Research to Support Management of Visitor Carrying Capacity of Boston Harbor Islands ROBERT MANNING 1, YU-FAI LEUNG 2,*, AND MEGHA BUDRUK 3 Abstract - Visitor carrying capacity has been a long-standing issue in management of parks and protected areas. Contemporary carrying capacity frameworks rely on formulation of indicators and standards of quality to define and manage carrying capacity. This paper describes a program of research to support management of carrying capacity of the Boston Harbor Islands national park area, a recent addition to the national park system. Research included: (1) an inventory and analysis of recreation-related resource impacts on selected islands, and (2) surveys of visitors to islands open to public use. Study findings are being incorporated into a visitor carrying capacity management plan through formulation of indicators and standards of quality for the park’s natural resources and visitor experience. Introduction Visitor carrying capacity The question of how much public use can be accommodated in a park or protected area is often framed in terms of carrying capacity. Indeed, much has been written in both the scientific and popular literature about the carrying capacity of parks and protected areas (e.g., Manning 2001, Mitchell 1994, Stankey and Manning 1986, Wilkinson 1995). The underlying concept of carrying capacity has a rich history in the natural resource professions. In particular, it has been applied in wildlife and range management where it refers to the number of animals that can be maintained in a given habitat (Dasmann 1964). Carrying capacity has obvious parallels and intuitive appeal in the field of park management. However, the first rigorous applications of carrying capacity to management of parks and related areas did not occur until the 1960s. These initial scientific applications suggested that the concept was more complex in this new management context. At first, as might be expected, the focus was placed on the relationship between visitor use and natural resource impact. The working hypothesis was that increasing numbers of visitors cause greater impact on natural resources as measured by soil degradation, vegetation damage, and related variables 1Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, VT 05405. 2Department of Parks, Recreation and Tourism Management, North Carolina State University, Raleigh, NC 27695-8004. School of Community Resources and Development, Arizona State University, Tempe, AZ 85287. *Corresponding author - leung@ncsu.edu. Boston Harbor Islands National Park Area: Natural Resources Overview 2005 Northeastern Naturalist 12(Special Issue 3):201–220 202 Northeastern Naturalist Vol. 12, Special Issue 3 (Hammitt and Cole 1998). However, it soon became apparent that there was another critical dimension of carrying capacity dealing with social aspects of the visitor experience. Wagar (1964), for example, in his early and important monograph on the application of carrying capacity to outdoor recreation, reported that his study “was initiated with the view that carrying capacity of recreation lands could be determined primarily in terms of ecology and the deterioration of areas. However, it soon became obvious that the resource-oriented point of view must be augmented by consideration of human values.” Wagar’s point was that as more people visit a park or related area, not only can the environmental resources of the area be affected, but the quality of the visitor experience as well. Again, the working hypothesis was that increasing numbers of visitors cause greater social impacts as measured by crowding, conflict, and related variables. Thus, as applied to parks and protected areas, carrying capacity has two components: resource and social. The early work on carrying capacity has since blossomed into an extended literature on the resource and social impacts of visitor use and their application to carrying capacity (e.g., Graefe et al. 1984; Haas 2001; Hammitt and Cole 1998; Kuss et al. 1990; Leung and Marion 2000; Lime and Stankey 1971; Manning 1985, 1999; Shelby and Heberlein 1986; Stankey and Lime 1973). But despite this growing scientific literature, efforts to determine and apply visitor carrying capacity have sometimes failed. The principal difficulty lies in determining how much impact, such as trail erosion and crowding, is too much. Theoretical development, backed up by empirical research, generally confirms that increasing visitor use leads to increased resource and social impacts. But how much impact should be allowed? This basic question is often referred to as the “limits of acceptable change” (Frissell and Stankey 1972). Given substantial demand for public use of parks and related areas, some decline or change in the quality of natural resources and the