Sustainability and Competitiveness in the Tourism Industry and Tourist Destinations: A Bibliometric Study

: This article presents an analysis of the state of the art on the relationship between tourism, sustainability and competitiveness (TSC); moreover, the analysis also includes a focus on tourism destination (TSCD). To that end, information on the publications in these ﬁelds from recent years was sourced from the Web of Science database. In addition, the VOSviewer software program was used to enable a more in-depth bibliometric study, allowing the results to be clustered by authors, institutions, countries, and journals. The study carried out revealed that 808 articles have been published on aspects relating to TCS and 409 regarding TSCD. The results obtained underscore the greater scientiﬁc output on aspects related to sustainability than on aspects related to competitiveness and also that there has been a signiﬁcant and exponential increase in both cases in recent years. These three ﬁelds of study (tourism, sustainability and competitiveness) are rarely combined in the literature, highlighting the lack of a comprehensive overview of this trinity. The evidence reported here suggest that the trend identiﬁed represents a future line of work for the coming years.


Introduction
The new challenges facing the tourism industry, as well as its specific characteristics and structure, are creating a need for more in-depth knowledge about the factors that affect and shape this industry [1,2]. The importance of the tourism industry to the economy has been extensively demonstrated [3][4][5][6][7]. Thus, a richer understanding of the relevant competitive factors is becoming crucial for helping companies in this industry to create strategies that improve their competitiveness [8,9].
In the same vein, it is vital to gain a better understanding of the influence that the concept of sustainability may have on the tourism industry [10,11]. When it comes to defining the tourism company's strategy, the elements to take into account should include a sustainable environment in the tourism destination, the prosperity of local communities, the appropriate use of natural resources, and the impact on the population and the environment [12][13][14][15].
As a consequence, a growing body of literature has focused on the relationship between tourism and competitiveness on the one hand, and between tourism and sustainability, on the other. However, this dichotomy or separation could give rise to questions about the extent to which sustainability and competitiveness can coexist [16]; this is particularly critical in the tourism industry, given its

Methodology and Data
WoS, owned by Thomson & Reuters, is one of the most commonly-used databases in bibliometric studies, tracking scientific output in a way that provides more consistent, standardized data [28]. It is widely accepted that WoS provides the highest quality, most extensive output for academic and scientific studies [30]. Keyword studies are a good approach to identify and track scientific progress [31,32].
The data retrieval was performed on 31 July 2019, in order to retrieve the maximum numbers of publications and ensure the most up-to-date study possible. Hereafter, when we refer to 2019, we are referring to the part of the year up to this date.
Initially, in order to gain a more general overview of the publications, we conducted two searches. The first search was on the terms "tourism" AND ("sustainability" OR "sustainable"), yielding 10,651 results. The second was on the terms "tourism" AND ("competitiveness" OR "competitive"), yielding 3392 articles. Thus, we obtained what we call level TS and level TC respectively.
In line with the key aim of our study, we then added those terms together, creating the sample of TCS (T+C+S), obtaining 808 articles; and going further, we created the sample TCSD (adding the term "destination" OR "cluster"), obtaining 409 articles. Cluster was added due to fact that it can be assimilated to the destination. The cluster is included in the Tourism Destination's definition [33]. The final sample retrieved was composed of 808 publications for TCS and 409 for TCSD.
In order to perform our analysis, we use the most widely-accepted measures in bibliometric research [24,34,35]: total number of papers and citations, the h-index, the number of publications above a specific threshold, the impact factor, and the ratio of article citations per year (CY).
The analysis of the networks can be used to create a graphic map of the relationships between the data [32]. We use VOSviewer software [36] to visualize the bibliometric analysis of the publications. The technique is based on the use of nodes and lines (links). Nodes can represent authors, journals, countries, institutions, references, and keywords. Links between nodes can show citations, co-citations, collaboration, and co-occurrence. The analysis of clusters may show networks, helping to reveal groups and trends [37]. We thus display keyword co-occurrence; co-citation of cited references; journal co-citation; author co-citation; bibliographic coupling of authors; and co-authorship links between researchers, countries, or institutions.
