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Abstract

I measured the rate of immigration by female common brushtail possums (Trichosurus vulpecula) in response to the removal of resident breeding females, in a landscape with no physical barriers to dispersal. I removed 10 residents from one 36 ha study grid and 9 from another, and monitored immigration over the next two years. Only one immigrant settled in one of the two removal areas. Sixteen breeding females resident on the periphery of the removal areas expanded their ranges into the removal areas. The one immigrant was a subadult that did not give birth in the breeding season following her arrival. Parentage analysis using microsatellite DNA indicated that the immigrant had moved only one home range away from her putative mother's home range (= 200 m). All of the known daughters of resident females settled beside their mothers. Parentage analysis indicated that 39 % of adjacent pairs of resident females were putatively mother and daughter, which is close to the 42 % expected if daughters always settle beside their mothers. The sex ratio of pouch-young was significantly male-biased, as predicted by the 'local resource competition' hypothesis, if most males disperse and most females settle beside their mothers. A deterministic, stage-based model of demography indicated that the birth rate was insufficient to balance the death rate (r = - 0.1), suggesting that the site was a 'dispersal sink'. However, even with immigration, the projection was that density would decline by 84 %. Neither 'old age', starvation, predation or disease could explain 11 of the 24 deaths among resident females. Most of these females demonstrated prior symptoms of stress. Females that were captured and handled more frequently had a significantly lower probability of survival, and the estimated adult survival rate was significantly lower than that expected from the observed age distribution. I suggest apparent 'dispersal sinks' may often be of our own making. Deaths due to capture and handling are analogous to removals. Consequently, since there were evidently more than 19 'removals', I conclude that the results of the experiment likely overestimate the importance of immigration in replacing losses among breeding females.