Historically, research on carrying capacity originated from considering the capacity of a pasture to sustain reindeer in Alaska in the 1920s (Hadwen and Palmer, 1922). With the depletion of natural resources, environmental pollution and ecological degradation, people gradually began to realize the interdependent relationship between man and nature during the 1960s-1970s (Arrow et al., 1995). The publishing of The Limits to Growth (Meadows et al., 1972) aroused the society consciousness greatly to protection the limited natural resources and environment. The importance of ecological carrying capacity research became more and more prominent. Carrying capacity research developed and formed into some new branches such as land carrying capacity, water resources carrying capacity, tourism carrying capacity and ecological carrying capacity etc. (Simón, et al., 2004). In the 1980s, ecological carrying capacity (ECC) was defined as the maximum population that a given area could carry without damaging the environment (Dhondt, 1988). In addition to natural resources and environment base, the maximum number of people a given region can support depends on their lifestyle and level of consumption (Xiang and Meng, 2012).

The core problem of ECC is how many people with some specific consumption level a given region can sustain. So, models to evaluate ECC and its dynamic is the primary method in the domain. ECC research methods include ecological footprint method, comprehensive index system method, ecological system service consumption evaluation method, and human appropriation of net primary productivity method etc. (Zhao et al., 2019).

ECC evaluation based on the ecological footprint method. In the early 1990s, the concept of ecological footprint (EF) was proposed (Rees, 1992; Wackernagel, 1996). The EF is calculated by estimating the area of bioproductive land needed to production goods for human consumption and to contain and decompose waste emitted during the process of production and consumption. The human appropriation of ecological capacity in a specific region can be evaluated by comparing the EF with the biocapacity (BC) in the unit of bio-productive area (hectare). Based on the ecological footprint method, the supply-demand balance of ECC was analyzed, and the ecological carrying capacity index (ECCI) was derived by the formula as ECCI = EF /BC. The ECCI was calculated for each county of China in 2007, so as to bring some scientific references and policy-making supports to the population spatial distribution planning,primary function zones and ecological security regionalization of China (Liu et al., 2012)．

ECC evaluation based on comprehensive index system method. Carrying capacity assessments model a population's potential self-sufficiency. A crucial step in the development of such modelling is to examine the basic resource-based parameters defining the population's production and consumption habits. These parameters include basic human needs such as food, water, shelter and energy together with climatic, environmental and behavioral characteristics. Considering humans basic need, A replicable approaches for carrying capacity assessment—Carrying Capacity Dashboard has been built at multiple spatial scales, including national, state and region scales, and with it the carrying capacity of 60 zones in Australia being evaluated efficiently (Lane et al., 2014). Peng et al. (2016) has explored a conceptual framework for ECC in view of coupled human and natural systems, and constructed an index system by integrating ecosystem vigor, resource and environmental carrying capacity, and social development ability. They identified the constraints factors of different counties in a low mountains and hilly areas in southwest of China, evaluated the ECC, and proposed a zoning management strategy. That is, priority areas for conservation, priority areas for development, areas suitable for short-term conservation but long-term development, areas suitable for short-term development but long-term conversation, and areas reserved for future appropriate development.

Ecosystem-Service-based Ecological Carrying Capacity (ESECC) method. ESECC is the population and the scale of the economy supported by ecosystem services which is determined by ecological structures, processes and spatial pattern. ESECC can be acquired by selecting the minimum from all the carrying capacities of every ecosystem service closely related to human beings as regional ecological carrying capacity. Seven consumable-direct ecosystem services were chosen to assessment ESECC, including food supply, biomaterial supply, fuel wood supply, freshwater supply, sewage purification, waste gas absorbability, and solid waste accommodation (Cao et al., 2015). From the view of balancing of supply-consumption of ecosystem services, impacts of ecosystem service consumption patterns on ECC were studied (Yan et al., 2012; Zhen and Du, 2017). The dynamic changes of ECC were explored under the condition of the tempo-spatial heterogeneity of ecosystem services and the ecosystem services consumption characteristic driven by social and economic development in open systems (David et al., 2010; Zhen et al., 2010).