visitor experience appears inevitable. But how much decline or change is acceptable or appropriate before management intervention is warranted? As illustrated in Figure 1, two hypothetical relationships between visitor use and natural resource and social impacts (represented by line A and curve B) show that visitor use and impacts are related: increasing amount of use causes increasing impact. However, it is not clear at what point carrying capacity has been reached. The hypothetical relationships in Figure 1 suggest that some impact is inevitable, given even relatively low levels of visitor use. Thus, some level of impact must be tolerated if parks and related areas are to remain open for public use. For the relationship defined by line A, X 1 and X 2 represent levels of visitor use that result in differing levels of impact as defined by points Y 1 and Y 2 , respectively. But which of these points—Y 1 or Y 2 , or some other point along this axis—represents the maximum amount of impact that is acceptable? Again, the difficulty in 2005 R. Manning, Y.-F. Leung, and M. Budruk 203 carrying capacity determination lies in deciding how much impact is acceptable. Empirical relationships such as those in Figure 1 can be helpful in making informed decisions about carrying capacity, but they must be supplemented with other information. To determine acceptable levels of visitor carrying capacity, some have suggested distinguishing between descriptive and evaluative (or prescriptive) components of carrying capacity (Shelby and Heberlein 1984, 1986). The descriptive component focuses on factual, objective data such as the types of relationships in Figure 1. For example, what is the relationship between the number of visitors entering a park and the number of encounters that occur among groups of visitors? Or what is the relationship between the level of visitor use and visitor perceptions of crowding? The evaluative or prescriptive component of carrying capacity determination concerns the seemingly more subjective issue of how much impact or change in resource conditions and the quality of the visitor experience is acceptable. For example, how many contacts between visitor groups are acceptable? What level of perceived crowding should be allowed before management intervention is needed? Recent experience with carrying capacity suggests that answers to the above questions can be found through formulation of management objectives and development of associated indicators and standards of quality (Belnap 1998; Graefe et al. 1990; Manning 1997, 1998, 2001; Manning et Figure 1. Hypothetical relationships between visitor use and ecological and social impacts. XX11 X2 204 Northeastern Naturalist Vol. 12, Special Issue 3 al. 1998; National Park Service 1997; Stankey et al. 1985). This approach to carrying capacity puts principal emphasis on defining the degree of resource protection and the type of visitor experience to be provided and maintained, monitoring conditions over time, and adopting management practices to ensure that acceptable conditions have been maintained. Management objectives are broad, narrative statements that define the degree of resource protection and the type of visitor experience to be provided. They are based on the purpose and significance of the area under consideration. Indicators of quality are measurable, manageable variables that reflect the essence or meaning of management objectives; they are quantifiable proxies or measures of management objectives. Indicators of quality may include elements of both the natural and social environments. Standards of quality define the minimum acceptable condition of indicator variables. Examples of management objectives and indicators and standards of quality may be helpful. Review of the US Wilderness Act of 1964 suggests that areas of the national wilderness preservation system are to be managed to provide opportunities for visitor solitude. Thus, providing opportunities for solitude is an appropriate management objective for most wilderness areas. Moreover, research on wilderness use suggests that the number of visitors encountered along trails and at campsites is important to wilderness visitors in defining solitude. Thus, trail and camp encounters may be good indicators of quality and help to make the general management objective of solitude more operational. Further research suggests that wilderness visitors may have normative standards about how many trail and camp encounters are acceptable before the quality of the visitor experience declines to an unacceptable degree (Heberlein et al. 1986; Lewis et al. 1996; Manning et al. 1996a, 1996b, 1999; Roggenbuck et al. 1991; Shelby and Vaske 1991; Vaske et al. 1986; Whittaker and Shelby 1988). Such data may help define standards of quality. A