The principle of biological productivity requires that the ecosystem services to be included in accounting must be renewable services that can be continuously produced by biological production. This means that any services produced by pure human labor, as well as the services produced by physicochemical processes in the natural world should be excluded from GEP accounting. All ecosystem services are produced by the biological production processes of ecosystems, including primary production and secondary production. Biological production constitutes the foundation of the values of ecosystem services (Zhang et al., 2021), and can be classified into direct biological production and indirect biological production. Ecosystem services produced by direct biological production mainly include food being produced by plants through photosynthesis (e.g., grains and fruits), raw materials (e.g., timber and rubber), and other agricultural, forestry, animal husbandry, and fishery products produced predominantly by secondary production (e.g., meat, eggs, milk, and honey). The remaining ecosystem services are mostly produced by indirect biological production. That is, they are produced indirectly by biological production action for humans through improving the biological, physical, and chemical conditions of ecosystems. For instance, the earth's water supply is not directly produced by photosynthesis or secondary production. However, ecosystems can alter the underlying surface conditions of terrestrial water cycling to conserve water. Biological production plays an indispensable role in terrestrial water cycling. The production of fresh air is usually accompanied by a sound eco-environment, vegetation also creates a favorable living environment through the actions of carbon fixation and oxygen release as well as the provision of negative air ions. Soil retention services are attributable to rainfall interception, rain wash mitigation, root soil fixation, and other effects of ecosystems.

The principle of human benefits requires that the ecosystem services to be included in GEP accounting must be the final services producing direct benefits for human well-being, or ecosystem services that reduce the costs of human protective inputs. In contrast, the indirect services that are produced by ecosystem intermediate processes and services that require no human protective inputs should be excluded from GEP accounting (Zhang et al., 2019). In addition, services that are without economic scarcity, are insensitive to human activities, or those that are free of human control should all be excluded from GEP accounting (Gao et al., 2019). When ecosystem functions and processes are affected or damaged, the quality of ecosystem services is lowered. In this case, humans will have to invest extra in ecological protection. Specific regulating services and supporting services reduce the costs of human inputs and conform to the principle of human benefits; thus, these services should be included in GEP accounting. For instance, the CO₂ changes in the atmosphere exerts no direct effect on human well-being, but they indirectly affect human economy and society through the greenhouse gas effect. Ecosystem carbon fixation offsets human carbon emissions and yields benefits for humans. Soil erosion causes a loss of soil resource stock, but the hydrological regulation action of ecosystems reduces the costs of human soil erosion control. Species conservation produces no direct benefits to humans, but low biodiversity endangers ecosystem security. Therefore, close attention should be paid to species conservation by humans by setting up nature reserves and taking other measures.

The principle of production territoriality requires that the ecosystem services to be included in GEP accounting must be ecosystem services produced and provided by local ecosystems. This means that ecosystem services that are produced non-locally and circulated and transported to a different destination should be excluded from GEP accounting. Depending on the spatial characteristics of production and consumption, ecosystem services can be classified into in-situ services, non-local transported services, and global shared services (Fisher et al., 2009). The term ‘in-situ services' refers to ecosystem services, the production area and benefitting area of which are basically consistent. For instance, the negative air ions produced by a forest through photosynthesis ionization exist for a very short period and cannot benefit populations living in distant areas through atmospheric diffusion. That is, the ions can only exert their effects in the same forest or in the immediately surrounding areas. Spiritual and cultural services can be realized only when people visit ecosystem landscapes. The term ‘non- local transported services' refers to ecosystem services, the production area and benefitting area of which are inconsistent. These services present a definite circulation-transport relationship. Ecosystem services with non-local transport characteristics generally include water supply, purification of atmospheric pollution, purification of marine pollution, and pelagic fishery. The term ‘global shared services' refers to globally inclusive ecosystem services that are free of geographical restrictions. For instance, the species conservation services of nature reserves maintain the stability of the entire natural ecosystem and benefit human society at large.

