Tourism and Sustainability: A Bibliometric and Visualization Analysis

Sustainability is a growing research topic in tourism due to the importance of environmental and social issues, and the maintenance of patrimony and other facilities to conserve the potential of tourism destinations. Specifically, sustainability in tourism is crucial in order to guarantee a consistent development of destinations, measured by growth in income and employment. This relevance has been translated into an explosive growth in the sustainability literature regarding tourism, income, and employment. However, there is a lack of bibliometric and visualization research on tourism sustainability (TS), and specifically on its relationship with income and employment. This paper aims to present a bibliometric overview of TS research, and specifically TS related to income and employment. The current work analyzed 2279 references collected from the Web of Science (WoS) Core Collection database and used the visualization of similarities (VOS)viewer program to graphically map the material. The study used co-occurrence of keywords, co-citation, bibliographic coupling, and co-authorship analyses. The results identify the development status and the leading trends in terms of impact, main journals, papers, topics, authors, institutions, and countries. The analysis and graphical presentations are relevant, as they can help researchers and practitioners better understand the state of the art of TS.


Introduction
Sustainability, and specifically the need to understand the nature and limits of growth, has developed into an important policy issue in tourism literature in the last decade [1], becoming an "integral component of tourism policy and strategy" [2] (p. XIX). The explanation of the relevance of tourism sustainability in the literature is, essentially, due to the interdependence of tourism pressure and sustainability [3], and at the same time the difficulty of governing sustainable tourism [4], in-depth research is required. This can be explained by the close relationship between sustainable tourism and sustainable development and, in particular, topics related to population, peace, ethics, prosperity, poverty, pollution, protection, and conservation [5]. Consequently, the analysis of sustainability in tourism literature, and specifically its connection with income and employment growth, has become an important topic, and a growing phenomenon. The aim of this paper is to illustrate and described the bibliographic analysis carried out on the tourism sustainability (TS) literature, focusing on its relationship with income and employment. This relationship emphasizes the economic and managerial dimensions of sustainability, mainly represented by the effect of sustainability issues on the growth of income and the generation of employment. In order to accomplish this purpose, this section

Status and Evolution of Sustainability and Tourism, Employment, and Income in the Literature
The first paper related to sustainability appeared in the Web of Science (WoS) in 1933. However, regular publications about this topic started in the 1970s with four papers. Since then, growth was substantial in the 1980s, and especially in the 1990s with some thousand articles in the decade. From 2003, more than 1000 papers were published a year, and since 2015, the number of papers a year has surpassed 10,000 ( Figure 1). This trend was translated to the literature of sustainability and tourism, and to the literature about sustainability related to income and employment. Focusing on tourism research, although the first paper related to sustainability appeared in 1990, in 2001, the number increased to 17 annual papers, since 2010 there were more than 100, and in last 2 years surpassed 400 papers annually. However, only 3.55% of papers about sustainability were related to tourism in 2017. Regarding sustainability related to income and employment, the increment has been higher: in 1991, the first 7 papers appeared, the amount increased to 42 annual papers in 2001, 219 in 2010, and more than 600 annually in 2016 and 2017, this last year representing 5.31% of the papers regarding sustainability. Finally, focusing on the literature about tourism sustainability related to income or employment, the first paper appeared in 1995; in 2001, the annual publications were also 1 paper, an amount that increased to 12 annual publications in 2010 and more than 30 annual publications in the last 2 years, with an increment of more than 700% in the period 2008-2017, but representing only 7.12% of the papers in the tourism literature related to sustainability. Figure 1 illustrates the annual trends of publications.   The relevance of the diverse documents can be also shown through the analysis of the number of citations that published articles have in this field. We have concentrated on the analysis of TS research, and on TS research related to income and employment. The most cited paper about sustainability is the one by Yusuf Chisti [59], with more than 4000 citations in the WoS. Focusing on TS, the most cited paper is an article by Dimitrios Buhalis [60] with more than 750 citations in the WoS. More specifically, within the research in TS related to income or employment, the most cited paper is that by Loumou and Giourga [61], with 125 citations in the WoS. Table 1 illustrates the general citation structure in TS, and TS related to income and employment. Following the WoS, in TS only the paper by Buhalis received more than 250 citations (0.04%), and 13.43% of papers have more than or equal to 10 citations. Specifically, for TS related to income or employment, only 1 paper (0.56%) has more than 100 citations, while 14.04% of articles exceed 10 citations. In addition, looking at the h-index [55], which provides a holistic analysis of the field [53], for all the articles related to TS it is 66 (66 papers have 66 or more citations), which is reduced to 20 for the TS papers of TS related to income or employment.

