4.1 | Effect of habitat fragmentation on population
genetic structure
Habitat fragmentation has been identified as one of the threats to
biodiversity of many terrestrial species. In fragmented habitats, the
roads, villages and farmland surrounding the populations are generally
considered to be barriers to gene flow, which often result in reduced
genetic diversity, enhanced inbreeding within populations and genetic
structure (Templeton, Robertson, Brisson, & Strasburg, 2001; Byrne,
Elliott, Yates, & Coates, 2008; Vranckx, Jacquemyn, Muys, & Honnay,
2012; Betancourth-Cundar, Palacios-Rodríguez, Mejía-Vargas, Paz, &
Amézquita, 2020). For most plants, habitat fragmentation often reduces
gene flow and genetic diversity by disrupting the movement of seed
(Sebbenn et al., 2011; Browne & Karubian, 2018; Ony et al., 2020).
Despite orchids have dust-like seeds, some studies investigating genetic
structure of orchids in fragmented landscape have found a significant
pattern (Cozzolino, Noce, Musacchio, & Widmer, 2003; Chung et al.,
2014; Mallet, Martos, Blambert, Pailler, & Humeau, 2014; Minasiewicz,
Znaniecka, Górniak, & Kawiński, 2018). The gene flow introduced via
seed dispersal often depends on the morphological features of seeds.
Orchid seeds are known to be small, light and easily transported by the
wind, perhaps the habitat fragment cannot prevent the seeds from being
transported for long distances. However, orchid seeds need the presence
of a fungus to germinate and grow (McCormick, Whigham, Sloan, O’Malley,
& Hodkinson, 2006), and the patches with variable fungi are shrinking
and being replaced by unfavorable environment. A study in a landscape
experiencing habitat fragmentation showed that although Chilean tree can
produce a large number of seeds, seedling establishment was uncommon
(Lander, Harris, Cremona, & Boshier, 2019).
Moreover, orchid species depend on insect pollinators such as honeybees
for seed production and dispersal. In continual forest area, these
pollinators are normally capable of flying one to several kilometers, so
gene flow via pollen was expected to be not restricted. In contrast, in
a fragmented landscape, pollen dispersal will become very difficult.
Suni & Brosi (2012) analyzed the genetic structure of two orchid bee
species in fragmented habitat, and found genetic differentiation for two
species was significant. Although biparentally inherited molecular data
are not shown here, it is certain that pollen-mediated gene flow can be
affected by habitat fragmentation.
Many forest areas in the Qinling Mountains have affected negatively by
human disturbances. From 1984 to 2014, human activities greatly
disturbed the forest in the mountains, and the degree of fragmentation
has increased obviously (Cui et al., 2018). In our study, the
populations SZA and SXX are surrounded by roads, railway line and
villages, which inevitably lead to alterations in genetic structure of
wild species. Our analysis revealed high genetic differentiation betweenC. faberi wild populations distributed within fragmented land and
others in continuous forest. Furthermore, a decreased level of genetic
diversity within SZA and SXX populations surrounded by roads was
detected. This suggested that seed-mediated gene flow between SXX, SZA
and other populations may be limited by habitat fragmentation. In
contrast, extensive gene flow detected in populations located in eastern
mountains can prevent or reduce genetic divergence, help maintain
overall genetic variation by reducing genetic drift.