3. Result
3.1 Basic characteristics of the literature
3.1.1 Quantity of articles and citations
From 1985 to 2020, the number of articles published in the field of seed
dispersal worldwide increased, indicating that this field has received
extensive attention from scholars and has strong developmental potential
(Fig. 2). From the perspective of the number of articles, there were two
turning points in the publication of seed dispersal research. One
turning point was in 1993, when the number of articles published
exceeded 31 for the first time; prior to 1993, the number of articles
published annually grew very slowly. The other turning point was in
2002, when the number of articles published exceeded 47 for the first
time. Since 2002, the number of articles published has increased
steadily every year. From 2002 to 2020, an average of 123.33 articles
were published each year, with an average annual growth rate of 4.21%.
This result indicates that research on seed dispersal is in the growth
stage and that this field has very large research potential. During the
period from 1985-2020, the total number of citations was 72,460; the
number of citations per article in this period was 37.49. The average
number of citations was highest in 2000, reaching 146.63. Subsequently,
the average number of citations showed a downward trend.
3.1.2 Analysis of subject categories
The 10 most frequent subject categories according to our research are
shown in Table 1; these subjects include ecology (1,199 articles,
accounting for 61.84% of the total), plant science (424 articles,
21.87%), evolutionary biology
(204 articles, 10.52%), forestry (168 articles, 8.66%), biodiversity
conservation (159 articles, 8.20%), zoology (141 articles, 7.27%),
biology (101 articles, 5.21%), multidisciplinary sciences (99 articles,
5.11%), environmental sciences (89 articles, 4.59%), and genetics and
heredity (86 articles, 4.44%). The number of publications in each
category reflects the trends of seed dispersal research in different
domains. As shown in Table 1, we found that the number of publications
in the fields of ecology and plant science accounted for the vast
majority of the literature included in this analysis.
Fig. 3 displays a co-occurring subject category network that includes 62
nodes and 227 links. Through the subject category network, we found that
seed dispersal research is a discipline that involves various subjects,
such as ecology, plant science, forestry, zoology and biology. Fig. 3
shows that the 3 most frequent categories in seed dispersal research are
ecology, environmental science and plant science. Environmental science
has high centrality because it connects many different subject
categories, including atmospheric sciences, ecology, environmental
chemistry and geoscience. Ecology also connected many subject
categories, including environmental science, plant science, forestry and
biodiversity conservation.
3.1.3 Journal analysis
The 1,939 articles on seed dispersal analysed in this study appeared in
371 journals. Table 2 lists the 10 journals with the highest numbers of
publications on seed dispersal. The number of articles published by
these 10 journals in the field of seed dispersal encompassed 27.7% of
all the analysed publications. Biotropica was the most popular journal
(88), followed by Oecologia (66), Journal of Tropical Ecology (65),
Journal of Ecology (62) and Ecology (61). The number of total citations
of a journal (TC) and the average number of citations per paper of a
journal (TC/P) reflect the impact of that journal (Ji et al. 2014). The
impact factor (IF) and H-index can also measure journal impact. As shown
in Table 2, among the literature included in this study, Biotropica,
Oecologia, Journal of Tropical Ecology, Journal of Ecology and Ecology
each published more than 60 articles about seed dispersal. Ecology had
the highest impact in the field of seed dispersal, with the highest
H-index (41), TC (5614) and TC/P (92.03).
3.2 Research power of seed dispersal
3.2.1 Analysis of countries and institutions
(1) Quantity of articles and citations
The papers included in this study came from 82 countries around the
world. The 10 most productive countries are listed in Table 3. As shown
in Table 3, the USA had the largest number of publications (555
articles) and total citations (26,942 citations). Spain had the second
highest number of publications (214 articles), followed by Brazil (156
articles), Germany (149 articles), Australia (109 articles) and China
(96 articles). Fig. 4 shows the distribution of countries/territories
and the relationships of international cooperation with impact on seed
dispersal research. The USA had the earliest publication on seed
dispersal and was the most productive country during the study period.
The USA had the highest degree of centrality (0.60), followed by France
(0.21), Germany (0.20) and Australia (0.16); these countries play an
important role in collaboration networks. The analysis revealed that
there was strong international cooperation between North America and
Europe; however, China’s international cooperation was weak.
