The literature review was performed to collect all available published works that address the main objectives of the study. The overview of the ES research in Bulgaria necessitates the finding of all literature sources that contributed to the development of this research. The review was conducted in several steps: 1. Identifying the geographical institutions in Bulgaria; 2. Identifying geographers (researchers) who work in ES-related fields; 3. Searching through institutions’ official web pages and authors’ scientific profiles (Scopus ID, WoS researcher ID; Google Scholar profile; ResearchGate profile) for ES-related papers. However, apart from the research papers published in scientific journals, many grey literature sources reveal valuable information for the overview we were aiming at. For instance, several methodological guidelines publications play an important role in shaping ES research in the country. It is also important to reveal the full scope of the Bulgarian geographers’ publication activity to analyze its strengths and weaknesses and make recommendations for future development.

The initial search resulted in a total of 145 publications. Most of them we authored or co-authored by geographers but there were also some publications with no geographer contribution. A specific part of these publications is related to the MAES process including the methodologies for mapping and assessment of ecosystems in Bulgaria and follow-up publications concerning the mapping of ecosystems itself. As there are methodologies and mapping-related publications with geographers’ participation, we decided to keep all publications from this group. All other publications without a geographer as an author or co-author were omitted from the list. The remaining publications were the subject of preliminary screening for relevance to the topic of the study. Those that do not have any kind of ecosystem services research were also omitted from the list. Thus, the final number of publications to be reviewed was reduced to 123.

To systematize and characterize the content of the reviewed papers, a special database with a standard nomenclature was constructed (Nedkov et al. 2022). A template in the form of a structured MS Excel sheet that enables easy and convenient data entry was developed. Its design follows the common structure of such templates and comprises five main parts: 1) general characteristics of the publication; 2) ecosystem services; 3) methods; 4) case study; 5) mapping. When possible, the variables in the table were entered using the binary numerical system, otherwise, inputs were made in the form of text (Nedkov et al. 2022). The first part reflects the second specific aim of the study to analyze the publications on ES and the distribution of the authorship among the geographical units in the country. It contains bibliographic data for the publication (authors, journal, year, DOI, and journal or publisher), data for the authorship (distribution between geographical entities and international collaboration), the language of the publication, the case study, the purpose, and the dimensions of the study. The second and third parts are designed to support the achievement of the third specific objective of this review to analyze the distribution of ES and the methods used in the reviewed studies. In the second part, for each publication, we count the number of ES and their distribution among the three main ES groups (provisioning, regulating, and cultural). The supply-demand side of ES research is an important aspect of any assessment therefore the template contains columns that indicate if the study concerns supply, demand, or both. In the third part, the publications were reviewed according to the three main methods (biophysical, social, and economic) used for ES assessment. The fourth and fifth parts contain data entries reflecting the fourth specific objective to analyze the spatial aspects of the ES studies with a focus on the maps and the mapping methods. The fourth part considers the case studies of the publications: presence of a case study, number of case studies, location, and scale of the case study. The fifth part is dedicated to the mapping of ES. The presence of maps in the publications is an important indicator to evaluate the geographical aspects of the studies. The use of an established ES mapping approach is an indicator of the integrity of the internationally accepted methodologies. The next five categories enable us to define the methods used to develop the ES maps. The predefined categories include matrix approach, expert score, use of statistical data, spatial proxy methods, and modeling methods. There is also an option to select the “other” option and add the name of the method that does not fit the previous options. Columns for mapping of ecosystem types and ecosystem condition are also presented in this part.

