Economic valuation of ecosystem services in Africa

. Ecosystems provide vital services that improve and support livelihoods and human well-being. Consequently, scientific research on ecosystem services (ES) has increased, over the past two decades globally, as well as in Africa. This study provides an overview of existing economic ES valuation methods in Africa using the Web of Science databases. The results highlighted that South Africa recorded the high number of ES valuation publications. The most evaluated ES category was provisioning then regulating services. In terms of economic valuation methods, the market price was the most popular, followed by the contingent valuation and the choice experiment methods. Recommendations are provided for future research in this filed.


Introduction
Ecosystems provide humans a diversity of services known as "Ecosystem services" [1]. They are generally classified into provisioning services such as food and raw materials, regulating services such as water and climate regulation, supporting services such as soil formation, and cultural services such as recreation and tourism. These categories are linked directly to human well-being [1,2]. The benefits provided by ES are crucial in maintaining human survival and contribute to poverty alleviation [2,3]. Therefore, The loss and disruption of these services, particularly in rural areas of developing countries [4], can negatively affect human well-being [5].
Africa hosts a high number of Least Developed Countries [6], and more than 50 percent of its rural population depends on ecosystem services [7]. However, the ecosystems of this continent are facing various anthropogenic pressures [8]. Common drivers are rapid population growth, increasing demand for food, water and energy, climate change, and overexploitation [9]. This shows the importance of assessment and valuation of such services in safeguarding and managing the loss of ecosystems [9,10].
Several approaches and methods have been developed [11]. According to [12,13], three main approaches can be distinguished: economic methods for estimating monetary values, biophysical methods for mapping and modeling ES, and socio-cultural methods for understanding social values of ES.
Many studies were conducted to assess and review the valuation of ES at different scales, such as: McDonough et al. [14] at the global scale, Rendon et al. [15] for Europe, Schuhmann and Mahon [16] for the Caribbean, Jiang [17] for China, Pittock et al. and Alamgir [18,19] for Australia, Van den Belt and Blake [20] for New Zealand, and Perez-Verdin et al. [21] for Mexico.
In Africa, several reviews were carried out by Vihervaara  Expressing the value of ES in monetary terms in Africa can help to face the challenge of biodiversity loss through the following two ways [9], [28][29][30][31]: -Raise awareness about the relative importance of ES; -Improve decision making on the use and management of ES.
In this context, we conduct a literature review on the economic valuation of ES in Africa. It summarizes the existing studies of ES research and provides key issues for future research. This study focuses on the publication trend between 2005 and 2019, the methods that are highly used and the ES category that is most frequently performed and with which methods.

Methods
Based on [32], the search process for our literature review consists of four phases: data collection, selection criteria, content analysis, and statistics analysis (Fig. 1).

Data collection
The ISI Web of Knowledge database (http://www.isiknowledge.com) was used for our data collection search using numerous combining terms: "Ecosystem service*" AND "Africa*" AND (valu* OR "Economic analysis" OR "Monetary assessment") in the titles, the abstracts, and as keywords. We gathered all English academic publications from 2005 to 2019. The starting date was the publication year of The Millennium Ecosystem Assessment (MEA) report [33]. Since then, a significant number of ecosystem services research have been published [12].
This ISI website search allowed us to identify 389 studies over the past 15 years. We suppose that our review covers a majority part of the literature, given that the ISI Web of Science indexes 11,877 major journals from 81 different countries across the world [34].

Selection criteria
The abstracts, titles, and methodology sections of the original database were reviewed to decide the final papers to be considered in our economic valuation of ES in Africa. In this study, we retained only the papers that: -Were conducted in an African country; -Provided an ES that can be categorized according to the MEA framework [33]; -Used an economic valuation method.
This set of criteria resulted in 28 scientific articles that were summarized and reviewed in detail (Appendix A lists all case studies with their characteristics).

Content analysis
The content of the 28 ES studies selected was classified according to the following aspects: -Publication year; -Geographic location; -Ecosystem type; -Ecosystem service category; -Economic valuation method.

Statistics analysis
Descriptive statistics was used in the statistical analysis of the information collected. The results are, generally, organized and presented as a number of case studies and publications for each feature mentioned above. All analysis and graphs were conducted using Excel software. Geographic distributions of economic valuation studies were mapped using ESRI ArcGIS 10.7.
3 Results and discussion 3 Results and discussion 3 Results and discussion

Publication trends
The results show an increase trend in the number of publications since 2012 (Fig. 2). An average of one publication per year was noticed from 2005 to 2012, and it increases to an average of three Step 1:

Data collection
Original database WoS search: "Ecosystem service*" AND "Africa*" AND (valu* OR "Economic analysis" OR "Monetary assessment")

Data classification
Publication year Geographic location Ecosystem type Ecosystem service category Economic valuation method Step 2: Selection criteria

Results presented in the section below
Final database n = 28 papers N=390 papers Step 3: Content analysis Step 4: Statistics analysis publications per year after 2012. No publication has been selected for 2005, 2006, and 2009. This global progress can be explained by the release of several initiatives and projects around ES and biodiversity, including the Millennium Ecosystem Assessment (MEA), The Economics of Ecosystems and Biodiversity (TEEB), Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES), and the Ecosystem Services Partnership (ESP). They provide crucial information on the ES concept and attract significant attention of the researches from all over the world [12], [35], including Africa.

