4 DISCUSSION
4.1 Influencing factors of network
stability
Our study shows that the structure
and function of the ESN in the UAHB had improved significantly in the
past 20 years. But how about the anti-jamming ability of the network?
What were the factors that affected the network’s connectivity and
stability?
The network stability was closely related to the number of deleted
nodes. In terms of the number of deleted nodes (Figure 7a,b,c,d),R 1 and R 2 under different
disturbance scenarios were correlated with the number of deleted nodes,
and both showed a decreasing trend. The overall trend ofR 1 and R 2 in the same year
was roughly the same, but there were local differences. In 1995 (Figure
7a,c), when the node failure ratio
(NFR, the ratio of the number of
the deleted nodes to the total number of the initial nodes in the
network) did not exceed 15% (12 nodes), the values ofR 1 and R 2 almost remained
unchanged and the number of the connected components did not change,
either (Figure 7e). The results show that the network structure was
relatively complete, while the network was in a relatively stable state.
When the NFR was at 15%-46%, R 1 andR 2 took different values under the two different
disturbance scenarios (Figure 7a,c) and the number of the connected
components increased, indicating that the degree of network
fragmentation increased (Figure 7e).
When the NFR was between 46% to
95%, the network complexity declined significantly with bothR 1 and R 2 reducing to less
than half of their initial values and the network was extremely
sensitive. When the NFR exceeded 95%, the number of the connected
components in the two disturbance scenarios dropped to 0. At this time,
there were only a few isolated nodes in the network, and the exchange
and transmission of information and energy could not be carried out
between the nodes, and the network was in a state of paralysis. Compared
to 1995, the network structure in 2015 was relatively complete and the
network was in a relatively stable state (Figure 7b,d,f). When more than
25% of nodes were deleted, the disturbance of the network structure
became obvious, and the network started to break down. With increasing
NFR, network stability declined, the number of the connected components
increased rapidly, and the network fragmentation increased until the
network was finally paralyzed.
In summary, it was found that the number of nodes influenced the
integrity and complexity of the network and had an impact on the
stability of the network. In 2015, the NFRs in the network
characteristics including stable state, sensitive state, and paralytic
state were all greater than that in 1995, indicating that the stability
of the ecological security network in 2015 was better than that in 1995.
Table 5 further describes the relationship between network robustness
and the number of deleted nodes.
The way of nodes deletion
was related to the stability of the network . As discussed above, the
disturbance scenarios represented different ways of removing nodes from
the network, i.e., NHD representing random deletion, whereas HD deleting
the nodes with the higher level
comprehensive importance first. As
shown in Figure 7a,b,c,d, although the values of bothR 1 andR 2 eventually
dropped to zero in both
disturbance scenarios, the curves of the HD scenario had more
fluctuation. For example, R 1 andR 2 of the HD scenario decreased more rapidly than
that of the NHD scenario (Figure 7), when more than 15% of the
important nodes were deleted. The results indicate that the stability of
the ESN was more sensitive to human disturbance than to nonhuman
disturbance. Especially, the nodes with a high level of comprehensive
importance had a greater influence on R 2, meaning
a greater impact on network efficiency.
Table 6 further illustrates the relationship between the network
robustness and the way of node deletion. The stability of the ESN
depended on the number of interactions of network nodes. The process of
nodes deletion based on the level of comprehensive importance of the
nodes usually represented the impact of the purposeful disturbance that
was mainly caused by human activities. The network of good spatial
structure can maintain high stability under any disturbance scenarios.
4.2 Ecological security network optimization
System scientists believe that
restructuring is to reframe the system’s structure to promote an optimal
combination of the system’s internal elements and to achieve the
system’s fundamental transformation (Zhou, Xu, & Lin, 2016). The
optimization and reconstruction of the ESNs provide spatial planning
methods to integrate ecological processes, spatial scales, and
ecosystems. The complexity and uncertainty of the network’s structure
should be considered in the reconstruction of the ESNs. Combined with
the reconstruction of the ESNs and the difference in ecological space
protection strategy, we propose five measures to improve the regional
ESN of the UAHB based on our research findings.
The first measure is to protect the important ecological sources.As the important network nodes, the ecological sources are the important
habitats for the living beings in a region. Increasing their quantities
and improving their quality is particularly important for the protection
of the regional ecological environment and biodiversity. Compared with
1995, the area of ecological sources of the UAHB increased significantly
in 2015 (Figure 3), but the increment was mainly concentrated in
Hangzhou, while the areas of the sources in Jiaxing and Huzhou were
small, and the corridor length between the sources in these two cities
and other sources and nodes was considerable long. Therefore, we suggest
that the integrity of national or provincial natural reserves, forest
parks, large forest land, and wetlands should be strictly protected, and
the forest land around them should be considered as part of the network,
to increase sources area, to enrich the biological species, to improve
the quality of the habitat, and to increase the suitability of the
habitat (Liang, Liu, Liu, Qi, & Liu, 2018; Yin et al., 2011).
