Demographic history of Cycas bifida and C. micholitzii and conservation implications
In the long course of history, plants would expand or shrink to cope with environment (e.g., glacial cycles) and habitat changes (e.g., habitat destruction). What’s more, some plants become endangered or extinct. According to our field survey, C. micholitzii has more populations and larger distribution ranges than C. bifida(Supplementary Table S1). Our IMa simulation analysis also revealed that the effective population size of C. micholitzii was larger than that for C. bifida (Figure 5a), and the EBSP results further revealed a larger effective population size in C. micholitziithan in C. bifida (Figure 7). In addition, the effective population sizes of the both species were larger than that of the ancestral species (Figure 5a), which indicated that they had been experienced population expansion once. Based on the EBSP analysis both species have experienced small-scale population expansion during the Upper - Pleistocene (Figure 7). The Quaternary climatic oscillations made deeper impacts on the population dynamics of C. bifida which experienced constant small-scale expansion, then small-scale contraction and a recent re-expansion, while little impacts on C. micholitzii . According to previous researches (Y. C. Chiang et al., 2009; Xiu Yan Feng et al., 2017; Xiu Yan Feng, Wang, & Gong, 2014; X. Y. Feng et al., 2016; Y. Q. Gong et al., 2015; Jian Liu et al., 2015; Zhan, Wang, Gong, & Peng, 2011; Y. Zheng et al., 2016), the glacial and interglacial climatic fluctuations have had varying degrees of influences on some Cycas species, and distinct demographics were detected in them. Of those, Cycas revoluta (Y. C. Chiang et al., 2009), Cycas taitungensis (Y. C. Chiang et al., 2009) andCycas guizhouensis (X. Y. Feng et al., 2016) have experienced population expansions during the most recent glacial period, whileCycas debaoensis (Zhan et al., 2011), Cycas simplicipinna(Xiu Yan Feng et al., 2014), Cycas diannanensis (Jian Liu et al., 2015), Cycas multipinnata (Y. Q. Gong et al., 2015), Cycas dolichophylla (Y. Zheng et al., 2016) and Cycas segmentifida(Xiu Yan Feng et al., 2017) have experienced population contractions. Improving knowledge about the demography of Cycas species will help us to predict how they will react to environmental fluctuations in the future and to draft conservation strategies for them. Considering that C. micholitzii had higher genetic diversity and accumulated more variation than C. bifida , it is capable to adapt to the changeable climate and environment in the future (DeWoody, Harder, Mathur, & Willoughby, 2021). On the other hand, massive illegal digging of C. bifida for trading or ornaments was occurred, and its habitat was destroyed more seriously for planting commercial or medicinal crops, which reduced its number. Therefore, more attention should be paid to protecting the genetic diversity of C. bifida . Of course, C. micholitzii cannot be ignored.