VOSviewer software offers different graphic ways of conveying information. It uses a node to represent the number of papers in which a keyword is used, such that the bigger the node, the greater the weight of the keyword. Each node represents an item (author, paper or journal). The same colour represents a group of connected keywords or cluster. The connections or lines depict the number of articles in which a specific keyword appears in conjunction with another. Thicker lines indicate more regular co-occurrence; in other words, the intensity of cooperation. The distance between nodes reflects how many articles the two keywords are in together compared to co-occurrence with other keywords.

Results
First, this paper analyses the current situation and the evolution of tourism research as it relates to sustainability (TS) and also to competitiveness (TC). The analysis includes information on the number of publications per year, the number of citations per field and per document, the countries with the most publications, the journals (sources) with the greatest scientific output, and the authors who are most representative of the area.
Second, the paper analyzes the current situation and the evolution of tourism research related to sustainability and competitiveness (TCS), and as an additional element focuses on the TCS research related to tourism clusters or destinations (TCSD). In addition, it analyses the citation structure of the publications. Third, this study focuses on the most-cited articles on TCS and TCSD. The fourth part analyzes the most representative journals in these subjects. The fifth examines author keyword co-occurrence in TCS and TCSD. Sixth, the article explores the co-citation of references, journals and authors in the areas of TCS and TCSD. The seventh part investigates the bibliographic coupling between authors. Finally, the paper considers the networks of co-authorship linking countries and research institutions in the areas of TCS and TCSD.

Trends in Tourism Research Related to Sustainability (TS) and Related to Competitiveness (TC)
The first two articles on TS, [38,39], appear in WoS in 1990; since then, publications on the subject have been continuously generated (Figure 1). From 2002 onwards, more than 100 articles were published per year, with the exception of 2003 and 2004, in which there were 93 and 94 articles, respectively. And from 2015 on, more than 1000 articles were published per year, with as many as 1446 publications in 2018. Although fewer publications are observed for 2019 (due to the fact that the data search and retrieval was carried out on 31/07/2019), when analyzing the rate of publications per month, this rate is higher in 2019 (145.28 = 1017/7) than in 2018 (120.5 = 1446/12).
In the case of research on TC, the first article [38] appears in 1990 ( Figure 1). From 2000 onwards, more than 10 articles were published per year. And from 2008 on, more than 100 articles were published per year, with as many as 458 publications in 2018. Analyzing the rate of publications per month, it can be seen that it is higher in 2019 (40.28 = 282/7) than in 2018 (38.16 = 458/12). Regarding citations on TS, the 10,651 published articles have registered a total of 88,667 citations. Looking at Figure 2, only 1.28% of the articles (136) have had more than 100 citations; 17.64% of the articles (1879) have had between 10 and 99 citations; 37.61% of the articles (4006) have had fewer than 10 citations; and 43.47% of the articles (4630) have had no citations at all. The three articles with the highest number of citations are Buhalis (2000) [17] with 994 citations, Gössling (2002) [40] with 361 citations and Choi and Sirakaya (2006) [41] with 346 citations. On the other hand, in the case of TC, the 3392 published articles have registered a total of 30,133 citations. Only 1.68% of the articles (57) have had more than 100 citations; 14.95% of the articles (507) have had between 10 and 99 citations; 34.14% of the articles (1158) have had fewer than 10 citations; and 49.23% of the articles (1670) have had no citations at all (Figure 2). The three articles with the highest number of citations are Buhalis (2000) [17] with 994 citations, Crouch and Ritchie (1999) [42] with 583 citations and Gallarza and Saura (2006) [43] with 470 citations. If we compare the average number of citations per article in TS and TC research, we see similar figures: In the case of TS, it is 8.32, and in the case of TC, it is 8.88.  According to Figure 4, the journals with the greatest scientific output on the topic of TS are Journal of Sustainable Tourism (Impact Factor: 3.4) with 617 publications, Sustainability (Impact Factor: 2.592) with 468 publications, and Tourism Management (Impact Factor: 6.012) with 335 publications, representing 5.79%, 4.39% and 3.15% of the total publications in this field, respectively. The journals with most publications on TC include Tourism Management (Impact Factor: 6.012) with a total of 149 publications, Tourism Economics (Impact Factor: 1.098) with 72 publications, and Current Issues in Tourism (Impact Factor: 3.395) with 66 publications, representing 4.39%, 2.12% and 1.95% of the total publications in this field, respectively ( Figure 4).