similar example for the natural resource component of carrying capacity relates to the wilderness management objective of preserving ecological conditions. Research on the ecological impacts of recreation suggests that destruction of ground cover vegetation and compaction of soil is a common impact of recreation at wilderness campsites (Hammitt and Cole 1998, Leung and Marion 2000). Therefore, bare soil exposure at campsites may be a good indicator of resource quality. The selection of a standard of quality in this case is guided primarily by visitor impact research, but is also influenced by social science research, particularly on the effect of resource impacts on the quality of the visitor experience (Shelby et al. 1988). While the above examples of management objectives and associated indicators and standards of quality apply to wilderness areas, recent studies have extended this research and management approach to a variety of more developed parks and related areas (Manning et al. 2002, 2003). Research at Boston Harbor Islands was designed to further extend this work. 2005 R. Manning, Y.-F. Leung, and M. Budruk 205 By defining indicators and standards of quality, carrying capacity can be determined and managed through an associated program of monitoring and management. Indicators of quality can be monitored and management actions taken to ensure that standards of quality are maintained. If monitoring suggests that standards of quality have been violated (or may soon be violated), then carrying capacity has been exceeded and management actions (such as modifying type and amount of use, visitor education or regulation) may be required. This basic approach to carrying capacity is central to contemporary park and protected area management frameworks, including Limits of Acceptable Change (LAC) (Stankey et al. 1985), Visitor Impact Management (VIM) (Graefe et al. 1990), and Visitor Experience and Resource Protection (VERP) (Manning 2001, National Park Service 1997). Application to Boston Harbor Islands The concept of carrying capacity was applied to Boston Harbor Islands national park area through a program of research and planning. This program included three components: (1) natural science research, (2) social science research, and (3) preparation of a carrying capacity plan incorporating research findings. In keeping with the contemporary approach to carrying capacity described above, this program of research and planning was directed at formulating a series of indicators and standards of quality for park resources and the visitor experience. The natural resources of the national park area and its cooperative administration are described elsewhere in this issue. Visitation to the park has been increasing due to its growing popularity among local communities, improved transportation, and promotion by the Boston Harbor Islands Partnership. In 2002, the park recorded 262,000 recreational visits (Boston Harbor Islands Partnership, unpubl. data). Methods Natural science research This component of research focused on identifying and developing resource-based indicators of quality (or resource impact indicators) and related monitoring procedures as well as assisting in the formulation of standards of quality for selected indicator variables. Major research activities included identification of potential indicators from various sources, inventory and condition assessment of trails and recreation sites in 2001, evaluation of other ecological indicators of quality in 2002, and evaluation and selection of indicator variables in 2003. First, official and unofficial sites and trails were identified. Official sites and trails are those designated and clearly identified by the management agency, while unofficial sites and trails (also referred to as social trails) are visitor-created and are not identified by the management agency. While locations of all official recreation sites were known, unofficial sites 206 Northeastern Naturalist Vol. 12, Special Issue 3 were searched extensively on all islands subject to public use as suggested by park staff and local experts. For official and unofficial recreation sites, a multiple-indicator assessment approach was adopted (Cole 1989, Leung and Marion 2000, Marion 1991, Marion and Leung 1997). The procedures began with a delineation of recreation site boundaries within which impact indicators were assessed. Inventory information, including GPS coordinates, site position on landscape, distance to water, distance to trail, and vegetation canopy cover, were quantified. Impact indicators assessed included a five-point condition class rating, area of disturbance based on site size measurement, vegetative groundcover on-site and at off-site controls, mineral soil exposure, tree damage (three categories), root