Day Use Social Carrying Capacity
ABSTRACT
Estimating Day Use Social Carrying Capacity in Yosemite National Park
Carrying capacity has been a long standing issue in management of parks and
outdoor recreation. Contemporary carrying capacity frameworks rely on formulation of
indicators and standards of quality of the recreation experience to define and manage
carrying capacity. This paper describes a program of research to support application of
carrying capacity to Yosemite Valley, the scenic heart of Yosemite National Park, USA.
Research included 1) a series of visitor surveys at selected sites within Yosemite Valley
to identify indicators and standards of quality, 2) development of computer simulation
models of visitor use at study sites to estimate maximum daily use levels without
violating standards of quality, and 3) a park exit survey to determine the percentage of
day users at study sites. Study findings are used to estimate a range of day use carrying
capacities at study sites and for Yosemite Valley as a whole.
Keywords: Carrying capacity; indicators and standards of quality; Yosemite National
Park
Estimating Day Use Social Carrying Capacity in Yosemite National Park
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Day Use Social Carrying Capacity
Carrying Capacity
The question of how much public use can be accommodated in a national park or
related area is often framed in terms of carrying capacity. Indeed, much has been written,
in both the scientific literature and popular press, about the carrying capacity of national
parks (Stankey and Manning 1986; Mitchell 1994; Wilkinson 1995; Manning 2001). 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. In fact, it was first suggested in the mid-1930s as a park management
concept in the context of national parks (Sumner, 1936). However, the first rigorous
applications of carrying capacity to management of parks and outdoor recreation 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 environmental conditions. The working hypothesis
was that increasing numbers of visitors cause greater environmental impact as measured
by soil compaction, destruction of vegetation, and related variables. It soon became
apparent, however, 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,
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Day Use Social Carrying Capacity
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, 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 and related variables. Thus, as applied to national
parks, carrying capacity has two components: environmental and social.
The early work on carrying capacity has since blossomed into an extended literature
on the impacts of outdoor recreation and their application to carrying capacity (e.g., Lime
and Stankey, 1971; Stankey and Lime, 1973; Graefe et al., 1984; Manning, 1985; Shelby
and Heberlein, 1986; Kuss et al., 1990; Hammitt and Cole 1998; Manning, 1999). But
despite this growing scientific literature, efforts to determine and apply carrying capacity
to areas such as national parks have sometimes failed. The principle difficulty lies in
determining how much impact, such as crowding, is too much. Theoretical development,
backed up by empirical research, generally confirms that increasing use levels and
encounters among visitors leads to increased environmental and social impacts. But how
much impact should be allowed in a national park? This basic question is often referred
to as the “limits of acceptable change” (Lime, 1970; Frissell and Stankey, 1972). Given
substantial demand for public use of a national park, some decline or change in the
quality of park resources and the visitor experience appears inevitable. But how much
decline or change is acceptable or appropriate before management intervention is
needed?
This issue is illustrated graphically in Figure 1. This figure addresses the social
impact of crowding. In this figure, a hypothetical relationship (line A) between visitor
use and crowding is shown. It is clear from this figure that visitor use and crowding are
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Day Use Social Carrying Capacity
related: increasing numbers of visits cause visitors to feel increasingly crowded.
However, it is not clear at what point carrying capacity has been reached. The
hypothetical relationship in Figure 1 suggests that some crowding is inevitable, given
even relatively low levels of visitor use. Thus, some level of crowding must be tolerated
if national parks are to remain open for public use. For the relationship defined by line
A, X1 and X2 represent levels of visitor use that result in differing levels of crowding as
defined by points Y1 and Y2, respectively. But which of these points— Y1 or Y2, or some
other point along this axis—represents the maximum amount of crowding that is
acceptable? Ultimately, this is a value judgment. Again, the principal difficulty in
carrying capacity determination lies in deciding how much crowding (or of some other
impact) is acceptable. Empirical relationships such as that in Figure 1 can be helpful in
making informed decisions about carrying capacity, but they must be supplemented with
other information and, ultimately, management judgments.
To emphasize and further clarify this issue, some writers have suggested
distinguishing between descriptive and evaluative (or prescriptive) components of
carrying capacity (Shelby and Heberlein, 1984; Shelby and Heberlein, 1986). The
descriptive component of carrying capacity 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 appropriate? What level of
perceived crowding should be allowed before management intervention is needed?
