3.1. Collaboration Status of China, Japan and South Korea
First, we analyzed the collaboration strength quantitatively among the three countries in terms of the biodiversity research, which is presented in Table 2
. We collected bibliometric data (Web of Science) from the years 1996–2015 and analyzed the co-author relationship. China collaborated with South Korea on 56 articles and with Japan on 134 articles. Japan and South Korea collaborated on 66 articles. We converted this into percentages of the total number of articles for each country. South Korea collaborated on 7.9% of the total articles with China and on 9.3% with Japan, which is a relatively higher ratio than that of the collaboration with the other countries by both Japan and China. Japan produced 3.1% of its total articles with South Korea and 6.4% with China. China produced 1.8% of its articles with South Korea and 4.3% with Japan (Table 3
). South Korea exhibited a high percentage of collaboration with China and Japan. We also analyzed the number of authors who worked on collaborated articles in Table 4
. Japan had the lowest average number of authors working on collaborated articles, and China had the highest average number of authors working on collaborated articles.
We subsequently investigated the collaboration status of China, Japan and South Korea in relation to the rest of the world through a collaboration map using a co-authorship network. Japan showed the most diverse collaboration with other countries in the first period (Figure 1
). Japan collaborated with 49 countries. China collaborated with 36 countries, and South Korea collaborated with only 13 countries. The United States of America (U.S.A.) indicated the strongest collaboration strength with all three countries among any other countries. In collaboration frequency, as well, Japan indicated the most frequent collaboration with other countries.
In the second period, a significant improvement is detected in China and South Korea (Figure 2
). Japan continues to reveal the highest variety of collaboration (86 countries). However, China exceeded Japan in the frequency of collaboration. The number of countries that collaborated with China is only slightly lesser than Japan (83 countries). South Korea did not collaborate with South East Asian countries in the first period, but collaborated with most of South East Asian countries, such as Thailand, Indonesia, Philippines, Vietnam, Bangladesh, Malaysia and Singapore, in the second period. This is noteworthy because it implies that South East Asia is becoming an increasingly prominent region in the marine biodiversity field. Particularly, the southern Philippines and central Indonesia are in the 10% richest locations for fish, corals, snails and lobsters [30
]. Aquaculture in Thailand and Vietnam are also noteworthy in that the aquaculture industries in Thailand and Vietnam have become major industries requiring serious consideration for sustainable development [31
]. Moreover, economic collaboration with three countries in East Asia and South East Asia is increasing. In this context, a deeper analysis of this area will lead to significant knowledge benefit. In addition, all OECD countries are now collaborating with China, Japan and South Korea. One notable thing is that eight of the countries among the 14 countries that collaborated only with Japan are small island countries. This appears to be because of the influence of the geological location of Japan.
3.2. Analysis of Research Trends Using Topography Maps
We analyzed research trends using a research topography map using the author keywords of publications. We extracted phrases from the title and abstract of all articles for bibliometric mapping analysis. We divided the WoS database into two periods (first period: 1996–2005; second period: 2006–2015) to observe changes in research trends. In the VOSviewer topography map, the color of each area is dependent on the number of items at any given point. One characteristic of VOSviewer is that the size of the distribution does not match the range or diversity of distribution. This means that if some keywords are distributed far from the main cluster, other clusters become smaller in the whole map because VOSviewer zooms out to show all clusters of the topography map. This topography map is designed to provide an overview of the general structure and to emphasize important areas of the map [26
The research trend in China has greatly expanded in 20 years. In the first period, only production-related research formed the major cluster. This cluster includes topics such as content, unit and report. These topics are closely related with aquaculture. The other research cluster also represents aquaculture-related topics, such as capacity, uptake and grain size. This means that aquaculture research was a major topic in the first period. Genetic research was only beginning (DNA sequencing in (a)). Compared to the first period, there was a dramatic increase in research diversity in the second period.
Four major research clusters are found in the second period (Figure 3
). The first one is a cluster related to South China. This region is closely associated with mass extinction research. A cluster of words, such as black shale, oil and deposit, was also made. This cluster is clearly related to basic research on resource excavation in the South China Sea. The second cluster is related to bacteria community research. This cluster includes bacteria-related research topics, such as Actinobacteria, strain, bacterial community, Streptomyces and marine sponge. All aforementioned topics are related to bacterial community research. This cluster showed the most vibrant research activity in China. The third cluster is the population research cluster. Population research is foundational research for biodiversity research. Population research is required to identify the specific characteristics of many different areas in the marine environment. The fourth cluster is related to genetics. Haplotype, molecular variance and loci are all keywords related with gene expression. Over 20 years, research on marine biodiversity in China developed and became greatly diversified.
In Japan, several research topics showed vibrant activity in the first period (Figure 4
). Genetic research was a strong topic in the first period. Genetic differentiation, allele and haplotype showed that advanced genetic research was ongoing in the first period. Compared to China and South Korea, this was advanced in the first period. Bacteria-related research was also strong in the first period. Coral research only appeared in Japan, such as coral reef fish, coral reef and rocky reef. This reflected the discovery of the world’s northernmost coral reef in Japan. Research topics included ribotype, proteobacteria and isolate. Additionally, vibrant research was detected in various research topics in the first period. In many aspects, research in Japan was more advanced than China and South Korea in the first period.
