Conservation and genetics in the fragmented monsoon rainforest in the Northern Territory, Australia: a case study of three frugivore-dispersed species

Abstract

Monsoon rainforest in the Northern Territory exists as an archipelago of tiny patches thought to be previously more continuous but having had retracted to a fragmented state by the Pleistocene. However, there is evidence of some more recent rainforest expansion, and many species have seeds that are dipsersed by small frugivores (birds and bats). Carpentaria acuminata and Ptychosperma bleeseri are endemic to the Northern Territory and Syzygium nervosum is endemic to the Northern Territory within its Australian distribution. Carpentaria acuminata and S. nervosum are common through moist rainforest, but P. bleeseri is rare, found in eight locations near Darwin. All have frugivore-dispersed seeds. The three species were surveyed across their geographic range to compare their population genetics. The results from C. acuminata and S. nervosum were analysed to investigate the distribution of allelic diversity, the effect of patch size and isolation on genetic diversity, and the effectiveness of seed dispersal by vagile frugivores. Genetic diversity (expected heterozygosity) was correlated with mean annual rainfall in both S. nervosum and C. acuminata. Genetic diversity (alleles per locus, percentage of polymorphic loci, expected heterozygosity) was also negatively correlated with population isolation but not patch size in C. acuminata. There was significant heterogeneity and low gene flow among C. acuminata populations (FST 0.379, Nm = 0.39) but not among S. nervosum populations, which had substantial evidence of gene flow (FST 0.118, Nm = 1.67). The distribution of rare alleles in C. acuminata was consistent with the theory of Pleistocene retraction to refugia. Ptychosperma bleeseri was genetically uniform across all sites. Only four individuals differed in their genotypes. The results suggest that the present populations originated from a single founder population rather than having independently contracted from a more extensive distribution. All three species had evidence of some more recent population expansion.