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Day Use Social Carrying Capacity
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 (Stankey et al., 1985; Graefe et al., 1990;
Manning, 1997; National Park Service 1997; Manning, 1998; Manning et al., 1998). This
approach to carrying capacity focuses 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
largely on review of the purpose and significance of the area under consideration.
Formulation of management objectives may involve review of legal, policy and planning
documents; consideration by an interdisciplinary planning and management team;
historic precedent; local, regional, national or international context of the park; and public
involvement.
Indicators of quality are measurable, manageable variables that reflect the essence
or meaning of management objects; they are quantifiable proxies or measures of
management objectives. Indicators of quality may include elements of both the
biophysical and social environments. Standards of quality define the minimum
acceptable condition of indicator variables.
An example of management objectives, indicators and standards may be helpful.
Review of the U.S. Wilderness Act of 1964 suggests that areas of the national park
system contained in 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
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Day Use Social Carrying Capacity
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; Vaske et al., 1986; Whittaker and Shelby, 1988; Roggenbuck et al., 1991;
Shelby and Vaske, 1991; Lewis et al., 1996; Manning et al., 1996a; Manning et al.,
1996b; Manning et al., 1999). Such data may help to define standards of quality.
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, then carrying capacity has been exceeded and management action is
required. This basic approach to carrying capacity is central to contemporary park and
outdoor recreation 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) (National Park Service 1997).
When feasible, the above approach to carrying capacity might be supplemented by
computer simulation modeling. Computer simulation models of park use have been
developed to estimate the relationship between park use levels and selected indicators of
quality, such as the number of encounters among hiking groups (Smith and Krutilla,
1976; Schechter and Lucas, 1978; Manning and Potter, 1984; Potter and Manning, 1984;
Wang and Manning, 1999). Such models could be used to estimate the maximum use
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Day Use Social Carrying Capacity
level that could be accommodated in a park (or sites within a park) without violating
specified standards of quality. In this way, carrying capacity could be estimated.
Study Objectives
The purpose of this study was to gather information to support application of the
concept of carrying capacity to Yosemite National Park. The study was focused on social
carrying capacity of Yosemite Valley for day use. The study had three specific
objectives:
1. Formulate indicators and standards of the quality of the visitor experience at
selected sites within Yosemite Valley.
2. Develop a series of computer simulation models to estimate the maximum daily
visitor use levels of each study site without violating crowding-related standards
of quality.
3. Estimate maximum daily visitor day use levels of Yosemite Valley based on the
percentage of day users who visit each of the study sites.
Study Methods
Setting
Yosemite Valley in the scenic heart of Yosemite National Park, arguably the first
national park in the United States, and certainly one of America’s best known and most
popular national parks (Runte, 1987; Runte, 1990). Yosemite Valley is a glacially carved
area of approximately seven square miles and features sheer granite walls of up to five
thousand feet and several of the world’s highest waterfalls. Yosemite National Park
draws over four million visits annually, and carrying capacity has been a longstanding
and controversial issue. Day use carrying capacity is of special concern because
overnight accommodations in Yosemite Valley are fixed at current levels by park policy.
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Day Use Social Carrying Capacity
Indicators and Standards of Quality
Three elements of research were conducted. The first focused on gathering data
that would help formulate indicators and standards of quality for the visitor experience at
strategic locations in and around Yosemite Valley. Seven sites were studied, including
the base of Yosemite Falls, the trail to the base of Yosemite Falls, the base of Bridalveil
Fall, the trail to the base of Bridalveil Fall, the trail to Vernal Fall, the trail to Mirror
Lake, and Glacier Point. These sites were chosen in conjunction with park staff and
represent a diversity of places that are important to visitors and to the park. At each of
these locations a survey of a representative sample of visitors was conducted.
Respondents were selected and contacted by a trained surveyor, and questionnaires were
self-administered. On each sampling day, the surveyor approached the first visitor who
completed his or her visit, briefly described the study, and asked the visitor if he or she
would be willing to participate in the study. Nearly all visitors agreed to participate.
After the initial respondent completed the questionnaire, the next visitor to complete his
or her visit was selected for participation, and this process continued for the duration of
the sampling day. Sampling dates and sample sizes are shown in Table 1.
