0 Introduction

Dialectical materialism holds that everything has two sides. Ecosystems can provide services for human well-being with no exception, and can produce both positive and negative services. With the acceleration of urbanization process, the malpractices in the process of urbanization development are increasingly apparent. With the severe situation of tight resource constraints, serious environmental pollution, and ecosystem degradation, the concept of ecological civilization must be established, which respects, conforms to, protects nature, and takes a sustainable development path. To alleviate the negative impact of urbanization and improve the sustainability of cities, we should take ecosystem services (ES) and disservices as the basis, and consider the services and disservices provided by urban ecosystem in a balanced way. Although there are many researches and reviews on ES, there are few researches on ecosystem disservices (EDS) in China. It is one-sided to ignore EDS to study ES, because firstly, the ecosystem will produce disservice at the same time, thus we should face up to the problem and see the objective existence of the problem in the research process; secondly, ignoring EDS will limit the service provided by the ecosystem and human well-being; thirdly, ignoring EDS means development of the methods to alleviate EDS will be restrained, which is detrimental to human welfare; finally, only focusing on ES for the assessment of ecosystem imbalance without considering EDS cannot maximize the function of ES. Therefore, it is necessary to study EDS.

There is no unified definition about the concept of EDS. The existing research on EDS mainly focuses on the forest ecosystem, urban ecosystem, wetland ecosystem, and other large-scale areas, while research on the small and medium-sized park green space ecosystem are rarely involved. Therefore, this paper takes urban park green space as the research object, and studies urban park green space ecosystem disservices (PEDS). First of all, it puts forward the concept of urban PEDS, analyzes the causes, influencing factors, and basic types of urban PEDS, and further points out the main problems of urban PEDS. Secondly, it points out that it is necessary to balance the service and disservice provided by urban park green space and explores the necessity of disservice evaluation of urban park green space ecosystem. Finally, it concludes that we should study urban park green space from a comprehensive perspective to provide guidance for park planning, design, and management.

1 Concept and Connotation of EDS 1.1 Concept of Ecosystem Service (ES)

ES are all benefits that human beings directly or indirectly obtain from the ecosystem^[1]. It refers to products, resources, and environment that a healthy natural ecosystem and its components provide to support and meet the needs of human survival and development^[2], which includes four categories, namely, supply services, regulation services, support services, and cultural services^[3].

1.2 Concept of Ecosystem Disservices (EDS)

The concept of EDS can be traced back to some viewpoints in the 19th century. Early studies based on the concept of ecosystem damage such as agricultural damage and health risk are related to negative effects, but were not discussed in the framework of EDS^[4]. One of the earliest definitions of EDS was proposed by Lyytimäki and Sipilä^[5], who defined EDS as "ecosystem function considered to be averse to human welfare". The concept of EDS changed over time, so there are a wide range of definitions, such as "the function or nature of ecosystems that causes harmful, unpleasant or unwanted impacts", which was proposed by Lyytimäki^[6], or harmful consequences of ecological change caused by inadequate ES. In 2015, von Döhren and Haase^[7] proposed the definition of "loss of biodiversity". Wang et al.^[4] summarized EDS as "the function, process or attribute of the actual or perceived negative impact of ecosystem on human welfare". The research of EDS has been in-depth, and the understanding of its concept has been comprehensive (Table 1).

As mentioned above, EDS is a concept opposite to ES.In this paper, EDS can be understood as a kind of benefit of the process and function that results in actual or potential negative impacts on ecosystem and human welfare due to the action of ecosystem itself or human activities involving ecological elements. EDS can be either generated by the ecosystem or received by the ecosystem, i.e., the EDS produced by the ecosystem, or the unique EDS that affect human well-being and act on the ecosystem. The interpretation of its concept can be summarized as follows:

1) EDS is the concept opposite to ES. For example, carbon sequestration and oxygen releaseare ES in the ecosystem, while some greenhouse gas emissions such as carbon dioxide, methane, or N₂O are EDS in the ecosystem.

2) EDS is directly related to ecological features or functions and is directly generated by the ecosystem. Some EDSs, for example, connect to species that endanger human health (pathogens, parasites).

