**1. Introduction**

Sustainable development requires human systems to function within specific "green" limits to ensure the sufficient supply of goods and services both to current and future generations [1]. The achievement of sustainability in an area depends on whether the impact of anthropogenic activities are within the "green" range, including those activities that take place outside the examined area but whose impacts affect its environmental status [2]. For this reason, the planning and natural resources management in all spatial scales is essential to aim toward sustainability [3].

Focusing on protected areas, the integration of anthropogenic activities within the boundaries of protected areas has led to increasing concerns regarding the appropriate use levels of parks, forests, lakes, and other environmentally sensitive areas [4]. Alongside the significant ecological habitats within protected areas, extensive human activities are developed such as households, tourism, agriculture, light industry, and transportation. The harmonious coexistence of those diverse activities is a basic concern of the management bodies responsible for protected areas [5]. More and more relevant authorities need to quantify and assess the maximum level of human activities that can be developed to satisfy current and future needs, while in parallel sustaining the environmental and ecological health.

The sustainability of protected areas is associated directly with Carrying Capacity for two reasons: (a) the idea of sustainability reflects a limit, similarly with the concept of Carrying Capacity, and (b) both concepts share the same challenges in formulating the objectives, practices, and actions of improvement [6]. From the early 1960s, due to the fact that public visits were the major threat for protected areas [7–10], research on outdoor recreation has utilized the concept of Carrying Capacity to address the resource and social impacts of visitors [7,11,12]. A number of frameworks [5,13–16] have been developed in order to provide management bodies with a basis for decision-making about the Carrying Capacity of national parks and protected areas, defining it as "*the maximum number of visitors an area can sustain without unacceptable deterioration of the physical environment and without considerably diminishing user satisfaction*" [9,15,17]. Therefore, in the current article, the concepts of Environmental Sustainability and Carrying Capacity are identical.

In Greece, there are twenty (20) protected areas [18] known for their great ecological and educational value, when at the same time, apart from tourism, several other human activities are traditionally and/or legally established within their boundaries. Thus, the assessment of the Carrying Capacity becomes more complex, since activities such as agriculture, livestock, aquaculture, households, light industry, and transportation have to be taken into account. Therefore, the Carrying Capacity assessment of an area must take into account the impact of all the activities that take place within its boundaries.

Serving this challenge, the aim of the specific study was to provide an applicable framework that is able to improve the evaluation and monitoring of the environmental sustainability of protected areas. The proposed framework takes advantages from the results of an extensive literature review we have conducted in a previous work [19]. In this work, 13 methods selected from a pool of 61 methods from a literature review were analyzed, categorized, and were finally evaluated based on specific criteria [19,20]. This analysis pointed out key conclusions related to the efficiency and the applicability of environmental sustainability assessment methods of protected areas. More specifically, the "Resource Availability Assessment" category of methods and especially the "Ecological Footprint" method have been indicated as the most appropriate method for the evaluation of environmental sustainability of protected areas. Moreover, the need for the improvement of the ability of methods to incorporate new activities within the environmental boundaries of the protected area, together with the necessity of the methods to provide environmental sustainability thresholds, in order to evaluate quantitatively whether the performance is sustainable, was also pointed out. An effective environmental sustainability assessment method should take into account the spatial characteristics of the examined area and ensure an adequate balance between the level of complexity and the coverage of key sustainability issues [19,20].

This paper consolidates key findings from our previous work on the evaluation of existing environmental sustainability assessment methods [19] and progresses a step forward by integrating all the information into an applicable framework that focuses on protected areas, taking into consideration all anthropogenic activities within its boundaries. The proposed framework is expected to provide a more holistic approach for the assessment of the environmental status and for the development of a sustainable strategy for a protected area.

#### **2. Method Description**

In this specific section, the theoretical background and the steps that compose the proposed methodological framework are briefly presented.

#### *2.1. Theoretical Background*

The concept of Carrying Capacity in general expresses an upper limit of the ability to sustain a living system, whereas beyond that limit, instability, degradation, or irreversible damage will subsequently occur [21]. Therefore, Carrying Capacity can be utilized as a supportive tool of policy and decision-making, in order to resolve the aforementioned challenges.

The assessment of the Carrying Capacity of an area is a case-specific procedure and depends on the nature of the problem to be solved and the objectives set by the researcher. As a result, various Carrying Capacity definitions are available in literature. A widely known definition was introduced by Rees [22], according to which the Carrying Capacity of an ecosystem is "*the maximum population of certain species that can be accommodated in an environment without permanent damage to the productivity of the environment"*.

All human populations need natural resources. The use of resources leads to physical outputs that affect global and local areas and has environmental impacts, such as waste generation and impact on climate change. The availability of natural resources and the environmental impacts from their utilization are the two restrictive parameters regulating the size of the population that can be sustained in a given area [23].

Thus, the majority of existing Carrying Capacity methodologies focus on the environmental constraints of resource consumption to determine population limits [24]. The resource-consumptionfocused methodologies [25,26] are universally applicable, reasonably comprehensive, and their data and methodology have been made publicly available [27]. The most common current existing examples of environmental modeling [24,28,29] are based on the Ecological Footprint, *"the amount of land and*/*or water that is necessary to a population or activity, in order to produce, in a sustainable way, all the natural resources it consumes and assimilate the waste it produces, using the available technology"* [30].

