Can foxes regulate rabbit populations?

I report on the second part of a e-phase experimental test of the predator regulation hypothesis. I examined the effect of predation by red foxes (Vulpes vulpes) on the population dynamics of European rabbits (Oryctolagus cuniculus) in montane Australia. Foxes were permitted to reinvade 2 sites where indices of rabbit numbers had increased 10.3- and 23.3-fold after 20 months of fox removal, and compared trajectories of these rabbit populations with those at 2 other sites where fox populations were not controlled. Over 16 months, foxes returned to both removal sites, reaching levels comparable to those on nonremoval sites, but lower than preremoval densities. Rabbit populations declined immediately after foxes reinvaded and remained low for 16 months on one site, suggesting that fox predation was effective at regulating numbers. However, the rabbit population sn another higher-density site recovered and increased another 23% over the following 16 months, suggesting they were not regulated by predation. Rabbit numbers at nonremoval sites continued to be suppressed. These results did not provide consistent support for the predator regulation hypothesis, but provided evidence that rabbit populations may escape predator regulation once they exceed a critical density.