(2) Collaboration network
Table 4 lists the 10 most productive institutions in terms of relevant
articles. Consejo Superior de Investigaciones Científicas (CSIC) was the
most productive institution, followed by the Chinese Academy of
Sciences, Universidad Nacional Autonoma de Mexico, the University of
Florida and Kyoto University. Fig. 5 shows the network of academic
collaboration on seed dispersal research between institutions. The
strongest collaborations were identified between CSIC and the University
of Oviedo. Kyoto University collaborated with the Forestry & Forest
Product Research Institute. The University of Oviedo had the highest
centrality (0.16) and cooperated closely with other institutions, such
as CSIC and the University of Sao Paulo.
(3) The development of seed dispersal research in selected countries
To evaluate the changes in the academic impact of the 10 abovementioned
countries in terms of seed dispersal research, the AI and the AAI were
used. It should be noted that because there is usually a lag between the
publication time and the citation time of an article(Glänzel and Persson
2003, Qiu and Chen 2009), the time range of the AAI was set 2 years
behind that of the AI in this study.
Quadrant diagrams of the changes in the two indexes are shown in Fig. 2,
with quadrants I to IV representing 4 different situations. The points
in quadrant I represent the years in which the country’s AI and AAI
indexes were both higher than the global average levels; the points in
quadrant II represent the years in which the country’s AAI and AI
indexes were higher and lower than the global average levels,
respectively; the points in quadrant III represent the years in which
the country’s AI and AAI indexes were both lower than the global average
levels; and the points in quadrant IV represent the years in which the
country’s AI and AAI indexes were higher and lower than the global
average levels, respectively.
In general, with the exception of Australia, the AI and AAI indexes of
the 10 selected countries showed an upward trend during the study
period. As shown in Fig. 2, the research power and academic impact of
the USA in most years were higher than the global average level; in
contrast, although Spain had a relatively large number of published and
cited articles, its academic impact was lower than the global average
level in most years. The research power and academic impacts of Brazil,
China and Japan were lower than the global average level in most years,
but in recent years, their academic impact was higher than the global
average level, indicating that the impact of these countries on seed
dispersal research is continuously increasing.
3.2.2 Author analysis
(1) Author collaboration network
Table 5 lists the 10 most productive authors according to the
publications included in this study. As shown in Table 5, A Traveset, M
Nogales and P Jordano had the highest numbers of publications. These
authors were followed by XF Yi (19 articles), M Galetti (19 articles), D
Garcia (18 articles), R Nathan (17 articles), SB Vander Wall (15
articles), MA Pizo (15 articles) and MB Soons (13 articles). The 10 most
productive authors all came from different research institutions (Table
5); 4 of these authors were in Spain, and the remaining 6 were in
different countries.
Fig. 7 is the collaboration network map of the authors of seed dispersal
research. Five author groups were identified. (1) A team consisting of A
Traveset, P Jordano and M Nogales (circle A) focused their research on
long-distance seed dispersal (Nogales et al. 2012, Traveset et al. 2014,
Pedro and Jordano 2017). (2) A group of authors led by XF Yi (circle B)
focused on animal-seed interactions (Steele and Yi 2020, Xianfeng et al.
2020). (3) M Huynen and A Albert formed a core team (circle C), and much
of their research focused on the analysis of feeding ecology and seed
dispersal of pigtail macaques (Macaca nemestrina ) (Latinne et al.
2008, Aurelie et al. 2013). (4) A team consisting of R Heleno and S
Timoteo (circle D) studied ecological networks (such as food webs and
pollinator networks) (Timóteo et al. 2018, Heleno et al. 2020). (5) A
group of authors led by M Schleuning (circle E) focused on plant-animal
interactions and ecological networks (Schleuning et al. 2015, Schleuning
et al. 2016).
(2) Author co-citation network
The results of the author collaboration analysis can reflect an author’s
contributions to the field of seed dispersal and the cooperative
relationship between authors, but it cannot reflect an author’s impact
on seed dispersal research. Therefore, we used author co-citation
analysis to provide further information. We selected the 10 most cited
project themes from each topic to ensure that we targeted the most
prominent authors.
On the basis of the author co-citation network analysis results, the
authors with high citation frequency and their countries of origin were
determined. As shown in Table 6, the author with the highest citation
frequency is R Nathan (3,612 citations, Israel), followed by P Jordano
(2798 citations, Spain), DJ Levey (1273 citations, USA), A Traveset
(1055 citations, Spain), M Galetti (976 citations, Italy), MB Soons (933
citations, Netherlands), SB Vander Wall (853 citations, USA), D Garcia
(832 citations, Spain), M Nogales (637 citations, Spain) and PR
Guimaraes (634 citations, Brazil). Four of these authors are from Spain,
two are from the USA, and the others are from Israel, Italy and the
Netherlands.