Trends in Tourism Research Related to Sustainability and Competitiveness (TCS) and TCS Related to
Cluster/Tourism Destination (TCSD) Figure 6 highlights two important issues when we compare TCS with the publications related to Tourism and Sustainability (TS) or Tourism and Competitiveness (TC). The first is that the trend in publications related to TCS started later, just before 2007; and the second is that the level of annual publications is lower compared to the two previous concepts (TC y TS), going from 20 publications in 2010 to 120 in 2018 [29]. We could interpret these differences as reflecting the emerging concern about sustainable and competitive policies and issues in the tourism industry.
The first article related to TCS [38] appears in WoS in 1990; after that year, articles appeared intermittently (not every year saw an article published). However, from 1999 onwards, articles are published every year, starting with a single article in 1999 and reaching a total of 120 in 2018 ( Figure 6). Analyzing the rate of publications per month, this rate is higher in 2019 (12.42 = 87/7) than in 2018 (10 = 120/12). A growing interest in research on the subject can be observed; although the search on TCS generated only 808 results, if we determine the increase over the last 10 years (2009 to 2018), we can see that TCS has increased by more than 300%, a greater increase than the other topics analyzed (TS: 290%, TC: 278%).
The first article related to TCSD [44] appeared in WoS in 1994; after that year, articles appeared intermittently (not every year saw an article published). However, from 2008 onwards, articles have been published every year, starting with 7 articles in 2008 and reaching a total of 63 in 2018 ( Figure 6). Analyzing the rate of publications per month, this rate is higher in 2019 (8.14 = 57/7) than in 2018 (5.25 = 63/12). A growing interest in research on the subject can be observed; although the search on TCSD generated only 409 results, if we determine the increase over the last 10 years (2009 to 2018), we can see that TCSD has increased by more than 380%, a greater increase than the other topics analyzed (TS: 290%, TC: 278%, TCS: 300%).
The influence of articles can be shown by using the number of citations as a measure of their relevance. Table 1 displays the citation level of all the papers published on TCS and TCSD at the date of the retrieval. When we focus on TCS, only one article received more than 500 citations; that is, 0.1% of the total. Analyzing all the articles, only 15.8% received more than 10 citations. This number is quite low when compared to other areas such as business, economics, physics, and chemistry [34].
Regarding citations of TCSD articles, it can be observed that only 3.67% (15) of the total articles (409) have more than 50 citations, 3.42% (14) of the articles have between 25 and 49 citations, 9.54% (39) have between 10 and 24 citations, and 83.37% (341) have fewer than 10 citations. As for their h-index, for TCSD articles it is 28 (i.e., 28 articles have at least 28 citations).   Table 2 displays the most cited articles and some specific characteristics. This information helps to identify which of all the published articles are the most influential [32] in both TCS and TCSD.  The variables we use to show the influence and quality of the articles are the total number of citations together with the number of citations per year. The most cited article on TCS is that by Buhalis (2000) [17], with 994 total citations and with the highest number of citations per year (49.70). The aim of this paper is to combine competitiveness and environmental management. The second most cited paper is that by Kiss (2004) [45], with 249 citations and 15.56 citations per year. Although this paper analyzes the so-called "Community-based ecotourism-CBET", the approach it takes is to study the link between economically successful projects and sustainable actions. Finally, the article by Mihalic (2000) [13] is the third most cited, with 243 citations in total and 12.15 citations per year. In that paper, the concept of environmental management in tourism destinations is studied along with tourism competitiveness.

Analysis of the Most Cited Articles on TCS and TCSD
As for TCSD research, 80% of the top 10 most cited articles were published in Tourism Management ( Table 2). The three most cited articles on the TCSD topic are Buhalis (2000) [17] with 994 citations, Mihalic (2000) [13] with 243 citations, and Aguiló et al., (2005) [46] with 165 citations.
On both levels, the most cited paper is by Buhalis (2000) [17], whose research emphasizes the need to combine strategic objectives with the sustainability of the destinations' resources and is therefore aligned with our TCS and TCSD approaches.

