Tourists’ valuation of nature in protected areas: A systematic review

In the face of biodiversity loss, it is crucial to broaden the arguments for conservation of protected areas by acknowledging diverse values of nature. We systematically reviewed empirical studies to investigate tourists’ values of nature in protected areas over time and across regions. To do so, we explored (1) the main ecological and social characteristics of the case studies; (2) methodological approaches; and (3) value types. Based on the review of 152 articles, we found that economic valuation has received the most scientific attention, while socio-cultural valuation approaches have recently increased. Values were primarily elicited and analyzed quantitatively and in monetary metrics, although valuation methods and frameworks have diversified over the past two decades. However, considering the role of valuation methods and frameworks as value-articulating institutions, we suggest that future research on nature valuation also applies qualitative and non-monetary methods, elicits diverse values, and conducts plural valuation. Supplementary Information The online version contains supplementary material available at 10.1007/s13280-023-01845-0.


INTRODUCTION
focus of the study were excluded a priori. No date restrictions were applied.

Data screening
Based on an iterative process, numerous articles were screened to define inclusion criteria.
Definition of inclusion criteria.

Data cleaning
Screening of title and abstract of 2526 articles, guided by the following five inclusion criteria: 1. Subject: Are tourists sampled? 2. Object: Does the article elicit tourists' (contextual) value(s) through a valuation method? 3. Focus of value: Is the (contextual) value of (any element of) nature collected? 4. Study area: Was the study area a protected area? 5. Case study: Does the article provide empirical data on the research topic? Inclusion criteria are described in detail in Supp. Material S3. If the abstract was missing or information insufficient, articles were accepted for full text screening.

Supplementary Material S3
Description of inclusion criteria

Inclusion criterion: case study
Case study: Does the article provide empirical data on the research topic?
Search string terms: We intentionally did not include any search term related to case studies because of the selection rule specified below.
We only included studies that provide empirical data on the research topic, whereas we excluded, for example, purely theoretical and conceptual articles, systematic literature reviews, meta-analysis, and gray literature to avoid including the same empirical data more than once and to assure scientific quality of the reviewed articles. Hence, we did not include any (redundant) search terms related to the inclusion criterion case study in the search string because we did not want to restrict the search of potentially relevant articles due to missing search terms. This inclusion criterion is rather a matter of reading and interpreting title, abstract, and main text. Case study and empirical study, for example, were indicator words for this inclusion criterion.

Inclusion criterion: value
Object: Does the article elicit tourists' (contextual) value(s) through a valuation method?
Search string terms: *valu* -positively identifiable words in title, abstract, and key words, e.g., (to) value, value, valuation; singular and plural versions and different tenses of some search terms.
There are many definitions of and uses for the term value found in the literature (Kenter et al. 2015). To be consistent in this review, we included contextual values only according to the definition stated in Kenter et al. (2015, p. 92), that is, "opinions about the importance or worth of something". Therefore, we developed the following four selection rules: 1. Articles on, e.g., transcendental values, value-belief-norm, new environmental paradigm, pro-environmental behavior and attitude, and those that used buzzwords such as values of nature-based tourism or ecotourism and proxies such as number of tourists or financial benefit did not meet our inclusion criterion value and hence, were excluded. Because of this selection rule, the following article was excluded, for example, Vásquez Lavín et al. (2016). 2. We only included articles in which the authors clearly stated what they referred to as value and/ or applied a valuation method, i.e., valuation exercise. We did not interpret additional data nor extract data beyond the author's definition. For example, Abu Bakar et al. (2016), whose ranking / rating exercise can be used to elicit contextual values but was not stated by the authors as a valuation method, was excluded from the final set of relevant articles.
3. We only included articles, which displayed the results of tourists' valuation exercise. Because of this selection rule the following article was excluded, for example, Heslinga et al. (2017). 4. Articles were excluded if they were found in the Scopus search due to the appearance of the terms 'valuable' or 'evaluation' in title, abstract and/or key words but did not refer to a contextual value or valuation method / exercise. Because of this selection rule the following articles were excluded, for example, Kulczycki and Halpenny (2014) and Mills et al. (1980).
Our search string term for the inclusion criterion value might have overlooked research on tourists' values of nature in protected areas because we decided to not use synonyms. Two reasons explain this decision. First, since the aim of this research was to explore how the scientific literature has approached nature valuation, we deliberately targeted those articles that had used the concept of value / valuation. Former systematic reviews have applied a similar approach by narrowing their search string to value / valuation and not adding synonyms for valuation (e.g., Acharya et al. 2019). Second, the use of other terms in the search that could potentially refer to value, such as perceptions, can lead to many articles that do not focus on nature valuation but on related yet different concepts such as beliefs, attitudes, and norms.