Main Cited Papers in TS and TS Related to Employment or Income
In order to identify the most influential papers in the field of TS and specifically in its relationship with income or employment, we went for the top 30 and 15 papers with the most citations. The examination of the number of citation reveals the quality of a document [11], and also the popularity and influence of a paper within a research field [53]. Table 2 illustrates the highly cited papers and their characteristics.   The article by Buhalis [60] ranks first in number of citations (752) in the area of TS, and also in number of citations per year (44.24). This article studies the destination concept and synthesizes several models for strategic marketing and management of destinations. The second most cited article (237) is a paper by Choi and Turk [62], although it is the seventh in number of citations per year (21.55). This paper develops indicators to measure community tourism development within a sustainable framework, using the Delphi technique. The paper by Saarinen [63] ranks third in number of citations (234), but it is the eighth in number of citations per year (21.27). This paper analyzes the nature of the limits of growth, and how these limits are approached and evaluated in discussions on a local scale.
Focusing on TS related to income or employment, the most cited paper is the paper by Loumon and Giourga [61] (125 citations), which ranks second in number of citations per year (8.93). The paper analyzes olive cultivation, and how it contributes to the sustainability of natural resources and especially agriculture. The second most cited paper in this area (91) is the article by Turpie, Heydenrych, and Lamberth [63], although it ranks fifth for citations per year (6.5). This document analyzes the contribution of the biodiversity of the Cape Floristic Region to the regional and national economy, the income generated by natural resource-based tourism, and some conservation strategies. Another relevant document is the one by Trung and Kumar [64], which ranks third in number of citations (89) and in citations per year (7.42). The paper analyzes the expansion of the hotel industry of Vietnam and the impact on income and environment, analyzing efficient practices to use and manage resources for the sustainability of the tourism sector. The rest of the papers are very diverse in authors and in content.

Leading Journals in TS and TS Related to Employment or Income
The 2279 and 178 publications about TS and TS regarding income or employment were published in 757 and 133 journals, respectively. The main categories of publications were Hospitality, Leisure Sport and Tourism (41.71%), Environmental Sciences (19.56%), and Green Sustainable Science Technology (19.47%) in TS, and Hospitality, Leisure Sport and Tourism (29.61%), Environmental Sciences (24.58%), and Environmental Studies (18.99%) regarding TS related to income or employment.
When analyzing TS, 60% of the sources just published 1 paper; 16 published 20 or more; 25 between 10 and 19; 52 between 5 and 9; and 94 published 3 or 4 (116 published 2 papers). Focusing on TS related to income or employment, only 21% of the sources published more than 1 paper. Only two journals published 10 or more papers, 1 published 4 papers, 6 published 3 papers, and the rest 2 papers.
One can observe that 28% of the papers (639 publications) in TS and 21% of documents (38 papers) in TS related to employment and income were published in the top 10 journals in these areas (Table 3). Concretely, the three top journals by number of publications in TS were: Journal of Sustainable Tourism with 3.35% of the total publications, Sustainability with 4.17% of total publications, and Tourism Management with 4.08% of total papers. However, the h-index for TS is led by Tourism Management (36), Journal of Sustainable Tourism (28), and Annals of Tourism Research (25). As for the three top journals by number of papers in TS related to income or employment, they are again the same journals as in TS, and in the same order, with 7.30%, 5.62%, and 2.25% of the papers published, respectively. They are also leading the h-index, although in reverse order, with an amount of 7, 3, and 4, respectively.
Considering the top 10 journals, if we focus on the sources that dedicate the highest number of their published articles to the topic of TS, they are Journal of Sustainable Tourism with more than 33% of its publications dedicated to TS, followed by Tourism Management Perspectives (10.96%) and Current Issues in Tourism (7.71%). As for the journals concentrating on the topic of TS related to income or employment, from the top 10 journals, the ones dedicating the highest number of their publications to TS related to income or employment are Journal of Sustainable Tourism (2.05% of its publications), Worldwide Hospitality and Tourism Themes (1.81%), and Current Issues in Tourism (0.53%). Finally

Keywords Analysis
This analysis studied the distribution of the most frequent keywords, investigated through keywords co-occurrence (keywords that appear together in the same paper). The aim is to highlight the most relevant research topics in the area of TS (as TS manuscripts related to income and employment are very scarce, we only concentrate on general TS publications) by focusing exclusively on the author keywords appearing below the abstract. This technique counts the number of papers in which two keywords appear together (keywords highlighted by the authors in each paper); considering the 2279 TS-related publications, VOS viewer software revealed the existence of 5552 keywords. Figure 2 illustrates the main keywords and the size of the nodes. (The larger the node and the keyword, the greater the weight is (how many papers a keyword appears in). Thicker lines mean more frequent co-occurrence (how many papers a keyword appear in together with another keyword). The smaller the distance between the nodes, the stronger the relationship they have (how many papers these two keywords appear in together, and relatively comparing co-occurrence with other keywords)). The same color of the nodes and keywords means that they belong to the same cluster (group of related keywords). The program created nine clusters. Figure 2 considers a threshold of 10 occurrences, representing the 96 keywords with most frequent co-occurrences. The most common keywords leading the main clusters are: "sustainability" (orange), "tourism" (green), "sustainable tourism", and "sustainable development" (both in the same purple cluster), "ecotourism" (brown), and "climate change" (pink). Table 4 shows the top 30 keywords, including frequencies and total link strengths.