3) EDS may be triggered by ES due to indirect effects of management. The disservices brought about by the side effects of human activities are managed by human beings on the functions and processes of the ecosystem, so that the ecosystem produces disservices for the well-being of human beings, which are considered as EDSs. For example, health problems after pesticide spraying are a disservice to the formation of agro-ecological management (a direct effect of management); the weed resistance invasion after pesticide spraying is an EDS induced by management (indirect effect of management).

4) Whether an ecosystem produces ES or EDS depends on the views of the stakeholders being considered, as well as on specific time and space. The same function or species can provide ES and EDS to different individuals at the same time. For example, plants provide shade, reduce temperature, and increase humidity in ES, while plant pollen is another group of allergens.

1.3 Influencing Factors of EDS

By definition, EDS is mainly from the direct generation of natural systems and human intervention in natural systems. However, ecosystem will also produce disservices while providing services. For example, carbon fixation and oxygen release formed by photosynthesis of plants are services, but at the same time, plants will also generate EDS that release carbon and produce volatile organic compounds (VOCs). Another example is plants have the function of conserving soil and water, while they need irrigation to maintain plant growth, and the water irrigation demand is an EDS. On the other hand, in order to obtain more services, human beings will act on the ecosystem, thus inducing a series of EDSs. For example, in order to prevent and control pests so as to promote agricultural production, spray pesticides are used to suppress pests, whose threat to human health is an EDS.

Although natural and artificial intervention will naturally produce EDS, pure natural disasters and human behavior cannot be defined as EDS. As for natural system, direct natural disasteris not EDS, while natural disaster triggered or intensified by biological process is EDS^[4]. As for human interference, simple crime is not categorized as EDS, but private space induced crime in natural system is an EDS.

2 Classification of EDSs

Previous attempts to classify EDSs rely on preestablished ES classification. For example, Ma et al.^[8] introduced the term "supply and regulation of EDS" to consider soil erosion in agricultural ecosystems. Price^[9] used "supporting and regulating EDS" in forestry. Based on the origin and consequence of EDS, other researchers put forward hybrid typology. Escobedo et al.^[10] classified EDS into monetary cost (economic cost caused by EDS), social hazard (impact on human health and fear), and environmental pollution (impact on intrinsic ecosystem attribute). Lyytimäki and Sipilä^[5] classified EDS according to its origin (social, socio ecological or ecological) and affected social actors (individuals, communities or humans). Shackleton et al.^[11] classified EDS according to its impact on aesthetic and cultural issues of economy, physical and mental health or human well-being. Through literature investigation, Vaz et al.^[12] proposed a detailed typology of EDS: the first is health EDS, the second is material EDS, the third is safety EDS, the fourth is culture and aesthetics EDS, and the fifth is recreation EDS.

There are different perspectives on the classification of EDSs. This paper aims to classify EDSs on the basis of relevant research. According to occurrence, EDS can be divided into two categories: actual perceived EDS and potential EDS. According to the nature of EDS, EDS can be divided into ecological disservice (EDS1), economic disservice (EDS2), health disservice (EDS3), and social and cultural disservice (EDS4).

2.1 Classification by Occurrence or Absence

The classification of EDS can be measured from two dimensions, namely, occurrence and absence, as shown in Table 2. The generalized EDS should include the potential disservices as well as the actual perceived disservices of the ecosystem. It is meaningful to study the actual perceived EDS and potential EDS, which not only recognizes the existence of EDS, but also proposes corresponding countermeasures against EDS, so as to reduce or inhibit EDS and improve ES.

2.2 Classification by Nature

Since EDS is ecosystem or human beings acting on the ecosystem and adversely affecting human well-being, the emergence of EDS involves several factors of ecology-economic-social-stakeholder, so the classification of EDS can be divided into ecological disservices (EDS1), economic disservices (EDS2), health disservices (EDS3), and socio-cultural disservices (EDS4).

1) Ecological disservices (EDS1): EDS will have a negative impact on the ecosystem structure, process or services provided. Examples of ecological impacts include increased water use by street trees and emissions of VOCs^[10] or greenhouse gas emissions from constructed stormwater wetlands^[13];

2) Economic disservices (EDS2): EDS will also affect the economic structure and process accordingly, such as the damage to infrastructure caused by plant growth and microbial activities, and the maintenance cost related to urban vegetation^[14];

3) Health disservices (EDS3): EDS will also have a negative impact on human health, which is an aspect of life quality. For example, allergies caused by street trees^[15], vector borne diseases in urban wetlands^[16], animal attacks^[17], and the occurrence of criminal acts^[18];

4) Social and cultural disservices (EDS4): EDS can cause negative emotions (anxiety and discomfort) in some people. Examples of this type include animal waste, plant waste^[19], or discomfort and fear caused by densely grown urban green space^[20].