Questionnaires administered at these sites addressed both indicators and standards
of quality. Indicators of quality were addressed through a series of open- and closeended questions. Open-ended questions probed respondents for what added to or
detracted from the quality of their visit to Yosemite Valley. Questions included the
following:
We would like to know what added to or detracted from the quality of your visit to
Yosemite Valley.
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Day Use Social Carrying Capacity
a. What have you enjoyed most about your visit to Yosemite Valley today?
_____________________________________________________________________
_____________________________________________________________________
b.What have you enjoyed least about your visit to Yosemite Valley today?
_____________________________________________________________________
_____________________________________________________________________
c. If you could ask the National Park Service to improve some things about the way
visitors experience Yosemite Valley, what would you ask managers to do?
_____________________________________________________________________
_____________________________________________________________________
Close-ended questions asked respondents to rate the seriousness of several potential
problem issues. Using a response scale that ranged from 1(“Not a problem”) to 3
(“Big problem”), respondents were asked to rate the seriousness of the following
potential problems: traffic congestion on roads, difficulty finding a parking place,
inconsiderate drivers, too many tour buses, too many people on trails, too many
people at places like the base of Yosemite Falls, too much noise, too many rules and
regulations.
Standards of quality focused on crowding-related issues, including the number of
people on trails and at attraction sites. A series of questions measured visitor crowding
norms. Since use levels are relatively high in Yosemite Valley, a visual approach was
used to measure crowding norms (Manning et al. 1996). A series of computer-edited
photographs was prepared for each study site showing a range of visitor use levels. Study
photographs are shown in Figure 2. Respondents were asked to rate the acceptability of
each photograph on a scale that ranged from +4 (“very acceptable”) to -4 (“very
unacceptable”) and included a neutral point of 0 (only the anchor points of this numerical
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Day Use Social Carrying Capacity
scale were labeled narratively). In addition, respondents were asked to judge the
photographs using several other dimensions of evaluation to gain additional insights into
how respondents judge alternative use levels (Manning et al. 1999). These included
“preference” (“Which photograph shows the level of use that you would prefer to see?”),
“tolerance” (“Which photograph shows the level of use that would be so unacceptable
that you would no longer visit [this site]?), and “management action” (Which photograph
shows the highest level of use that the National Park Service should allow? In other
words, at what point should people be restricted from visiting [this site]?”).
Computer Simulation Model
The second element of research focused on developing a series of computer-based
simulation models of visitor use at all study sites. The models were built using the
commercial simulation package, Extend by Imagine that, Inc. Extend is an objectoriented, discrete-event dynamic simulation package that has been used extensively in
business, manufacturing, and electronics applications to improve quality and efficiency.
The object orientation makes code writing unnecessary, and the programming algorithm
can easily be expressed by the graphic display of objects and connections. For example,
Figure 3 shows the top layer of the simulation model developed for Glacier Point. The
model was built with a series of “blocks” to represent components of this study site. The
block labeled “Hiker Generator” in the top left section of the model is where simulated
visitors are generated. Connected to it are groups of blocks that provide data concerning
empirical visitor arrival rates and group sizes. (These data are derived from field
observation.) The block “Total Use” allows the researcher to specify the daily total visit
use level to be simulated. The simulated visitors then spend a randomly assigned length
of time (based on field observations) at the “Viewing Area” block which represents the
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Day Use Social Carrying Capacity
Glacier Point visitor attraction. The associated blocks (those underneath the Viewing
Area block) measure the number of people-at-one-time (PAOT) at the Glacier Point
viewing area. A more detailed description of this computer simulation modeling
approach can be found in Wang and Manning (1999).
Model input was based on detailed counts and observations of visitor use at each
study site during the summers of 1998 and 1999. Variables included length of trails,
length of typical trail viewscapes (how far along the trail a hiker can typically see),
number of visitors arriving per hour, visitor group size, length of time visitors stop at
attractions, and the speed at which visitors hike trails. Model output was the number of
people-at-one-time (PAOT) at attraction sites and the number of people-per-viewscape
(PPV) along trails, and these model outputs corresponded to the crowding-related
normative standards measured using the study photographs described above. Resulting
simulation models were designed to estimate PAOT and PPV for alternative total daily
use levels. In this way, maximum total daily use levels could be estimated for each study
site without violating alternative standards of quality. In effect, these maximum total
daily use levels are estimates of social carrying capacity for each study site.
