Introduction

With the development of open access (OA) publishing and the success of PLOS ONE, a growing number of publishers have created journals termed “mega journals” (MJs) that aim for annual publication volumes of hundreds if not thousands of articles. According to Björk’s research (2015), to qualify as a mega-journal, a title should have a large volume, broad scope, soundness-only peer review and OA based on article processing charges (APCs). It is widely acknowledged that MJs are closely related to a considerable journal size (Busby 2015), which implies that a MJ features “a magnitude larger than an average journal in a particular field” (Zhang 2006), although other subjective criteria are under discussion. The emergence of MJs has attracted the attention of the academic community and received many comments on their impact on scholarly communication, which refers to the system through which research and other scholarly writings are created, evaluated for quality, disseminated to the scholarly community, and preserved for future use (Finlay et al. 2015). However, it is essential to recognize that publishing and citation practices vary significantly between different academic disciplines. Thus, further investigation is warranted to understand the specific impact of MJs in diverse disciplines on scholarly communication within those fields. Moreover, it is crucial to note that there exists notable diversity in the disciplinary composition of various MJs, leading to faster diffusion in certain scientific fields (Siler et al. 2020). This diversity and its implications prompt us to focus our research on a distinct category of MJs termed “discipline-specific mega-journals” (DSMJs). DSMJs are characterized by their substantial publication volume within a particular subject area, setting them apart from the broader scope of previous studies on general MJs.

The assessment of scientific research is an important facet of the entire academic communication system. In the context of research evaluation, citation-based indicators or metrics are applied to measure and compare the scientific performance of various scientific entities. Examples include the use of the journal impact factor (JIF) in the assessment of communication artifacts such as scholarly journals (Borgman and Furner 2002), cross-disciplinary citations in the analysis of interdisciplinary influences of a specific subject (Truc 2022) and citation performance to illuminate the scientific impact of nations (Smith et al. 2014; Li et al. 2023). More specifically, despite the controversy about their capacity to indicate research quality, these citation-based metrics are adopted as tools for authors to make decisions about publication outlets and are used by institutions in review, promotion, and tenure processes (Althouse et al. 2009; Niles et al. 2020; Pontika et al. 2022). Taking journal evaluation as an example, a journal that performs well in an evaluation (likely manifested as highly ranked in a field) benefits itself substantially and reputationally. Overall, when a citation is generated, a “benefit” is generated and “gained” by the cited journal, and its discipline and affiliated country/region in terms of a citation-based evaluation. In bibliometrics, the relationship between the generation and acquisition of citation benefits is often expressed as “referring/citing” and “cited”. Nevertheless, a gap in present studies to date is that MJs in a specific discipline exist as a separate and significant source of citations, which we believe is an important aspect of their impact on scientific communication. A great deal of research has focused on the impact of MJs, but a new concern is how the existence of DSMJs, despite being in a specific discipline, impacts assessment in the scholarly communication system. From this perspective, this study aims to answer the following questions:

  1. (1)

    Who benefits and to what extent from mega-journals with a specific disciplinary scope (DSMJs) at the discipline/region level?

  2. (2)

    Within a specific discipline, if there are journals with considerable outputs, who (which journal) has benefitted from them and to what extent?

Given field differences in citations (Ioannidis and Thombs 2019; Larivière and Sugimoto 2019) and the wide usage of established subject categories such as the WoS category in the evaluation of science, we selected a representative field from the WoS category list and screened DSMJs. The DSMJs in this paper meet the first principle of general MJs, i.e., publication outputs ranking at the top position within a discipline. Therefore, in this paper, the DSMJs studied are all relative to the full list of journals in a specific WoS category. This approach allows us to investigate factors related to MJs’ influence on the citation-based evaluation, with a specific focus on the disciplinary context. In terms of benefits, this study treats citations as benefits generated from DSMJs and “gained” by the cited journal, and its discipline and affiliated country/region that epitomizes the return of evaluation indicators. Moreover, at the journal evaluation level, the JIF score is calculated as the quotient of citations to citable items published in the two preceding years during the index year divided by the total number of citable items published in the two preceding years (Fischer and Steiger 2018). This citation-based metric, which prevails in scholarly communication, offers a measurable way to identify benefits by quantifying the increase in indicators. Bibliometrics provides the possibility of quantifying the impact of DSMJs on academic evaluation systems, specifically citation benefits to other scientific entities, i.e., journals, disciplines and regions. Therefore, a bibliometric and citation analysis method was used to serve the following two purposes: (1) first, to examine what disciplines and countries/regions achieve increased scores from the high rate of citations generated by DSMJs and to what extent and (2) to ascertain what journals benefit from being cited by DSMJs within the same discipline and to what extent.

