You’re driving down a long straight roads of the Mackenzie country. Beautiful mountains and vast tussock land looking all too photogenic and some would even say majestic. And yet there are hectares upon hectares of land that are not being utilised.
You wonder, is it caused by a lack of rainfall? Probably, but that’s not the full answer. Maybe, the Department of Conservation or the government owns the land? This is generally correct but still not what we’re are looking for. Maybe, sheep or cattle just don’t do well in the extreme environments of the high country, with freezing cold nights and scorching hot days? But they do well enough…
The real and completely destructive reason as to why these gorgeous paddocks with a view aren’t being used is because of rabbits (Oryctolagus cuniculus). Yep, you heard me right. The wee fluffy bum hippity hoppity rabbits that many people have as pets, are destroying OUR natural landscape and are changing it for the worse.
Now, I know what you are thinking… But Brittany what are you talking about? Cute little rabbits surely can’t be ruining such a vast amount of land? That’s impossible! Well let me give you a bit of background here.
Rabbits were introduced by the Europeans to New Zealand in the 1830’s for food and sport. But unknown to the Europeans (you’d have to think that they didn’t know, right?) rabbits reproduce at an alarming speeds, and so did the population that was introduced. This turned into plague populations in many parts of the country, including the Mackenzie High Country. It was nothing short of an ecological disaster.
Rabbits are a major pest in New Zealand as they graze down pastures which means farmers and stock can’t productively function on the land. Many of the worst affected areas were once well covered with tussock, grasses and small shrubs, but the rabbits reduced the vegetation cover, leading to soil erosion by wind and rain. The loss of soil has left areas where only the hardiest colonizing plants will now grow. Burrowing by rabbits in some soil types and on steep slopes has increased the soil erosion.
Michael P. Scroggie and colleagues found that, in the absence of all herbivores (rabbits, hares and sheep) in arid parts of Otago, the vegetation was predicted to grow in all seasons apart from winter and in all but the most degraded areas.
In the absence of sheep, but in the presence of up to 10 lagomorphs (rabbits and hares) seen per kilometre, accumulation of vegetation biomass follows the same seasonal pattern as in the absence of herbivores. As lagomorph counts reach 50 or more/km, vegetation biomass accumulates only in spring and summer in the least degraded sites.
In the least degraded sites some sheep can be grazed while maintaining positive pasture growth even when there is a moderate presence of lagomorphs (~30/km). However, in the most degraded sites only a very low density of sheep can be maintained during in spring and summer with a small presence of lagomorphs (~10/km).
There have been many attempts at controlling rabbit populations, from hunters doing spotlight hunts, rabbit fences, to poisons and biological control, such as introducing viruses to wipe out the population. There are some strains of viruses that have been legal and some spread illegally.
One of these legal strains is called the rabbit haemorrhagic disease (RHD) and is characterized by acute necrotising hepatitis, with haemorrhages found in other organs, in particular the lungs, heart, and kidneys due to the spread of intravascular coagulation.
Ben Reddiex (Lincoln University PhD at the time) and colleagues looked at the impact of predation and RHD on population dynamics of rabbits and the survival of juvenile rabbits. Hefound that rabbits born at the start of the breeding season had very high rates of post-emergence deaths, as they appeared to be susceptible to the RHD virus later in the breeding season.
The age at which juvenile rabbits become susceptible to RHD, the timing of RHD epidemics, and the abundance of predators are likely to be important in determining survival of juvenile rabbits. Reddiex’s study demonstrated that predation can reduce rabbit populations to low levels, but only in combination with other factors such as RHD.
James Ross, who helped with this paper on juvenile rabbits and predators, commented “as the original virus RHDV1 virility declines – areas with low predator abundance combined with favourable breeding conditions (semi-arid) should see big increases in numbers going forward – which we are – hence the release of a new strain RHDV1 K5 in 2018 – which has had some success – but others forms of control will be needed”. This suggests that rabbit abundance in favourable Mackenzie (semi-arid) country will always be a problem and require control of rabbits. Multiple measures will need to be in place as any one control method won’t be enough.
Rabbits are a pest mammal that is going to stick around regardless of all attempts of eradication by farmers, DOC or anyone. So, if we can just decrease the population sizes to somewhere balanced where they aren’t affecting productivity of land or erosion of land then I think that could be a win for us and for them.
Brittany Graham is a postgraduate student completing a Master of Science (Conservation and Ecology). She wrote this article as part of his assessment for ECOL 608 Research Methods in Ecology.