Percentage of Day Users
The final element of research focused on applying carrying capacity estimates
specifically to day users. As noted earlier, overnight visitor capacity of Yosemite Valley
is fixed at current levels by park policy. Application of carrying capacity estimates to
day users required two types of information. First, the percentage of day users at each
study site was determined by a short series of questions in the visitor questionnaires
described above. Respondents were simply asked if they had spent last night or were
planning to spend tonight in Yosemite Valley. Second, the percentage of day visitors to
Yosemite Valley who visit each of the study sites was determined through a park exit
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Day Use Social Carrying Capacity
survey. Visitors exiting the park were randomly selected and administered a short
questionnaire that asked if they were day visitors to the park and , if so, had they visited
each of the study sites. Exit surveys were conducted by personal interview with visitors
during August, 1999.
Study Findings:
Estimates of Day Use Carrying Capacity
Indicators and Standards of Quality
The first element of research yielded information on potential indicators and
standards of quality for the visitor experience in Yosemite Valley. Crowding-related
issues, particularly the number of visitors on trails and at attraction sites, emerged as
potentially important indicators of quality. In the open-ended questions, crowding was
reported by respondents as the least enjoyable aspect of their visit to all study sites.
Moreover, in the close-ended questions, between 45.8 and 68.1 percent of respondents
across the five study sites judged “too many people on the trails” to be a “small” or “big”
problem, and between 49.4 and 59.2 percent of respondents judged “too many people at
places like the base of Yosemite Falls” to be a “small” or “big” problem.
Data on crowding-related standards of quality are shown in Table 2.
“Acceptability”-based standards of quality were derived by plotting average acceptability
ratings for each of the visitor use levels shown in the six photographs for each study site.
PAOT and PVV standards of quality shown in the table are points at which average
acceptability ratings cross the “0” or neutral point on the acceptability scale (i.e., fall out
of “acceptable” range and into the “unacceptable” range). “Preference”, “management
action”, and “tolerance”-based standards of quality were derived by calculating the
average number of people in the photographs selected by visitors in response to the
questions described earlier.
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Day Use Social Carrying Capacity
These data suggest a potential range of standards of quality for each study site.
None of the points within these ranges for each study site is more “valid” than any other.
Each point has potential strengths and weaknesses. For example, standards of quality
based on preference-related norms may result in very high quality recreation experiences
but would restrict access to a relatively low number of visitors. In contrast, standards of
quality based on acceptability, management action, or tolerance allow access to greater
numbers of visitors but may result in recreation experiences of lesser quality. Findings
that offer insights into multiple evaluative dimensions provide a rich base of information
and may lead to formulation of the most thoughtful and informed standards of quality.
Such data allow more explicit understanding of the potential trade-offs between use level
and quality of the recreation experience.
Computer Simulation Model
The second element of research developed a computer-based simulation model of
visitor use at each study site. These models were used to estimate the maximum total
daily use levels (i.e., daily carrying capacities) that could be accommodated at each study
site without violating the normative crowding standards shown in Table 2. Model output
could be generated in several graphic and numerical forms. For example, Figure 4 traces
minute-by-minute PPV levels along the trail to Bridalveil Fall over the duration of a day.
This particular model run was generated using an average summer day total use level of
1,415 visitors (derived from the counts of visitor use taken to help construct the model).
All models were run multiple times (to “average out” the randomness associated with
each individual model run) to estimate the maximum total daily use level that could be
accommodated at each study site without violating each of the crowding norms shown in
Table 2 more than 10 percent of the time. (This 10 percent allowance is discussed later in
this paper.) The range of daily carrying capacities for each study site is shown in Table
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Day Use Social Carrying Capacity
3. It is clear from the table that daily carrying capacities vary substantially across the
range of normative standards and across study sites.
Percentage of Day Users
The third element of research determined the percentage of visitors at each study
site who where day users, and the percentage of Yosemite Valley day users who visited
each study site. The former was obtained from the survey of visitors at each study site,
and the latter was obtained from the park exit survey. Findings are shown in Table 4. It
is clear these figures vary substantially from site to site.