Reference, Journal, and Author Co-Citation Analysis
Apart from the previous analysis of the quality of the papers by the number of citation, in this section, we use co-citation, analyzing references, journals, and authors. The co-citation analysis examines the simultaneous citation of two items (paper, journal, or author) by a third document's citation (they appear together in the reference lists of other papers) [57]. This mechanism divides the bibliometric material into clusters, through a network analysis, permitting visualization and analysis of the relationships, characteristics, structure, and development of a field, in this case TS.
We start with the reference co-citation network. In this analysis, the nodes show the connections between the diverse papers, illustrating the investigation themes strongly related to a research domain, in this case TS [11]. The chart ( Figure 3) and the cluster analysis reveal that two articles by Butler [65,66] were cited 138 and 99 times in the 2279 papers related to TS, and hence led this ranking. The first of these papers is the one with most citations, although it ranks fifth in total link strength (608). A second ranking was led by the second paper by Butler, with a link strength of 882 (although these two papers are not included in the list of 2279 documents regarding TS, as the journals of these papers did not belong to the WoS when these papers were published). These two papers by Butler led the cluster in red, of the six clusters shown. The third most cited paper is the one by Saarinen [1] (97 citations), although it ranks seventh in total link strength (661), this paper is also included in the same cluster as the two papers by Butler. The second ranked paper according to link strength is the one by Choi and Turk [62] (734), ranking fourth in number of citations (95 citations). This paper led the second main cluster, violet in the image. Completing both ranks, the fifth most cited paper is the one by Hunter [67] with 83 citations, ranking third in link strengths (733), also in the main red cluster. As for the other main clusters, the green cluster is led by the papers by Gossling et al. [68]

Reference, Journal, and Author Co-Citation Analysis
Apart from the previous analysis of the quality of the papers by the number of citation, in this section, we use co-citation, analyzing references, journals, and authors. The co-citation analysis examines the simultaneous citation of two items (paper, journal, or author) by a third document's citation (they appear together in the reference lists of other papers) [57]. This mechanism divides the bibliometric material into clusters, through a network analysis, permitting visualization and analysis of the relationships, characteristics, structure, and development of a field, in this case TS.
We start with the reference co-citation network. In this analysis, the nodes show the connections between the diverse papers, illustrating the investigation themes strongly related to a research domain, in this case TS [11]. The chart (Figure 3) and the cluster analysis reveal that two articles by Butler [65,66] were cited 138 and 99 times in the 2279 papers related to TS, and hence led this ranking. The first of these papers is the one with most citations, although it ranks fifth in total link strength (608). A second ranking was led by the second paper by Butler, with a link strength of 882 (although these two papers are not included in the list of 2279 documents regarding TS, as the journals of these papers did not belong to the WoS when these papers were published). These two papers by Butler led the cluster in red, of the six clusters shown. The third most cited paper is the one by Saarinen [1] (97 citations), although it ranks seventh in total link strength (661), this paper is also included in the same cluster as the two papers by Butler. The second ranked paper according to link strength is the one by Choi and Turk [62] (734), ranking fourth in number of citations (95 citations). This paper led the second main cluster, violet in the image. Completing both ranks, the fifth most cited paper is the one by Hunter [67] with 83 citations, ranking third in link strengths (733), also in the main red cluster. As for the other main clusters, the green cluster is led by the papers by Gossling et al. [68] (67 citations, 332 link strength) and Buckley [5] (64,334), and the blue cluster by Murphy [69] (58 and 455) and Jamal and Getz [70] (52 and 429). The authors of the other two clusters are not ranked in the top papers. The first paper by Butler [65] relates the concept of a tourist area cycle of evolution, while the second one [66] is a state-of-the-art review of sustainable tourism. The paper by Hunter [67] studies the evolution of the concept of sustainable tourism. The one by Choi and Turk [62] is about sustainable indicators. A review of other papers in this violet cluster also reveals some tourism sustainability assessment procedures or critiques of sustainable tourism developments. The main papers in the green cluster are strongly connected to the previous papers, especially to the one by The first paper by Butler [65] relates the concept of a tourist area cycle of evolution, while the second one [66] is a state-of-the-art review of sustainable tourism. The paper by Hunter [67] studies the evolution of the concept of sustainable tourism. The one by Choi and Turk [62] is about sustainable indicators. A review of other papers in this violet cluster also reveals some tourism sustainability assessment procedures or critiques of sustainable tourism developments. The main papers in the green cluster are strongly connected to the previous papers, especially to the one by Choi and Turk [62]. Hence, the paper by Gossling et al. [68] provides a methodological framework for the calculation of ecological footprints as a tool to assess tourism sustainability, while the one by Buckley [5] is a review of social and environmental impacts, responses, and indicators of tourism sustainability. Finally, the book by Murphy [69] and the paper by Jamal and Getz [70] are related to community tourism.