The second measure is to improve the effectiveness of connectivity
between nodes . Nodes are the key to ensuring network connectivity.
Their interactivity, importance, and quantity are important factors to
maintain the integrity and complexity of the network structure. As
displayed in Figure S2, there were two isolated nodes in Jiaxing (see
also Figure S1). Although there were potential ecological corridors in
Jiaxing, Huzhou, and other areas (see also Figure 3), only a fewkey ecological corridors existed due to large landscape
resistance, so the ESN was not fully connected. On the one hand, the
protection of the key nodes will not only help to improve landscape
connectivity but also promote the virtuous cycle of logistics and energy
flow in the network. On the other hand, we should implement spatially
distributed control and protection measures based on the distribution
characteristics of the important nodes to change the formation mechanism
of the network and to realize the reconstruction of the ESN. At least
three reconstruction strategies may be adopted based on the node
importance: individual protection, general protection, and extensive
protection. According to the needs of biological diffusion and the
possibility of ecological construction, we suggest planning corridors to
connect the two currently isolated sources in Jiaxing to promote their
connection with the nearest nodes and to improve the ESN’s structure.
The third measure is to restore ecological breaking points .
Ecological corridors are an important part of the regional ESN, which
can improve the overall quality of the regional ecological environment
(Li, Han, & Tong, 2009). However, the more landscape types the
ecological corridors cross, the greater the accumulated resistance of
the landscape will be, which, in turn, will reduce the effectiveness of
the functions of the ecosystem. Roads, especially high-grade road
networks, have a certain barrier effect on the transfer of material and
energy flows in the ESN, which may cause ecological breakpoints and
habitat fragmentation. We conducted the overlay analysis of railway,
expressway, national road, provincial road, and important ecological
corridors in the network of the UHAB. The intersections of railways,
expressways, and corridors were regarded as the main breaking points,
while the intersections of national roads, provincial roads, and
corridors were regarded as secondary breaking points. Overall, 36 main
breaking points and 23 secondary breaking points were extracted (Figure
8). Currently, many scholars have called to include wildlife passageways
such as underground passageways, tunnels, and overpasses in the
construction codes of the high-grade roads (Chen, Yin, Kong, & Yao,
2015; Yin et al., 2011). According to the current economic and social
development level of the UAHB, it is suggested that the high-grade roads
network should avoid the areas where wildlife activities are frequent
and provide more access to wildlife by restoring ecological breakpoints.
The fourth measure is to strengthen the protection and planning of
steppingstones . The steppingstones can provide temporary habitats for
migrating species, especially for the species with long migration
distances. The quantity, quality, and spatial locations of the
steppingstones are important factors that affect the time, frequency,
and success rate of species migration and increase the regional
biodiversity (Yin et al., 2011).
Therefore, it is necessary to strengthen the protection of the existing
steppingstones and the planning and construction of future
steppingstones. When combined with the intersection of important
ecological corridors and the nodes’ comprehensive importance in the
network, the nodes with high comprehensive importance are selected as
steppingstones. As a result, 21 steppingstones are identified and
selected in our study (Figure 9).
The fifth measure is to
connectthe ESNs with the surrounding areas at different scales . The
interconnection of the networks inside and outside the region can
contribute to the material exchange and energy transfer, enhancing the
ecosystem stability (Yin et al., 2011). At present, the surrounding
areas of the UAHB, such as Jiangsu, Anhui, Jiangxi, Fujian, and
Shanghai, also have a good ecological environment. Therefore, in the
process of improving the internal network of the UAHB, it is suggested
to strengthen the connection of the ESNs between the UAHB and the
surrounding areas at different scales. This will not only help to
improve the stability of the ecosystems but also enable the ESNs to have
longer buffer zones and recovery times when they are damaged by external
disturbances.
4.3 Limitations
Our case study in the UAHB may provide empirical evidence and support in
terms of space optimization, planning, and protection of the ESNs in
other regions where are also experiencing rapid urbanization. However,
our study is not without limitations. First, the assessment indexes of
ESNs can be further improved. The area and shape of ecological sources
have important influences on the structure and function of an ESN, but
these influences cannot be reflected merely based on the analysis of
network topology. Second, we assume that nodes and edges are not
functioning when they are disturbed and ignore the situations when they
may partially or fully recover from disturbance.