Population responses to sterility imposed on female European rabbits

Published source details Williams C.K., Davey C.C., Moore R.J. Hinds L.A., Silvers L.E., Kerr P.J., French N., Hood G.M., & Krebs C.J. (2007) Population responses to sterility imposed on female European rabbits. Journal of Applied Ecology, 44, 291-301.
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Summary

In Australia, the introduced European rabbit Oryctolagus cuniculus is considered a pest species which damages crops, pastures and native flora. Most control efforts aim to kill rabbits or destroy their burrows but the scope of such measures are restricted as rabbits infest vast areas and resources to control them are limited. Two rabbit-specific diseases, myxomatosis (introduced in 1950) and rabbit haemorrhagic disease (RHD, introduced in 1995), suppress rabbit populations, but additional methods are needed. A control option under development, immunocontraception, is intended to suppress the rabbit's high fertility. It would spread contagiously via genetically modified myxoma virus and European rabbit fleas Spilopsyllus cuniculi. An experiment with field populations of rabbits assessed whether suppressing fertility reduces their abundance.

Study sites: The study was undertaken at 12 sites on grazing land in the Southern Tablelands of New South Wales, each with an average of 31 rabbit warrens (range 20-46) and an expected minimum of 40 female rabbits. At each a warren was fenced with wire netting.

Treatments: In February 1993, four levels of sterility treatment were assigned randomly in three replicates to the 12 sites. 0%, 40%, 60% or 80% of females (over 0.5 kg) were surgically sterilized. In February, March and July of 1993-96, new female recruits (over 0.5 kg) were sterilized in the proportions initially assigned.

Monitoring: Rabbits were live-trapped in cage traps (then released) in September 1992 and annually in February, March, July and November until July 1996. Each rabbit was weighed, ear-tagged and assessed for status of pregnancy and lactation or testis development. Population abundance (estimated by capture-mark-recapture), flea abundance and Myxoma virus transmission were assessed. Active entrances of burrows were counted at about 6-weekly intervals, including five comparative non-fenced, non-trapped warrens.

Rainfall and growth of forage strongly influence rabbit breeding in Australia. Annual rainfall during the study was highly variable (383 to 800 mm). It was above average in 1992, normal in 1993, very low in 1994, low and then high in 1995 and above average in 1996. Warren surveys showed a progressive decline in percentage of active burrows. Populations declined during the 1994-95 drought , increasing in late 1995, and declining after the summer of 1995-96. The untrapped sites behaved identically.

Population sizes and survival: The sterilized populations produced fewer young but the average adult population size remained unchanged in all treatments. Sterilized adult females survived much better than fertile females. Immature rabbits and unsterilized adults of both sexes also survived better in the sterilization treatments; this improved survival compensated for reduced reproductive output.

Flea counts: Fleas were fewer on both adult females and males in the sterilized populations but this did not impede transmission of myxomatosis.

Conclusions: Imposing sterility reduced breeding-season peaks of abundance but improved survival compensated for female sterility even up to 80% and this sustained populations, even in the presence of detrimental effects of drought and myxomatosis. The authors conclude that immunocontraception alone has poor prospects for controlling rabbits.

Note: The compilation and addition of this summary was funded by the Journal of Applied Ecology (BES).If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-2664.2006.01264.x