence the ecological impacts of the disturbance and the recovery process, but managers can influence both severity and pattern of effects. What lessons can managers learn from LIDs? First, there are some natural as well as anthropogenic disturbances that we cannot do anything about. Thus, managing the disturbance per se is not always an option. Second, regardless of how ‘‘natural’’ or ‘‘unnatural’’ a disturbance may be, its impacts and the recovery of the system are often amenable to management actions based on ecological principles. Third, the position of the disturbance on the ‘‘naturalness’’ continuum is not an indication of the potential success at managing impacts and recovery. Fourth, management actions should complement the natural repair mechanisms as opposed to inhibiting recovery. Thus, managers need some understanding of natural recovery processes. Fifth, to improve the effectiveness of management actions, managers should focus on the interface between the LID and the normal ecosystem processes. Organisms or systems that receive the brunt of the disturbance force might require managerial priority. In each individual case, managers must evaluate how the disturbance interacts with biota. Deciding how to respond to LIDs and LID-affected systems is a good example of ecosystem management, for it requires managers to take a systems perspective and to consider the ecosystem, the disturbance event, and recovery processes together with effects on the landscape and the human community. ACKNOWLEDGMENTS The workshop at which these ideas were developed was held at the National Center for Ecological Analysis and Synthesis sponsored by the National Science Foundation. Reviews of the manuscript by Stephen Carpenter, Dennis Knight, Monica Turner, and two anonymous reviewers are greatly appreciated. Fred O’Hara and Gay Marie Logsdon edited the manuscript. V. H. Dale’s contribution was, in part, sponsored by the Strategic Research and Development Program (SERDP) through the US Department of Defense, Military Interagency Purchase Requisition W74RDV53549127, and the Office of Biological and Environmental Research, US Depart-
Ecosystem Management and Disturbances ment of Energy, under contract DE-AC05– 96OR22464 with Lockheed Martin Energy Research Corp. A. E. Lugo acknowledges the National Science Foundation Long-Term Ecological Research Program in the Luquillo Experimental Forest (grant BSR-8811902 to the University of Puerto Rico and the USDA International Institute of Tropical Forestry). S. Pickett acknowledges National Science Foundation grant BSR 9107243. This is Environmental Sciences Division Publication 4706.
REFERENCES Adams AB, Dale VH, Kruckeberg AR, Smith E. 1987. Plant survival, growth form, and regeneration following the May 18, 1980, eruption of Mount St. Helens, Washington. Northwest Sci 61:160–70. Adler T. 1996. Healing waters: flooding rivers to repent for the damage done by dams. Sci News 150:188–9. [AFPA] American Forest and Paper Association. 1994. Ecosystem management and private lands. Washington (DC): AFPA. Agee JK, Johnson DR. 1998. Ecosystem management for parks and wilderness. Seattle: University of Washington Press. Aide TM, Zimmerman JK, Rosario M, Mercado H. 1996. Forest recovery in abandoned cattle pastures along an elevational gradient in northeastern Puerto Rico. Biotropica 28:537–48. Arnold GW. 1995. Incorporating landscape pattern into conservation programs. In: Hansson L, Fahrig L, Merriam G, editors. Mosaic landscapes and ecological processes. New York: Chapman and Hall. p 307–39. Baker WL. 1992. The landscape ecology of large disturbances in the design and management of nature reserves. Landscape Ecol 7:181–94. Bleser CA. 1992. Status survey, management and monitoring activities for the Karner blue butterfly (Lycaeides melissa samuelis) in Wisconsin, 1990–1992. Bureau of Endangered Resources, Wisconsin Department of Natural Resources; Final Report submitted to the US Fish and Wildlife Service Office of Endangered Species Per Cooperative Agreement E-1–13, Study 320.