Estimates of Day Use Carrying Capacity
Based on findings from the three elements of this program of research, a range of
daily day use carrying capacities can be estimated for Yosemite Valley as shown in Table
5. These estimates are the maximum daily number of day use visitors that can be
accommodated in Yosemite Valley without violating PPV or PAOT standards of quality
at each study site. The estimates are based on findings from the three elements of
research described above, and are applied in three steps or mathematical calculations.
First (step 1), the number of overnight visitors in Yosemite Valley at all study sites was
estimated using findings from the visitor surveys and the counts of visitor use. Second
(step 2), the number of overnight visitors to each study site was then subtracted from the
maximum total daily use levels that can be accommodated at each study site. This leaves
the maximum number of day visitors that can be accommodated at each study site. Third
(step 3), this number was then expanded to account for the fact the park exit survey found
that only certain percentages of all day visitors to Yosemite Valley visit each of the study
sites.
An example will help illustrate how these estimates were calculated. Table 2
indicates that the “preference-based” standard of quality along the trail to Bridalveil Fall
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Day Use Social Carrying Capacity
is 7 PPV. This number was derived from the survey of visitors to this site and
respondents’ assessment of the photographs showing a range of visitor use levels. Table
3 indicates that a maximum of 1,200 visitors could use this trail per day without violating
this preference-based standard of quality. This number was derived from the computerbased simulation model of this site. The three steps or mathematical calculations
described above are then applied to derive the estimate of the daily day use carrying
capacity of Yosemite Valley based on the findings for the trail to Bridalveil Fall. The
number of overnight visitors at Bridalveil Fall was estimated (step 1). This was done by
multiplying the percentage of day users at this site ( 77 %) (taken from the first row of
Table 4) by the average daily number of all visitors to this site (3,501) (taken from the
counts used to develop the computer simulation model of visitor use). From this
operation, it is estimated that an average of 805 visitors to the trail to Bridalveil Fall are
overnight visitors to Yosemite Valley on an average summer day. This leaves a capacity
of 395 day visitors at this site (1,200 minus 805)(step 2). Finally, the number of day
users that can be accommodated on the trail to Bridalveil Fall needs to be expanded to
estimate the number of day users that can be accommodated in Yosemite Valley (step 3).
Since only 43.4 percent of all day users who visit Yosemite Valley visit the trail to
Bridalveil Fall, (taken from the second row of Table 4), this means that 910 day users can
be accommodated in Yosemite Valley (395 divided by .434) without violating the
preference-based standard of quality for the trail to Bridalveil Fall.
Discussion
The program of research described in this paper raises a number of issues
regarding estimation and management of carrying capacity of parks and related areas. As
noted at the beginning of this paper, an emerging principle is that carrying capacity
decision-making must be guided by management objectives and associated indicators and
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Day Use Social Carrying Capacity
standards of quality. A corollary of this principle is that there is no one, inherent carrying
capacity of a park or recreation area. Rather, each park (or even site within a park) has a
range of capacities depending upon the degree of resource protection and type of
recreation experience to be provided.
Data developed in this study illustrate this point. The number of people
encountered at all study sites is important to most visitors, therefore measures of PPV and
PAOT are good indicators of quality. However, standards of quality for this indicator
vary substantially depending upon the evaluative dimension used in the study (i.e., the
type of recreation experience). Crowding norms ranged consistently (with one small
exception) from a low associated with “preference” (a high quality experience) to a high
associated with “tolerance” (a lower quality experience) for all study sites. The carrying
capacities of these sites, and ultimately Yosemite Valley, vary accordingly.
A related principle of carrying capacity is that some element of management
judgment must be exercised. Again, the data developed in this study illustrate this
principle. What point (or points) along the range of standards of quality and associated
carrying capacities should be selected for management purposes? This is ultimately a
judgment that should consider a variety of other factors inherent in carrying capacity,
including the purpose and significance of the area (as may be defined in law or policy),
the fragility of natural and/or cultural resources, financial and/or personnel resources
available for management, historic precedent, and interest group politics. Management
judgments about standards of quality and associated carrying capacities are not
necessarily “either/or” decisions. In fact, it may be highly desirable to provide a
spectrum of recreation opportunities among parks within a region.