Compared to the first period, research in the second period of Japan lost vibrancy in many areas. However, research topics become more advanced than in the first period. Gene flow research was a major topic in genetic research. This means that genetic studies in Japan focused not merely on the allele itself, but allele transfer from population to population. Bacteria-related research also remained a major research topic, but remained focused on bacterial community research. It is noteworthy that research focus moved from the individual to the community.
We would like to note that the decrease in research diversity in Japan in 20 years does not imply a reduction in the amount of or interest in research. It can be perceived as the maturation of technology and focus for more advancement. A deeper analysis is required to identify the reasons for the change in the research trend.
Research in the first period of South Korea indicated that South Korea did not have an appropriate research cluster, except genetic research (Figure 5
). However, there was a significant change in the second period. Four research clusters are found in the second period. First, research on allele and polymorphism created a huge research cluster; second, research conducted for discovering new bacterial strains is also strong; the third important cluster is the one on gene expression and regulation. Fourth and lastly, various ecosystem research created a research cluster.
Genetic research made two big research clusters in the second period: (1) allele and polymorphism research; and (2) gene expression and regulation. It is impressive that marine ecosystem research made a big advancement even though there were no appropriate research topics on ecosystem or marine environment in the first period.
China, Japan and South Korea have common research areas: (1) general diversity; (2) genetic diversity. First, the general diversity research area contains research topics that seek to find new species and determine the traits of marine populations. It also contains research topics related to habitats, the ecosystem and the environment. Second, research topics related to genetic diversity, such as microsatellite, phylogeny and sequencing, are found. Both research areas are inter-connected, because genetic research is necessary for a deeper understanding of the status of the sea and ecosystem.
A research trend analysis of the world was performed for comparing the world and the three countries (Figure 6
). Interestingly, the most vibrant research cluster of the world in the first period included topics, such as forest, conservation, epifauna and integration. This cluster showed us that the environment around the sea and coast was a major research topic in the first period. In addition, deposit and basin also created one cluster of research. Permian, progradation and mudstone are also located close to this cluster. It is a research cluster on the estuary environment. Both clusters are related to environment research around the sea. Genetic diversity was identified as the third biggest cluster in the first period. Bacteria research was diverse, but vibrancy was not high at the moment.
In the second period, population research emerges as a major cluster. The microbial community also became a big research topic in microorganism research. This means that research focus moved from the individual to the community. This is a natural movement that we can identify: the shift of the research focus from the specification of the individual organism to interaction and communication in the community. Conservation and management research became a bigger research cluster than the first period. This cluster contains research topics, such as policy, planner and model prediction. It implies that a systemic approach to protecting the marine environment became vibrant in the second period.
We briefly investigated the top research topics of China, Japan and South Korea to observe specific research topics and found that each country has its own topics in addition to the topics on which it collaborates with the other countries. We attempted to figure out collaboration topics through this analysis. We removed research topics that occurred less than two times from 1996–2015. We did not consider those topics as major research topics. We then generated a country and research-wide top 20 frequently-appearing topics map, indicated in Figure 7
. If keywords appeared with the exact same frequency, we selected all of them. Therefore, some groups have more than 20 topics. In addition, some groups only have a few research topics because topics in this group did not cross the threshold.
China, Japan and South Korea have common research topics about microorganism research (grey color group). It is not surprising that the three countries’ major research topic was microorganism research for 20 years. Bacteria, microbial community, SP-NOV, identification and 16S ribosomal RNA are topics from the field of microorganism research. Sediment research was also of common interest to the three countries.
China and South Korea collaborated on research on the marine environment (purple color group). The aquatic environment, Gigartinales and reef sponge are important parts of the marine habitat for marine organism. Other topics, such as estuarine, CO2 and pathogen, are also closely related to the environment of the sea and coast. From these research topics, we can conclude that China and South Korea collaborated on the general environment of the sea and coast.
China and Japan collaborated on environment research. Anaerobic ammonium oxidation, water quality and restoration revealed that the two countries are more focused on pollution and environment. Deep-sea research topics, such as cold seep and deep-sea sediment, were also revealed. It is noteworthy that China and Japan collaborated with each other despite their political conflicts.
Japan and South Korea shared only one topic in common: bacterial systematics (green color group). Bacterial systematics indicates that both countries were working together for taxonomy research.
We also found country-specific research topics. The research topics were reflective of each country’s endemic circumstances. China indicated a greater number of regional topics. Many region names are found in research topics, such as Daya Bay, Guizhou, Southwestern China, Changjiang estuary and Pearl River estuary. This means that China vibrantly performs region-specific research.
South Korea has a few research topics that are country-specific. However, it is impressive that South Korea is the only country that has a climate change-related topic as a research topic: (cold) tongue El-Niño. In addition, aquaculture-related topics are identified, such as prey concentration and diversicolor supertexta (Haliotis diversicolor, Haliotis supertexta
). Recent research also suggested that lactic-acid bacteria have the potential to become an alternative for anti-biotics [32
Japan revealed clear research topics in terms of country-specific topics, namely deep-sea research. We found several research topics related to deep-sea research, such as hermatypic corals, Suiyo Seamount, Mid-Okinawa Trough, black smoker chimney, Izu-Bonin Arc and thermophilic bacterium. Given Japan’s contact with the deep trench (Japan Trench), which is a part of the circum-Pacific belt via the Mariana Trench, it is only a natural result that deep-sea research should be a unique research topic in Japan. This area is a valuable research area not only for biodiversity, but also for earthquake research.