The second co-citation study analyzes the journal co-citation network on TS (Figure 4). In this analysis, the size of a node shows the number of published papers and hence its activity, and a short distance between two journals reveals a greater citation frequency. The program revealed three main clusters plus one residual. The first one, in green, includes Tourism Management as the journal with most citations (5330) and highest link strength (4457) Choi and Turk [62]. Hence, the paper by Gossling et al. [68] provides a methodological framework for the calculation of ecological footprints as a tool to assess tourism sustainability, while the one by Buckley [5] is a review of social and environmental impacts, responses, and indicators of tourism sustainability. Finally, the book by Murphy [69] and the paper by Jamal and Getz [70] are related to community tourism. The second co-citation study analyzes the journal co-citation network on TS (Figure 4). In this analysis, the size of a node shows the number of published papers and hence its activity, and a short distance between two journals reveals a greater citation frequency. The program revealed three main clusters plus one residual. The first one, in green, includes Tourism Management as the journal with most citations (5330) and highest link strength (4457)  The third co-citation analysis studied the network of the main authors. The author analysis, plotted in Figure 5, illustrates the existence of six clusters. By number of components, the first cluster is the red one with 43 authors; the second with 38 authors is the green cluster; the third cluster is the dark blue in the image, with 20 items; the fourth one, with 19 authors, is the yellow; the fifth, with 15 items, is the violet; while the last one in light blue contains only 14 items. However, if we focus on the citations, the main cluster is the yellow one, led by Colin Michael Hall (706 citations) (the main cited author in the label; tourism at Google Scholar) and Stephan Gossling (633 citations). This is the most central cluster and includes the most cited authors. A review of its main authors reveal that the yellow cluster has essentially an environmental and ecological perspective, although this behavior is observed from many viewpoints, as is shown by its relationship with the other clusters. Hence, while Hall analyzes environmental problems highlighting business, planning, and geographic perspectives (and Moscardo (162 citations) concentrates on ecotourism and community problems), Gossling focuses more on the pure environmental management perspective (for instance, with the calculation of ecological footprints, and the analysis of environmental problems and consequences). The third co-citation analysis studied the network of the main authors. The author analysis, plotted in Figure 5, illustrates the existence of six clusters. By number of components, the first cluster is the red one with 43 authors; the second with 38 authors is the green cluster; the third cluster is the dark blue in the image, with 20 items; the fourth one, with 19 authors, is the yellow; the fifth, with 15 items, is the violet; while the last one in light blue contains only 14 items. However, if we focus on the citations, the main cluster is the yellow one, led by Colin Michael Hall (706 citations) (the main cited author in the label; tourism at Google Scholar) and Stephan Gossling (633 citations). This is the most central cluster and includes the most cited authors. A review of its main authors reveal that the yellow cluster has essentially an environmental and ecological perspective, although this behavior is observed from many viewpoints, as is shown by its relationship with the other clusters. Hence, while Hall analyzes environmental problems highlighting business, planning, and geographic perspectives (and Moscardo (162 citations) concentrates on ecotourism and community problems), Gossling focuses more on the pure environmental management perspective (for instance, with the calculation of ecological footprints, and the analysis of environmental problems and consequences). Between both of them in this cluster are Buckley (297 citations) and Mckercher (196 citations), who focus on environmental and ecological impacts, and sustainable development. Far from the center, Scott (285 citations) concentrates on climate and biology questions, and Becken (262) on climate and energy consumption. The second important cluster according to citations is in red. This is also a multidisciplinary cluster, although it focuses on destinations and has a mainly geographical perspective. The red cluster is led by Richard Butler (562 citations), whose work (located very centrally on the green cluster) highlights questions related to cycle tourism, reviews of sustainable tourism, tourism geography, and urban management. The contribution of Bill Bramwell (419) is also important, at the heart of the red cluster, his work is centered on local tourism policy making, rural tourism governance, and community participation; Tosun (147) concentrated on community participation, and Getz (217) (close to the light blue cluster) centered on community tourism. However, this cluster includes well-known researchers in tourism, such as Dwyer (186) with a planning and economic perspective, Buhalis (122), and Cooper (113). Finally, only one of the other four clusters, the green one, includes authors ranking in the top 10 cited authors: Weaber (419), professor of tourism research focusing on ecotourism and tourism management, is ranked 5; Sharpley (248), ranked 9 (and between the two main clusters: yellow and green), centered on environmental, economic, social, and sustainable development. Important authors in the green cluster are Scheyvens (185), focusing on ecotourism, local communities, and poverty; Cohen (176), with a sociological perspective; Sarinen (172), who analyses limits of growth, and natural resources at local and regional level; Jamal (130); and Ryan (136). The other three clusters are light blue-including Graham Miller (186), Hunter (186), Choy (115), World Tourism Organization (111), and OECD (107)-with a business and policy-making perspective, concentrating also on sustainable concepts and sustainable indicators; the dark blue cluster, with an economic, strategic planning, and growth perspective, with the analysis of physical and social impacts and institutional diversity and self-organizations, includes Ostrom (148 citations), Wall (127), World Bank, United Nations, World Commission on Environment, and UNESCO; finally, the violet cluster, led by Xabier Font (183) and Bohdanowicz (146), focuses on the sustainability of organizations, such as hotels and destinations, and on questions related to marketing, attitudes, impacts and awareness of managers and organizations, and corporate social responsibility.