Analysis of the Journals on TCS and TCSD
A list of journals with the highest number of influential papers is presented in Table 3.   In the case of TCS research, the main categories of these publications are Hospitality, Leisure Sport and Tourism (46.27%); Environmental Studies (18.07%); Management (17.83%); Green and Sustainable Science and Technology (14.70%); Economics (14.46%); Environmental Sciences (13.25%); and Business (10.60%).
The three main journals by number of publications on TCSD are: Sustainability, which focuses on experimental and theoretical research related to natural sciences, social sciences and humanities that examines the impact of global change and development; Tourism Management (the journal with the highest number of citations), which focuses on the planning and management of travel and tourism; and Journal of Sustainable Tourism, which focuses on the analysis of the relationships between tourism and sustainable development. The journal with the highest h-index is Tourism Management (15). In addition, 23 (Table 3).

Keyword Analysis
As the keywords selected by the authors indicate the core content and subject of the article, they can reveal trends in research and the most significant themes in a specific area. In this section, we show the keywords that appear together in the same article, or keyword co-occurrence. When working with keywords, the Occurrences attribute indicates the number of documents in which a keyword occurs ( [48], p. 32).
By examining the 808 papers in TCS research, the analysis reveals the existence of 2922 keywords. VOSviewer treats this information graphically, as can be seen in Figure 7. The most frequent keywords by cluster are: Tourism (yellow cluster), Competitiveness (red cluster), Sustainable tourism (purple cluster), Management (green cluster), and Model (blue cluster).  Table 4 shows the top 20 keywords in TCS, as well as occurrences (frequency) and co-occurrences (total link strength).
In the case of TCSD research, the analysis reveals the existence of 1158 keywords. Figure 8 shows the main keywords, which are organized into six clusters, where the most important compounds are Sustainable Development (red cluster), Destination (cluster in green), Sustainable Tourism (dark blue cluster), Competitiveness (yellow cluster), Tourism (purple cluster), and Sustainability (light blue cluster). Table 5 shows the top 10 keywords in TCSD, as well as occurrences (frequency) and co-occurrences link (total link strength). In terms of occurrence and co-occurrences, the most important keywords are Tourism, Sustainability and Competitiveness.   3.6. Reference, Journal, and Author Co-Citation Analysis If two items (whether author, paper or journal) are cited together by a third (all of them are present in the reference sections of other articles), there is a co-citation relationship between them. The correlation between the items can thus be identified [49]. Again, by using the concept of nodes, clusters and lines, VOSviewer helps to graphically visualize the existing relationships in an area of research.
The distance between two journals, authors or references indicates the relatedness of the journals, authors, or references in terms of co-citation links. In general, the closer two journals, authors, or references are to each other, the stronger their relatedness. The strongest co-citation links between journals, authors or references are also represented by lines [48].
The first approach is co-citation network analysis, where every node represents an individual article and the lines indicate the collaboration relationship and the most frequent research topics relating to TCS. Figure 9 shows the leading articles on TCS in this regard. There are three clusters. In the most cited cluster (red), the paper by Ritchie J.R.B. [50] has been cited 78 times. The second is by Dwyer, L. [51] with 73 citations. And the third, with 71 citations, is by Crouch, G.I. [42]. In the second most important cluster (green), with 66 citations we find Buhalis D. [17], who had the most cited paper in Table 2. Following some way below, is Barney, J.  [42] examines the link between social prosperity and tourism using a competitiveness frame of reference.  The second network we are studying is journal co-citation for TCS ( Figure 11). The bigger the node, the more articles a journal has published; also, the distance between the nodes represents the frequency of citation between them. Thus, when two nodes are very close, they have cited one another more times.
As we can see, the program identifies three main clusters and one residual. Tourism Management is the journal with the most citations (2548) in the green cluster and also the highest link strength (79,693) followed by Annals of Tourism Research (1372 and 48,607, respectively). Regarding the topics of the journals in this cluster, we can see that they are mainly focused on tourism and travel. Journal of Sustainable Tourism (606 and 23,034, respectively) leads the blue cluster, which is mostly focused on sustainability; and in the red cluster, which is made up of management journals, we have International Journal of Hospitality Management, with 422 citations and a total link strength of 16,555.