According to the classification of ES, there are also dialectically unified disservices in the ecosystem. Table 3 shows the corresponding relationship between ES and EDS. It shows that the EDSs are not one to one, but cross with each other. It shows that when studying the ecosystem, we should face up to the negative services of the ecosystem, so as to improve the ESs and prevent EDSs.

3 Urban PEDS 3.1 Urban Park Green Space

Urban park green space is one of the main contents of urban construction, which is an important part of urban ecosystem and urban landscape. It is a place to meet urban residents' leisure needs, provide rest, sightseeing, exercise, communication, and hold various collective cultural activities^[21]. As a unique ecosystem in urban ecosystem, urban park, like other ecosystems, is also composed of nonlife part and life part. The nonlife part is composed of two parts: humanized natural environment (such as topography, climate, soil, water, etc.) and artificial environment (such as winter landscape architecture, infrastructure, recreation facilities, etc.) in the park. The life part is composed of three parts: producers (various garden plants, some algae in water), consumers (urban residents, insects, birds, fish, etc.), and decomposers (bacterial and fungal communities)^[22]. The urban park green space is taken as the research object. Our study shows EDS plays an important role in the sustainable development of the ecosystem.

3.2 Concept and Types of Urban PEDS

Urban park green space ecosystem is an artificial ecosystem dominated or controlled by human beings.EDS formed by the park ecosystem is called urban park green space ecosystem disservices (PEDS). The formation of EDS in urban park green space is caused by artificial nature and human activities. On the one hand, it is produced by artificial ecosystems themselves. For example, plants and their pollens in urban parks may cause allergies or poisoning. On the other hand, it is disservice caused by human activities, such as the damage of park infrastructure caused by human activities, and the cost of later maintenance of park infrastructure, as shown in Table 4.

On the basis of the classification of EDS, urban PEDS types can be divided into the park green space ecological disservice (PEDS1), park green space economic disservice (PEDS2), park green space health disservices (PEDS3), and park green space social culture disservice (PEDS4).

3.3 Analysis on Factors of Urban PEDS

Based on the classification of urban PEDS, research on urban PEDS can be analyzed from four aspects: park green space ecological disservices (PEDS1), park green space economic disservices (PEDS2), park green space health disservices (PEDS3), and park green space social and cultural disservices (PEDS4). Tables 5-8 are the analysis of the specific factors.

3.4 Correlation between ES and EDS of urban park green space

The term EDS derives from ES, which are conceptually related. The same ecosystem of urban park green space can produce both ES and EDS. For example, carbon sequestration and oxygen release formed by plant photosynthesis is a service, while plants also generate EDS by carbon release and VOCs. Plants have the functions of soil and water conservation, but irrigation water demand to maintain plant growth is an EDS. On the other hand, in order to obtain more services, human beings will act on the urban park green space ecosystem, which causes a series of EDSs. For example, in order to maintain the urban environment, people will build parks to provide ES, while the economic cost of the later maintenance and management of parks is an EDS.

The attributes and functions of urban park green space ecosystem have both positive and negative effects on human welfare. However, ES and EDS are not completely opposite but rather complementary. Only in terms of their advantages and disadvantages can they be opposite to each other. The types of ESs of urban park green space and the types of EDSs are not one-to-one correspondence but cross-integration, which indicates that the evaluation of the ecosystem of urban park green space should consider both ES and EDS, respect the ecosystem functions and manage them at the same time, and limit the negative impacts but recognize that they also play a positive role.

4 Balanced Consideration of Urban Park Green Space ES and EDS 4.1 Necessity of studying EDS

First of all, theoretically speaking, the study of EDS makes up for the lackin the EDS theory, and is a new research perspective to provide theoretical support for the sustainable development of the ecosystem. Secondly, the relationship between EDS and ES is positive and negative, which can affect the ecosystem at the same time. Therefore, the study of EDS is of great significance for ecosystem protection. In a word, only when the harm of EDS is recognized and the real positive and negative aspects of ES are taken into account, can EDS be identified and reduced, and the benefits of ES and human well-being improved.

