Riparian woodrat and black rat competition: investigating the role of an exotic species in the decline of a native endangered species

Abstract

Competition from non-native species constitutes a significant threat to numerous native species worldwide.  Endangered riparian woodrats (Neotoma fuscipes riparia) may be restricted to a single population occupying approximately 100 ha at Caswell Memorial State Park (CMSP) in central California.  This population is vulnerable to a number of threats including from non-native black rats (Rattus rattus) that co-occur at CMSP.  Black rats potentially engage in both interference and exploitative competition with woodrats.  From September 2001 to September 2004, we investigated interactions between riparian woodrats and black rats to determine whether competitive interactions were reducing woodrat abundance or reproductive success.  Two sites with riparian woodrats were identified at CMSP.  Between January 2003 and September 2004, 179 black rats were removed from one site.  Abundance of both species and woodrat reproductive success were assessed through live-trapping and radio-telemetry.  Mean litter size, mean number of young for litters with emerged young, and mean number of young per female were all higher on the black rat removal site compared to the control site.  Also, mean number of young for litters with emerged young increased on the removal site from 2003 to 2004 and decreased on the control site between years.  Woodrat abundance trends were more equivocal and actually were higher on the control site during most of the period of black rat removal.  The results of this investigation suggest that black rats may indeed suppress reproductive success of riparian woodrats, and that black rat removal could benefit woodrats.  Black rats have been implicated in the declines and even extinction of other native rodents, including other woodrat species.  Thus, black rats may constitute a significant threat to riparian woodrats, particularly in concert with other threats such as flooding, wildfires, and continued habitat loss and degradation.  Therefore, we recommend that black rats be removed quarterly from highly suitable woodrat habitat in the CMSP.  We also recommend that surveys be conducted to identify additional riparian woodrat populations and that black rat removals be conducted in those populations as well.