Geographical distribution
As illustrated in Fig. 3, the spatial distribution of economic ES studies shows that more than 50% of the studies have been conducted in South Africa and Kenya alone (13 and 6, respectively). The remaining papers are distributed between Mozambique, with a total of four studies; Tanzania, with three publications and a single study per country for Senegal, Ghana, Madagascar, and Benin. The first African study related to ES issues was conducted in South Africa in 2005, using a spatial mapping of provisioning and supporting ES [27]. Since then, it has become the leading African country in terms of ES articles published between 1987 and 2017 [36].
Moreover, Southern and Eastern Africa gather almost all case studies, while there were no papers from the northern part of the continent, even in the review provided by [27]. Although it is difficult to explain the lack of research in these countries, it might be due to the limited human and financial resources or the exclusion of non-English studies [37].

Ecosystem types
Within the 28 publications, eight ecosystem types were investigated (Fig. 4). The most dominant types were wetland and forest ecosystems (5 studies for each), followed by marine, agroecosystem, and urban (3 studies for each). Freshwater and dryland are also considered (2 studies for each). The rest of the studies cover more than one ecosystem (e.g., Grassland, woodland, wetland). However, one study [38] did not clearly define the type of ecosystem and it uses the geographical name of a region (The Orange River Basin in the Lesotho Highlands). We should note that some ecosystems are difficult to classify. For example, the urban forest is not clear whether to be categorized into a forest or an urban ecosystem. Consequently, future studies should adopt an international framework for a consistent defi MEA or TEEB) [17].

Ecosystem services categories
In terms of ecosystem services studied (Fig. 5), provisioning s followed by regulating (12) and cultural services (11). H least attention from researches (2).
Overall, 19 of 28 studies assessed only a multiple services, of which the combination of common. [39] and [40] applied trade-offs and synergie this type of analysis was lacking in those studies based Regardless of the number of ES, more than half of the 82 ES categories are provisioning services (food, water, raw materials, etc.), confirming the findings by provisioning ES were the most services studied fact that many African countries depend directly on this category of services to meet their basic needs [25], [41][42][43]. , provisioning services were the most assessed (13), followed by regulating (12) and cultural services (11). However, supporting services received the assessed only a single ES. The remaining nine studies examined multiple services, of which the combination of provisioning and regulating services was the most offs and synergies between different types of ES. However, in those studies based on one category. e than half of the 82 ES categories are provisioning services (food, water, raw materials, etc.), confirming the findings by [27], [36] who reported that were the most services studied. This particular attention can be explained by the depend directly on this category of services to meet their basic

Economic valuation methods
This study identified nine main valuation methods, which include revealed and stated preferences ( Table 1). The number of papers using each method is shown in is the most used, followed by the contingent valuation and frequency). The number of ES categories by each economic method is experiment and benefits transfer were applied the other methods. In contrast to [44], [45], the market price method has been frequently used to evaluate the provisioning services. The next sections focus on presenting two of the most used methods per approach.

Approach Method
Revealed preference  This study identified nine main valuation methods, which include revealed and stated preferences using each method is shown in Fig. 6. In general, the market price contingent valuation and the choice experiment (used at a similar by each economic method is shown in Fig.7. Choice applied to assess large categories of services compared to the market price method has been frequently used to two of the most used methods per approach.

Description from reviewed studies
Derives economic values of ES using prices in markets.

Production function
Relates the marketed output to the inputs used in production. Ecosystem service is considered as an additional input.

Replacement cost
Uses costs as a proxy for the value of ES, including methods based on the costs to substitute services, the costs of avoiding damages, or the travel and time costs for access to ecosystems.
Estimates economic values of ES using the prices of similar properties, usually a house.
Transfers ES values from existing benefits from the literature to another context.

Regulating Cultural Supporting
Summary of the reviewed economic valuation methods

Revealed preference methods
Seven revealed preference methods, found in seven studies, were identified, of which the market price dominates the database (Table 1, Fig. 6). It directly provides the value of services from markets [46], [47]. For example, [48] calculated the value of grazing service based on the commercial value of the grass. In addition, four papers adopted the travel cost method to provide the value of recreational activities. As an example, to estimate the recreational value of a freshwater system, [49] calculated the time it takes to get to the site and the time spent there.

Stated preference methods
Contingent valuation and choice experiment are the two stated preference methods founded (Table  1, Fig. 6). The contingent valuation measures people's willingness to pay (or to accept) [50]. For instance, this method was used to estimate the economic value of non-market benefits of an urban forest in Ghana [51]. Furthermore, choice experiment asks respondents to choose their preferred option between different attributes [9]. [52] investigated the Mozambique population' preferences for water, firewood, and land, applying the choice experiment method. Therefore, stated preference methods had received more attention than the revealed preference methods. This result is consistent with [45], [53].

Combination of methods
Four case studies used a combination of economic methods. For example, the ES valuation of [39] was based on market pricing and benefit transfer methods. We also found that three studies used economic and mapping methods. For instance, [54] combined economic valuation (market price and benefit transfer) and mapping assessment to provide the values of key northern Mozambique's marine ecosystem. Integration of economic and biophysical models needs also to be considered as in [55].

Limits of the review
This study was limited to English peer-reviewed studies and it is based exclusively on the databases of the Web of Science. Thus, all conclusions presented here should be interpreted with caution.

Conclusion
The main objectives of this study were to present an overview of economic valuation of ES in Africa and to offer some recommendations for future research. Twenty eight studies conducted in different parts of the continent were analyzed. Most the studies was conducted in South Africa and focused on services provided by wetland and forest ecosystems. Provisioning services was the most assessed category and the market price was the frequently cited method.
Recommendations for future research on ES in Africa should: -Improve the knowledge of ES and their incorporation into decision-making and the management of natural resources; -Combine multidisciplinary approaches, especially economic and mapping methods; -Explore trade-offs and synergy between different ES; -Adopt an international framework for a solid classification of ES; -Assign equal attention to different ES category, especially those that are neglected (e.g., cultural diversity, knowledge); -Conduct further economic valuation studies in Africa, especially in the northern region of the continent using English language.