The principle of current increment requires that ecosystem services to be included in GEP accounting must be the product and service increments provided by ecosystems for humans within a certain period. In other words, only products and services produced by ecosystems in the specified current period can be included in accounting; ecological resource stocks and service flows already converted to stocks should be excluded. ‘Stocks' are defined as the aggregate amount of services at a particular point in time, while ‘increment' is defined as the change of service flows within a certain period (Xie et al., 2001). Ecosystem service flows are derived from ecological resource stocks, and unconsumed ecosystem service flows can turn into or increase ecological resource stocks. Ecosystem service flows can accurately reflect the benefits and well-being provided by ecosystems for humans, while ecological resource stocks may not be able to mirror the current real status of ecosystems in specific cases. Ecological resource stocks are the aggregate amount of services formed by the natural world on a long-term basis. In certain cases, a very large ecological resource stock may end up having an undesirable real ecosystem status and providing extremely poor services. For instance, a severely degraded alpine grassland only offers very limited ecosystem services, but nevertheless stores a huge amount of peat. In contrast, a mature economic eucalyptus forest has a large forest stock, but offers poor ecosystem services in terms of soil retention and biodiversity maintenance. All ecological resource stocks should be excluded from GEP accounting. For instance, biological species resources constitute ecological resource stocks, but only the annual increments of species populations are to be included in accounting as ecosystem service flows (Hu et al., 2018). Ecosystem services (e.g., water supply) have both stock and flow attributes, so they can be included in GEP accounting as service flows (Liu et al., 2019).

The principle of actual effectiveness requires that the ecosystem services to be included in accounting must be actual services provided for humans. This means that potential but unrealized services, as well as services that fail to meet benchmark conditions should be excluded from GEP accounting. According to this principle, the following two categories of ecosystem services should be excluded from GEP accounting: The first category covers potential, unrealized services, e.g., existence value, option value, bequest value, and other non-use values. Services of this category either have not been realized or may be realized in the future, and generally refer to the stock values of ecosystems. The second category covers services that fail to meet benchmark conditions. For specific accounting subjects, a natural background exists even in the absence of ecosystems; only services beyond this natural background can be counted as services provided by ecosystems. For instance, even extremely degraded bare land in the natural world will yields runoff regulation and soil retention effects. Certain ecosystem services must meet preconditions before they occur. The natural background in the absence of ecosystems or the precondition for the actual occurrence of ecosystem services constitutes the benchmark for the accounting of ecosystem services. The accounting of ecosystem services should exclude circumstances where benchmark conditions are unmet; otherwise, the values of ecosystem services will be overestimated.

The principle of physical metrizability requires that the ecosystem services to be included in GEP accounting must be services with a definitely measurable physical quantity under current scientific and technological conditions. This means that GEP accounting should exclude both: 1) Services whose physical quantity cannot be measured by physical, chemical, biological, or other scientific or technological methods; 2) Services without suitable physical quantity indicators. First, GEP accounting must take a relatively precise physical quantity as a premise for value quantity accounting. Under current scientific and technological conditions, the physical quantity of most ecosystem services can be measured, estimated, and simulated with relatively high precision. However, the spiritual and cultural services provided by ecosystems (e.g., cultural heritage, spirit, inspiration, religion, arts, and complex nostalgia) usually lack physical manifestations (e.g., chemical, biological, or physical). Therefore, their value quantity accounting can only be performed via subjective and biased methods such as the simulated market method. In particular, the simulated market method measures values using an artificially constructed hypothetical market, which in turn yields accounting results with relatively low effectiveness, reliability, and objectivity. Thus, ecosystem services with immeasurable physical quantity should be excluded from GEP accounting. Additionally, ecosystem disaster mitigation is an ecosystem service that contributes tremendously to human well-being. However, because of the lack of basic data and methods required by accounting under current scientific and technological conditions, accurately measuring the physical quantity of ecosystem disaster mitigation is impossible.