exposure, trash, human waste, and vandalism parameters. A simplified version of the assessment procedures was applied to rest/viewing areas which are usually small with only benches or no formal facilities. For condition assessment of official trails an integrated approach was adopted, combining sampling-based point measurements and a census of problem events (Leung and Marion 1999, Marion and Leung 2001). Assessments were conducted for the entire length of each trail segment under study. Field staff pushed a measuring wheel along the trail while stopping every 60.6 m (200 ft) to perform point-based measurements such as width, incision depth, and tread substrate composition. Field staff also documented each occurrence of pre-defined problem events such as excessive soil erosion, root exposure, and muddy soil. A more rapid approach was adopted for social trails, on which only length and overall condition rating were collected (Cole et al. 1997, Leung et al. 2002). A series of field measurements was conducted in the summer of 2002 to evaluate additional ecological indicators commonly used in recreation ecology research (Hammitt and Cole 1998, Leung and Marion 2000). Quadrat-based and continuous line-transect measurements were applied within twelve circular sampling plots on Georges, Peddocks, and Grape Islands, six of which were indicative of high-use areas and the other six representing relatively low-use areas. Initial measurements were conducted in June 2002, with remeasurements performed in August and October 2002. Major ecological indicators assessed included percent cover of ground vegetation, plant litter and exposed soil, soil compaction as measured by penetration resistance (Lowery and Morrison 2002), and soil stability as measured by the slake test in which soil aggregates/fragments are subject to rapid wetting cycles (Doran and Jones 1996). Results from site/trail assessments and ecological indicator evaluations provide the baseline data for the park, and they were utilized to inform selection of resource-based indicators of quality based on criteria developed in previous carrying capacity research (Belnap 1998, GYWVU 1999) and the park’s specific concerns. Monitoring protocols were also developed for the selected indicator variables. 2005 R. Manning, Y.-F. Leung, and M. Budruk 207 Social science research Social science research included two surveys of park visitors. The first survey was designed to identify potential indicators of quality of the visitor experience. In summer 2000, randomly selected park visitors returning on ferries from the islands, were administered a questionnaire that included a series of questions on what they enjoyed most and least during their visit to Boston Harbor Islands. Following this series of questions, respondents were asked to judge the degree to which a series of issues (e.g., crowding, litter) were problems at the park. The survey resulted in 695 completed questionnaires. The second survey, conducted in summer 2001, was designed to measure normative standards of quality of visitors for selected indicator variables. Norms in parks and outdoor recreation are generally defined as standards that individuals and groups use for evaluating behavior and social and environmental conditions (Donnelly et al. 1992, Shelby and Vaske 1991, Vaske et al. 1986). If visitors have normative standards concerning relevant aspects of park conditions and recreation experiences, then such norms can be studied and used as a basis for formulating standards of quality. Developed in sociology, normative theory and methods have attracted considerable attention as an organizing concept in park and outdoor recreation research and management. In particular, normative research has special application in helping to formulate standards of quality for both resource and social conditions in parks and related areas. Application of norms to standards of quality is most fully described by Shelby and Heberlein (1986), Vaske et al. (1986), and Manning (1999), with these applications relying on the work of Jackson (1965), who developed a methodology to measure norms. Using these methods, the personal norms of individuals can be aggregated to determine social norms. Normative research has focused largely on the issue of crowding (e.g., Heberlein et al. 1986; Manning et al. 1996a, 1996b, 1998; Patterson and Hammitt 1990; Shelby 1981; Vaske et al. 1986; Williams et al. 1991; Whittaker and Shelby 1988), but also has been expanded to include other potential indicators of quality, including ecological impacts at campsites (Shelby et al. 1988). Traditionally, norms have been measured through a numerical approach. For example, respondents are asked to evaluate the acceptability of alternative use levels, such as zero, five, or ten encounters with