Figure 6 The relationships are given, (a) between species
asynchrony and logarithm (with base 10) of N addition rate and (b)
between the former and CV complexity (standard variance of network
complexity divided by the mean. Solid line isv the linear least square
regression. Note: * < 0.05.
Nitrogen enrichment significantly reduced species asynchrony
(\(r^{2}=0.268,\ \ P<0.05\), in Fig. 6a), because it changed species
competitive advantage and mobility, weakened complexity and network
degeneracy. The increase of CV complexity had a tendency to reduce
species asynchrony, but it was not significant
(\(r^{2}=0.844,\ \ P>0.05\), in Fig. 6b). So, the external
environment was the main factor affecting the species asynchrony rather
than network complexity.
4. Discussion
4.1 Effects of network structure on plant community
attributes under nitrogen
enrichment
The theoretical and practical approaches on species interactions ignores
the more complex chain of indirect interactions that may occur in
different natural communities
(Gallien
2017a). Gallien (2017b) defined four forms of interspecific interaction
in theory and found that intransitive loops can both stabilize and
destroy species coexistence, but the length and topological structure of
intransitive loop significantly affect intensity and importance of
intransitive competition. In reality, species-competition network often
is not just a single structure, but multiple simple structures
interweaving, controlled the vegetation community. We found that there
were the five simple forms of intransitive loop, among which the
nested-loop has a widest distribution range in the sample plots,
accounting for 76.96%, always along with other forms except for the
short loop.
Mathematical studies have shown that intransitive loops with an odd
number of species stabilize species coexistence, while loops with an
even number of species destabilize species abundance (Vandermeer 2011).
Ulrich (2018) found a significant positive correlation between the
nesting degree of network structure and the Alpha diversity in European
salt marsh plant communities. Our results showed that, in terms of
long-term effects on maintaining species diversity, nested network was
always in an absolute advantage at the small (Alpha) and large-scale
(Gamma) diversity, but had a certain delay effect on the Beta diversity,
indicating that the effect of the ecosystem needed time to offset the
negative effect of nitrogen deposition. In addition, nested network was
also conducive to introduction of new species, suggesting that more
complex competition can provide more chance to new species. There was no
significant difference in species emigration between the two networks,
while there may be yet other factors affecting on the difference.
Intransitive competition is likely to be the main reason for the
construction of plant community. High intensity habitat disturbance
results in less structured competition as well as less intransitive
competition loops in plant communities (Ulrich et al. 2018). Our results
showed that environmental filtering affects not only the attributes of
the steppe communities, but also the competitive structure within the
communities. With the increase of nitrogen enrichment, the complexity
and degeneracy of network decreased simultaneously, indicating that high
N-addition rates will destroy the soil environment on which vegetation
depends for survival and then interfere with the interspecific
interactions, which simplify the network structure and gradually clarify
the competition levels, or expand the difference in the levels (Sun et
al. 2021). Expansion of differences in the levels also lead to an easy
natural selection among species, which generally existed in communities.
The difference between the two kinds of networks was mainly
characterized by colonization and emigration. The plots with nested
networks had more resource utilization and stronger colonization
potential for new species, resulting in the transition from the
competition effect to the promotion effect.
The productivity of the plots treated with mowing was higher than that
with no mowing (in Fig. S3be), so mowing management can partially offset
the negative effects of nitrogen enrichment. This is mainly because
mowing will increase soil nutrient loss and cause nutrient imbalance
(Giese et al. 2013; Liu et al. 2015), thus intensify the competition for
nutrients by lessening the competitivity of the dominant species. Mowing
can also increase soil surface temperature by increasing ground
irradiance (Wan et al. 2002), thereby increase the synchronicity of
species responses to climatic conditions during growing season
(Shestakova et al. 2016). In the temperate steppe, nutrients and light
are important resources for plant growth, and mowing leads to
redistribution of available nutrients and light, this may increase
interspecific competition (Zhang et al. 2017). So, mowing can remove
part of nitrogen enrichment, relieve the environmental pressure and
promote plant growth. On the other hand, mowing disrupts the competitive
balance of species networks, weakens the competitiveness of dominant
species, resulting in reducing the differences in the levels of
competition, promoting balanced growth of species, and maintaining the
level of species diversity.
4.2 Effect of network complexity on plant community
function under nitrogen
enrichment
Empirical studies have shown that Intransitive competition, which
usually occurs in communities along with direct interactions, has
important effects on community structure and functions (Maynard et al.