These considerations may suggest valid reasons to formulate certain standards of
quality that in turn define carrying capacities. For example, there may be valid reasons
16
Day Use Social Carrying Capacity
for the trail to Mirror Lake to be managed for a PPV standard of quality that is close to
the “preference” end of the range of crowding norms identified in this study. This trail is
less accessible than the other trails included in this study, has relatively low historic use
levels, and traverses areas containing fragile cultural resources. The trail to the base of
Yosemite Falls, however, is easily accessible, highly used, does not contain especially
sensitive natural or cultural resources, and leads to an “icon” feature of the park that most
visitors want to experience. Thus, it may be reasonable to manage this trail for a PPV
standard of quality that is more toward the “management action” or even “tolerance” end
of the range of standards, and therefore, for a relatively high carrying capacity. In this
way, management judgments can lead to a spectrum of recreation opportunities that serve
a diversity of public desires and appropriately balance competing carrying capacity
considerations. However, such management judgments should be as informed as
possible, based on data such as those developed in this study, and fashioned deliberately
within the structured, rational context of a framework such as LAC or VERP. This
approach to carrying capacity is most likely to lead to thoughtful park and outdoor
recreation management that serves the needs of society and can withstand the inevitable
test of public scrutiny.
The carrying capacity-related data gathered in this study has potentially important
implications to other components of park and outdoor recreation management. For
example, Yosemite National Park is one of many national parks being considered for new
or expanded public transit systems. Data on social carrying capacity of alternative sites
within Yosemite Valley are instrumental in designing a transit system that will deliver the
“right” number of visitors to the “right” locations at the “right” times.
The program of research described in this paper relies on several distinctive
methodological approaches. First, visitor surveys provide the empirical foundation for
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Day Use Social Carrying Capacity
the indicators and standards of quality developed, and the carrying capacities ultimately
derived. This is important as it involves those who are most directly interested in the
park and who have much to gain or lose as a result of park managing decisions. It may
be wise, when and where feasible, to expand this research to other interest groups as well,
including residents of local communities and even the general population (Manning et al.
1999).
Second, a visual approach is used to measure crowding norms. A visual approach
may be more realistic and “valid” than conventional narrative and numeric approaches
(i.e., asking respondents to evaluate encountering selected numbers of other people),
especially in relatively high use contexts (Manning et al. 1999). Research suggests that
visitors may process some encounters with other people at a subconscious level,
especially when such people are perceived to be “like” the respondent in terms of
recreation activity, behavior, or other appearance. A visual approach to measuring
crowding norms allows for such subconscious processing, while narrative/numerical
approaches call explicit attention to all persons encountered. When and where feasible,
visual approaches might be extended to normative evaluation of other recreation-related
impacts such as resource damage to trails and campsites (Manning et al., 1996).
Third, multiple evaluative dimensions (preference, acceptability, management
action, and tolerance) were employed in asking respondents to evaluate alternative use
densities. Resulting data are complex, but provide empirical, detailed insights into how
visitors feel about alternative use levels, including how use levels affect the quality of the
visitor experience, and tradeoffs visitors might make between solitude and maintaining
reasonable public access to park attractions (Manning et al., 1999; Lawson and Manning,
2001).
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Day Use Social Carrying Capacity
Fourth, respondents to the surveys conducted at each study site were asked to
assess the questionnaire items designed to measure crowding norms (Manning et al., In
Press). Most respondents agreed that they 1) understood the questions that were asked, 2)
thought the photographs realistically represented different numbers of people at study
sites, 3) had confidence in their responses, and 4) agreed that the National Park Service
should use data from studies like this in deciding how to manage the park. These
findings support the validity of study data and suggest public support for carrying
capacity decisions based on study findings.
Fifth, computer simulation modeling was used to estimate carrying capacities at
each study site. Contemporary carrying capacity frameworks such as LAC and VERP
suggest that carrying capacity is determined through monitoring of indicators of quality.
When monitoring suggests that standards of quality have been violated, carrying capacity
has been reached. However, computer simulation modeling allows estimation of visitor
use levels (i.e., carrying capacities) that will violate selected standards of quality. This
facilitates a more proactive approach to defining and managing carrying capacity.
Sixth, the study focused primarily on carrying capacity for day use. Many park
and related areas have established carrying capacities for overnight visitors, and these are
managed through mandatory permit systems. However, day use is often the dominant
and fastest growing component of park visitation, may be responsible for substantial
resource and social impact and warrants added management attention (Roggenbuck et al.
1994).
Clearly, the program of research described in this paper has important limitations.