other groups per day along trails. Resulting data are aggregated to determine social norms. More recently, visual approaches to measuring norms have been developed (Hof et al. 1994; Manning et al. 1995, 1996a, 1996b). In this technique, computer software is used to manipulate photographs to depict alternative use levels and associated resource and social impacts. Both numerical and visual approaches to norm measurement were incorporated into questionnaires administered to random samples of 208 Northeastern Naturalist Vol. 12, Special Issue 3 Figure 2. Frequency distribution and descriptive statistics for site size (area of disturbance) of official recreation sites. visitors to the seven islands where visitor services were provided in the summer of 2001. The survey resulted in 724 completed questionnaires. Results Natural science research Field assessment procedures of potential recreation site and trail impact indicators were developed and applied to 22 islands (and former islands) with established or possible recreational use. A total of 144 sites were assessed in 2001, including 82 official picnic or camping sites, 21 unofficial recreation sites, and 41 rest/viewing areas. The majority of recreation sites had modest site size or area of disturbance (Fig. 2). Official sites contributed to more than 90% of the cumulative area of disturbance (46,680 m2 or 4.7 ha; 11.5 acres) for the entire park. There were several very extensive official sites on Georges, Lovell, and Bumpkin Islands, with the largest one being 6970 m2 in size. Unofficial sites, most of which were created for overnight use, were generally small, ranging from 16 to 265 m2 with a median of 49 m2. A limited number of large official sites may be necessary on public use islands to support high levels of use with large groups. These sites, if appropriately located and maintained, are not generally considered an unacceptable resource impact issue unless they show signs of inappropriate degradation as indicated by site expansion, excessive loss Site Size (m2) Official Recreation Sites (%) Mean = 531; median = 92; S.D. = 1191; min. = 14; max. = 7061; N = 82 *(No. of sites, cumalitve frequency) 2005 R. Manning, Y.-F. Leung, and M. Budruk 209 Table 1. Assessment results of unpaved official park trails on the Boston Harbor Islands. Trail resource No. of % of trails Length of problem segment (m) impact occurrences surveyed++ Mean Min. Max. Std.Dev. Sum Bare patches* 5 1.90 11.9 2.7 21.8 8.3 59.4 Soil erosion+ 15 1.92 24.8 1.5 82.1 27.3 372.1 Root exposure 10 0.56 10.8 5.5 21.2 6.1 107.9 Muddy soil 12 0.28 4.5 1.5 7.3 1.9 54.5 *On grassy trails of Grape and Great Brewster Islands only (total length of trails surveyed = 3126 m). +Recordable incidents are defined as trail segments that are incised more than 0.3 m (1 ft) for at least 3 m (10 ft) in length. ++Total length of unpaved official trails surveyed was 19,422 m. of groundcover vegetation, and increased soil exposure. The existence and size of unofficial sites, however, is indicative of unacceptable visitorinduced resource impacts that require management attention. Mineral soil exposure appears to be more pronounced on unofficial recreation sites, with about 30% of unofficial sites exhibiting 63% or more exposed soil on site. This indicates a potential impact concern to natural resources associated with soil degradation and specifically soil erosion. In contrast, about 30% of official sites had 38% or more exposed soil. However, when the areal extent of soil exposure was considered (recreation site size x percent soil exposure), official sites exhibited more cumulative impact with a total of 4288 m2 of exposed soil while the sum of unofficial sites was only 632 m2. Damage to tree trunks was found to be more common on official recreation sites than their unofficial counterparts. The typical official site has two damaged trees (median value), with just one damaged tree on the typical unofficial site. A total of 91 official trails (30 km) and about 36 km of visitorcreated social trails were identified and assessed in 2001. Results suggest that soil erosion, root exposure, and other resource impacts existed on unpaved park trails, but the extent of these problems was limited (Table 1). For example, the cumulative length of eroded trail treads amounted to 372 m, which was only 1.92% of total length of trails surveyed. Despite this, several soil erosion and root exposure segments were rather long and they may have compromised the trail’s transportation function and visitor experience. The density of social trails was highest on Worlds End with 209 meters per hectare of unit area (277 feet per acre), followed by Georges (170 m/ha; 226 ft/ac) and Raccoon (154 m/ha; 204 ft/ac) Islands. High densities of social trails may suggest proliferation