Demography in relation to population density in two herbivorous marsupials: testing for source-sink dynamics versus independent regulation of population size

We compared demography along gradients of population density in two medium-sized herbivorous marsupials, the common brushtail possum Trichosurus vulpecula and the rufous bettong Aepyprymnus rufescens, to test for net dispersal from high density populations (acting as sources) to low density populations (sinks). In both species, population density was positively related to soil fertility, and variation in soil fertility produced large differences in population density of contiguous populations. We predicted that if source-sink dynamics were operating over this density gradient, we should find higher immigration rates in low-density populations, and positive relationships of measures of individual fitness - body condition, reproductive output, juvenile growth rates and survivorship – to population density. This was predicted because under source-sink dynamics immigration from high-density sites would hold population density above carrying capacity in low-density sites. The study included 13 populations of these two species, representing a more than 50-fold range of density for each species, but we found that individual fitness, immigration rates and population turnover were similar in all populations. We conclude that net dispersal from high to low density populations had little influence on population dynamics in these species; rather, all populations appeared to be independently regulated at carrying capacity, with a balanced exchange of dispersers among populations. This study has implications for our understanding of the causes of decline of ‘critical-weight-range’ marsupials (of which these species are good examples), because it has previously been argued that source-sink dynamics provides part of the explanation for their high extinction rates