555
Clark JS, Royall PD, Chumbley C. 1996a. The role of fire during climate change in an eastern deciduous forest at Devil’s Bathtub, New York. Ecology 77:2148–66. Clark JS, Stocks BJ, Richard PJH. 1996b. Climate implications of biomass burning since the 19th century in eastern North America. Global Change Biol 2:433–42. Coffin DP, Lauenroth WK. 1988. The effects of disturbance size and frequency on a shortgrass plant community. Ecology 69:1609–17. Collier MP, Webb RH, Andrews ED. 1997. Experimental flooding in Grand Canyon. Sci Am 276:82–9. Covich AP, Crowl TA. 1990. Effects of hurricane storm flow on transport of woody debris in a rain forest stream (Luquillo Experimental Forest, Puerto Rico). In: Krishna JH, Aponte VQ, Gomez-Gomez F, editors. Tropical hydrology and Caribbean water resources. San Juan (Puerto Rico): American Water Resources Association. p 197–205. Covich AP, Crowl TA, Johnson SL, Varza D, Certain DL. 1991. Post–Hurricane Hugo increases in Atyid shrimp abundance in a Puerto Rican montane stream. Biotropica 23:448–54. Covich AP, McDowell WH. 1996. The stream community. In: Reagan DP, Waide RB, editors. The food web of a tropical rain forest. Chicago: University of Chicago Press. p 434–59. Dahm CN, Cummins KW, Valett HM, Coleman RL. 1995. An ecosystem view of the restoration of the Kissimmee River. Restor Ecol 3:225–38. Dale VH. 1991. The debris avalanche at Mount St. Helens: vegetation establishment in the ten years since the eruption. Natl Geogr Res Explor 7:328–41. Dale VH, Mann LK, Olson RJ, Johnson DW, Dearstone KC. 1990. The long-term influence of past land use on the Walker Branch forest. Landscape Ecol 4:211–24. del Moral, R. 1993. Mechanisms of primary succession on volcanoes: a view from Mount St. Helens. In: Miles J, Walton DH, editors. Primary succession on land. London: Blackwell Scientific. p 79–100. Denslow JS. 1980. Patterns of plant species diversity during succession under different disturbance regimes. Oecologia (Berl) 46:18–21. Elfring C. 1989. Yellowstone: fire storm over fire management. BioScience 39:667–72.
Bradshaw FJ. 1992. Quantifying edge effect and patch size for multiple-use silviculture: a discussion paper. For Ecol Manage 48:249–64.
Fitzsimmons AK. 1996. Sound policy or smoke and mirrors: does ecosystem management make sense? Water Resour Bull 32: 217–27.
Brown DA, Brown KD. 1996. Disturbance plays key role in distribution of plant species. Restoration Manage Notes 14: 140–7.
Foster RB. 1980. Heterogeneity and disturbance in tropical vegetation. In: Soule ME, Wilcox BA, editors. Conservation biology: an evolutionary–ecological perspective. Sunderland (MA): Sinauer. p 75–92.
Bureau of Land Management. 1994. Ecosystem management in the BLM: from concept to commitment. Washington (DC): US Government Printing Office. Christensen NL, Agee JK, Brussard PF, Hughes J, Knight DH, Minshall GW, Peek JM, Pyne SJ, Swanson FJ, Thomas BF, and others. 1989. Interpreting the Yellowstone fires of 1988. BioScience 39:678–85. Christensen NL, Bartuska AM, Brown JH, Carpenter SR, D’Antonio C, Francis R, Franklin JF, MacMahon JA, Noss RF, Parsons DJ, and others. 1996. The report of the Ecological Society of America Committee on the scientific basis for ecosystem management. Ecol Appl 6:665–91. Clark JS. 1991. Disturbance and tree life history on the shifting mosaic landscape. J Ecol 72:1102–18.
Foster D. 1995. Land-use history and four hundred years of vegetation change in New England. In: Turner BL II, Go´mez Sal A, Gonza´lez Berna´ldez F, Di Castri F, editors. Global land use change: a perspective from the Columbian encounter. Madrid: Consejo Superior de Investigaciones Cientificas; and Paris: SCOPE (Scientific Committee on Problems of Environment). p 253–319. Foster DR, Knight DH, Franklin JF. 1998a. Landscape patterns and legacies resulting from large, infrequent forest disturbances. Ecosystems 1:497–510. Foster DR, Motzkin G, Slater B. 1998b. Land-use history as long-term broad-scale disturbance: regional forest dynamics in central New England. Ecosystems 1:96–119.
556
V. H. Dale and others
Foster DR, Zebryk T, Schoonmaker P, Lezberg A. 1992. Postsettlement history of human land-use and vegetation dynamics of Tsuga canadensis (hemlock) woodlot in central New England. J Ecol 80:773–86. Franklin JF, MacMahon JA, Swanson FJ, Sedell JR. 1985. Ecosystem responses to the eruption of Mount St. Helens. Natl Geogr Res 1:198–216. Franklin JF, Swanson FJ, Harmon ME, Perry DA, Spies TA, Dale VH, McKee A, Ferrell WK, Gregory SV, Lattin JD, and others. 1992. Effects of global climatic change on forests in northwestern North America. In: Peters RL, Lovejoy TE, editors. The consequences of the greenhouse effect for biological diversity. New Haven: Yale University Press. p 244–57. Glenn SM, Collins SL. 1992. Effects of scale and disturbance on rates of immigration and extinction of species in prairies. Oikos 63:273–80. Goldstein B. 1992. The struggle over ecosystem management of Yellowstone. BioScience 42:183–7. Grumbine RE. 1994. What is ecosystem management? Conserv Biol 8:27–38. Haeuber R. 1996. Setting the environmental policy agenda: the case of ecosystem management. Nat Resour J 36:1–28. Harmon ME, Bratton SP, White PS. 1983. Disturbance and vegetation response in relation to environmental gradients in the Great Smoky Mountains. Vegetatio 55:129–39.