The remainder of this paper is organized as follows. In Section 2, we provide materials and methods, i.e., the journal selection, the data collection and the equation designed to quantify how DSMJs contribute to JIFs and Immediacy Index scores. In Section 3, the empirical results are introduced. In Section 4, we discuss our results and offer some directions for further investigation. The final section is the conclusion.

Materials and methods

DSMJ selection

“Environmental Sciences” (ES), one of the subject fields from the WoS categories that reported 126,235 citable items and ranked second in all 254 categories in Journal Citation Reports™ (JCR) in 2021, was chosen in this paper. The top five journals with the most outputs in ES were selected and examined. Specifically, according to the number of ‘Web of Science Documents’ in 2021 from Clarivate InCitesTM, we chose the top five journals, i.e., Sustainability (Sustainability), International Journal of Environmental Research and Public Health (IJERPH), Science of the Total Environment (STOTEN), Environmental Science and Pollution Research (ESPR) and Journal of Cleaner Production (JCP). In the preliminary retrieval from JCR, five DSMJs published 43,210 articles in 2021 and 2,547,355 references, with more than 58.95 references per paper (see Table 1).

Table 1 Articles and references from five DSMJs in 2021.
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Data collection

To analyze the citing data, we obtained articles published in 2021 and their corresponding references by retrieving the five DSMJs in “Publication Titles” in the Web of ScienceTM (WoS) core collection, which includes the Science Citation Index Expanded (SCIE), the Social Science Citation Index (SSCI), the Arts & Humanities Citation Index (AHCI) and the Emerging Sources Citation Index (ESCI). We filtered out journal articles published between 2019 and 2021 in references (see Table 2) and exported them with author information (affiliated countries/regions), field information corresponding to journal classification, and journal information. At the discipline/region level, the quantification of the degree of benefits from DSMJs was based on the total number of citations that entities gained, while for a single journal, we chose two representative metrics in the JCR, the JIF and Immediacy Index (II), as the calculated basis. The II indicates how quickly articles in a journal are cited and can provide a supplemental perspective for evaluating journals specializing in cutting-edge research. Therefore, we matched the amount to 13,725 cited journals with their total citations, total articles, counts of citable items, and JCR data, including JIFs and II scores (for calculations, see Eqs. (1) and (2).

Table 2 The yearly distribution of references published in 2019–2021.
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From ClarivateTM, the 2021 JIF is calculated using Eq. (1), and the 2021 II is calculated using Eq. (2).

$${{{\mathrm{JIF = }}}}\frac{A}{B}$$
(1)

A = Citations in 2021 to items published in 2019 and 2020; B = Number of citable items in 2019 and 2020

$${{{\mathrm{Immediacy}}}}\,{{{\mathrm{Index = }}}}\frac{C}{D}$$
(2)

C = Citations in 2021 to items published in 2021;

D = Number of citable items in 2021

Quantitation method

Figure 1 illustrates a reference-citation relationship between papers of the five DSMJs and cited journals. Taking a cited journal, the Journal of Hazardous Materials, as an example, we use Eqs. (3) and (4) to calculate how much DSMJs benefitted the JIF and the II of this journal in 2021, respectively.

