DISCUSSION
We recorded 2235 interactions events between 14 bird species and 13
plant species in our study area (Fig.2), suggesting that even in urban
green spaces with relatively small biodiversity, interaction networks
can be highly complex and dynamic. Passerine birds occupied a dominant
position in the whole interaction network (Fig.2), which may be related
to their wide feeding niche, foraging on a variety of fruits, and high
adaptability to disturbed urban habitats. Thus, the existence of
passerine birds with different body traits in cities can increase the
cumulative connection of the plant-frugivore bird networks in urban
ecosystems. However, compared with the random network generated by the
null model, the observed network had a lower connectance (C), lower
interaction diversity (H2 ), lower interaction
evenness (E2 ),
higher nestedness, and higher
specialization (H´ ) (Fig.3). The results indicated that the
structure of the observed network was simpler than that expected by the
null model, and these shifts in the network structure can be explained
by the intrinsic mathematical behavior of the null model tends to
generate more connected matrices than that observed (Dormann et al.,
2009; Costa et al., 2016). The higher degree of specialized interaction
in the null model, which is consistent with other studies (Blüthgen et
al., 2008), but these highly specialization interactions are more easily
lost from the networks because extinction of one of the species may have
fatal consequences for its specialist interacting partners
(Sebastián-González et al., 2015). Hence, disturbed urban habitats may
show more nestedness and less modular structures because the most
specialization interactions have been lost due to human activities.
We found that the characteristics of the interaction network varied with
season, and most interactions were detected in autumn and winter
(Fig.4), which is consistent with our prediction (i). This difference
could be explained by the decrease in other food sources such as insects
and the arrival of wintering populations, which intensifies competition
for food resources (Cruz et al., 2013; Yang et al., 2013). The number of
ripe fruit species and participating frugivore birds were positively
correlated with the interaction connections in different seasons, and
other studies have also shown that an increase in fruit abundance
significantly improves bird richness and network complexity
(Ramos-Robles et al., 2016;
Schneiberg et al., 2020). The connectance (C), nestedness, and
interaction diversity (H2 ) for the autumn and
winter networks were higher (Table1), indicating more efficient use of
resources (i.e. higher number of realized links among all possible
links). Interaction evenness (E2 ) was higher in
spring and summer, which means that there was a more homogeneous
distribution of the frequency of links in these networks; whereas, in
autumn and winter a few highly frequent interactions dominated the
network, particularly Cayratia japonica and C. camphora .
Higher specialization in spring and summer may arise as a response of
the higher trophic level to low plant diversity (He et al., 2022), which
also means a high degree of niche differentiation (Blüthgen, 2010;
Sebastián-González et al., 2015) and decreasing competition between
different birds to facilitate species coexistence (Silva et al., 2016).
There was no significant correlation between species traits and their
network roles, except for the effect of fruit color on species strength
(Fig.5; Fig.6). This is contrary to the results of previous studies
(Saavedra et al., 2014; Pigot et al., 2016) and is inconsistent with our
prediction (ii). These differences can be explained by the following two
factors: First, individual species play important roles, and their
contribution to the network is much higher than the average value of
species with the same traits; therefore, the differences cannot be shown
in the analysis. Second, the small sample size for the weighted analyses
may have caused this contrasting result (Costa et al., 2016). Plants
with black fruit have greater species strength, which means they can
attract more frugivorous birds. This, could be explained by the black
aril of mature fruits that can create a strong visual contrast with the
green background of the surrounding plants attracting birds to feed on
their fruit (Duan et al., 2014; Zhang et al., 2022), and there is a
significant positive correlation between the lipid nutrients and color;
thus, color may be a signal of seed maturity and nutritional rewards
(Schaefer et al., 2014). Finally, the evergreen fruit plants, such asC. camphora and B. javanica can provide temporary shelter
when birds forage in highly disturbed urban habitats and the long-term
interaction adaptation between these
plants and frugivore birds has
resulted in bird foraging preferences for these species.
Our results suggested that there are significant seasonal differences in
the structure and characteristics of the
interaction network between
plants and frugivore birds in urban green space, and none of plant and
bird traits were significantly related to the functional roles of
species in the network structure, except for the effect of fruit color
on species strength. This result may be caused by the sample size and
the area limitation. Therefore, it is necessary to expand research area
in the next research to further understand the impact of urbanization on
the characteristics of the interaction network between plants and
frugivore birds.