Abstract:
Invasive mammals have had significant negative effects on New Zealand’s biodiversity, but their interactions and impacts in urban environments are poorly known. Domestic cats (Felis cattus), ship rats (Rattus rattus), and Norway rats (R. norvegicus) prey upon native and exotic bird species. However, cats also prey on rats and therefore may convey some benefits for birds. There have been some policy and management concerns that removing either domestic cats or rats from urban fragments may have indirect negative effects on birds, by either increasing rat populations (via cat removal), or by cats prey-switching to birds (via rat removal). While policy to remove cats from reserves is unlikely to happen in the near future, community groups are removing rats from urban reserves, with unknown effects on cat behaviour. Therefore, I investigated the effect of reduced rat populations on cat visitation to urban reserves by conducting an M-BACI experiment across eight urban forest fragments. Through ground based trapping at four treatment sites I reduced rat trapping rates by 83% from an average of 8.5 rats 100 ctn-1 (S.E. = 2.7) to 1.7 rats 100 ctn-1 (S.E. = 1.3). During the five night trapping period prior to rodent control, camera traps recorded 241 instances of 49 individual cats visiting my eight sites. Neither number of cats visiting nor the frequency of visits significantly changed in response to the reduction of rat trapping rates. Although it appeared that rodent control elicited a shift towards more daytime visits, high inter-site variation made determination of causation difficult. Further research is required to investigate whether the hunting success or prey composition of cats changes following rodent control.