Genetic variation in fragmented populations of an Australian rainforest rodent, Melomys cervinipes

Abstract

Small, isolated populations of an Australian rodent, Melomys cervinipes, occur in rainforest fragments on the Atherton Tableland in northeastern Queensland. We studied the genetic structure of four of these populations: one island (4.3 ha; isolated in 1960), three fragments (2.5, 7.5, 97.5 ha; isolated between 1920 and 1930) and a control population in continuous rainforest. The relative density of M. cervinipes did not differ among the populations, hence population size was approximately proportional to the forest area. Electrophoresis was performed on blood samples taken from the populations. Average heterozygosity (H) was estimated from the allelic distribution of 24 loci for each population and varied from 0.01 to 0.05. The island population had reduced heterozygosity compared to the control population, but the fragment populations were not significantly less heterozygous than the control. Although the fragment populations were markedly different in size, they did not differ in heterozygosity among themselves. The ability of fragment populations to maintain genetic viability is probably due to migration. The rainforest fragments exist in an extensive grassland containing a variety of habitat corridors which could facilitate movement and gene flow. Following the electrophoretic work, spool-and-line and radio tracking and live-trapping were carried out in the corridors; the presence of M. cervinipes in the corridors and its use of the corridors for movement was confirmed. The island population appears to be substantially more isolated than the fragment populations as water is likely to be a much more effective barrier to movement in M. cervinipes than is heterogeneous grassland. The genetic viability of the island population has probably been reduced through drift, leading to fixation of alleles: six of eight polymorphic loci being fixed in the island population. We therefore suggest that retention or establishment of habitat corridors is an important means of sustaining the genetic variability of populations in fragmented systems.