By comparing Table 6 and Table 5, we determined that there is a strong
relationship between the 10 most cited authors and the 10 most
productive authors. Only DJ Levey, M Nogales and PR Guimaraes appear in
only Table S2; the remaining authors appear in both tables.
3.3
Intellectual base
The number of citations of an article represents the degree of
recognition it received in a particular research field or its academic
impact (Ebrahim et al. 2013, Yoshikane et al. 2013). The 10 most highly
cited references are listed in Table 7. The most highly cited reference
was published by R Nathan, with a citation count of 1,343. Through this
review, the author shows that with the growing interest in spatial
ecology, new methods for researching seed dispersal, which is one of the
key processes in determining the spatial structure of plant populations,
are emerging. Seed dispersal methods vary among plant species,
populations and individuals; with distance from the parent plant; and by
microsites and times. The development and improvement of mathematical
models is expected to produce a deeper and more mechanical understanding
of the seed dispersal process and its consequences. The remaining
documents with a high citation rate were published in the 21st century,
mostly between 2000 and 2011. The content of these papers shows that the
intellectual base focuses on long-distance seed dispersal and dispersal
patterns in different habitat conditions. These publications have laid a
good foundation for future research directions.
3.4 Research hotpots
3.4.1 Keyword co-occurrence
Keywords reflect an author’s intention and interests and summarize the
main contents of an article. People can obtain useful information, such
as research goals, methods, and important opinions, from the keywords of
an article (Xu et al. 2018). Therefore, keyword frequency analysis and
period change analysis are essential to discussing research hotspots and
development changes within a research field (Wang et al. 2018).
We use the keyword co-occurrence analysis in CiteSpace software to
create a keyword co-occurrence network diagram of the seed dispersal
research analysed and identify the main research hotspots in this field
(Fig. 8). Each node in the graph represents a keyword; larger nodes
indicate keywords that appear more frequently. Similarly, the width of
the links indicates the frequency of keywords appearing together; a
thicker link indicates that the two connected keywords appear together
more frequently. A total of 624 keywords were obtained, among which 14
appeared more than 100 times. There is no doubt that “seed dispersal”,
the main keyword of this article, is the largest node in the Fig. 8;
this keyword appears 324 times and is closely related to “pattern”
(245 times), “plant” (165 times), “frugivory” (160 times),
“recruitment” (160 times), “ecology” (152 times) and “forest” (152
times). Other keywords, such as “germination” (136 times), “bird”
(122 times), “predation” (136 times), “rainforest” (101 times) and
“distance” (100 times), also had high frequencies.
3.4.2 Keyword burst analysis
Keywords burst analysis identifies those keywords with a sharp increase
in frequency. Burst detection is a useful analytical method for finding
keywords that are of particular interest to the relevant scientific
community over a given period of time (B et al. 2018). Therefore,
bursting keywords can be used as indicators of research frontiers and to
forecast research trends. Fig. 9 shows the 20 keywords with the
strongest citation bursts from 1985 to 2020. In this figure, the
strength of the burst represents the intensity of focus on a given
topic. In the field of seed dispersal research, wind dispersal has been
a research hotspot since 1989, with the focus subsequently diversifying
to include “selection” (strength: 4.55), “bird” (strength: 5.56),
“dispersal” (strength: 5.26), “environment” (strength: 5.21),
“tropical rainforest” (strength: 8.2) and “dynamics” (strength:
5.01). We found that in the last ten years (2010-2020), studies related
to “conservation” and “spatial pattern” were emerging active topics
(Fig. 9).
3.4.1 Keywords in different countries
Due to differences in climate, geographical features, and historical and
economic conditions, the development of research in a certain field in
different countries or regions may not be balanced. Fig. 10 shows the
high-frequency keywords, main research types and regional distribution
of relevant research in the 10 countries with the highest numbers of
publications. The results showed that in the field of seed dispersal,
the differences between countries or territories are not very large and
that the study of the relationship between ”seed dispersal” and
”frugivorous animal” received the most attention. For example, from 1985
to 2020, the most frequently used keywords in the USA were “seed
dispersal” (162 times) and “frugivory” (75 times); the most
frequently used keywords in Spain during this period were “seed
dispersal” (41 times) and “frugivory” (26 times).