The analyses of ES research in the reviewed publications were performed in three main aspects: i) the character of the ES research; ii) revealing what ES is/are studied; iii) the methods applied in ES studies. The character of the ES research was analyzed on one hand the presence or absence of specific ES and research on supply/demand and on the other hand by revealing if the study is focused on supply, demand, or both. The presence of the specific ES and supply/demand is an indicator that the publication presents results of real ES assessment. The papers that do not fulfill the requirements of this indicator could be either conceptual or editorial which cover general aspects of ES research or studies on broader environmental topics that mention ES in text but do not perform real ES research. The ES in the review template were reduced to the three main groups of services (provisioning, regulating, and cultural) following the CICES classification (Haines-Young and Potschin 2018). The analyses were focused on the distribution of the ES groups in the studied publications and the number of individual ES studied in them. The methods applied in the ES studies were systematized and analyzed using the most common classification of the methods into three major groups: biophysical, social, and economic).

The spatial dimensions of the ES research in Bulgaria were analyzed in two main aspects: the case studies in the publications and the mapping of ES. The presence or absence of a case study is an important indicator of the character of the ES studies, namely the geographical element in them. The mapping is a very important aspect of ES research as maps are very useful for raising awareness about areas of ecosystem goods and services supply and demand, and to provide information about interregional ES flows (Burkhard and Maes 2017). The analyses on mapping aspects of the publication were performed at two levels: for the whole set of publications and for research papers. Metrics about the number of publications with ES maps, general ES methods used in ES mapping, specific methods used for ES mapping, and methods applied with the matrix approach were calculated.

The literature search resulted in 123 publications in total that were authored or co-authored by Bulgarian geographers. Their list consists of various types of peer-reviewed publications and grey literature sources published between 2007 and 2023. Ten major types of papers were identified during the review process (Table 1) where research and review papers have the highest share of 40% (49 publications). One type of publication is not presented in the reviewed publications—norm documents (Category 7). Category “10”, other types of publications refer to one discussion paper and one blog post. The highest number of papers were published in 2017 and 2021 (Fig. 1A). Those papers refer to two national projects that ended in the reference years—the nine national mapping projects under the MAES framework (ESMERALDA project) and the project Heritage.BG. The only year without any publications for the studies period of 2007–2023 is 2009. Out of the 123 publications 43% are indexed in Scopus or Web of Science while most of them are not indexed 57% (Fig. 1C). Analysis of the accessibility of publications shows that 66% of all documents are available online with open access, 19% are with paid access, and 15% are counted as grey literature since they do not appear online (Fig. 1E). All publications are published in English, Bulgarian, or both languages (Fig. 1I). The English language is the primary publishing language with 69% of all, followed by Bulgarian language (21%). Only 10% of the publications are bilingual. The top three of the most impactful journals or publishers are the Journal of the Bulgarian Geographical Society with 16 papers, One Ecosystem with 13 published papers, and Clorind Publishing House with 11 published methodologies (Fig. 1J). Six out of ten are periodical journals, two are book series, one is conference proceedings, and another is a publishing house.

Figure 1.

Characteristics of the publications and authors A number of publications per year B top 10 Bulgarian lead authors C number of indexed publications D number of publications with international collaboration E number of accessible publications online F number of publications per institution G global distribution of foreign co-authors H number of authors per publication I language of the publication J most contributing journals and publishers to the ecosystem services research in Bulgaria. Abbreviations (1F): NIGGG-BAS—National Institute of Geophysics, Geodesy and Geography - Bulgarian Academy of Sciences, SU—Sofia University “St. Kliment Ohridski”, SRTI-BAS—Space Research and Technology Institute - Bulgarian Academy of Sciences, UVT—“St. Cyril and St. Methodius” University of Veliko Tarnovo, SWU—South-West University “Neofit Rilski”, ShU—“Konstantin Preslavsky” University of Shumen.

