Short-term ecological effects of rabbit haemorrhagic disease in the short-tussock grasslands of the South Island, New Zealand

Rabbit haemorrhagic disease (RHD) has reduced populations of rabbits (Oryctolagus cuniculus) across most rabbit-prone short-tussock grasslands of New Zealand, at scales rarely seen there before. Flow-on effects to other parts of these ecosystems will be inevitable. We report evidence for increases in pasture biomass, increases in abundance of other exotic herbivores, declines in abundance of rabbit-specialist predators, and short-term increases in predation rates of some native birds by these predators. At one site in Central Otago, RHD reduced an index of rabbit abundance by 88%, and an index of their grazing impacts by 77%. Recovered biomass consisted mostly of fast-growing exotic pasture species of moderate palatability to livestock. Spotlight counts and hunters' returns suggest increases in possum (Trichosurus vulpecula) and hare (Lepus europeaus) abundance, but their grazing pressure is unlikely to replace that originally imposed by rabbits. The apparent increase in possum numbers poses an increased risk from the spread and maintenance of bovine tuberculosis (Tb), although this risk may be offset by declines in the counts of ferrets (Mustela furo), which also carry Tb. Declines in predator numbers (including feral cats, Felis catus) may also, in the longer term, benefit some native fauna that are secondary prey of these predators. There is evidence for increased predation of some native birds' eggs since RHD arrived. It is not possible at this stage to generalise the effects of RHD-induced declines in rabbit abundance on New Zealand ecosystems. Effects are highly variable, and their implications for pastoral production, management of bovine Tb, and conservation of native species are likely to vary locally according to the suite of plant and animal species originally present.