ECC evaluation based on the conception of HANPP. With the population and economy increasing, the relationship between humans and environment remains far from being sound. Nearly 40% of potential terrestrial net primary productivity is used directly, coopted, or foregone because of human activities. We are interested in human use of bio-resource both for what it implies for other species, which must use the leftovers, and for what it could imply about limits to the number of people the earth can support (Vitousek et al., 1986). Human beings should not take up all the net primary production of the terrestrial ecosystem (NPP0), but must leave part of the material and energy to nature and other species (NPPt) to maintain ecosystem services. Thus, the concept of human appropriation of net primary production (HANPP) was proposed. Global HANPP was 15.6 PgC yr^-1, accounting for 23.8% of NPP0 (Haberl et al., 2007). Global human appropriation of the net primary production of the ecosystem doubled in the 20th century (Krausmann et al., 2013), and it has been approaching the “threshold” of global biological resources (Nash et al., 2017). Some studies have paid attention to the human availability of ecological carrying capacity. For examples, the World Commission on Environment and Development suggested that 12% land suitable for biodiversity conservation should be deducted when calculating ecological carrying capacity (WCED, 1987). Dai et al. (2006) distinguished the concepts between the total regional ecological carrying capacity and the available carrying capacity for mankind.

Due to the openness of ecosystem, the complexity of ecosystem resilience, and the scarce of knowledge in aspects of the impact of regional trade flows on ECC (Hubacek and Giljum, 2003), and the interaction between human activities and nature, it is still difficult to quantify the ECC scientifically, which hampered it from being an effective management tool reliably.

In future, research on ECC should improve the evaluation index system,strengthen the temporal and spatial dynamics assessment of ECC, couple the ecosystem services flow factors into the evaluation system, and clarifying the interaction between the ecosystem process and human activities (Zhao et al., 2019)．A technical system and management platform integrating ECC assessment, early warning and regulation should be build and development into an effective tool in environmental management (Feng et al., 2017). We should take effective measures to protect and restore area where is approaching to even surpass the threshold of overload, to making surely the impacts of human activities within the threshold of environment and ecosystem can withstanding.

In recent years, a series of research programs on ECC in countries along the BRI region has been launched, and some progress has been made. Organized by the Resource Ecology Research Committee of the China Society of Natural Resources, this special issue (SI) on the ecological carrying capacity and green development in the countries along the BRI is published. This SI includes 14 papers covering ecological carrying capacity and green development in the BRI countries from different perspectives, and they can be classified into four complementary families. The first group has five papers which assess ecological carrying capacity. This group of papers contributes to the methodology development and application of ECC assessment and evaluation, and can also contribute to the sustainable use and management of ecological resources of the BRI region and the countries under consideration. Zheng and Xu (2019) analyzed water carrying capacity of Belt and Road initiative countries using virtual water theory. Jin and Xu (2019) assessed flows of livestock products between China and BRI countries from the perspective of ECC. Zou et al. (2019) analyzed the ECC in central Asian countries and the impact on animal husbandry development. The current situations, problems and prospects of ECC accounting in China are proposed for future consideration (Li et al., 2019b). Ecological carrying capacity can also be evaluated on the basis of the ecosystem’s provision and human consumption of ecosystem goods and services. Thus, the land carrying capacity in China’s Xilin Gol grassland in Inner Mongolia was accounted by the perspective of food consumption (Yang et al., 2019). Each of these papers used different sources of data and analytical methods and in different spatial temporal scale. The second group has three papers which focus on ecosystem services. The consumption of ecosystem goods and services is assessed for the BRI region as a whole (Zhang et al., 2019), and the Laos (Liang et al., 2019b). The ecosystem products and services of community forest and its role to poverty alleviation in Nepal was evaluated by means of participatory management (Dhruba Bijaya et al., 2019). The third group has five papers which provide clear pictures on resources use and environment change in the BRI region. Grassland productivity in the eastern part of Mongolia (Li et al., 2019a), grassland ecological pressures in Kazakhstan (Wen et al., 2019), and the climate suitable for travel in the cross border regions between China and Russia (Zhou et al., 2019) were evaluated by remote sensing and GIS (Geographical Information System) method. The methods applied for spatial temporal pattern and change of grassland productivity and pressure evaluation can be of significance for other studies, and the analysis on potential impacts of economic development on the environment is helpful for guiding future investment. The effects of foreign investment on CO₂ emissions in Laos (Xiong and Wang, 2019) was measured with an interdisciplinary viewpoint. The ultimate goal of ECC study is to promote sustainable development. Chen et al. (2019) emphasized the importance of sustainable agriculture for realizing sustainable development goals in the BRI region by analyzing multi-sources. The fourth group only have one paper focus on the big data on the desertification in China (Liang et al., 2019a). This SI is expected to strength and deepen the knowledge of ecological management in the BRI region, and will be helpful for decision-making on the harmonized relationship between man and nature.