Inclusion criterion: nature
Focus of value: Is the (contextual) value of (any element of) nature elicited?
Search string terms: We intentionally did not include any search term related to natural elements because they are diverse, and we did not want to restrict the search due to potentially missing search terms. We specified the following three selection rules: 1. We referred to nature as any abiotic, biotic, tangible and intangible element of nature and any nature-mediated element, e.g., landscape, animal, species composition, sound, ecosystem, clouds, and rocks, scuba-diving, and Ecosystem Services / Nature's Contributions to People. 2. Articles on management and conservation plans and/or scenarios that considered elements of nature in the valuation exercise met the inclusion criterion nature (e.g. Iranah et al. 2018). 3. Articles on general management and/or conservation plans and/or scenarios, that is, they did not target natural elements specifically but only non-natural management options of protected areas (e.g., maintenance of trails, educational programs, facilities, and number of benches) through the valuation exercise (mainly, e.g., choice experiments and willingness to pay exercises), did not meet the inclusion criteria nature and hence, were excluded. Table S3 provides an exemplary overview with arguments for inclusion or exclusion of valuation methods to elicit the value of natural and non-natural elements of nature.

Inclusion criterion: study area
Study area: Is the study area a protected area?
Search string terms: "protected area*" OR "protected landscape*" OR "biosphere reserve*" OR "nature reserve*" OR "natural monument*" OR "national park*" OR "natural park*" OR "biosphere area*" OR "conservation area*" OR "biosphere region*" -positively identifiable words in title, abstract, and key words: singular and plural versions of the search terms The definition of the term protected area was broad and context-specific because it was based on any sub-national, national and/or international (e.g., IUCN categories of protected areas) legal definition of a natural area that is under protection qua law. Furthermore, we specified three selection rules: 1. Articles that scrutinized tourists' value of nature in a protected area, nature of a network of protected areas, and/or of multiple protected areas met the inclusion criterion study area. Because of this selection rule the following articles were excluded, for example, Pereira et al. (2019) and Vianna et al. (2012). 2. Articles on a hypothetical protected area or a study area adjacent to a protected area did not meet the inclusion criterion study area. Because of this selection rule the following article was excluded, for example, Vianna et al. (2018). 3. If the study area included both protected and non-protected areas but the results were not separated for them, the article did not meet the inclusion criterion study area.