ME, Aber JD, editors. Restoration ecology. Cambridge: Cambridge University Press. p 221–37. MacMahon JA. 1997. Ecological restoration. In: Meffe GK, Carroll CR, editors. Principles of conservation biology. 2nd ed. Sunderland (MA): Sinauer. p 479–511. McCune B, Allen TFH. 1985. Will similar forests develop on similar sites? Can J Bot 63:367–76. McDonnell MJ, Stiles EW. 1983. The structural complexity of old field vegetation and the recruitment of bird-dispersed plant species. Oecologia (Berl) 56:109–16. Moritz MA. 1997. Analyzing extreme disturbance events: fire in Los Padres National Forest. Ecol Appl 7:1252–62. Nothnagle PJ, Schultz JC. 1987. What is a forest pest? In: Barbosa P, Schultz JC, editors. Insect outbreaks. New York: Academic. p 59–80. Odum HT. 1970. Rain forest structure and mineral-cycling homeostasis. In: Odum HT, Pigeon RF, editors. A tropical rain forest. Oak Ridge (TN): US Atomic Energy Commission. Chapter H-1. Otsamo A, A˚djers T, Hadi TS, Kuusipalo J, Tuomela K, Vuokko R. 1995. Effect of site preparation and initial fertilization on the establishment and growth of four plantation tree species used in reforestation of Imperata cylindrica (L.) Beauv. dominated grasslands. For Ecol Manage 73:271–7. ˚ djers G, Hadi TS, Kuusipal J, Otsamo A. 1996. Early Otsamo R, A performance of 12 shade tolerant tree species interplanted with Paraserianthes falcataria on Imperata cylindrica grassland. J Trop For Sci 8:381–94.
Holling CS, Meffe GK. 1996. Command and control and the pathology of natural resource management. Conserv Biol 10:328–38.
Paine RT, Tegner MJ, Johnson AE. 1998. Compounded perturbations yield ecological surprises. Ecosystems 1:535–45.
Keystone Center. 1996. The Keystone National Policy Dialogue on ecosystem management. (CO): Keystone Center.
Peters CS, Mangel M. 1990. New methods for the problem of collective ruin. SIAM J Appl Math 50:1442–56.
Knight DH, Wallace LL. 1989. The Yellowstone fires: issues in landscape ecology. BioScience 39:700–6.
Peterson CJ, Carson WP, McCarthy BC, Pickett STA. 1990. Microsite variation and soil dynamics within newly created treefall pits and mounds. Oikos 58:39–46.
Krause BA, DeSteven D. 1996. Effects of management and site history on plant succession and seedbank composition in oldfields at the UMW field station. Univ Wisc Milw Field Stn Bull 29:1–20. Krummel JR, Gardner RH, Sugihara G, O’Neill RV, Coleman PR. 1987. Landscape patterns in a disturbed environment. Oikos 48:321–4. Leach MK, Givnish TJ. 1996. Ecological determinants of species loss in remnant prairies. Science 273:1555–8. Leslie M, Meffe GK, Hardesty JL, Adams DL. 1996. Conserving biodiversity on military lands: a handbook for natural resource managers. Arlington (VA): Nature Conservancy. Ludwig DB. 1995. A theory of sustainable harvesting. SIAM J Appl Math 55:564–75. Lugo AE, Figueroa J Colo´n, Scatena FN. The Caribbean. In: Barbour MG, Billings DW, editors. North American terrestrial vegetation. London: Cambridge University Press. Chapter 16. Forthcoming. Lugo AE, Scatena FN. 1996. Background and catastrophic tree mortality in tropical moist, wet, and rain forests. Biotropica 28:585–99. Luken JO. 1990. Directing ecological succession. London: Chapman and Hall. Mack RN. 1981. The invasion of Bromus tectorum L. into western North America. Agroecosystems 7:145–65. MacMahon JA. 1987. Disturbed lands and ecological theory: an essay about a mutualistic association. In: Jordan WR, Gilpin
Peterson CJ, Pickett STA. 1990. Microsite and elevational influences on early forest regeneration after catastrophic windthrow. J Veg Sci 1:657–62. Pickett STA, Thompson JN. 1978. Patch dynamics and the design of nature reserves. Biol Conserv 13:27–37. Putz FE. 1997. Florida’s forests in the year 2020 and deeper into the Homogocene. J Public Interest Environ Conf 1:91–7. Rogers KH. 1997. Operationalizing ecology under a new paradigm: an African perspective. In: Pickett STA, Ostfeld RS, Shachak M, Likens GE, editors. The ecological basis of conservation: heterogeneity, ecosystems, and biodiversity. New York: Chapman and Hall. p 60–77. Romme WH, Despain DG. 1989. Historical perspective on the Yellowstone fires of 1988. BioScience 39:695–9. Romme WH, Everham EH, Frelich LF, Moritz MA, Sparks RE. 1998. Are large, infrequent disturbances qualitatively different from small, frequent disturbances? Ecosystems 1:524–34. Scatena FN, Moya S, Estrada C, Chinea JD. 1996. The first five years in the reorganization of aboveground biomass and nutrient use following Hurricane Hugo in the Bisley Experimental Watersheds, Luquillo Experimental Forest, Puerto Rico. Biotropica 28:424–40. Schullery P. 1989. The fires and fire policy. BioScience 39: 686–94. Slocombe DS. 1993. Environmental planning, ecosystem science, and ecosystem approaches for integrating environmental and development. Environ Manage 17:289–303.