of unmanaged recreational trampling with potential threats to sensitive habitats. Spatial proximity of social trails and unofficial sites to sensitive habitats was evaluated using geographic information systems. Field 210 Northeastern Naturalist Vol. 12, Special Issue 3 researchers identified 141 m of social trails and three unofficial sites within 50 m of known habitats of rare, threatened and endangered species, including the seabeach dock plant (Rumex pallidus Lesson), Least Tern (Sterna antillarum Bigelow), and Barn Owl (Tyto alba L.). Findings of the 2002 assessment exhibited a considerable variability in soil compaction and ground cover measurements, while soil stability (slake test) measurements showed considerable homogeneity among sampling sites, indicating non-responsiveness to visitor use. The degree of soil compaction, as measured by penetration resistance, at Boston Harbor Islands was less pronounced than that reported in previous impact studies in the East (Marion and Cole 1996). This may be partly attributed to higher background soil compaction levels, which are in turn influenced by glacial parent materials and a long history of human activities on these islands. The average penetration resistance (pocket penetrometer) values ranged from 1.5 kg/cm2 on Grape Island to 3.0 kg/cm2 on Georges Island. Soil compaction was higher on use areas than off-site controls, with penetration resistance increased by 53% on Georges Island’s low use plots and by 144% on Grape Island’s low use plots. Social science research The first visitor survey identified seven potential indicators of quality: number of people-at-one-time (PAOT) at selected attraction sites, number of groups encountered per hour while hiking, environmental impacts to trails, environmental impacts to campsites, amount of litter, amount of graffiti, and amount and quality of information about the park. These variables are related to both the resource and social conditions of the park and were reported by visitors as important in determining the quality of the visitor experience. The second visitor survey identified a range of potential standards of quality for these seven indicator variables. Two of these indicator variables will be used to illustrate study findings: visitors to Little Brewster Island were asked to judge the acceptability of a series of five computeredited photographs showing a range of visitors to the island (Fig. 3), and visitors who camped on Bumpkin and Grape Islands were asked to judge the acceptability of a series of five computer-edited photographs showing a range of ecological impact at a representative campsite (Fig. 4). For the Little Brewster Island photos, the response scale was anchored at -4 (“very unacceptable”) and +4 (“very acceptable”). The mean response for the sample as a whole for each photograph is graphed in Figure 5 and results in a “social norm curve.” The point at which the social norm curve crosses the neutral point of the response scale (approximately 95 PAOT) represents an “acceptability”-based standard of quality, as this is the minimum level of acceptability for PAOT for most respondents. 2005 R. Manning, Y.-F. Leung, and M. Budruk 211 Figure 3. Study photographs of people- at-one-time (PAOT) at Little Brewster Island. Figure 4. Study photographs of campsite impacts at Bumpkin and Grape Islands. 1 2 3 4 5 1 2 3 4 5 212 Northeastern Naturalist Vol. 12, Special Issue 3 Table 2. Summary of PAOT study findings for Little Brewster Island. Standards of quality N Mean PAOT Preference 107 49 Acceptability 107 72 Management action 103 89 Displacement 103 174 Existing conditions 101 47 Figure 6. Social norm curves for campsite impacts at Bumpkin and Grape Islands. Figure 5. Social norm curve for people-at-one-time (PAOT) on Little Brewster Island. Table 3. Summary of campsite impact study findings for Bumpkin and Grape Islands. Bumpkin Island Grape Island Standards of quality N Mean photo number N Mean photo number Preference 32 1.4 37 1.4 Acceptability 33 3.2 33 3.0 Management action 33 2.8 35 2.7 Displacement 33 4.0 35 4.4 Existing conditions 28 1.8 34 1.9 PAOT in photographs 2005 R. Manning, Y.