Results of the affiliation of the lead author show that most publications were authored by researchers from the National Institute of geophysics, Geodesy and Geography at the Bulgarian Academy of Sciences (NIGGG-BAS) with 41% (56) and Sofia University with 26% (35), while only 9% (12) were authored by foreign authors (Fig. 1F). Only one of the studied institutions—Shumen University, has zero authors publishing about ES. The most active lead author is Nedkov with 19 publications, followed by Assenov with 10. Five out of the ten most active authors are affiliated with the NIGGG-BAS, four with Sofia University, and only one is affiliated with a non-geographical institution (Fig. 1B). Results on the international collaboration show that only 18% (22) of the publications have foreign co-authors, and the majority of the publications 82% (101) were authored entirely by Bulgarian scholars (Fig. 1D). The geographical distribution of the foreign authors shows that the most collaborative country is Germany with 15 publications, followed by Spain with six publications (Fig. 1G). Most of the foreign co-authors are based in countries in Europe such as Finland, Italy, Sweden, Romania, and France. Authors from only two countries besides Europe appear in the publications—those are Australia with four publications and the USA with three publications. The most frequent number of authors per publication varies between one (24 publications) and three (20 publications). The highest number of authors per publication is 83 and appears for only one publication, which is out of the range (Fig. 2H).

Figure 2.

Characteristics of the ecosystem services research A assessed ES types in studied publications B distribution of the assessed ES C assessed supply-demand D ES assessment methods E mapping methods F matrix mapping methods G international case studies H Bulgarian case studies I distribution of all publications with ES maps between assessment methods J distribution of research papers with ES maps between assessment methods.

The results on the character of the ES research showed that 64% (80), of the reviewed publications have assessed ES whereas the rest 36% (43) had no ES assessment. Further analyses of the latter group revealed that some of these papers are either editorial papers (five) from special issues containing ES assessment papers (such as Nikolova et al. 2021a) or conceptual papers (nine) that deal with methodological aspects of ES research. Another group contains five publications that present ES assessment without focusing on particular services. Four publications deal with ecosystem mapping (such as Hristova and Stoycheva 2021; Petkova et al. 2022) and assessment of ecosystem conditions (Uzunov et al. 2017). The largest group (21) consisted of publications that deal with broader environmental topics mentioning ES in the text but do not perform real ES research. Although these publications are evenly distributed between the four periods, their share during the first two is much larger (33% and 31%) than in the third (12%) and the fourth (17%) period. The number of studied ES (provisioning, regulating, cultural, or a bundle of ES) differs within each study.

The most studied ES group is the regulating (50% overall), followed by provisioning (38% of all papers) and cultural (35% of all papers) ones (Fig. 2A). The publications that focus only on one of the ES groups number 36 and those assessing all three ES groups number 30. The publications with two ES groups number 13. Almost all of them contain the combination provisioning-regulating, while only one contains the combination provisioning-cultural. The distribution of the number of ES in each paper varies from mostly studying one ES (mainly regulating ES) to 38 in two methodological publications that address 38 ES (Karamfilov et al. 2017; Kostov et al. 2017). The biggest difference is within regulating ES, which ranges from one to 21 ES within one paper (Fig. 2B). Research papers mainly study a bundle of ES from each ES group, varying from five up to 21 ES. The studies of the ES supply-demand side have a well-defined trend for assessing mainly ES supply (59%). The lack of research on ES demand, as well as studies assessing both supply and demand, is evidence knowledge gap (Fig. 2C). Biophysical methods are the most used ones for ES assessment (62%), while the social and economic have a similar presence in the studied papers (19% each) (Fig. 2D).

