Conserving dryland lizards by reducing predator-mediated apparent competition and direct competition with introduced rabbits

Authors


Grant Norbury, Landcare Research, PO Box 282, Alexandra, New Zealand (fax + 64 3448 9939; e-mail norburyg@landcare.cri.nz).

Summary

  • 1Native skinks Oligosoma spp. in New Zealand’s dry grasslands have declined in range and abundance since the arrival of humans. I hypothesized that introduced rabbits Oryctolagus cuniculus exacerbate this decline by supporting introduced mammalian predators for which they are primary prey; by sudden declines in abundance that cause predators to switch to skinks; by grazing vegetation, thereby reducing skink refuges from predators; and by reducing skink food and shelter from climatic extremes. The first three effects cause enhanced skink predation and represent indirect or apparent competition. The fourth effect represents direct competition.
  • 2Interactions between two introduced predators (ferrets Mustela furo and cats Felis catus), rabbits and two secondary prey (McCann’s skink O. maccanni and common skink O. nigriplantare polychroma) were studied. By measuring skink consumption, and predator, rabbit and skink numbers, offtake of skinks by predators under varying rabbit and skink densities was calculated.
  • 3Predation by ferrets and cats was inversely density-dependent because predation accelerated at low skink densities. As skink densities rose, predation became an increasingly less important source of mortality. Therefore, predation could potentially exterminate skink populations if densities fell below some critical range.
  • 4Predation of skinks increased markedly after sudden declines in rabbit abundance, because predators remained abundant but switched to feeding on skinks. Although a temporary effect, repeated cycles of intense rabbit control and population recovery may have chronic detrimental effects on skink population viability.
  • 5Optimal rabbit management for maintaining viable skink populations is likely to require avoidance of large swings in rabbit abundance by maintaining populations at low, stable levels. Fewer rabbits mean fewer predators and greater refuge (less apparent competition), and improved food and shelter (less direct competition). If large swings in rabbit abundance cannot be avoided, the effects of prey-switching could be reduced by controlling predators when rabbit numbers decline.
  • 6Better understanding and management of primary–secondary prey systems, where the primary prey consume the habitat of secondary prey, will require recognition of the extra complexities that underlie these systems.

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