$${{{\mathrm{DSMJ}}}}^\prime {{{\mathrm{s}}}}\,{{{\mathrm{contribution}}}}\,{{{\mathrm{to}}}}\,{{{\mathrm{the}}}}\,{{{\mathrm{JIF = }}}}\frac{{{{\mathrm{E}}}}}{{{{\mathrm{F}}}}} \ast 100\% = 10.07\%$$
(3)

E = Citations in 2021 to citable items published in 2019 (1647) and 2020 (3228) from DSMJs;

F = Total citations in 2021 to citable items published in 2019 (13216) and 2020 (35217)

$${{{\mathrm{DSMJ}}}}^\prime {{{\mathrm{s}}}}\,{{{\mathrm{contribution}}}}\,{{{\mathrm{to}}}}\,{{{\mathrm{the}}}}\,{{{\mathrm{Immediacy}}}}\,{{{\mathrm{Index = }}}}\frac{G}{H} \ast 100\% = 10.47\%$$
(4)

G = Citations in 2021 to citable items published in 2021 (1967) from DSMJs;

H = Total citations in 2021 to citable items published in 2021 (18782)

Fig. 1: Illustration of a reference-citation relationship of MJs.
figure 1

We give an example of how citations of the cited journals are contributed by papers of MJs. Limited to the citing data in Fig., the bibliometric analysis is as follows: Journal A wins 3 citations from MJs; Journal B wins 3 citations from MJs; Journal C wins 2 citations from MJs; and Journal D wins 3 citations from MJs. Therefore, MJs contributed 11 citations in 2021 to Journals A, B, C and D during 2019–2021.

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Analysis and results

In our research, citations are a proxy for benefits in quantitative scientific evaluation, which are generated by DSMJs and gained by cited publications, and indirect “beneficiaries”. In 2021, the five DSMJs referenced amounted to 658,687 times to articles published in 2019–2021, corresponding to 13,725 journal beneficiaries. In the following analysis, we first identified the discipline beneficiaries that corresponded to research categories provided by the WoS by presenting the distribution of DSMJs’ citation benefits among subject fields and the contribution to the total citation of each field. Then, the distribution of DSMJs’ citation benefits among countries/regions was displayed to trace country/region beneficiaries. Finally, to identify beneficiaries at the journal level within this ES field and to further quantify the degree of benefits, we utilized the contribution indicators pertain to journal assessment.

Discipline beneficiaries

The cited documents can be divided into different research fields based on the WoS category to which their source publications belong. Those categories reflect the different subject content of the articles and correspond to the journal categories in the JCR. Note that 5073 journals were classified as “multiple”, which means multiple categories. We extracted their “most representative” category, where they obtained a better JIF percentile, for analysis at the disciplinary level. Those 658,687 citations to publications in 2019–2021 were distributed across 247 different categories out of the 254 indexed by WoS (see Supplementary Table S4 online). Specifically, in Table 3, we display the top 20 WoS categories with the number of journals and their citation counts based on aggregation by category. ES, with 172 cited journals and 148,102 citations contributed by DSMJs, is the highest-ranking subject field, followed by Public, Environmental & Occupational Health and Green & Sustainable Science & Technology. In addition to the categories related to the environment, DSMJs cited substantial multidisciplinary literature, accounting for 3.26% of the citation benefits to the “Multidisciplinary Sciences”.

Table 3 Top 20 WoS categories of cited journals ordered by “Citations from DSMJs”.
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In the second step, we performed the contribution calculation, calculating the percentage of citations from the five DSMJs to the total citations received by each category of journals (PCC). Specifically, journals A, B and C were classified as “Energy & Fuels” and indexed in WoS. If papers published in 2019–2021 from A, B and C were cited 1000 times in total in 2021 and among these, 10 were cited by the five DSMJs, the PCC of “Energy & Fuels” was 1%. This calculation is a proxy indicator of DSMJs’ contribution to the citation benefits that each discipline gained. Because the number of citations has been extensively transformed into other various bibliometric indicators in research performance evaluation, it is often applied at the field level. Similarly, we considered the 2021 citations gained by publications published in 2019–2021. There were 4658 journals missing values in the JIF, the II or citable item fields because of the database or our collection process and resulting missing total citations in 2019–2021, which are required for benefits calculation. Thus, the remaining 600,231 references (91.13% of the 658,687 references) and corresponding 9067 journals were left for analysis at the discipline level. In Table 4, ES with 802,169 total citations, of which 18.33% benefitted from five DSMJs, lagging behind the 25.55% gained by Environmental Studies.