-F. Leung, and M. Budruk 213 Respondents were also asked to select the photograph that best represented (1) the PAOT condition they preferred (“preference”- based standard of quality), (2) the PAOT condition that was so unacceptable that they would no longer visit Little Brewster Island (“displacement”-based standard of quality), (3) the maximum PAOT condition they thought the National Park Service should allow before restricting visitor use (“management action”-based standard of quality), and (4) the PAOT condition they experienced on the day of their visit (“existing conditions”). All standards of quality findings are summarized in Table 2 and provide an empirical basis for selecting PAOT standards of quality for the island. For the Bumpkin and Grape Islands campsite photographs, the same response scale as described above was used. The points at which these social norm curves cross the neutral point of the response scale (mean response values of approximately photo number 3.2 for Bumpkin Island and photo number 3.0 for Grape Island) represent “acceptability” based standards of quality (Fig. 6). “Preference,” “displacement,” “management action,” and “existing conditions” questions were also asked as described above, and all standards of quality findings are summarized in Table 3, providing an empirical basis for selecting standards of quality for ecological impacts at park campsites. Discussion Planning component After completion of the natural and social science research components, a series of planning workshops were held in the winter and spring of 2003 to formulate indicators and standards of quality for the park. Workshop participants included the planning committee of the Boston Harbor Islands Partnership and research staff, including principal investigators and graduate students. The workshops were facilitated by the park assistant superintendent. The National Park Service, in conjunction with the Boston Harbor Islands Partnership, recently completed a general management plan (GMP) to guide park management (National Park Service 2002). This plan established six zones into which all park lands are assigned. The management objectives for each of the zones provided initial guidance concerning appropriate indicators and standards of quality for natural and social conditions. Study findings from the natural and social science research provided a more empirical basis for formulating indicators and standards. For example, management objectives developed in the park’s GMP specified that the Natural Features Emphasis zone of the park would provide opportunities for visitor solitude, while the Visitor Services and Park Facilities Emphasis zone would provide opportunities for a more social 214 Northeastern Naturalist Vol. 12, Special Issue 3 experience. Study findings from the social science data outlined a range of potential standards of quality for visitor density (from “preference” to “displacement”), including PAOT at attraction sites and encounters along trails. For the Natural Features Emphasis zone (emphasizing solitude), a standard of quality near the preference end of the range was selected (to ensure high levels of solitude), while a standard of quality near the management action or displacement end of the range was selected (because solitude is not important in this area)for the Visitor Services and Park Facilities Emphasis zone (emphasizing a social experience). Findings of the natural science component of the study were also utilized in the formulation of standards of quality. Frequency distribution charts and tables of impact indicators were provided to the planning committee in order to determine the level at which a realistic standard of quality can be set given the current levels of resource impact. For instance, previous research has shown that soil erosion rates were substantial: beyond 20–30% of mineral soil exposure on the ground (Kuss and Morgan 1984, 1986). The planning committee eventually selected the lower end of 20% as the standard. This decision was informed by the body of visitor impact research and the current resource conditions which indicate that only a minority of sites would exceed this standard and require corrective management actions. A third example involves both natural and social science research findings. Natural science research found that visitors were causing impacts at campsites through destruction of ground cover vegetation and resulting sheet erosion of exposed bare soil. From an ecological standpoint, this process can lead to degradation of the park’s natural condition. However, natural science data provides only a partial picture to suggest an appropriate standard of quality for this indicator variable. Social science data may complement natural science data by suggesting a standard of quality for campsite condition from an aesthetic standpoint based on visitors’ perceptions. The social science findings described earlier for Bumpkin and Grape Islands suggest the point at which campsite impacts are judged as unacceptable, and these data can be used to help set a socially acceptable standard of quality for the ecological indicator variable of campsite impacts. Workshop sessions resulted in sets of resource and social indicators and standards of quality for each zone on each island. These indicators and standards of quality are designed to meet the management objectives for each zone as defined in the GMP and to provide for an appropriate range of resource conditions and visitor opportunities throughout the park. Indicators and standards of quality that were selected are summarized in Table 4. Indicators and standards of quality for the social science component were recommended for only those islands open to public access in 2001. 