Predation by introduced foxes on native bush rats in Australia: do foxes take the doomed surplus?

1. Introduced vertebrate predators are one of the most important threats to endemic mammal species. Prey naivety can lead to heavy losses to alien predators, which may be additive to 'natural' sources of mortality that limit prey populations. Alternatively, predators may take only individuals that are surplus to the population, and hence predator control may have little benefit for susceptible native prey populations. 2. A field-based predator removal experiment was used to test the predator limitation and doomed surplus hypotheses on the impact of introduced red foxes Vulpes vulpes on populations of native bush rats Rattus fuscipes in south-eastern Australia. 3. Poison baiting was used in July 1993 to reduce fox numbers in two fox-removal sites from 2.8-3.4 km(-1) (spotlight counts) to less than 0.5 km(-1) within 6 months. Fox density in two non-removal sites remained typically five times higher than that in removal sites. 4. Bush rat numbers on replicated trapping plots showed no response to fox removal, and rodent numbers fluctuated seasonally in all sites over 22 months of fox control, which represented two breeding seasons for rats. 5. Fox removal also had no effect on rat persistence time, adult body weight, captures of juveniles or immature animals during the breeding season, nor captures of immigrant or transient animals. 6. The general lack of response by rat populations to fox removal supported the doomed surplus hypothesis, that fox predation operated as a compensatory source of mortality rather than an additive one. Consequently, there was no measured benefit to native rat populations of intensive short-term fox control. 7. The results suggest that where predation pressure is low, not all predation mortality will be additive to prey populations even if it results from a predator introduced to the ecosystem. Hence, indiscriminate control of introduced predators is unlikely to produce uniform benefits for all the species they prey upon. Feral predator control should therefore be targeted for native species known to be predation limited or for species where any mortality threatens persistence.