The principle of data availability requires that the ecosystem services to be included in GEP accounting must be services whose physical quantity can be simulated or calculated using actual monitoring data of ecosystems. For this reason, ecosystem services, the physical quantity accounting of which can only be performed in reference to the empirical parameters of other areas (because of a lack of actual monitoring data), should be excluded from GEP accounting. The ecological monitoring data used for physical quantity accounting generally include service monitoring data, resource inventory data, and scientific investigation data. Preference should be given to the routine service monitoring data of various industry sectors and the data obtained from regular resource inventories (e.g., routine meteorological, hydrological, and environmental monitoring, forest resource inventories, land and resource surveys, national geographical surveys, and pollution source censuses). In the case of specific ecosystem services, because of the lack of ecological monitoring data required by physical quantity accounting, physical quantity can only be estimated by referring to empirical parameters, which leads to inaccurate and highly arbitrary accounting results. For instance, urban landscaping has an obvious noise reduction effect, which is closely related to vegetation types, plant characteristics, noise source positions, and affected populations. However, regarding parameters measuring the noise reduction effect of urban landscaping, there is often a lack of routine monitoring data. Ecosystem services without routine monitoring data of these parameters can be measured with data obtained from a single-round monitoring survey. However, such data should be excluded from GEP accounting when supplementary monitoring surveys are unavailable. When actual monitoring data are available, they can be included in GEP accounting.

The principle of harmlessness requires that the ecosystem services to be included in accounting must be services that are beneficial or harmless to ecosystem functions, or they must be services capable of sensitively reflecting human protection, restoration, or damage activities. This means that services which may harm the ecosystem's bearing capacity should be excluded from GEP accounting. In fact, specific ecosystem services may even harm ecosystems themselves after exceeding a certain scale and scope. Normally, ecosystems can provide fresh air, clean water, and other ecological products for humans by accommodating, absorbing, and degrading pollutants. However, when the pollutants that humans discharge into the environment exceed the ecosystem bearing capacity, ecosystems will inevitably be harmed. For instance, services such as seafood fishing, timber supply, and grassland grazing all pose the problem of exceeding ecosystem bearing capacity, as over-fishing, over-logging, and over-grazing are bound to harm ecosystems. When such services are included in GEP accounting, the accounting results will increase in line with increasing pollutant discharge, seafood fishing, timber logging, and livestock farming, which contradicts the normal logic of eco-environmental management.

In addition, GEP accounting is a continuously developing and updating process. Some principles should be observed under any circumstances, and this study refers to such principles as “solid principles: no-violation in any circumstance”. The solid principles include biological productivity, human benefits, production territoriality, current increment, actual effectiveness and harmlessness. The other principles that can be broken through are in a category referred to as “soft principles: evolve with research progress”. In this paper, soft principles include physical metrizability and data availability. With the innovation of model methods and the improvement of data monitoring systems, some ecosystem services that cannot be accounted for at present will be able to be achieved and be subjected to data acquisition and physical measurement in the future.

Among the provisioning services provided by ecosystems, mineral raw materials, gene resources, solar energy, tidal energy, water power generation, wind power generation, ocean shipping, inland river shipping, land reclamation by flushing, sediment transport, pelagic fishery, pleasant climate, and human inputs in agricultural product production (Costanza et al., 2017) should all be excluded from GEP accounting. First, mineral raw materials (e.g., coal, petroleum, natural gas, and salt resources) are non-ecological natural resources produced in the natural world under long-term geological processes. They involve no biological production action; they also gradually decrease or damage the eco-environment over the course of development and utilization by humans. Their unsustainability deviates from the principles of biological productivity and current increment. Gene resources constitute a major category of resources among ecosystem services (USEPA, 2009), and current scientific and technological conditions determine that their physical quantity and value cannot be accurately measured. The formation of gene resources through long-term evolution and accumulation in the natural world does not agree with the principle of current increment. Second, services such as solar energy, tidal energy, water power generation (Mitchell and Carson, 1989), shipping (Coffin et al., 2013), and land reclamation by flushing involve no biological production action, and are mainly derived from pure physicochemical actions. They represent the economic products produced by human labor. Pleasant climate is mainly related to geographical location, which is completely outside the realm of human control. The goal of pelagic fishery is the cross-regional supply of products, which contradicts the principle of production territoriality. Pesticide and fertilizer inputs in agricultural product production are non-biological production inputs (Zhang and Gao, 2020), which should be excluded from GEP accounting.