Between both of them in this cluster are Buckley (297 citations) and Mckercher (196 citations), who focus on environmental and ecological impacts, and sustainable development. Far from the center, Scott (285 citations) concentrates on climate and biology questions, and Becken (262) on climate and energy consumption. The second important cluster according to citations is in red. This is also a multidisciplinary cluster, although it focuses on destinations and has a mainly geographical perspective. The red cluster is led by Richard Butler (562 citations), whose work (located very centrally on the green cluster) highlights questions related to cycle tourism, reviews of sustainable tourism, tourism geography, and urban management. The contribution of Bill Bramwell (419) is also important, at the heart of the red cluster, his work is centered on local tourism policy making, rural tourism governance, and community participation; Tosun (147) concentrated on community participation, and Getz (217) (close to the light blue cluster) centered on community tourism. However, this cluster includes well-known researchers in tourism, such as Dwyer (186) with a planning and economic perspective, Buhalis (122), and Cooper (113). Finally, only one of the other four clusters, the green one, includes authors ranking in the top 10 cited authors: Weaber (419), professor of tourism research focusing on ecotourism and tourism management, is ranked 5; Sharpley (248), ranked 9 (and between the two main clusters: yellow and green), centered on environmental, economic, social, and sustainable development. Important authors in the green cluster are Scheyvens (185), focusing on ecotourism, local communities, and poverty; Cohen (176), with a sociological perspective; Sarinen (172), who analyses limits of growth, and natural resources at local and regional level; Jamal (130); and Ryan (136). The other three clusters are light blue-including Graham Miller (186), Hunter (186), Choy (115), World Tourism Organization (111), and OECD (107)-with a business and policy-making perspective, concentrating also on sustainable concepts and sustainable indicators; the dark blue cluster, with an economic, strategic planning, and growth perspective, with the analysis of physical and social impacts and institutional diversity and self-organizations, includes Ostrom (148 citations), Wall (127), World Bank, United Nations, World Commission on Environment, and UNESCO; finally, the violet cluster, led by Xabier Font (183) and Bohdanowicz (146), focuses on the sustainability of organizations, such as hotels and destinations, and on questions related to marketing, attitudes, impacts and awareness of managers and organizations, and corporate social responsibility.

Bibliographic Coupling of Authors
Another way of analyzing the degree similarity of a subject or, in this case, of authors' work, is by analyzing bibliographic coupling, a concept introduced by Kessler [58]. Bibliographic coupling is used in the literature to complement the co-citation analysis, offering another perspective of a

Bibliographic Coupling of Authors
Another way of analyzing the degree similarity of a subject or, in this case, of authors' work, is by analyzing bibliographic coupling, a concept introduced by Kessler [58]. Bibliographic coupling is used in the literature to complement the co-citation analysis, offering another perspective of a subject or authors' relatedness. While co-citation indicates that two documents appear together in the reference list of another document, bibliographic coupling counts the number of references a group of papers have in common (paper A and paper B are coupled if both of them cite document C). Figure 6 illustrates our analysis. According to the strength and number of documents, the list is led by Hall (1078 link strength, 19 documents), Gossling (975 link strength,13 documents), Scott (742 link strengh, 9 documents), Pulido-Fernandez (486 link strengh, 9 documents), and Weaver (461 link strengh, 15 documents). There are six main clusters. The dark blue one is led by the three main authors mentioned before, and also Peeters (437 link strengh, 7 documents). Pulido-Fernandez and Lopez-Sanchez (7th in link strength) led the yellow one. Wall (8th) and Su (11th), light blue; Higham (10th) and Lusseau (19th), violet; and Gonzalez (15th) and Caballero (16th), green, are the only authors from three other separate clusters. Finally, the central cluster is led by Weaver (5th) and Buckley (9th) and includes 34 other authors, located centrally in the figure. Previously, we explained the works of most of these authors. Moreover, this analysis shows the existence of three new perspectives of studying sustainability issues: the main one, led by Pulido-Fernandez and Lopez-Sanchez, from University of Jaen, analyzes sustainable tourism, mainly in order to improve tourism polices, from an applied economic perspective, focusing on relevant areas such as income and employment (the differentiated topic of this paper). Gonzalez and Caballero, from University of Malaga, concentrate on sustainable tourism indicators, programming, and optimization, with a mathematical perspective. Finally, Higham and Lusseau focus on a biological and ecological perspective of managing environmental tourism sustainability. subject or authors' relatedness. While co-citation indicates that two documents appear together in the reference list of another document, bibliographic coupling counts the number of references a group of papers have in common (paper A and paper B are coupled if both of them cite document C). Figure 6 illustrates our analysis. According to the strength and number of documents, the list is led by Hall (1078 link strength, 19 documents), Gossling (975 link strength,13 documents), Scott (742 link strengh, 9 documents), Pulido-Fernandez (486 link strengh, 9 