4.3 Different effects of nonforest landscapes on gene flow and
dispersal
Urban landscapes are generally considered to be barriers to gene flow,
and an increase in farmland landscapes can also directly or indirectly
negatively affect the ecological processes and dispersal of many species
(Butler, Vickery, & Norris, 2007; Johnson & Munshi-South, 2017;
Lane-Degraaf, Fuentes, & Hollocher, 2014; Ruell et al., 2012; ).
According to the Mantel test of the correlation between the LCP of
multiple resistance surfaces and the genetic distance, they were
significantly correlated only when the urban and farmland landscapes
were given high resistance values (Figures 4A, B). By contrast, no
significant correlations were obtained with the other three resistance
surfaces in which low resistance was given to urban and farmland
landscapes (Supporting information Figure S3). Thus, by acting as
barriers, urban and farmland landscapes played an important role in the
dispersal and the gene flow of M. alternatus . This effect is
possibly due to the limited food sources of the species in cities and
farmland. Moreover, the use of pesticides in farmlands can also affect
insects (Sonoda, Izumi, Kohara, Koshiyama, & Yoshida, 2011), likely
affecting the identification of hosts by M. alternatus and
inhibiting dispersal.
Notably, the LCTA, in which the resistance value was objectively
defined, also indicated that urban and farmland landscapes had a
negative effect on gene flow (Tabel 3). Especially in Xiapu, the best
three models in the LCTA showed that urban landscapes inhibited gene
flow. In addition, the GAM model also suggested that urban and farmland
landscapes were negatively correlated with genetic diversity at the 800
to 1,000-m scale (Figure 6). Urbanization is one of the important
factors that cause the loss of species habitat as well as forest
fragmentation and the gradual but eventual disappearance of small areas
of forest (Caspersen & Olafsson, 2010), and the distribution of
farmland destroys the continuity of host distribution. Because of the
decline in forest coverage and the disappearance of host continuity, the
population of M. alternatus may be separated to form isolated
populations, thus reducing the gene flow. Indeed, farmland can directly
affect the distribution and spread of insects and even hinder insect
migration, which has important effects on population dynamics (Ge, Ou,
& Men, 2016). In a study on other species of Monochamus , the
flying time of this species was short, sometimes only 1 km (David,
2014), so the farmland and urban landscapes at the scale of 800 to 1,000
m would most likely be barriers due to the unwillingness to fly. Thus,
at a fine-scale, urban and farmland landscapes distributed are important
barriers to dispersal and migration in this species.
Corridors in landscapes can promote the dispersal of species among
different landscape types (Lapolla, 1993; Nicholls, Parrella, &
Altieri, 2001). Among nonforest landscapes, roads are very important
corridors, and especially in areas with high landscape diversity, road
patches can often play a role in connectivity. The fourth model of the
LCTA in Shunchang showed that roads were the most suitable dispersal
habitat for this species (Table 3). The LCP model showed that roads had
the same low resistance value as other host landscapes (Figure 4C). The
db-RDA and GAM models also indicated that roads were positively
correlated with genetic diversity (Figure 5, 6;Table 4, 5). These
results all suggest that roads are important and promote the dispersal
of M. alternatus in a heterogeneous landscape. Although the
existence of road networks can inhibit the dispersal and gene flow in
some other species (Garcia-Gonzalez, Campo, Pola, & Garcia-Vazquez,
2012; Hartmann, Steyer, Kraus, Segelbacher, & Nowak, 2013), M.
alternatus has the characteristic of the human-mediated movement, such
as in the transport of infested wood from one area to another, including
lumber and wood packaging material (Haack, 2006; Ye, 2019). This type of
movement may cause the dispersal of this species in heterogeneous
landscapes and may even lead to long-distance transmission. In a
previous study on the genetic structure of M. alternatus , the
road transportation system was also found to promote the dispersal of
this species in mainland China (Hu, Ning, Fu, & Haack, 2013).
Therefore, roads can facilitate dispersal and gene flow of M.
alternatus in heterogeneous landscapes by promoting long-distance,
human-mediated transport.