The residual cluster has only three items. It is located at the bottom of the network, shown in yellow. Two of the three items are journals related to an economics perspective. , which focuses on the study of tourism destinations related to aspects of marketing and management, placing them in specific policy, planning, economic, geographical, and historical contexts. Lastly, the light blue cluster includes Tourism Economics (Citations 89, Link Strength 2780), which focuses on the economic and commercial aspects of tourism. These journals are mainly focused on the categories of Hospitality, Leisure, Sport and Tourism, and Environmental Studies. It can also be seen that all clusters are very close to each other, indicating a strong relationship between them.
We can see the third co-citation analysis in Figure 13. The main author network reveals the existence of three clusters. The small differences between the top authors are indicative of the homogeneity in terms of most cited authors, regardless of the cluster they belong to. For instance,   Examining the main author of each cluster, we can determine their typical perspective. Dwyer's research focuses on destination competitiveness, most specifically in Serbia. His articles also analyze performance and tourism business strategies. Buhalis' top article on this issue synthesizes models for the strategic marketing of destinations; he explains that the "marketing of destinations should balance the strategic objectives of all stakeholders as well the sustainability of local resources" [3]. In the green cluster, we find Hall, C.M., whose research focuses on international tourism and climate change.
The author co-citation network for TCSD ( Figure 14) illustrates the existence of three clusters. The red cluster consists of 11 authors, of which the predominant author is Buhalis, D. (143 citations and Link Strength 1134). He has 102 publications in WoS, and his research focuses on smart tourism and smart hospitality, social media and mobile marketing, augmented reality, and the management and personalization of experiences enhanced with technology.

Bibliographic Coupling of Authors
Whereas in the analysis of co-citation we looked for whether Article A cited Articles B and C, with bibliographic coupling we are looking for whether A and B cite Article C. In other words, we are counting the number of references that a group of papers has in common. Figure 15, which depicts this analysis, shows that there is no single author that stands out above the others, regardless of the cluster. This is because almost all the authors have only three publications in common. The top four authors do have a few more, but the differences are not substantial: Pulido-Fernandez, J.I. with seven publications; Dwyer, L. with five; and Buffa, F. and Sainaghi, R. with four. The program has identified seven clusters. The main one is the red cluster, which has 15 items and is led by Pulido-Fernandez, J.I. The second most important cluster is the green one, which has seven items. The next cluster is the dark blue one, comprising four elements. What is striking about this cluster is that its four items are on the right-hand side, far away from the rest and almost superimposed over each other. All four of these authors are located in Malaysian universities and worked together on the articles. The yellow and purple clusters have three items each. Lastly, the light blue and the orange clusters have two elements.
If we examine the predominant authors in the larger clusters, we can see the different perspectives of their studies. In the red cluster we have Pulido-Fernandez, J.I., whose work focuses on sustainable tourism, especially its relationship with performance and growth. In the green cluster we have Buffa, F., focusing on sustainability from the perspective of management practices and business models. In the case of the dark blue cluster, the studies are closely focused on event greening. Although Dwyer, L. is the author with the second highest number of publications, he is located in one of the smallest clusters, namely the yellow one.
According to the analysis of TCSD, there are five clusters ( Figure 16

Country and University Co-Author Analysis
Finally, we carry out an analysis of co-authorship, which depicts the networks of collaboration between countries and organizations for the two levels that we are studying.
The size of the items (nodes) in this case reflects the relevance of the country or organization in question, and the distance between them indicates the degree of collaboration.
For the TCS level (Figure 17), the program presents us with seven clusters, of which the red one contains the most items and is led by Portugal. We see a fair degree of dispersion in the cooperation between countries. In general, the clustering is quite heterogeneous at first glance, although we do find two clusters of countries in which the relationships are more evident. This is the case with the orange cluster, which brings together the Spanish-speaking countries of Spain, Mexico and Ecuador. The green cluster, on the other hand, contains countries related in terms of their geolocation: Egypt, India, Indonesia, Iran, Malaysia, and Thailand.