documents), and Weaver (461 link strengh, 15 documents). There are six main clusters. The dark blue one is led by the three main authors mentioned before, and also Peeters (437 link strengh, 7 documents). Pulido-Fernandez and Lopez-Sanchez (7th in link strength) led the yellow one. Wall (8th) and Su (11th), light blue; Higham (10th) and Lusseau (19th), violet; and Gonzalez (15th) and Caballero (16th), green, are the only authors from three other separate clusters. Finally, the central cluster is led by Weaver (5th) and Buckley (9th) and includes 34 other authors, located centrally in the figure. Previously, we explained the works of most of these authors. Moreover, this analysis shows the existence of three new perspectives of studying sustainability issues: the main one, led by Pulido-Fernandez and Lopez-Sanchez, from University of Jaen, analyzes sustainable tourism, mainly in order to improve tourism polices, from an applied economic perspective, focusing on relevant areas such as income and employment (the differentiated topic of this paper). Gonzalez and Caballero, from University of Malaga, concentrate on sustainable tourism indicators, programming, and optimization, with a mathematical perspective. Finally, Higham and Lusseau focus on a biological and ecological perspective of managing environmental tourism sustainability.

Country and University Co-Author Analysis
Finally, the bibliometric literature also highlights the so-called co-authorship analysis. This analysis helps to interpret the structure of research collaboration networks in a specific field. Hence, it contemplates the endogenous and self-organizing behavior of research teams [71]. The nodes in this analysis reveal the influential countries or institutions, while the thickness and distance between them show the degree of collaboration [11].
Starting with the analysis of countries, the VOS viewer program shows a dispersion of the literature. Led mainly by four nations (Figure 7

Country and University Co-Author Analysis
Finally, the bibliometric literature also highlights the so-called co-authorship analysis. This analysis helps to interpret the structure of research collaboration networks in a specific field. Hence, it contemplates the endogenous and self-organizing behavior of research teams [71]. The nodes in this analysis reveal the influential countries or institutions, while the thickness and distance between them show the degree of collaboration [11].
Starting with the analysis of countries, the VOS viewer program shows a dispersion of the literature. Led mainly by four nations (Figure 7

Discussion and Conclusions
This paper has studied the relevance of tourism sustainability theoretically, concentrating on its relationship with employment and income growth. The article analyzed and defined the concept of TS and focused on the previous analyses of the structure of the field of sustainability in general (and its relationship with diverse areas such as management, marketing, and economy), concentrating on the use of bibliometrics.
Moreover, the paper examined the previous bibliometrics regarding tourism in general and TS. Due to the lack of studies in the literature, and the importance of the bibliometric approach, we developed a bibliometric study and visualization of TS-related documents, also focusing on their connection with income and employment. From the results, we can extract the following conclusions. Debates, discourses, and criticism about the term TS have been a consistent feature of the literature [5,8,9] and continue with authors noting a raft of shortcomings [48,72]. In this respect, this paper highlights the complexity of the process and shows the interdisciplinary nature of TS research, with the need to integrate and bring together economic, social, and environmental-ecological dimensions of sustainability [1,73], and the relevance of focusing on tourism-specific characteristics in TS conception [5]. Nevertheless, and in the light of previous bibliometric studies, the results highlight that research into TS questions is maturing, moving away from definition and conceptual papers to more applied and empirical research [48].
Furthermore, research about sustainability, although being developed regularly since the 1970s, has shown a huge growth in recent years, with more than 10,000 papers annually since 2015. This increase has been translated to the literature on sustainability and tourism (more than 400 papers annually in the last 2 years), and to the literature about sustainability, income, and employment (more than 600 annually in 2016 and 2017). Results show that the growth in TS research continues to be remarkable, as observed some years ago [48]. The literature about tourism sustainability related to income or employment is still scarce, although it has increased more than 700% in the last 10 years. This paper is pioneering in analyzing this aspect.
The relevance of the subject was also reflected in an important number of citations. Hence, the most cited paper about sustainability, TS, and TS related to income and employment had more than 4000, 760, and 125 citations in the WoS, although all of them were published recently. These results agree with the relevance of TS outside the academic field. Hence, TS was recognized by the UN in the 2030 agenda for sustainable tourism, where in 3 of the 17 goals featured tourism. They indicate the need to: "devise and implement policies to promote sustainable tourism that creates jobs and promotes local culture and products"; "develop and implement tools to monitor sustainable development impacts for sustainable tourism with creates jobs, promotes local culture and products"; and "increase the economic benefits of Small Island Developing Spaces and LCDs from the sustainable use of marine resources, including through sustainable management of fisheries" [74].