The analyses showed that 86 (70%) of the publications have clearly defined and spatially outlined case studies while the rest 37 (30%) had no case study. The case studies are predominantly in Bulgaria but there are also several case studies in other countries such as Germany, USA, Finland, North Macedonia etc. (Fig. 2G). The scale of the case studies is predominantly local with 57 of the publications dealing with such cases. Regional and national level case studies number 12 and 14 respectively. There are only two multiscale studies in Bulgaria combining local–regional and local–national scales, and one international study combining all three scales. Apart from the multiscale studies, six local studies cover between two and four separate sites. Thus, the whole number of studied sites in the above-mentioned 86 publications became 109. 97 of them are in Bulgaria and the rest are located in ten other countries. The local scale case studies in Bulgaria form several visible clusters that can be divided into three groups (Fig. 2H). The first one contains only one but the far biggest cluster located in the area around the Central Balkan National Park combining 19 cases. The second group contains three moderate size clusters combining between nine and 12 cases. The third group contains 12 clusters combining between two and five cases. The highest number (20) of local-scale case studies were municipalities. Natural objects were chosen as research objects in 18 case studies, which comprise seven mountain areas, four lakes, and six other natural objects. River basins as a spatial unit were used in 16 case studies, while urban areas (city/town) were used in 13 cases.

The publications with ES maps were 58 which comprise almost half (47%) of the reviewed sources. The figures in the research paper are slightly different with 59% of the papers presenting some kind of ES maps and 41% without ES maps. The respective distribution between the main groups of ES assessment methods showed a significant difference in the number of publications containing ES maps (Fig. 2E). The studies that use biophysical methods are predominantly with ES maps, which are presented very well in the research papers with 78% share of the publications containing ES maps (Figs 2I, 2J). The respective distribution for the publications applying social methods is almost equal but for research papers, the share of no ES maps publications is even higher which is opposite to the general trend defined for the total dataset. The ES maps in the publications applying economic methods are even lower with only one-third presenting by maps the results of their studies.

The ratio between publications with and without maps changed during the different periods. During the first period, the publications with no maps are predominant (83%). During the second period, the predominance is not so pronounced (62%). The third period is characterized by balance (52% to 48%) while during the fourth period, the share of no ES maps publications is no more predominant (47%).

The ES mapping methods applied in the studies were evenly distributed among the publications with a slight dominance of the matrix approach, expert score, and spatial proxy methods (Fig. 2F). The matrix approach and the spatial proxy methods were the most popular for the total dataset, while for the research papers, the expert score replaced the spatial proxy methods as the second best used. Modeling methods are also well represented in the total dataset but their use in the research papers rapidly declines. The studies with ecosystem types and ecosystem condition maps comprise about 25% of the mapping publications and such maps are presented predominantly in research papers.

The case studies used in ES assessment reveal three important topics for discussion. Firstly, the municipality and river basin appear as the most appropriate spatial units that ensure a clear extent and policy relevance. Both of them have clearly defined boundaries (administrative or natural) which ensure a strong rationale for the study and comparability to other studies. The river basin is the main spatial unit in water management, while the municipalities are the main objects of the local government. Therefore, both units ensure a direct relationship between research findings and decision-making. Secondly, natural objects such as mountains, lakes, and karst areas appear as quite popular case studies. However, they vary a lot in the dimensions and the definitions of their extent. Their boundaries are mostly case-specific and hardly correspond in extent to other studies. Such studies can have high research contributions but their uptake necessitates further work to be adjusted to the policy needs and decision-making. The methodological frameworks for mapping and assessment of ES give some guidance but their development at regional and local level is still in the beginning (Nedkov et al. 2021). Thirdly, the nature protection areas are rarely selected as case studies which is an obvious research gap. The recent studies on the provision of ES by the nature heritage in Rila mountain (Silvestriev et al. 2021) and the habitat maintenance ES in mountain-protected areas (Borisova et al. 2023) are positive examples but much more research in this direction is needed.

The predominance of local case studies and the limited number of multiscale studies show that the researchers prefer to focus on relatively small areas that can easily be provided by data and expertise. The efforts during the national mapping under the MAES process resulted mainly in methodological publication which applicability in the real research is still to be proved. The limited number of research papers in high-ranked journals presenting results from the national mapping (the only exception is Nedkov et al. 2017) is clear evidence of the need for further development of these methodologies. Therefore, despite the undoubtable achievements during these 20 years ES research in the country is still a relatively new field that is still to be fully come of age.