Table 4 Top 20 categories with their PCCs ordered by “Citations from DSMJs”.
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Region beneficiaries

The corresponding author’s region is often considered the home base or origin of the study or paper (Ho 2012), and it plays a crucial role in determining the regional impact of the research. In this study, the addresses of corresponding authors available from the WoS database were used to identify regions with which the corresponding authors were affiliated. Note that 63,628 paper entries (9.6% of the 658,687 references) lacked information in the corresponding author address field and were purged from this subsection. In addition, some corresponding authors listed more than one institutional affiliation in the publication data. Specifically, for every 36,136 citations (6.1% of the total), benefits were shared by multiple regions. At the region level, there were 634,669 citation benefits left to be analyzed. Table 5 displays the number of citation benefits that each region received (total and from the five DSMJs), the percentage and the PCC.

Table 5 Top 20 regions of cited journals ordered by the number of Citations from DSMJs.
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According to the affiliation data of the corresponding authors, 595,059 references pertained to 182 countries/regions, and 634,669 regional citation benefits were generated (see Supplementary Table S5 online). Table 5 partly describes the regional distribution of the 595,059 corresponding authors and displays the top 20 regional beneficiaries in terms of the comparative number of citations gained from DSMJs. The share of regions in citation benefits generated by DSMJs was skewed to China and the USA, which gained 26.66% and 11.35% of total DSMJ citation benefits in 2021. Note that when performing the quantification of the degree of benefits, we utilized the InCitesTM dataset to obtain the number of times cited of publications in 2019–2021 belonging to the regions with which corresponding authors were affiliated as “total citations” (updated through the week of January 31st, 2023). We performed the contribution calculation by calculating the percentage of citations from the five DSMJs to the total number of citations of each region recorded by the WoS (PCC). As shown in the PCC column, the four countries (Spain, Turkey, Poland, Portugal) in the top 20 citation benefits from DSMJs were the only four regions whose PCCs were greater than or equal to the average PCC, 1.25%.

Journal beneficiaries within a specific discipline

In the previous section, we showed the contribution of the five DSMJs to the different subject fields, which was highly beneficial to ES. Next, we quantified the contribution of DSMJs to other journals within the discipline to determine the specified journal beneficiaries. From the data collection of 9067 journals mentioned in the previous section, we selected 243 journals belonging to ES. To determine which journals benefitted the most from citations from the five DSMJs, we calculated the two elaborated metrics mentioned above, the “contribution to JIF” and the “contribution to Immediacy Index” (see Tables 6 and 7), to quantify how DSMJs contributed to the cited journals within the same discipline.

Table 6 Top 20 journals whose 2021 II benefitted the most.
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Table 7 The top 20 journals with 2021 JIF scores that benefited the most.
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In Table 6, we show the beneficiaries based on the II. Only citations to items published in 2021 were used for the analysis. Sustainability is the journal that was cited most often, receiving 6416 citations to citable items in 2021 from the five DSMJs. According to Eq. (4), the five DSMJs contributed to increasing the II of Sustainability by 44.46%. This was followed by ESPR, which received 4524 citation benefits from the five DSMJs. The five DSMJs contributed to increasing the II of ESPR by 60.81%.