The countries that stand out for their contributions to this area of study are Spain (106 publications), China (77), Italy (77), Romania (59), and Croatia (50). The analysis of the relationship between countries in terms of co-authorship on TCSD provides us with a network of 24 countries divided into six clusters ( Figure 18): The red cluster is composed of seven countries, with the most representative being Australia (22 publications, 215 citations), Italy (47 publications, 408 citations) and China (25 publications, 59 citations); the green cluster is composed of six countries, with the most representative being England (24 publications, 1384 citations) and Slovenia (13 publications, 359 citations); the dark blue cluster is composed of three countries, with the most representative country being Croatia (27 publications, 27 citations); the yellow cluster is composed of three countries, with the most representative country being Thailand (5 publications, 22 citations); the purple cluster is composed of three countries, where the most representative country is Spain (71 publications, 622 citations); and the light blue cluster is composed of two countries, with the most representative country being the United States (17 publications, 120 citations). Finally, it can be seen that the different clusters are fairly spaced out.
The countries that stand out in terms of the number of publications are Spain, Italy and Croatia. Conversely, if we look at the number of citations, the leading countries are England, Spain and Italy. And lastly, if we analyze total link strength, the stand-out countries are Australia, England and Spain. We then carry out the same analysis, but for organizations. In this case, of the total of 861 institutions, we can see in Figure 18 that only five of them are collaborating with each other. There are two distinct clusters: We find a clear collaboration between the University of Central Florida (with 7 publications) and the Hong Kong Polytechnic University (with 10); furthermore, in the red cluster we see the collaboration between the University of Novi Sad (7) in Serbia, the University of Ljubljana (7) in Slovenia, and Griffith University (9) in Australia.
However, although they have not collaborated with the other institutions and therefore are not reflected in the network in Figure 19, the most influential institutions in terms of number of publications are, first, the Bucharest University of Economic Studies, with 15 publications; and second, the University of Rijeka, in Croatia, with 14. Another relevant finding of the analysis of the most influential institutions is the notable presence of Spanish universities as this was the country with the most contributions (as can be seen in the analysis depicted in Figure 17). On an individual basis, these universities are not at the top of the ranking, but taken together they boost the relevance of the country. It is interesting to note that these universities have a sufficient presence to ensure that their country is the most influential, yet there is no collaboration between them. Specifically, these universities are the University of Las Palmas de Gran Canaria (10 publications), the University of Jaén (9), the University of Alicante (8), the University of the Balearic Islands (6), the University of Barcelona (6), the University of Extremadura (6), the Autonomous University of Madrid (6), and the University of Málaga (6).
Finally, when creating the co-authorship network by institution for TCSD (Figure 20), for the universities that comply with the restriction of a minimum of three publications, it can be seen that there is little collaboration between the different universities. Indeed, from the total of 457 universities, the largest group consists of only eight universities, which are organized into three clusters. The red cluster is composed of three universities: Griffith University in Australia

Summary
As a summary of the above mentioned analysis, from 3.2 until 3.8, which are the main contributions we make, we present Table 6. Emerging concern in TCS, mainly in 2007, although less than TC or TS. Only one article with more than 500 citations, and six with more than 100 citations. In TCSD, there is an important upward tendency in publications since 2007. Only one article with more than 500 citations, and three with more than 100 citations.    Tables 4 and 5 3.6.
To display the existing co-citation relationships between authors, papers and journals. In

Discussion and Conclusions
Using bibliometric analysis of publications in WoS, this article provides an up-to-date picture of the trends in research on sustainability and competitiveness in the tourism industry, together with a specific insight into tourism destinations. The significance of this article lies in the fact that, as far we are aware, there are no previous bibliometric studies on the tourism industry that have simultaneously focused on competitiveness and sustainability. Moreover, taking an additive approach, we have incorporated the concept of tourism destination.
First, the analysis focused on the results in TS and TC. Significant growth was observed, which demonstrates the relevance of these issues. If we compare the average number of citations per article in TS and TC research, we see similar figures: In the case of TS, it is 8.32, and in the case of TC, it is 8.88. The three articles with the highest number of citations in TS research are Buhalis (2000) [3], Gössling (2002) [4] and Choi and Sirakaya (2006) [5]. On the other hand, the three articles with the highest number of citations in TC research are Buhalis (2000) [3], Crouch and Ritchie (1999) [6] Gallarza and Saura (2006) [7]. The most representative journals in TS and TC are Journal of Sustainable Tourism and Tourism Management, respectively. In both TS and TC research, China is shown to be the country with the most publications, accounting for 14.40% and 12.64% of total publications on these topics, respectively.