4.2 Balanced Consideration of the Services and Disservices Provided by Urban Park Green Space

ES and EDS are integral parts of a system. Only when the EDS is recognized and evaluated in accordance with the service, the relevant concepts of ES and EDS can become an effective tool for urban park green space management. In short, this paper aims to propose a comprehensive assessment of ES and EDS. In order to achieve sustainable development of urban park green space system, it calls for more attention to urban PEDS, rather than ignoring these disservices. With the urban park green space becoming the focus of human life, the management of urban PEDS may become more complex and important. Ignoring the disservice of these ecosystems may seriously affect the general environmental management, especially the management of the urban ecological system. It is suggested to balance the service of the urban park green space ES and EDS to develop a system in an all-round way. It is necessary to evaluate all factors, and urban park green space as an artificial complex ecosystem is no exception.

4.3 Equilibrium Considering Urban Park Green Space System Corresponding Disservice Strategy

There are disservice factors in urban park green space. As long as the corresponding conditions are met, the disservice of urban park green space will emerge. Through research on urban PEDS, the impact of urban green space ecosystem on human well-being can be understood, the ability of the ecosystem to provide social services can be clarified, the short board of its social benefits can be found, all kinds of services provided by the urban park green space can be comprehensively considered, and specific strategies can also be made according to different causes, occurrence status, and impact degree system. Generally speaking, the strategies include the following:

1) Urban PEDS should be controlled through effective management and education of government departments. Urban PEDS will harm and disturb the ecosystem and natural space. Considering that many disservices are often caused by human ecosystem interference first, proper management and education can greatly reduce the risk and perceived risk, which are unlikely to exceed the many health benefits brought by urban ESs.

2) The media publicize and objectively report the harm of disservices to correctly guide the public not to ignore or fear the disservices. The publicity research on ES and EDS is not only to make the public understand the EDSs, but also to reflect on the process of urbanization.

3) A comprehensive evaluation framework should be established, an urban ecosystem framework that combines urban park green space ecosystem services with disservices should be put forward, ESs should be strengthened, EDSs should be reduced or prevented, and the positive effect of park green land ecosystem on human well-being should be maximized.

4) The management of park green space needs the participation of many stakeholders (residents, planners, environmentalists). Stakeholders should identify and weaken EDSs, protect and enhance ESs. For example, stakeholders should avoid the generation of allergy by selecting trees and genotypes without allergic potential, take appropriate protective measures for the prevention and reduction of accidents, and maintain high-quality natural space.

5) More sustainable human activities and lifestyles should be adopted to avoid EDS. By saving resources, protecting the natural environment, and reducing environmental pollution, people can maintain ecological balance, improve life quality and level, and reduce EDSs caused by human factors.

5 Conclusions and Discussion 5.1 Conclusions

This paper puts ES and EDS in the same evaluation status, aiming to explain that the ecosystem can produce both positive and negative impacts. It highlights the importance of EDS evaluation, and proposes that combining EDS with ES is a good way to understand the natural environment, integrate human understanding of the natural environment, and improve the perception of services.

5.2 Discussion

This study aims to help people from all walks of life realize ES and EDS coexist in the ecological system. In order to comprehensively assess the influence of urban park green space ecosystem on human well-being, the influence of urban park green space ecosystem services and disservice should be considered. The relationship between ES and EDS should also be considered from a dialectically contradictory perspective. In this paper, the research also has some shortcomings, for the concept of EDS as well as the interaction between natural and artificial ecological elements are uncertain. Investigation on the equilibrium relationship between ES and EDS has some limitations and one-sidedness. There has been more and more attention on EDS at home and abroad, and relevant researches are emerging. There is no clear standard for its evaluation framework and indicators, and comprehensive studies of various disciplines are still needed. It is very important to study the ecosystem, but the necessity of EDS research is controversial. In the future, it will become a research hot spot in the field of ecology. The following direction will need to be addressed in the future. Firstly, an EDS classification with strong operability and local adaptability should be established through in-depth research. Secondly, to realize the comprehensive evaluation of ES and EDS, it is necessary to further strengthen the quantitative evaluation of EDS. Furthermore, the tradeoff between ES and EDS also calls for further consideration.