Therefore, both Ecological Footprint and Carrying Capacity are based on similar procedures such as defining an area, selecting resources, and defining relevant indicators for qualification [28]. In order to assess the Carrying Capacity of an entity, its Ecological Footprint per person is compared with its Biocapacity, a term that represents the available biologically productive land that absorbs the impact of consumption along with subsequent waste [25]. The Ecological Footprint calculations refer to the estimation of annual consumption needs of anthropogenic activities that take place on the study area and their conversion to biologically productive land. According to Wackernagel et al. [31], the aforementioned consumption needs are classified into six (6) Ecological Footprint land use types, namely, agricultural products, livestock products, fishery and aquaculture, timber products, CO2 emissions, and built-up surfaces. The Ecological Footprint of consumption for each product (EFc) is calculated as:

$$\rm{EF\_C} = \rm{EF\_P} + \rm{EF\_{im}} - \rm{EF\_{ex}} \tag{1}$$

where EFP is the Ecological Footprint of production, and EFim and EFex are the Ecological Footprints embodied in imported and exported commodity flows, respectively.

In order to ensure that all the production procedures as well as the necessary materials both for product production or energy generation will be taken into account in the estimation of the Ecological Footprint, the Life Cycle Analysis (LCA) approach was innovatively applied, leading to a more thorough assessment of human activities and ensuring that all required processes and materials were taken into account in the calculations [32]. The Life Cycle Analysis is conventionally characterized as a "cradle-to-grave" or "closed-loop" approach, as it examines the overall environmental impact of a product, process, or system, taking into account every step of its life—from receipt of raw materials to its construction, its sale, usage, and final disposal into the environment [33].

The Biocapacity calculations refer to the estimation of the existing available biologically productive land. The available land is divided into five land uses, namely, cropland area, grazing land area, marine/inland water area, forest area, and infrastructure area [31].

The calculation of Biocapacity is implemented based on the accounting framework proposed by Wackernagel et al. [31]. The available productive areas were firstly converted into land in terms of world average productivity by multiplying the available hectares of each land use with the corresponding "yield factors" [34]. "*The Yield Factors (YFs) account for countries' di*ff*ering levels of productivity for particular land uses are country-specific and vary by land use type and year. They may reflect natural factors such as di*ff*erences in precipitation or soil quality, as well as anthropogenic di*ff*erences such as management practices*" [34]. Subsequently, the abovementioned lands were converted to Biocapacity by multiplying the world hectares of each land use with the corresponding "equivalence factor" [34]. "*The Equivalence Factors (EQFs) convert the areas of di*ff*erent land uses, at their respective world average productivities, into their equivalent areas at global average bioproductivity across all land use and they vary by land use as well as by year. The rationale behind the Equivalent Factor calculation is to weight di*ff*erent land areas in terms of their inherent capacity to produce human useful biological resources. The weighting criterion is not the actual quantity of biomass produced, but what each hectare would be able to inherently deliver*" [34]. The values of the mentioned parameters of yield factor and equivalence factor are retrieved by available references and databases [35,36]. Finally, the Biocapacities of land uses are summed to the total Biocapacity.

The proposed framework is based on the "Ecological Footprint" calculation framework described above, but goes a big step further by simplifying the Ecological Footprint calculations and introducing a new unit, namely, "equivalent person," in order to enable the evaluation of more than one activity.

#### *2.2. Methodological Framework Description*

The methodological framework has been developed for estimating the Carrying Capacity of an anthropogenic spatial entity, which refers to the spatial scale below a nation and usually includes a province or a municipality or parts of them that are administrated under a specific management scheme or other specific authority (e.g., National Park) and it is summarized in Figure 1.

**Figure 1.** The proposed framework.

Based on the above, a comprehensive quantitative definition of Carrying Capacity is given by the authors with the following formula:

$$\text{Carrying Capacity } (\text{max.equivalent population}) = \frac{\text{Bioconacity (available land)}}{\frac{\text{Ecalological Footprint (required land)}}{\text{P (existing equivalent population)}}} \tag{2}$$

where P is the existing equivalent population of the inhabitants of the spatial entity under study. The unit of "equivalent population" is introduced, since the proposed methodological framework besides the consumption needs of the real human population of the study area estimates the consumption needs of anthropogenic activities, which are not directly dependent on the size of the real population. For example, the annual energy consumption of buildings depends on their size and their use and not on the population size of the study area.

At first, the spatial and time parameters of the framework, namely the geographical boundaries of the study area and the reference year essential for the procedure of data acquiring, are clarified. Following, the calculations are separated in three sectors: (a) Biocapacity calculations, (b) Ecological Footprint calculations, and (c) Calculations of existing Equivalent population.

#### 2.2.1. Biocapacity's Accounts

In order to calculate Biocapacity, specific indicators per land use of the CORINE (Coordination of Information on the Environment) land cover methodology [37] were used. These indicators are presented in Figure 2. The available productive land was estimated with the application of a GIS (Geographic Information System) software compatible with the European databases for land uses. Then the available productive lands were converted to Biocapacity per land use by multiplying the available hectares per land use firstly with the yield factors and then with the corresponding equivalent factors, both known from the literature. The total Biocapacity is the sum of all Biocapacities per land use.

**Figure 2.** The indicators of Biocapacity per land use.