Regulating services refer to the benefits human society gains from the regulatory effects of ecosystem processes (MEA, 2005). In this paper, the components of specific indicators to be excluded from accounting are discussed according to the regulating service categories of ecosystems (Table 3). Regarding the accommodation and purification of pollutants (e.g., the purification of water by natural rivers and lakes and the absorbance of atmospheric pollutants by plants), the bearing capacity of natural ecosystems is limited. Once pollutants exceed the maximum accommodation and degradation capacity of ecosystems, ecosystems will be considerably damaged. Moreover, pollutants in the water and the atmosphere are usually diffused with the flow of media. The purification of pollutants is essentially completed on a very large spatial scale, which violates the principles of harmlessness and production territoriality. Therefore, these pollutants should be excluded from GEP accounting. Ecosystems have a temperature regulation function, by which they lower temperature and mitigate the “heat island effect” in hot summers. However, this function does not come into play in cold winters in the north. Furthermore, humans cannot benefit from the noise reduction service in empty, depopulated areas. It is also difficult to accurately measure the populations or scope of those benefitting from noise reduction in urban-rural areas. Ecosystem disaster mitigation and pest control are major regulatory functions of ecosystems that produce huge benefits for humans. However, it is difficult to obtain data on ecosystem disaster mitigation and pest control under current scientific and technological conditions. Moreover, because they are affected by many natural and human factors, it is impossible to accurately measure disaster mitigation and pest control. The dissolution and absorbance of marine greenhouse gases and the temperature regulation function are services that are dominated by physicochemical actions, which involve no biological production action; their scale is effectively out of human control.

Cultural services generally include tourism and recreation services, and specific spiritual and cultural services such as aesthetics, spiritual and religious value, inspiration, and the complex nostalgia. Tourism and recreation services are usually accompanied by the combined action of biological production and human production activities. In actual accounting, the construction costs of purely artificial recreational facilities should be excluded, as tourism and recreation services violate the principle of biological productivity. However, natural landscapes can be included in GEP accounting, as their values can be measured by land and real estate premiums (Chinese Academy of Environmental Planning, Research Center For Eco-Environmental Sciences, CAS, 2020)^①(① Chinese Academy of Environmental Planning, Research Center for Eco-Environmental Sciences, CAS. 2020. The technical guideline on gross ecosystem product (GEP) (1.0 version). (in Chinese)). Other services such as spiritual and religious value, inspiration, and the complex nostalgia cannot be accurately measured in terms of physical quantity or value quantity; thus, they should be excluded from GEP accounting.

The supporting services of ecosystems represent the supporting actions provided by ecosystems through their functions and processes for the production of other ecosystem services; these generally include soil formation, nutrient cycling, primary production, and lifecycle maintenance (MEA, 2005). Supporting services are essential for the maintenance of ecosystems themselves, but most supporting services do not directly produce benefits for human well-being. Instead, they indirectly serve or benefit humans through the support of provisioning services, regulating services, and cultural services. In other words, the contributions of supporting services to humans are largely embodied in provisioning services, regulating services, and cultural services (Müller and Burkhard, 2012). Moreover, supporting services have complex associations with other ecosystem service categories, and one specific supporting service may simultaneously support multiple other services. In this case, including intermediary services which produce no direct human benefits in GEP accounting will unavoidably cause the repeated calculation of GEP values, resulting in the overestimation of GEP. Plant pollination and nutrient cycling services are essential to produce food and raw materials, but the human benefits they produce are ultimately embodied in the agricultural and forestry products that are directly used by humans. Primary production is a self-maintaining function and an intermediary service of ecosystems. The service is either directly consumed by humans in the form of food, or indirectly utilized by humans in the form of timber, industrial raw materials, or other raw materials. The portion of primary production that is directly or indirectly utilized has already been included in GEP accounting via other subjects. In the unutilized portion of primary production, the fixed CO₂ has been included in accounting under the subjects of ecosystem carbon fixation (and so forth) and has become part of ecological resource stocks as the increment of biomass. Soil formation serves humans indirectly via agricultural, forestry, and animal husbandry products. It represents a stock formed by the weathering of rocks and the long-term accumulation of organics; thus, it violates the principles of human benefits and current increment.