Ecosystem Management and Disturbances Solem JC. 1994. Nuclear explosive propelled interceptor for deflecting objects on collision course with Earth. J Spacecraft Rockets 31:707–9. Sparks RE. 1996. Ecosystem effects: positive and negative outcomes. Changnon SA, editor. The great flood of 1993, causes, impacts, and responses. Boulder (CO): Westview. p 132–62. Starfield AM, Chapin FS. 1996. Effects of the Exxon Valdez oil spill on marine bird communities in Prince William Sound, Alaska. Ecol Appl 3:828–41. Swanson FJ, Franklin JF, Sedell JR. 1990. Landscape patterns, disturbance, and management in the Pacific Northwest, USA. In: Changing landscapes: an ecological perspective. New York: Springer-Verlag. p 191–213. Swetnam TW, Lynch AM. 1993. Multicentury, regional-scale patterns of western spruce budworm outbreaks. Ecol Monogr 63:399–424. Tilman D. 1989. Ecological experimentation: strengths and conceptual problems. In: Likens GE, editor. Long-term studies in ecology: approaches and alternatives. New York: SpringerVerlag. p 136–57. Turner MG, Baker WL, Peterson CJ, Peet RK. 1998. Factors influencing succession: lessons from large, infrequent natural disturbances. Ecosystems 1:511–23. Turner MG, Dale VH. 1998. Comparing large, infrequent disturbances: what have we learned? Ecosystems 1:493–96. Turner MG, Dale VH, Everham EE. 1997. Crown fires, hurricanes, and volcanoes: a comparison among large-scale disturbances. BioScience 47:758–68. Turner MG, Hargrove WW, Gardner RH, Romme WH. 1994.
557
Effects of fire on landscape heterogeneity in Yellowstone National Park, Wyoming. J Veg Sci 5:731–42. US Air Force. 1993. Natural resources management plan: Eglin Air Force Base 1993–1997. Eglin Air Force Base (FL): Department of the Air Force. Walker LR, Brokaw NVL, Lodge DJ, Waide RB. 1991. Ecosystem, plant and animal responses to hurricanes in the Caribbean. Biotropica 23:313–521. Walker LR, Silver WL, Willig MR, Zimmerman JK. 1996. Longterm responses of Caribbean ecosystems to disturbance. Biotropica 28:414–613. Walters C. 1997. Challenges in adaptive management of riparian and coastal ecosystems. Conserv Ecol 1(2):1. Weatherhead PJ. 1986. How unusual are unusual events? Am Nat 128:150–4. White MA, Mladenoff DJ. 1994. Old-growth forest landscape transitions from pre-European settlement to present. Landscape Ecol 9:191–205. Wiens JA. 1996. Oil, seabirds, and science: the effects of the Exxon Valdez oil spill. BioScience 46:587–97. Willig MR, Moorhead DL, Cox SB, Zak JC. 1996. Functional diversity of soil bacterial communities in the Tabonuco Forest: interaction of anthropogenic and natural disturbance. Biotropica 28:471–83. Wood CA. 1994. Ecosystem management: achieving the new land ethic. Renew Resour J Spring:6–12. Wootton JT, Parker MS, Power ME. 1996. Effects of disturbance on river food webs. Science 273:1558–61. Yaffee SL. 1996. Why environmental policy nightmares recur. Conserv Biol 11:328–37.