Examinations of the citations indicates that the most influential and cited papers in TS are the ones by Buhalis [60], with 44.24 citations per year, and the one by Loumon and Giourga [61] (125 citations), focusing on TS related to income and employment.
The top journals in the area were Journal of Sustainable Tourism, Sustainability, and Tourism Management, and these three journals, in the same order headed the specific area of TS related to income or employment. Moreover, results indicate that despite being a relatively young area of study, research on TS and the specific area related to income or employment has developed simultaneously across multiple academic disciplines and is expanding.
Focusing on the main topics in TS, the keyword co-occurrence analysis highlights "sustainability", "tourism", "sustainable tourism", "sustainable development", "ecotourism", and "climate change" as the most frequent keywords. This agrees with the appreciation that the main subjects in TS literature, with some exceptions, remain constant and stress the emerging area of climate change [5,48]. The results also indicate that areas such as population, prosperity, peace, pollution, protection [5], social responsibility [45], or ethics have yet to fully emerge, as posited in previous bibliometric studies [45,48].
Co-citation analysis reveals that two articles by Butler [65,66] led the rank of the most cited papers of the 2279 papers related to TS. Other relevant documents are one by Saarinen [1], which is included in the same central cluster, and another by Choi and Turk [62], which led another cluster. The cluster analysis reveals other clusters, surrounding the first central one, which are relatively dispersed.
The journal co-citation network indicates the existence of four clusters of journals. One, led by Tourism Management, comprises journals oriented to the management of organizations; another, led by Journal of Sustainable Tourism, is mainly orientated to the analysis and management of destinations; the third one, more dispersed from the previous two, led by Ecological Economics, with a mainly environmental-ecological orientation; and a reduced cluster associated with a geographical perspective.
The author co-citation analysis illustrates six clusters. The main, central one, led by Hall and Gossling, observes an essentially environmental and ecological perspective, although analyzed from many scientific viewpoints; its relationship with the other clusters is shown. Other clusters essentially deal with planning and geography (Butler, Bramwell, Getz); ecotourism and social and local development (Weaber, Sharpley); business and policy-making (Miller, Hunter, Choi); and economic, strategic planning, institutional (Ostrom, Wall), organizational, social responsibility, and marketing (Font, Bohdanowicz) perspectives.
The bibliographic coupling of authors indicates the existence of two central clusters of authors. The main one is led by the three authors with the highest link strength (Hall, Gossling, and Scott). Another, more central in the figure, is led by Weaver and Buckley (5th and 9th in link strength, respectively), which includes 36 of the 48 main authors considered. Moreover, the analysis discovered new perspectives of analyzing TS: applied economic (with Pulido-Fernandez and Lopez-Sanchez (4th and 7th in link strength)), mathematical (Gonzalez and Caballero), and biological and ecological (Higham and Lusseau) views. These new perspectives could be added to other previous ones, such as biosecurity or rural policies founded in previous research when studying circular-, green-, and bio-economy sustainability concepts in tourism [29], or others related to the use of information and smart technologies when focusing on urban sustainability [46,47].
Finally, the co-authorship analysis of TS reveals the prevalence of four countries (the USA, Australia, Spain, and England), leading the main clusters. The study shows that the top institutions analyzing our field are Griffith University, University of Queensland, and James Cook University. The analysis also shows the lack of collaboration between some Spanish universities, such as U. of Malaga and U. Pablo de Olavide, with the central ones. To sum up, the bibliometric and visualization study on TS-related documents shows that TS is a broad area that should integrate and reconcile very diverse perspectives and the main traditions and narratives of TS.
The paper also revealed the diverse traditions and main lines of research in TS and their relative importance in the literature, and it offers some interesting trends in TS literature. These questions are important for practitioners when considering different policies. They are especially relevant to researchers, as the study found differences and connections among the diverse areas studying TS and revealed the existence of new and growing conceptions of study that can open new areas of research. The literature also indicates the need of future research about the sustainability of tourism growth, its effects on questions related to indicators that measure this sustainability, and, specifically, economic indicators [75]. Let us focus on the main findings and contributions for practitioners and researchers.
