Optimal management of invasive pests can benefit from quantitative measures of rates of recruitment, and particularly, relative contributions of immigration and reproduction. However, these vital rates are difficult to estimate by trapping or observation. Recent studies have demonstrated that analyses with DNA markers may provide detailed information on the origin of immigrants into pest populations, but these studies have not provided comparable data on reproductive rates. We integrated genetic and demographic information from a unique longitudinal data set to comprehensively quantify recruitment during the past 15 years into an island population of red foxes (Vulpes vulpes) and to reveal relative contributions of immigration and reproduction. This population established 100 years ago and persists despite several decades of management aimed at population suppression. Primary source of recruits on the island was in situ reproduction (>95%/annum), although the number of foxes reproducing was small relative to the total number present. Immigration occurred at rates up to 3.6%/annum and was primarily by dispersing males, but is unlikely to be demographically important. We also show that although fox control effectively reduced fox density, there was evidence that control did not reduce the net number of recruits, most likely because the population exhibited a density-dependent release from reproductive suppression. Our results imply that fox control on Phillip Island should primarily focus on reducing on-island abundance and reproduction, but eradication will not be sustained unless immigration ceases.
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Vol. 74 • No. 8