First, it addresses social carrying capacity only. The resource component of carrying
capacity noted at the beginning of this paper also needs research and management
attention. However, the methodological approaches outlined in this paper may have
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Day Use Social Carrying Capacity
some application to the resource component of carrying capacity, or at least the
interaction between these components. For example, recreation impacts to soil and
vegetation at campsites have a potentially important aesthetic component, and the visual
approach to measuring normative standards for such impacts may be useful in
formulating standards of quality for these resource-related indicators of quality.
Second, even within the social component of carrying capacity, this study
addresses only crowding-related indicators and standards of quality. The visitor surveys
conducted at each study site suggest the importance of crowding-related indicators, but
also suggest other potential indicators of quality such as automobile traffic congestion,
noise, and lack of information. Expanded treatment of other potential indicator variables
and associated standards is warranted.
Third, data reported in Table 2 suggest that some of the standards of quality
derived in the study may be underestimated. Questions addressing the “management
action” and “tolerance” dimensions of standards of quality allowed respondents to report
that visitor use should not be limited at study sites or that none of the photographs used in
the study were so unacceptable that respondents would no longer visit study sites. These
responses could not be included in the calculation of standards of quality. The
percentage of respondents selecting these response options varied from 9 to 38 percent
depending upon the question and the study site.
Fourth, estimation of carrying capacities from the computer simulation models
allowed for standards of quality to be violated 10 percent of the time. This allowance
was factored into modeling because it may not be reasonable to prescribe that standards
of quality will never be violated. Park and outdoor recreation use has an inherent random
element that tends to result in occasional peaks or spikes of activity or encounters. The
graph in Figure 4 tracing minute-by-minute use and PPV levels for the trail to Bridalveil
20
Day Use Social Carrying Capacity
Fall is representative. Occasionally, by happenstance, several visitor groups may arrive
at the trailhead simultaneously, and this may result in isolated spikes in encounter levels.
Total use levels would have to be kept very low to ensure that these spikes never (or
rarely) occurred, and this may not serve public interests for reasonable access to parks. A
10 percent allowance was adopted in this study, but this figure was arbitrarily chosen.
Additional research is warranted to derive a more empirically sound basis for specifying
this allowance.
Conclusions
The program of research described in this paper was designed to support
application of the concept of carrying capacity to day use of Yosemite Valley. In
particular, it was designed to help provide an empirical foundation for application of a
contemporary carrying capacity framework such as LAC and VERP. Study findings
suggest crowding-related indicators of quality, a range of standards of quality for these
indicator variables, and associated estimates of carrying capacity for each study site and
for Yosemite Valley as a whole. Management judgments must still be rendered in
choosing among alternative standards and carrying capacities, or perhaps more
appropriately, in choosing a suite of standards of quality and associated carrying
capacities that facilitate a spectrum of recreation opportunities in Yosemite Valley.
However, study data help provide an informed basis for such management judgments.
This paper also outlines principles and frameworks that underlie the theoretical
foundation of carrying capacity, and describes and applies several research methods and
issues that can be used to help define and manage carrying capacity. This discussion, and
its application to Yosemite Valley may provide guidance for future applications of
carrying capacity and a program of research that is needed to support this work.
21
Day Use Social Carrying Capacity
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24
Day Use Social Carrying Capacity
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25
Day Use Social Carrying Capacity
Table 1. Sample Sizes and Sampling Dates for Visitor Surveys
Study Site
Number of Completed
Questionnaires
407
390
Trail to Vernal Fall
Yosemite Falls (included both
trail to Falls and base of Falls)
Bridelveil Fall (including both 295
trail to Fall and base of Fall)
Glacier Point
322
Trail to Mirror Lake
317
Dates Questionnaires
Administered
August, 1998
August and September, 1998
August, 1999
August, 1999
August and September, 1999
26
Day Use Social Carrying Capacity
Table 2. Alternative Crowding-Related Standards for All Study Sites
Normative
Standard of
Quality
Preference
Acceptability
Management
Action1
Tolerance2
1
Trail to Vernal Trail to
Fall (PPV)
Yosemite
Falls (PPV)
11
18
26
40
30
46
39
60
Base of
Yosemite
Falls (PAOT)
43
92
100
Trail to
Bridalveil
Fall (PPV)
7
18
20
126
26
Base of
Bridalveil
Fall (PAOT)
8
20
19
Glacier Point
(PAOT)
Trail to Mirror
Lake (PPV)
19
42
49
10
24
26
25
61
34
Number of respondents who reported that the NPS should not limit use:
Trail to Vernal Fall = 66
Tail to base of Yosemite Falls = 59
Base of Yosemite Falls = 57
Trail to Bridalveil Fall = 73
Base of Bridalveil Fall = 88
Glacier Point = 97
Trail to Mirror Lake = 74
2
Number of respondents who reported that none of the photographs were so unacceptable that they would no longer visit this site:
Trail to Vernal Fall = 37
Tail to base of Yosemite Falls = 40
Base of Yosemite Falls = 46
Trail to Bridalveil Fall = 95
Base of Bridalveil Fall = 113
Glacier Point = 99
Trail to Mirror Lake = 65
27
Day Use Social Carrying Capacity
Table 3. Range of Daily Carrying Capacities At All Study Sites1.