In Table 7, we mainly focus on the beneficiaries of JIF. Citations to items published in 2019–2020 were used to analyze the change in 2021 JIFs. From the five DSMJs’ citations, STONEN benefited the most, receiving 140,940 citations from the five DSMJs to its items published in 2019–2020. The five DSMJs contributed to increasing the JIF of STOTEN by 22.62%. This was followed by JCP, which received 99,460. The five DSMJs contributed to increasing the JIF score of JCP by 25.71%. Sustainability ranked third with 69,045 citations contributed by the five DSMJs, for which the JIF score was increased by 31.17%.

Self-citations and intracitations of DSMJs

From the above analysis, we can see that the five DSMJs’ citations widely contributed to different regions and disciplines and markedly increased the JIFs and II scores of journals within the same subject field. Taking a further step, we separately calculated the self-citations and coupling citations of the five DSMJs in 2021, as shown in Table 8 and Fig. 2. In Sustainability, there were 22,188 self-citations (accounting for 32.14% of the total citations) and 8.33% from the other four. In IJERPH, 17.99% of the total citations benefitted from self-citation and 6.43% from the other four. In STOTEN, there were 22,093 self-citations and 16,205 citations from others (accounting for 11.5% of the total citations) to items published in 2021. In ESPR, 9821 citations were from self-citation (accounting for 23.28% of the total citations) to items published in 2021 and 11.32% were from intracitations. In JCP, there were 15,241 self-citations (accounting for 15.32% of the total citations) and 15,083 citations from the other four DSMJ citations (accounting for 15.16% of the total citations) to items published in 2021.

Table 8 Self-citation and intracitation in the five DSMJs in 2021.
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Fig. 2: The proportion of self-citations, intracitations and other journal citations in the five DSMJs’ citations in 2021 to items published in 2019–2021.
figure 2

This figure focuses on the five DSMJs’ citations in 2021 to their own items published in 2019–2021, which include two types of notable citations, i.e., DSMJs’ self-citations and intracitations, represented by each slice of the bar chart.

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Discussion

In 2006, the launch of PLoS ONE by the Public Library of Science introduced the mega-journal as a new type of scientific publication, significantly impacting scholarly communication (Lăzăroiu 2017). Specifically, MJs have emerged as vital channels for disseminating academic research, partly owing to their favorable performance in bibliometric evaluations (Solomon 2014; Wakeling et al. 2017). JCR 2020 metrics reveal that MJs contribute a 26.5% article share and excellent distribution in the JIF quartile (Kim and Park 2022). MJs relying on bibliometric measures as proxies for scientific influence and research performance, noticeably impact scholarly assessment processes. However, it is essential to consider that this influence extends beyond the MJ category and affects other areas, including relatively traditional journals and preferred disciplines, with implications that extend beyond merely elevating their positions.

This paper adopts a new perspective to investigate the impact of MJs’ discipline-specific counterparts (DSMJs) on scholarly evaluation, specifically in terms of their citing contribution to citation-based indicators. Based on their shared characteristic of considerable publication volume, we conduct exploratory research with a disciplinary context. Through our analysis of five DSMJs in Environmental Sciences, we demonstrate that the substantial number of articles in DSMJs results in significant citation benefits, benefiting both the journals themselves and their subject fields. Specifically, self-citation in these DSMJs increased citation rates by 15.32–32.14%. Within the same field, 15 journals increased their 2021 JIFs by more than 20% because of citations that benefitted from the DSMJs. It is broadly accepted that because the mean JIF of the journals in each discipline and the range of values that the JIFs take can differ greatly and that JIFs are comparable only within the same discipline, but processes of evaluation sometimes normalize the JIFs for scientific disciplines for comparison (Podlubny 2005; Van Leeuwen and Moed 2002; Van Leeuwen et al. 2003; Sombatsompop and Markpin 2005; de Moya-Anegón et al. 2005). Hence, in the evaluation of science, as revealed in the scientific classification, a rise in the overall JIF of journals belonging to the same discipline enhances disciplinary visibility. The disciplinary scope of our sampled journals is further validated by the fact that Environmental Science received the largest share of DSMJs’ citations according to our analysis of the subject field distribution. On the other hand, the reference distribution also implied the multidisciplinarity of DSMJs corresponding to Environmental Science since there was a considerable share of Multidisciplinary Sciences. At the disciplinary level, for Environmental Science, 18.33% of citations received in 2021 were contributed by the DSMJs.

