Severe Habitat Fragmentation Leads to Declines in Genetic Variation, Mate Availability, and Reproductive Success in Small Populations of a Once-Common Australian Grassland Daisy

Abstract

Premise of research. Inference concerning the impact of habitat fragmentation on genetic variation and demography of plant species is a key theme in conservation biology. For once-common Australian temperate grassland species where there has been almost complete habitat loss, generalizations about habitat fragmentation are unclear because most prior study has focused on rare species.Methodology. We examine the relationship between population setting (density, isolation), reproductive population size (n = 26 to >8000 flowering plants), and genetic variation by using allozyme markers, field seed set, and laboratory germination and growth in fragmented populations of the grassland daisy Leucochrysum albicans var. tricolor, a species once common across southeastern Australia.Pivotal results. Allelic richness, observed and expected heterozygosity, percent seed set, and seed germination were all positively and significantly correlated with log population size (). There was no significant association, however, between log population size and the level of polymorphism, the inbreeding coefficient, or plant fitness (measured as biomass at 28 wk). Diallel crossing showed that small populations were mate limited, suggesting that erosion of allelic richness at self-incompatibility loci may be responsible for lower and variable reproductive output.Conclusions. The outcomes of habitat fragmentation on plant population persistence of common species are likely mediated by population size, as this impacts mate availability via loss of genetic diversity. This will likely elevate the extinction risk of small populations in the long term in ecosystems where there has been intensive long-term land use and population losses and may require conservation measures that include genetic rescue to recover small populations.