Conservation genetics and demographic analysis of the endangered cycad species Cycas megacarpa and the impacts of past habitat fragmentation

Abstract

The limited seed dispersal range of cycads, longevity of individuals, including a long juvenile phase, coupled with dioecy, suggest that the impacts of habitat fragmentation are not straightforward. Cycas megacarpa K.D. Hill. is an endangered cycad that occurs in a highly fragmented landscape derived from past agricultural land clearing. Current threats continue to be land clearing for both urban and rural developments and major infrastructure development for coal seam gas. This study investigated the effect fragmentation has had on C. megacarpa populations across its entire range. Twelve microsatellite loci were developed with next generation sequencing and used to analyse 992 samples from 33 populations. Geographic information system (GIS) analysis of the total reduction in suitable regional ecosystem (RE) habitat in Queensland revealed a 69% loss between 1960s pre-clearing records and 2009 with only 29% of remaining habitat occurring in protected areas. Populations are highly variable with many small populations, and there is evidence of possible declining recruitment of seedlings. The relatively low proportion of adult sized plants that reproduce synchronously promotes outcrossing; however, there is some evidence that reduced population sizes may lead to inbreeding. Genetic diversity is not correlated with population size but it is weakly correlated with isolation. There is evidence of considerable historic gene flow among populations and little genetic differentiation among broad geographic regions. The results show genetic clustering and are consistent with hypotheses that C. megacarpa existed as a set of metapopulations historically linked by gene flow. This has maintained diversity in small populations but suggests that fragmentation that increases population isolation has and will continue to reduce the species viability. The persistence of adult cycads through multiple generations may allow rare alleles to remain within populations over a longer period of time.