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.