Inclusion criterion: tourist
Subject: Are tourists sampled?
Search string terms: touris* OR recreation* OR leisure OR travel* OR trip* OR journey*positively identifiable words in title, abstract, and key words, e.g., tourist, tourism, touristic, recreation, recreational; singular and plural versions of some search terms.
Originally, we wanted to develop a precise definition of the inclusion criterion tourist, e.g., based on the distance travelled from home to the protected area. However, after screening potentially relevant articles to develop such a definition (see review step 2 data screening in Table S2), we found that the majority of the reviewed articles did not provide a precise definition and thus, it was not possible to develop a consistent definition and apply it across all potentially relevant articles. In fact, we also assumed the definition of tourist depends on the local context. Therefore, articles generally met the inclusion criterion 'tourist', if authors used the term tourist to describe their target group or one of their investigated target groups, e.g., local, non-local, national, and international tourists.
While screening the relevant articles, we learnt authors used synonyms for the term tourist, such as visitors, travelers, foreigners, and non-locals, to describe their target group. Therefore, we developed, first, our working definition (see (1) below) of the term tourist and, second, inclusion rules to account for synonyms of the term tourist aligned with our working definition (see (2) below). Finally, we could not apply our working definition and inclusion rules to two valuation methods that were used in the articles found in the Scopus search, that is, zonal travel cost and public method based on social media and photo-based data. At the same time, we did not want to neglect this information on values, as the rationale inherent to the valuation method did not allow us to apply our working definition and selection rules. Thus, we defined specific selection rules for these two valuation methods (see (3) below).
(1) Working definition of the term tourist: We defined a tourist as any person who travels to and visits the protected area in their free time for hours to days, that is, day and overnight tourists. We did not apply a minimum travel distance to classify a person as a tourist because this information was also not stated by the majority of the articles (with two exceptions see (3) below). People who are present in or in the surroundings of the protected area in the context of their daily life (e.g., (indigenous) residents, local people, local community, visitors from the region) and job duties (e.g., park managers and decision-makers) or we investigated through a household survey in or in the surroundings of the protected area were not considered tourists. Hence, articles that sampled these target groups only were excluded.
(2) Selection rules to account for synonyms of the term tourist: While screening the articles, we encountered potential synonyms for the term tourist, e.g., visitor, traveler, foreigner, and non-local. We did not want to bias our sampling process by just including articles that strictly use the term tourist, as we would have excluded many relevant articles that use synonyms. Thus, we interpreted the synonyms based on our working definition. To make consistent decisions, we developed standardized selection rules to account for the synonyms. The terms traveler, foreigner, and non-local, for example, generally met our working definition. The term visitor seemed to be more challenging because it is less specific than the term tourist: All tourists are visitors but not all visitors are tourists. The term visitor can include local people who visit the protected area, for example, but local people 2 do not meet our working definition of the term tourist. Therefore, the following six selection rules were developed and applied to avoid extracting data from mixed samples, i.e., mainly local people and tourists.
1. If authors used the terms tourists and visitors as synonyms, the article met the inclusion criterion. 2. If authors used the term visitors but not tourists and did not specify the geographical origin of the visitors, the article did not meet the inclusion criterion tourist and hence, was excluded to avoid having a mixed sample that included, e.g., local and foreign visitors. Because of this selection rule, the following articles were excluded, for example, Becker et al. (2005) and Englin et al. (2006). 3. If authors used the term visitor to describe a target group and distinguished it from local people as a separate target group, these visitors were considered tourists and hence, the article met the inclusion criterion tourist. 4. If the authors distinguished between local and foreign visitors, foreign visitors only met the inclusion criterion tourist. 5. If the authors distinguished between any of the three groups local/nearby(in-state), domestic/national/long-distance (out-of-state), and foreign/international visitors, local visitors did not but the other type of visitors met the inclusion criterion tourist, e.g., Richardson et al. (2006). 6. If the authors distinguished between the terms non-visitor and a visitor who visits the protected area in their leisure time, non-visitors did not but visitors who visited the protected area in their leisure time met the inclusion criterion tourist.
(3) Definition of the term tourist regarding specific methods: We defined specific selection rules for zonal travel cost method as well as social media and photo platform-based valuation methods. Articles applying these valuation methods usually use the term visitor to describe their samples.
• Zonal travel cost method: If the authors considered 'visitors' who travelled a minimum distance of 100km to the protected area and display their travel cost, the article met the inclusion criterion. Because of this selection rule the following article was excluded, for example, Becker et al. (2005). • Public method based on social media and photo-based data: Articles analyzed the social media content of users and (other) photo platforms, such as Flickr.com, to elicit visitor's values to protected areas. Authors did not personally consult the users or photographers. Hence, critical information on the role they took on in the protected area was missing. Thus, values investigated for different target groups such as locals and tourists can hardly be distinguished in the valuation method of public method (e.g., Martínez Pastur et al. 2016). Nevertheless, we decided that such articles met the inclusion criterion, as the method used did not allow the authors to provide this information. We are aware that values of locals might have been included in our literature review and see this decision as a limitation of our review. However, we recognized the value of this rather new valuation method, e.g., to elicit values from visitors based on a large database that is easily and globally accessible and wanted to demonstrate how the scientific community has been applying this method in the scientific field of tourists' values of nature in protected areas.       Table S10: Association between value types as identified in the literature review on tourist's valuation of nature in protected areas and the value types embedded in the three most commonly used valuation frameworks: the Total Economic Value (TEV), The Economics of Ecosystems and Biodiversity (TEEB), and the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) valuation framework. This table deepens the information presented in Figure 7 in the main text and shows the quotes found in the literature of this systematic review that prove the association between value types identified in the reviewed literature and those embedded in the three valuation frameworks. The background color represents the degree of association between each value type identified in the literature review and the different value types used in the three valuation frameworks: high association (black background), medium association (dark gray background), low association (light gray background), and no association (white background). Example quotes from articles of systematic literature review are presented in black and white (Black and white were chosen to improve readability against the background.). Examples in green are added by authors of the literature review (Different shades of green were chosen to improve readability against the background.). Biophysical values according to TEEB were not considered, as they cannot be elicited for tourists. We considered that use and non-use values (TEV) measured through non-monetary metrics can represent socio-cultural values (TEEB) in certain cases. For example, the direct use value can be elicited in non-monetary terms by the (additional) time a tourist is willing to travel to reach a protected area, that is, medium association Preference for a protected area expressed by a tourist because touristic activities such as wildlife watching nurture their relationship with nature.