The number of citations is a key variable often used for comparing scientific impact among countries. This metric is combined in indicators like citations per publication (Vinkler 2010) and the citation impact (the average number of citations a set of documents received) in InCiteTM, aiding the identification of countries and institutions with the best performance for funding strategies (Leydesdorff et al. 2019). Citations generated by the DSMJs are distributed across regions and hence also generate explicit gains for each region based on regional measures of research impact. Because publication authorship plays an important role as a measure of regional research capacity (Smith et al. 2014), we credited a region with a publication if the corresponding authors were affiliated with institutions from that region. Mainland China led with 169,176 citation benefits from DSMJs in 2021. However, a high share of DSMJ citations does not necessarily translate into significant citation benefits. Even though there was some skewness in the regional distribution of citations generated by DSMJs (for example, 26.66 and 11.35% of the references have corresponding authors affiliated with Mainland China and the USA, respectively), these contributions do not stand out due to these countries’ own scientific impact, accounting for 0.92 and 0.5% of total times cited, respectively. Furthermore, 1.59, 1.25, 1.25 and 1.65% of the total citations of Spain, Turkey, Poland and Portugal, four countries with substantial citations, were contributed by DSMJs, while none of the other 16 top regional beneficiaries exceeded the average PCC. The average PCC, 1.25%, also provides a specific grasp of the DSMJs’ citation contribution to all regions as a whole. However, it is important to note that our study has not fully addressed certain crucial issues related to using corresponding authorship as a measure of regional citation beneficiaries. Specifically, this measurement may not fully account for the role of international cooperation within research teams. The regional affiliations of other authors involved in an article, particularly in cases with multiple authors, have not been explicitly considered in our analysis. To gain a comprehensive understanding of regional citation beneficiaries in scholarly evaluation, we recognize the need for future research to explore this aspect and its implications further.

Overall, DSMJs, focused on a specific discipline have generated substantial citation benefits for scholarly evaluation, giving beneficiaries at the above levels increases in citation metrics. However, for the overall scholarly evaluation, the increase in overall citation volume raises concerns about the applicability of the present citation-based measure to compare the impact of papers, researchers, and journals (Fire and Guestrin 2019). Our study found that one scientific entity that receives the greatest share of DSMJs’ citation distribution does not gain the most benefit. For entities facing a large number of citations from DSMJs, their overall citation impact plays a role in stability. Another point worth noting is that, the rapid diffusion of MJs has occurred more in some scientific fields than others (Siler et al. 2020). Our research on distribution at the discipline level and its contribution to subject fields demonstrates that DSMJs significantly impact their respective disciplines, leading to potential bias in the evaluation system based on the advantages of specific disciplines. Although citations are not necessarily the same as scholarly contributions, they are a useful proxy for them (Björk and Catani 2016). Considering the rapid development of mega-journals, their emergence in specific subject areas (Gong et al. 2020), and the broad but skewed impact on citation-based evaluation systems, it is worth rethinking the MJ model from the perspective of what is most conducive to the development of science to improve the quality of academic exchanges, make benefits reasonable, and further address some of their critiques. Of course, empirical support from various disciplines is needed to further validate these concerns.

Conclusion

MJs have become an established part of the scholarly communication landscape, offering a nontraditional publishing model that distinguishes them from traditional journals. Our study is dedicated to understanding their impact on scholarly communication by quantifying the influence of DSMJs on scholarly evaluation. We carefully examined the benefits that many scientific entities derive from the citations generated by these journals at the journal, discipline, and region levels. MJs expanded into specific subject areas, and their broad yet skewed impact on citation-based evaluation systems. It becomes essential to focus on enhancing the quality of academic exchanges, ensuring equitable benefits, and addressing some of the critiques associated with this nontraditional publishing approach.