Normative
Standard
Preference
Acceptability
Management
Action
Tolerance
1
Trail to Vernal
Fall
2,100
6,000
7,000
Trail to Yosemite
Falls
4,500
9,500
11,000
Base of
Yosemite Falls
3,000
5,500
5,900
9,300
13,000
7,300
Trail to
Base of
Bridalveil Fall Bridalveil Fall
1,200
700
3,200
1,700
3,500
1,700
4,800
2,300
Glacier
Point
1,500
4,000
4,800
Trail to
Mirror Lake
1,800
5,000
5,500
6,300
7,700
Daily carrying capacities were calculated to allow normative standards of quality to be exceeded a maximum of 10 percent of
the time.
28
Day Use Social Carrying Capacity
Table 4. Percentage of Day Users at All Study Sites.
Percentage of
visitors at each
study site who
were day users
Percentage of
Yosemite
Valley day
users who
visited each
study site
Trail to
Vernal Fall
Trail to
Yosemite Falls
Base of
Yosemite Falls
42.9
36.8
36.8
77.0
10.2
35.4
35.4
43.4
29
Trail to
Base of
Bridalveil Fall Bridalveil Fall
Glacier
Point
Trail to
Mirror Lake
77.0
62.8
37.4
43.4
18.1
8.4
Day Use Social Carrying Capacity
Table 5. Estimated Daily Day Use Carrying Capacities of Yosemite Valley Based on All Study Sites.
Normative Standard
of Crowding
Preference
Acceptability
Management Action
Tolerance
Trail to
Vernal Fall
6,274
44,510
54,314
76,863
Trail to
Yosemite Falls
3,104
17,228
21,465
27,116
Base of
Yosemite Falls
0
5,929
7,059
11,015
30
Trail to
Bridalveil Fall
910
5,518
6,210
9,205
Base of
Bridalveil Fall
0
2,062
2,062
3,445
Glacier
Point
994
14,807
19,227
27,514
Trail to
Mirror Lake
10,774
48,869
54,821
81,012
Day Use Social Carrying Capacity
Figure 1. Hypothetical Relationship Between Visitor Use and Perceived Crowding
Perceived Crowding
A
Y2
Y1
X1
X2
Visitor Use
31
Day Use Social Carrying Capacity
Figure 2. Study Photographs for All Study Sites.
32
Day Use Social Carrying Capacity
Figure 2A. Trail to Vernal Fall
33
Day Use Social Carrying Capacity
Figure 2B. Trail to Yosemite Falls
34
Day Use Social Carrying Capacity
Figure 2C. Base of Yosemite Falls
35
Day Use Social Carrying Capacity
Figure 2D. Trail to Bridalveil Fall
36
Day Use Social Carrying Capacity
Figure 2E. Base of Bridalveil Fall
37
Day Use Social Carrying Capacity
Figure 2F. Glacier Point
38
Day Use Social Carrying Capacity
Figure 2G. Trail to Mirror Lake
39
Day Use Social Carrying Capacity
Figure 3. Schematic Diagram of Computer Simulation Model of Glacier Point.
40
Day Use Social Carrying Capacity
45
40
35
30
PPV
25
20
15
10
5
0
7:00 am
8:00 pm
Time of Simulation Day
Figure 4. Simulated Daily PPV Levels for the Trail to Bridalveil Fall.
41