The share of publications containing specific geographical elements in the form of ES maps and spatial analyses has gradually risen from the first to the fourth period. Two main reasons are behind this tendency. The first one came from the MAES process that stimulated the mapping studies all over Europe and the Bulgarian researchers were among the most active which was proven by the results in the MAES barometer indicating the leading role of the country (Burkhard et al. 2018a). The second one is the development of the spatial aspects in ES research inspired by the activities in the Ecosystem Services Partnership (ESP) working group on mapping and modeling of ES where the Bulgarian geographers were also present (Burkhard et al. 2013; Crossman et al. 2013). However, even in the last period the share of the publications without maps remains high (47%) which is not reasonable for the geographical community. Therefore, more attention on the spatial aspect of the ES research by the Bulgarian geographers is needed.

The disproportion between the ES supply and demand studies is another important finding of our review. Although it corresponds to the general trend defined in international studies (Campagne et al. 2020) the lack of studies focused on the demand side, especially on the supply/demand balance, remains a significant research gap. In the Bulgarian case, this disproportion corresponds also to the predominance of biophysical methods at the expense of social and economic. The latter (especially the social) are more appropriate for the studies on ES demand. This is a clear message on the need for the development and application of more social methods in ES research by geographers. This need is even more pronounced taking into account the existence of the social-economic branch of geography.

The findings from the review allow us to identify both achievements and research gaps in the ES studies conducted by Bulgarian geographers. This enables us to define the main research priorities of the coming years which can trace the future directions of ES research in the country. The main priority should be directed towards the development of the spatial aspects in the methodological frameworks for mapping and assessment of ES. This includes problems such as the identification of relevant scales for the implementation of particular indicators and the methods for their quantification, a precise statement on the spatial scale and extent of the ecosystem services assessment, a clearer and more precise definition of the spatial units used in the assessment, application of more spatially explicit indicators including those for assessment of ecosystem condition, include a sensitivity analysis to understand the effects of varying spatial resolutions, include potential trade-offs between different spatial scales and their implications on ecosystem service, etc. The achievement of such objectives will ensure both scientifically robust mapping of ES and relevance to policy and decision-making.

The next priority should be in the use of GIS-based tools for mapping ES which are developing very fast in recent years. Particularly the development of specialized ES tools such as InVEST, ESTIMAP, and ARIES ensures a variety of spatial options for a wider range of individual ES (Nedkov 2018). The tendency for models’ integration in the ES research is another aspect that can be used also to incorporate more social and economic methods and further development to ES demand and supply demand studies which would contribute to overcoming this research gap. Another important priority should be the improvement of the publication quality and increase of the papers published in highly rated indexed journals. This could be achieved by focusing on internationally recognized topics, application of modern methods, and abandonment of the grey literature as a publication option.

The authors have declared that no competing interests exist.

No ethical statement was reported.

This work was partially supported by the Bulgarian Ministry of Education and Science under the National Research Programme “Young scientists and postdoctoral students-2” approved by DCM 206/07.04.2022.

Conceptualization: SN, HP, VS. Data curation: HP, SN, IA, VS. Formal analysis: SN, VS, HP, YY, IA. Funding acquisition: HP, SN. Investigation: YY, HP, VS, IA. Methodology: VS, SN. Resources: HP, VS, SN. Supervision: SN. Validation: IA, HP, VS. Visualization: SN, HP. Writing - original draft: SN, VS, HP. Writing - review and editing: SN, VS, HP.

Stoyan Nedkov https://orcid.org/0000-0002-0052-9815

Vanya Stoycheva https://orcid.org/0000-0001-7354-1711

Hristina Prodanova https://orcid.org/0000-0003-2453-8975

Ivaylo Ananiev https://orcid.org/0009-0006-9924-5463

Yordan Yordanov https://orcid.org/0009-0004-6535-5926

All of the data that support the findings of this study are available in the main text or Supplementary Information.