2017a; Soliveres et al. 2015). Niche differences among species responds
asynchronously to environmental fluctuations, resulting in asynchronous
population dynamics and ultimately more stable overall ecosystem
attributes
(Loreau
and De Mazancourt 2013). Species asynchrony can stabilize communities by
compensating for growth in species or functional groups (Song and Yu
2015). Thus, species asynchrony is the main mechanism regulating the
temporal stability of the community (Zhou et al. 2020; Taofeek et al.
2021).
The directly positive effect of
nitrogen enrichment on species
asynchronism is due to dynamic compensation, which increases
interspecific competition for light resources (Schluter 1984; Micheli et
al. 1999; Grman et al. 2010) and ultimately result in higher species
asynchrony. The indirect effect of
nitrogen addition on species asynchronism is due to the fact that
nitrogen enrichment induces a decrease in species diversity, thus
weakening the stabilizing effect of species asynchronism on the
community (Yachi and Loreau,1999; Yang et al. 2012). High level of
nitrogen enrichment may reduce intensity of competition for nutrient
resources, thus reducing the asynchronous degree of species fluctuations
(Song and Yu 2015). In high nitrogen enrichment environment, competitive
order of dominant species changes and some rare species even disappear
completely (Zhou et al. 2020). Our results showed that species
asynchrony was synchronized with grassland ecosystem response to
N-addition rates, which was consistent with previous findings (Liu et
al. 2019). The effect of CV complexity on species asynchrony was not
significant, thus the external environmental disturbance was the main
factor affecting the species asynchrony rather than network complexity.
Although the fierce interspecific competition easily leads to species
replacement, a loop-structured network basically maintains the balance
of species asynchrony. Thus, species asynchronous compensation has less
effect on the stability in plot with loop structure.
In the sample plot of intransitive competition network, the damage of
high nitrogen enrichment (≥15 g
N·m-2·year-1) to the temporal
stability of the ecosystem was basically the same as that of all
samples. This indicated that under the high nitrogen enrichment, the
absorption of nitrogen by plants reaches saturation and the positive
effect on the ecosystem stability disappears (Zhou et al. 2020).
Excessive nitrogen enrichment also accelerates vegetation transpiration,
which leads to the decrease of water resources and thus aggravates the
community instability (Chen et al. 2016). Under medium nitrogen
enrichments (5, 10 g N·m-2·year-1),
the temporal stability of the plots with loop structure had a
significant increase, which inferred that the intransitive competition
under the nitrogen enrichments is conducive to enhance the ecosystem
stability and counteracts the negative effect of nitrogen enrichment
apparently. Intransitive competition increased local diversity and
small-scale species replacement, which may be the reason why network
complexity did not have significant negative effect on species
asynchronism. Therefore, in addition to the compensative effect of
species asynchronism on ecosystem stability, we believe that the
structural attributes of species-competition network may be another
important internal mechanism to maintain the stability of the grassland
ecosystem.
Community attributes have an obvious complementary relation in term of
complexity. The relationship between network complexity and plot biomass
was a significantly quadratic curve with down bending direction, while
that between plant abundance and complexity was also a quadratic curve
but with opposite direction. This shows that when plot biomass is low,
it will be compensated by high plant abundance, and vice versa. The
bending direction of the two curves also indicated that the competition
structure may be a mechanism for self-recovery of grassland ecosystem,
which can alleviate the high intensity of external interference.
5. Conclusion
In this paper, the response of plant communities to intransitive
competition structure under nitrogen enrichment was investigated from
the perspective of interspecific competition. We found that the overall
structure of a real species-competition network was always very complex,
weaving up of a variety of simple structures. Compared with short
network, nested network introduced new species with higher colonization
rate and had a long-term temporal mechanism to maintain the small-scale
Alpha diversity, but had a significant lag effect on regulating the
large-scale Gamma diversity. With change of network complexity, the
biomass and the plant abundance showed quadratic curves with opposite
bending direction,
demonstrating
a complementary relationship.
The
response of species asynchrony and grassland ecosystem stability
decreased with the increase of nitrogen enrichment synchronously.
Fortunately, at low and medium nitrogen enrichment, intransitive
competitive network offset the negative effect of nitrogen. Intransitive
competition structure is likely to be the internal mechanism to resist
the external environmental disturbace and maintain the grassland
ecosystem stability. In the future, we will focus on the changes in the
intensity of species competition within network and analyze the
ecological information behind them. We hope that the results of this
study can enrich the theory of nitrogen deposition affecting
interspecific interactions among species and provide theoretical support
for coping with the effects of nitrogen deposition.
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