Rivers as obstacles to home range expansion by the brushtail possum

Strategies for defending large tracts of land from mammalian pest incursion are urgently needed. We report on a study investigating whether brushtail possum (Trichosurus vulpecula) range expansion into a controlled area was restricted by a watercourse. The true left of the Orongorongo River valley was treated with 1080 poison baits, and a 250 ha area bordering the river on the true right was excluded from treatment. Nontoxic cereal bait containing pyranine biomarker was sown repeatedly over half of the excluded area for nine weeks after poisoning. Traps installed on the true left of the river caught no marked possums. This outcome suggests that the river acted as an obstacle to possum movement, specifically, home range expansion into an area of low conspecific density. Our study contributes to the body of evidence that watercourses can inhibit possum movement, supporting operational practice that aligns eradication boundaries with rivers to slow the rate of possum reinvasion.