Focusing on practitioners, the diverse perspectives included in TS, and knowledge of the industry and the tourism research literature [5], cannot be avoided if the aim is to improve sustainability during the planning and management of organizations or destinations. Hence, policy makers and practitioners should include, comprise, and integrate the complexity of the area. In particular, social, economic, and environmental-ecological impacts should be incorporated [29], but also landscape, culture and patrimony, and new perspectives facing TS. Practitioners should also consider the advances, effects, and implications (for both organizations and destinations) of changes in information technology for the sustainability of tourism-the impact of social media, for instance [76]-or the availability of new green techniques, smart governance, planning techniques, technological solutions, or even social and environmental applications and crowd-sourcing and open-source solutions affecting sustainability, as highlighted in recent bibliometric studies about urban sustainability [46,47]. For successful governance, the planning needs to include public and private participation, considering the involvement of the main stakeholders from all tourism interests, on various levels, as posited by Gössling et al. [72] and Buhalis [60] in the most cited paper about TS. Tourism polices need to be created to influence sustainable planning processes, for instance, with the promotion of socially responsible policies from organizations [45] or the promotion of social entrepreneurship [77]. In addition, the increased involvement of communities is needed to guarantee the equity of benefits to every group that will influence or will be influenced by tourism operations, "ensuring economic development that is also ethical and environmentally, socially, culturally and politically conscious", as stressed by Dos Santos et al. [45] (p. 223) in their bibliometric study of hotels. Moreover, the analysis of TS should be conceived as a multilevel dynamic process rather than a goal, as posited recently by Kristjánsdóttir et al. [73], by continuously redefining sustainability challenges in response to economic, social, or environmental situations. Furthermore, technological, individual, and political means, including regulation and participatory governance, are recognized as essential to generate gains in sustainability [5,47].
Focusing on the theoretical importance of the paper, bibliometric analysis can offer some answers to important questions that researchers should consider when focusing on developing a paper about TS. Specifically, the results could help researchers to better discover the reasons that promote new trends, or, specifically, the factors that influence these publication trends. Obviously, this is crucial for researchers when developing new research on TS. From our point of view, the new trends are motivated by three main reasons: the expansion of the topic (in this case, TS), that makes it more interesting for researchers; the development of new sciences or areas of study; and the existence of fashionable questions in research or society. Taking these perspectives into account, this bibliometric analysis can provide us with some points for further research. In this regard, and apart from the evolution of society, this paper shows that the evolution of TS is making its research more interesting for classical areas, such as applied economics, mathematics, biologics, and ecology. An example of this is, for instance, the explosive growth of TS papers related to income or employment, mainly associated with applied economics, as shown in our analysis. In addition, the explosive expansion of other areas, such as that related to marketing [60] or those linked to information technologies, social media, open innovation, or crowdsourcing [46,47,76], are also provoking the expansion of TS to topics related to these areas. For instance, the most influential paper on TS (the one with most citations in the general literature, of the 2279 papers analyzed) is the one by Buhalis [60]. However, this is not the paper most cited by the other 2278 TS-related papers analyzed in our study (the most cited by the TS literature are the ones by Butler [65,66] and Hunter [67]). Moreover, focusing on the paper by Buhalis [60], although it obviously deals with TS (the paper considers the relevance of the "sustainability of local resources", also in the abstract), it would be not ranked as a classical TS paper by many readers (unlike, for instance, the papers by Butler [66] or Hunter [67], which review TS or its conception). This is because the main focus of Buhalis's work is not TS, but "destination marketing". Nevertheless, the expansion and popularity of marketing-related areas in academia means this paper is the most cited, and, as a consequence, citation of this paper could provoke the appearance of papers on topics related to TS, such as marketing, as they will probably have more citations in the future. As a result, this example indicates that TS research will eventually evolve toward some of the most popular areas and subjects in the general academia. Moreover, the still scarce but incipient research in some important scientific fields (such as mathematics, programming, or optimization), observed in this chapter, or the still relatively low importance of some important keywords (some of them related, for instance, to tourism literature, such as the one by Buckey [5], or others relevant in academia in general) indicates that some areas of TS need more research. Specifically, and only concentrating on topics related to management and economy, the small amount of TS literature in fields such as finance, information systems, social media, and user-generated content, crowd-sourcing, entrepreneurship, ethics, social responsibility, and so forth, indicates that more research is needed in these fields. Finally, as pointed out before, results indicate the maturity of TS, the trend to more empirical papers, and especially the need for papers related to the development of TS indicators [48], and particularly economic TS indicators [75].
Nevertheless, the extent of the paper reveals some limitations, which, together with a further focused evaluation of the trends observed, can promote further interesting analyses. Firstly, bibliometric methodology has limitations, as it is based on the objective collection of keywords, which can produce confusing interpretations if it is not complemented with more qualitative analyses, as in the example of the paper by Buhalis [60]. In addition, the work used the WoS Core Collection database as a data source, considering only articles, reviews, letters, and notes. Although this method provides us with the most relevant and important works, other studies could complement this one by analyzing other kinds of secondary documents included in the database in order to detect other pioneering trends. Other possibilities could be extending the analysis with the use of databases such as Scopus or Google Scholar, or other sources and data sets that analyze other kinds of reports, such as doctoral theses or works in other languages.
Further analyses could also focus on some of the trends detected by analyzing some of the trends about sustainability (or TS) considered from specific scientific points of view (management, economics, geography, and so forth). Moreover, the software used in this paper permits further analysis, using more bibliographic or mapping tools (for instance, one can extend bibliographic coupling, co-citations, co-authorship, or other analyses), or other software or methodologies which could enrich this work, using other possible bibliographic studies. Finally, the study could be completed by refining the analysis with a deeper study of some of the clusters and themes detected and observed in this work.