In light of our findings, it can be concluded that there is a rising trend in TS and TC research; however, there has been far more focus on the former than the latter in the literature, with the gap between them widening over the last decade. About 20 years ago, the gap between them amounted to about 100 articles a year, but in 2018 and the first 7 months of 2019, the gap was 1000 articles. This data may be interpreted as reflecting the increasing concern about sustainability in tourism (TS), which is growing at a far faster rate than the rising trend in research on competitiveness in the tourism industry (TC).
Thanks to the novel approach of our research, we can highlight two important differences. First, the number of publications on TCS is quite low compared to either TS or TC. This relevant finding indicates that there is still a long way to go in understanding how to implement sustainable policies that simultaneously enable competitiveness in the tourism industry. In 2018, there were 120 publications on TCS, whereas the equivalent numbers for TS and TC were 1446 and 458, respectively. Second, much more research is still needed on sustainable and competitive tourism destinations, TCSD, on which there were 63 publications in 2018. The literature on TCSD remains scarce (only 409 publications), although it has experienced growth of over 380% in the last 10 years (2009 to 2018), outstripping the growth in the other subjects analyzed (TS: 290%, TC: 278% and TCS: 300%).
As described above, similar behaviour is observed for the number of citations in TCS and TCSD. Only one article has more than 500 citations in TCS and TCSD, while only six and three articles, respectively, got more than 100 citations. Given the importance of achieving competitive and sustainable tourism [54]-and, more specifically, competitive and sustainable tourism destinations-the numbers of publications and citations for TCS and TCSD indicate that there is still much to study, thus providing opportunities for researchers to explore this new field of research.
The  [10]. It is important to note that Buhalis (2000) [3], with a total of 994 citations, appears as the most cited article in the four searches carried out (TS, TC, TCS, TCSD), because it defines the concept of destination and examines the effect of destination marketing on the competitiveness of destinations.
The top journals in terms of the number of articles published on TCS and TCSD are Sustainability, Tourism Management and Journal of Sustainable Tourism. TCSD research represents only a small percentage of the total research presented in the top 10 journals (with coverage ranging from 0.02% to 1.45%).
The most frequent keywords reveal the main topics in a specific area of research. For TCS, our analysis emphasizes the keywords "tourism", "competitiveness", "sustainable tourism", "management", and "model". These keywords reflect the main trends followed by researchers. They indicate that the concept of a competitive and sustainable tourism industry goes, or should go, hand in hand with appropriate management systems, processes and models that enable and help to achieve the desired competitiveness and sustainability.
The most notable keywords for TCSD are "tourism", "sustainability", "competitiveness", "sustainable development", and "sustainable tourism". In addition, the co-occurrence network of author keywords features "Innovation", "Cooperation", "Strategy", and "Stakeholders". These keywords are related to the concepts of tourism clusters and tourism destination.  [51] with 68 citations, and Crouch G.I. (1999) [42] with 66 citations. The third cluster, in which the most important reference is Liu (2003) [53] with 23 citations is farther away in relation to the number of citations and also shows a lesser relationship with the other two clusters.
As for the analysis of the journal co-citation network for TCS, there are three main clusters. The first is headed up by Tourism Management, the second by Journal of Sustainable Tourism, and the third by International Journal of Hospitality Management. The topics of these journals are tourism and travel, sustainability, and management, respectively.
At the TCSD level, six clusters can be observed. The first is headed up by Tourism Management, the second by Journal of Sustainable Tourism, the third by International Journal of Tourism Research, the fourth by Annals of Tourism Research, the fifth by Journal of Destination Marketing & Management, and the sixth by Tourism Economics. These journals are mainly focused on the categories of Hospitality, Leisure, Sport and Tourism, and Environmental Studies.
In the author co-citation network analysis for TCS, we find three fairly homogeneous clusters in terms of the number of citations and link strength of the top authors. Nevertheless, the most notable author in relation to these two aspects is Dwyer, L. (202 and 1544).
In In terms of the number of authors belonging to the same cluster, the bibliographic coupling of authors in TCS reveals a main cluster of 15 authors, followed by a second group of 7 authors. As for the leading author in terms of the number of publications, we have Pulido-Fernandez, J.I. with seven, who belongs to the largest cluster. In second position is Dwyer, L. with five publications, but assigned to one the smallest clusters, in which only four authors are grouped.
