Mystery still surrounds exactly how a group of renegade farmers imported the calicivirus into New Zealand in 1997 and spread it around the South Island in a bid to kill the rabbits that were destroying their land.
But 17 years later, scientists have been able to give some indication of the impact the illicit introduction of the virus had on rabbit populations. They used data gathered over 23 years at Alexandra’s Great Easter Bunny Hunt where the rabbits shot during that day are counted up. It turns out that rabbit kills counted after the hunt fell 60 per cent following the introduction of calicivirus – or Rabbit Haemorrhagic Disease (RHD) as it is now known. Kill rates remained low for several years afterwards before gradually increasing again and then fluctuating year to year.
“The disease initially had a dramatic negative effect on kill rates, but this effect began to wane 4 years later. Similar results based on spotlight counts have been reported from the same area, and from the MacKenzie Basin nearby.”
Counting rabbits killed in the Bunny Hunt could be a good indication of the health of rabbit populations, indeed better than spotlight counts, where rabbits can easily be double-counted. But the researchers point out that not all farms in the region participate in the Bunny Hunt, so it’s not a true random sample. Many farmers down there believe that rabbits are so numerous in the region that the Bunny Hunt barely makes a dent in, or reflects rabbit population numbers.
This table from the researchers’ paper well illustrates the impact RHD had on rabbit populations:
However, conventional rabbit control in the few years after 1997 were at very low levels in the region, so the scientists are fairly confident the disease was the major influence on rabbit populations during the period.
RHD continues to be active in the rabbit population:
“RHD still appears to be killing rabbits (low rates of increase) but its efficacy as a biological control agent is waning, compared with the first outbreaks of the disease.”
So how many rabbits do they kill at the Great Easter Bunny Hunt? Well between 1991 and 2013 they counted 248,000 dead rabbits. That equates to 35.1 rabbits shot per team member in a 24 hour period.
Since the calicivirus controversy of 1997 the government has legalised use of RHD to control rabbit populations, however its effectiveness has decreased as rabbits have developed immunity to it. The main rabbit control measures remain shooting, trapping and poisoning.
This article has some good background on the issue.
A summary of the Wildlife Research paper…