2005 R. Manning, Y.-F. Leung, and M. Budruk 215 Table 4. Selected indicators and standards of quality for visitor carrying capacity on the Boston Harbor Islands (the notation “-” means not recommended as an indicator for the island). Eroded Official picnic Density Area Quality of Amount of People Tour Trail tread on sites & campsites of disturbed information information at one group encounters official Campsite Bare soil social unofficial (% visitors (% of visitors Management Zone time size (hikers/ trails impacts exposure Litter Graffiti trails recreation who have who have and Islands (photo #) (photo #) hour) (ft2) (photo #) (%) (photo #) (photo #) (ft/acre) sites (ft2) quality info) adequate info) Natural Features Emphasis - - - - - - 1–2 2 10 0 - - (Button, Calf, Green, Hangman, Langlee, Little Calf, Middle Brewster, Outer Brewster, Raccoon, Ragged, Sarah, Shag Rocks, Sheep, Slate, Snake) Managed Landscape Emphasis - - 5–15 50 a 20 1–2 2 10 0 75 a (Bumpkin, Gallops, Grape, Great Brewster, Webb Memorial Park, Worlds End, Rainsford) Historic Preservation Emphasis 60b 30b - - - 20 1–2 2b 10b 0b 75b a (Little Brewster, The Graves) Multiple Management Emphasis 185–200c 30 15–25 - 20 1–2 2 10–50 0 75 a (Deer, Georges, Long, Lovells, Moon, Nut, Peddocks, Spectacle, Thompson) Special Use Emphasis - - - - - - 1–2 2 - - - - (Nixes Mate) aApplicable to public use islands only b(Little Brewster only) c(Georges and Long only) 216 Northeastern Naturalist Vol. 12, Special Issue 3 Conclusions An integrated program of natural and social science research and associated planning was conducted at Boston Harbor Islands national park area to manage visitor carrying capacity. Contemporary approaches to carrying capacity emphasize the importance of formulating indicators and standards of quality that define minimum acceptable resource and social conditions in the park. This process entails a long-term commitment to monitoring indicator variables, and implementation of management actions to ensure that standards of quality are maintained. Management actions designed to control and/or reduce the resource and social impacts of visitor use include education on low-impact visitor behavior, hardening and maintenance of visitor use areas, and, ultimately, limiting visitor use. The research and planning process used at Boston Harbor Islands national park area provides an empirical, scientific basis for justifying implementation of such management actions, protecting the quality of park resources and the visitor experience, and managing the park within a defined visitor carrying capacity. A carrying capacity–related monitoring program for Boston Harbor Islands is now being designed and will focus on the indicators of quality outlined in Table 4. When and where monitoring data suggest that standards of quality are in danger of being violated, management action will be undertaken. For example, if visitor density or natural resource impact standards of quality are violated, adjustments can be made to ferry schedules so that fewer visitors are delivered to islands experiencing these problems or so that visitation can be temporally or spatially shifted. Experience at Boston Harbor Islands clearly demonstrates that carrying capacity has both resource and social dimensions, and that there can be a strong relationship between these dimensions. Visitor use of parks such as Boston Harbor Islands can impact both the natural resources of the park and the quality of the visitor experience. Moreover, resource impacts can also affect the quality of the visitor experience, and these aesthetic impacts can offer a potentially important justification and guidance for setting standards of quality. Carrying capacity analysis and management requires an integrated program of natural and social science research. The program of research outlined in this paper provides an important scientific basis for managing carrying capacity. Carrying capacity also requires the exercise of management judgment. For example, the natural and social science research outlined in this paper offers an empirically based range of potential standards of quality for selected indicator variables. However, selection of a standard of quality requires some element 2005 R. Manning, Y.-F. Leung, and M. Budruk 217 of management judgment. But such judgments should be as scientifi- cally informed as possible (Manning and Lawson 2002). Literature Cited Belnap, J. 1998. 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