For TCSD, the bibliographic coupling of authors reveals five clusters. There is a main cluster of seven authors, followed by the second most important cluster of four authors. The leading authors are Pulido-Fernandez, J.I. with seven publications; Dwyer, L. with five; and Buffa, F. with four. Two of the clusters, which are farther away from the other three, present emerging topics of research. One is made up of Kruger, M. and Saayman, M., and the other of Dwyer, L., Armenski, T. and Mihalic, T.
Regarding the analysis of co-authorship relationships between countries and institutions for TCS, we find that, at the country level, Spain leads the way in terms of number of contributions, followed by China and Italy. As for the institutions, we observe the interesting fact that although the Spanish universities are responsible for the most contributions (at country level rather than individually), they do not engage in any kind of collaboration with each other. The most important institutions are the Bucharest University of Economic Studies and the University of Rijeka. The most notable collaboration is between the University of Central Florida and the Hong Kong Polytechnic University.
Regarding the analysis of co-authorship relationships between countries for TCSD, there are six clusters, headed up by Australia, England, Croatia, Thailand, Spain, and the United States. It can be seen that the different clusters are relatively far apart from each other, indicating a less collaborative relationship. Finally, the analysis of the co-authorship network of institutions for TCSD shows only eight universities organized into three clusters, headed up by Griffith University in Australia, the University of Ljubljana in Slovenia and the University of Technology Sydney in Australia. Particularly notable is the role played by Australian universities both in collaboration with universities from other countries and among national universities.
Given the results of our bibliometric analysis, our study sheds light on the main research topics in the literature on TCS and TCSD, and, consequently, on potential trends. Our study can help researchers by highlighting different concepts and the links between them, thereby suggesting future lines of research. In this vein, our article reveals a number of clear trends. The first is the increasing importance of TS, as shown by the number of publications in WoS [19]. The second is the rise in the number of articles published related to TC. Third, the gap between TS and TC reveals a need for greater insight into how to improve competitiveness in this sector. Fourth, we found a substantial gap separating TCS from TC and TS, indicating that the coexistence of these two attributes in the tourism industry requires more in-depth knowledge, thus calling for greater attention from researchers [55]. It can be explained by the lack of interaction between the academic world and the tourism industry [14,56]. Fifth, in the case of TCSD, there has been an increase in the number of published articles, although the growth has been less explosive than in TS and TC. Furthermore, there is a gap between TCS and TCSD in terms of the amount of research, pointing to the need for further research on the management, governance and marketing of tourism destinations and their impact on the competitiveness and sustainable development of the destination [13,46]. Sixth, the keyword analysis also highlights a need to include new fields of knowledge in TS, TC, TCS, and TCSD [17,24,29]. Environmental concerns and related issues must be addressed in connection with strategy and management, thereby enabling the enduring success of companies and organizations in a sustainable manner. Seventh, regarding the research on TCDS, it is also worth highlighting the need for joint work between different universities (from different countries), as there has been little such collaboration to date. This collaboration will enable the exchange of knowledge and best practices of tourism destinations.
Our study is also helpful for practitioners. Business organizations should take into account the findings from the research literature. The complexity of achieving TSC and TSCD requires an innovative approach in which the transformation comes from a global perspective incorporating other disciplines [55]. In that vein, sustainability must go hand in hand with the management approach, marketing strategies, social issues, and technology, etc. Since stakeholders sometimes seem to view sustainable tourism as providing no kind of competitive advantage [16], the literature also helps practitioners to recognize the benefits of being sustainable. In addition, as governments play an active role in encouraging responsible behaviour [16], tackling the abovementioned complexity requires policies, incentives and regulations to encourage and motivate the private sector [57]. Regarding TCSD research, it highlights the importance of cooperation and creation of networks between the public and private sector, in order to boost the competitiveness of tourism. This may lead to further research collaborations between universities, tourism businesses and governments, creating networks within a region or country that enables the generation of knowledge and technological development, as well as their application.
Nevertheless, when it comes to limitations, our study is no exception. First, our findings could be extended by using other well-known databases such as Scopus [58]. Secondly, the use of keywords might be puzzling. Third, we do not consider other sources of information, such as articles in languages other than English.

Conflicts of Interest:
The authors declare no conflict of interest.