Tourism has experienced continuous growth and diversification for decades, representing one of the largest sources of income in a large number of countries. At present, tourism is closely related to development and is composed of an increasing number of destinations (UNWTO, 2015) which attempt to be more competitive, which requires these destinations to be sustainable. Global trends in the tourism sector indicate that tourists are looking for new vacation experiences related to sustainability (Buffa, 2015) and the demand for sustainability in tourism is growing. Sustainability is now taken into account when holidaymakers choose a tourist destination (Pulido-Fernandez and Lopez-Sanchez, 2014). Therefore, the development of sustainable tourism has become a strategic objective for any tourist destination (Federal Ministry for Economic Cooperation and Development [FMECD], 1995; Hassan, 2000; Moore and Carter, 1993; Simao and Partidario, 2012).
The sustainability of a destination can be understood as the improvement and long-term conservation of the destination's cultural and natural resources (Fennell, 2008). Furthermore, sustainability is considered to be a link between the development of society and economic agents that are part of the society (Garcia Mesanat & Sancho Perez, 2002).
The concept of sustainability was introduced into the tourism sector initially as the idea of sustainable development, followed by the well-known Brundtland report or the World Commission on Environment and Development Report (WDEC, 1987). According to this report, sustainable development is defined as "development that meets the needs of the present without compromising the ability of future generations to meet their own needs".
According to the United Nations World Tourism Organization (UNWTO), the development of sustainable tourism is defined as the type of tourism that " ... makes optimal use of environmental resources; respects the socio-cultural authenticity of host communities; ensures viable, long-term economic operations, providing socio-economic benefits to all stakeholders; requires the informed participation of all relevant stakeholders, as well as strong political leadership; and also maintains a high level of tourist satisfaction" (UNWTO, 2005; 11-12).
The relationship between the concepts of sustainability and competitiveness of a tourist destination has been discussed in the literature where sustainability is shown to improve competitiveness (Hassan, 2000; Ritchie & Crouch, 2003; etc.). The union between these two concepts was born from the belief that the business that pursues an improvement in environmental, economic, and social performance will benefit (Valeri, 2015). The competitiveness of tourist destinations not only has an economic aspect, but also a sustainable aspect related to the contribution to social cohesion and the preservation of resources or local natural and cultural values (Romao et al., 2014). Thus, Ritchie and Crouch (2000: 2) state that "competitiveness is illusory without sustainability". The United Nations World Tourism Organization also defines a clear link between competitiveness and sustainable tourism "to compete effectively destinations have to deliver excellent value to visitors. This depends on many aspects working together in unity. From the time that the visitor arrives at the destination until he/she leaves, visitor value is affected by many services and experiences, including a range of public and private services, community interactions, environment, and hospitality" (UNWTO, 2007: 9).
From our understanding of the factors that can determine the competitiveness of a tourist destination and especially the inclusion of sustainability as a performance indicator of competitiveness, we have developed models that interpret the tourism system. These models are not only focused on the environmental aspects, but seek to establish a new development threshold based on sustainability, aiming to meet the needs of communities, tourists, and entrepreneurs (Mazaro and Varzin, 2008).
This study aims to contribute to the literature through the analysis of the main models that have been developed for measuring sustainability in the tourism sector. To this end, a series of guidelines, rules, and recommendations will be given, based on the analysis of a series of sustainability measurement models existing in literature, in order to improve various aspects that have not yet been queried in the context of the sustainability of tourist destinations.
In addition, a series of examples of tourist destinations will be presented in which sustainable practices arise that are recommended or in which they are already being implemented.
In order to achieve this goal, firstly a number of models in the literature associated with the measurement of sustainability will be reviewed; secondly, once the weaknesses of the studied models are detected, a number of areas for improvement will be advised. This is followed by some examples of tourist destinations and practices or possible sustainable practices that arise from the literature. Finally, the study's conclusions are demonstrated and discussed.
Competitiveness models for a sustainable tourist destination
Research in recent years has shown that for a tourist destination to be competitive it has to be sustainable in the sense of "triple bottom line"; i.e., it has to be sustainable in an economic, environmental, and social way.
At present, in models that analyze competitiveness there is a stressed importance of economic, environmental, and social aspects that were hardly discussed in previous competitiveness models. Thus, several authors proposed models for measuring the sustainability of a destination with different types of indicators and from different perspectives.
A more thorough analysis of the model indicates that it is not enough to be competitive at a certain time, but continuous monitoring of the development of the destination and the development of competitors and emerging destinations is needed in order to remain competitive in the long term. Therefore, the development of sustainability strategies seems a successful way to maintain the competitiveness of the destination (Lloret, 2016).
As seen in Table 1, most models measuring sustainability of a destination consider the three basic aspects--economic, social, and environmental--mostly giving greater weight to the social and environmental aspects than to economic aspects within the model.
The following is an analysis of the characteristics of the two models developed to measure the sustainability of tourism in Spain. In the model by Blancas et al. (2012), 85 indicators were developed of which 35 are social and 29 environmental. These authors proposed a model to analyze the sustainability of rural tourist destinations. The methodology used was the creation of a composed index that allows the incorporation of statistical information through the mentioned indicators.
Thus, the index delivered many functions: 1) the characterization and comparison of rural tourist destinations; 2) identification of benchmarking practices; and 3) quantification of tourism sustainability goals.
This model aimed to identify strengths and weaknesses and thus proposed specific policies to achieve sustainability in a balanced manner, involving all stakeholders.
Navarro et al. (2013) created a synthetic indicator based on 175 indicators to measure the sustainability of coastal holiday destinations from the perspective of sustainable capacity. This model was based on three theoretical approaches: 1) capacity based on resources in order to protect natural resources from an ecological perspective of sustainability; 2) changes occurring in tourism are tracked using the development perspective (this perspective comes from the life cycle model of tourist destination by Butler); and 3) sustainability based on community involvement, i.e., all local actors are involved in the development of tourism in the area.
The community-based model necessitates the involvement of all actors in the community. Tao and Wall (2009) stated that for a tourist destination to be viable over time, it should develop strong capabilities of adaptation and involve all...