According to prior studies, the values of ecosystem services, besides the direct and indirect use values provided by ecosystems for humans, also include existence value, option value, bequest value, and other non-use values. Existence value means that ecosystems not only provide services, but also comprise an inner value. ‘Option value' refers to the ecosystem value optional for future use. ‘Bequest value' refers to the ecosystem value available for use by future generations. These non-use values of ecosystems are all potential, unrealized services, from which humans have not benefited so far. Thus, they violate the principles of human benefits, current increment, and actual effectiveness. See Table3 for full details.

Full consideration should be given to the relationship between biological production and human production activities when investigating ecosystem services. Under the premises of clearly distinguishing biological production from human production and establishing the dominant role of biological production, biological production alone can be included in GEP accounting. In cases where it is impossible to distinguish biological production from human production, but the dominant role of biological production has been established, the service can be included entirely in GEP ac counting when the proportion of human production is ignored. Examples in this regard include climate regulation, flood regulation, change of water quality, and change of air quality. In cases where it is impossible to distinguish biological production from human production, but where the dominant role of human production has been established, the service should be excluded from GEP accounting. Examples in this regard include purely artificial landscape tourism, ocean shipping, and inland river shipping.

To distinguish and identify the circulation-transport relationship and spatial differences of ecosystem services, efforts should be made in the following two aspects: First, the circulation-consumption relationship of ecosystem services with obvious non-local transport characteristics (e.g., water supply, purification of atmospheric pollution, and purification of marine pollution) should be fully considered. Water supply has prominent basin-wide characteristics. The headwater and upstream areas of rivers conserve water and supply clean water for downstream areas. Therefore, when calculating the provisioning services of local water resources, foreign water input from upstream areas should be deducted if relevant data are available. The purification of atmospheric pollution mainly relies on atmospheric diffusion, dilution, deposition, and other physical processes, all of which need to be completed on very large spatial scales (instead of being provided by local ecosystems). The purification of marine pollution is similar to the purification of atmospheric pollution. The pollutants discharged into the ocean are not only purified in local coastal waters but are also diffused, diluted, deposited, and degraded in the wider ocean. As far as these services are concerned, their spatial circulation should be prioritized in the accounting of regional GEP values. Second, global shared services are universally beneficial, and the areas they benefit are not confined within certain spaces. Therefore, global shared services should be included in GEP accounting. For instance, ecosystem carbon fixation reduces CO₂ concentration in the atmosphere, and affects greenhouse gases worldwide through atmospheric diffusion. The species conservation services provided by nature reserves maintain the stability of the entire natural ecosystem; they also benefit human society at large. Therefore, GEP accounting should include both the in-situ services produced and provided locally, as well as global shared services, while non-local transported services should be excluded.

It is also necessary to reversely consider the harms caused by ecosystem services to the ecosystem bearing capacity. For ecosystem services that may harm ecosystems beyond a certain scale and scope, three strategies can be adopted in GEP accounting. First, other subjects can be preferentially adopted as substitutes. For instance, the service of purifcation of pollutants only targets artificial wetlands and other projects built by humans. Services such as the purification of atmospheric pollutants and water by natural ecosystems are excluded from GEP accounting, and are replaced by clean water, fresh air, and other ecological products. Environmental quality is adopted as a substitute for pollutant discharge to measure the purification of environments by ecosystems. Second, in cases where it can be ascertained that a service would not affect or harm ecosystems themselves, the service can be included in GEP accounting. For instance, the growth of forest stock and the carrying capacity of pasture land can both be scientifically and accurately calculated. Timber logging and supply capable of satisfying the sustainable growth of forest stock and livestock farming within the theoretical carrying capacity of natural pasture land can be included in GEP accounting, while those exceeding the carrying capacity should be excluded. Third, ecosystem services whose bearing capacity cannot be scientifically estimated should be excluded from GEP accounting. For instance, estimating the bearing capacity of marine fishery resources is impossible. In addition, unplanned fishing and over-fishing are widespread all over the world, resulting in the gradual exhaustion of marine fishery resources on a large scale. This explains why marine fishery resources should be excluded from GEP accounting.