, Volume 143, Issue 1, pp 70–76 | Cite as

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

  • Christopher N. Johnson
  • Karl Vernes
  • Alison Payne
Population Ecology


We compared demography of populations 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. These two species have suffered recent reductions in range, and they are ecologically similar to other species that have declined to extinction in inland Australia. It has been argued that part of the cause of the vulnerability of species like these is that they exhibit source–sink dynamics, and disturbance to source habitats can therefore cause large-scale population collapses. The results of our study argue against this interpretation.


Marsupial Trichosurus Aepyprymnus Habitat preference Fitness 



We thank Julian Caley, Andrew Krockenberger and Euan Ritchie for comments and discussion; Euan Ritchie for help with data analysis; and Bob Congdon for help with soil analysis. For field assistance we thank Samantha Fox, Diana Mendez, Euan Ritchie, Paula Winkel, Karen Wright, and many volunteers. Dennis and Tom Sheahan gave permission to work on their land. Permits were provided by the Queensland Parks and Wildlife Service. This project was supported by a grant from the Australian Research Council.


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Copyright information

© Springer-Verlag 2004

Authors and Affiliations

  • Christopher N. Johnson
    • 2
  • Karl Vernes
    • 1
    • 2
  • Alison Payne
    • 2
  1. 1.Department of Ecosystem ManagementUniversity of New EnglandNew South WalesAustralia
  2. 2.School of Tropical BiologyJames Cook UniversityQueenslandAustralia

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