By Daniel Ramp, University of Technology, Sydney and Karl Vernes, University of New England
“Senator Rhiannon embarrasses herself by sharing conspiracy theories about kangaroo numbers being in terminal decline. She only has to drive between Winton and Longreach at night to see the plagues of roos.” Queensland Nationals Senator Barry O’Sullivan, press release, February 13, 2015.
Senator O’Sullivan’s comments, made after Greens Senator Lee Rhiannon raised concern over kangaroo populations being at risk, reflect a common view that Australia is swarming with kangaroos.
But how secure are kangaroo species? Are they in plague proportions, necessitating large-scale killing and a commercial industry, or are they in decline and at risk?
Are ‘roos at risk?
We don’t yet have clear evidence showing kangaroos are at risk of extinction. But we cannot afford to get too comfortable, given their historical decline.
The NSW Scientific Committee last year rejected a 2011 nomination that four species of kangaroos be listed as vulnerable. You can read their preliminary determinations for the four species here, here, here and here.
But despite what Senator O’Sullivan has seen on the night drive between Winton and Longreach (an area adjacent to a national park), Australia is not swarming with kangaroos. A plague is not biologically plausible because kangaroos are slow growing.
Juvenile mortality rates are high and female kangaroos tend to have only three or four joeys survive in their lifetime. Compare that to rabbits, which can produce up to 14 young in a litter. Kangaroo populations, as a whole, do not grow more than 10% in a year.
While it’s true there are large kangaroo populations in some parts of Australia, in other parts they are scarce. As population numbers are reported to fluctuate a lot within a few short years, it depends on which period you are looking at.
So we should be very careful about blanket statements about “plagues” of kangaroos. They do not represent the overall picture.
Six species in the kangaroo family have gone extinct since European settlement, with over 60 species remaining. Some species remain endangered, like the brush-tailed rock-wallaby, which was killed in the hundreds of thousands for its fur. Others remain in substantial numbers, but human history is littered with examples of causing the extinction of populous animals.
In 1982, CSIRO researchers Short and Grigg reported that 85% of kangaroo populations had been reduced to below one per square kilometre in western Victoria. In Tasmania, the kangaroo population is at 10% of pre-European settlement numbers.
What can we infer from recent surveys?
When asked by The Conversation for data to support her concern that kangaroos are at risk, Senator Rhiannon referred to government records collected over the past 30 years from commercial harvest zones in four states, estimated from aerial surveys of transects. Animals seen from planes are multiplied by correction factors and extrapolated to entire zones.
As the graph below shows, these records showed kangaroo numbers fell between 2001 and 2011. (The federal government stopped providing national kangaroo population numbers online in 2011, although they remain reported on a state-by-state basis.)
Referring to a 2011 nomination to list kangaroos as threatened, Senator Rhiannon highlighted a decline of 40% in national populations between 2001 and 2011.
However, population estimates were then reported to increase between 2011 and 2013 due to reasonable rainfalls.
Nationals Senator O’Sullivan has also said that populations in Queensland have more than doubled from about 12 million to more than 25 million over the past decade. The available data says that is true.
Can both be right?
Kangaroo populations grow slowly and decline rapidly during drought. Many localities in Queensland have recorded declines of over 50% since 2013.
Both senators appear to be looking at two different windows of population estimates, ranging between two peaks ending with the 2001 and 2014 droughts.
Senator O’Sullivan also recently said that kangaroos are “taking up to half of the pastures on some drought-stricken properties across Western Queensland”.
However, CSIRO research shows little direct impact on crops and little competition with livestock for pasture. Kangaroos do not benefit from artificial watering points, rarely drinking.
A spokesman for Senator O’Sullivan said that “heavy grazing pressures placed on properties by kangaroos across Western Queensland is anecdotal. However, we feel it is highly reliable.”
The elephant in the room is that the data isn’t really adequate to paint an accurate picture of kangaroo populations in Australia. Both Senator Rhiannon and the NSW Scientific Committee have noted frailties with the data.
We don’t yet have data showing that kangaroos are at risk, but we cannot afford to be complacent given their historical decline.
The senators are comparing apples and oranges, looking at different data sets from different time periods and extrapolating to paint a picture of either plague or near-endangerment.
There is less and less space for kangaroos: more kangaroos are killed for meat and skins than any other mammal species, they are killed for trespassing on farmland, shot illegally, hit by vehicles, caught in barbed-wire fences and persecuted by domestic dogs.
If we want kangaroos to remain a part of the Australian landscape and identity, we need better population data upon which appropriate policy can be based.
The article is based on the best available population data collected by the various states on commercially harvested kangaroo species, and the vast scientific knowledge that has been amassed on their biology and ecology.
Kangaroo populations definitely fluctuate with climate, a factor built into harvest models. For example, annual surveys since the early 1980s show dramatic declines (sometimes as high as 97%) related to several droughts from the 1980s to the 2000s, but then (albeit slower) recoveries of the populations during intervening times of better than average rainfall. However, because quotas reflect not only current population estimates but also the modelled impact of current and projected climate on numbers, quotas should protect populations from over-harvesting.
All available evidence points to the industry being sustainable; furthermore, quotas are almost never met, so even fewer kangaroos are harvested each year than sustainable quotas allow for, further buffering the populations from the possibility of over-harvesting.
The author’s comments on complacency, however, are also sensible; over-harvesting kangaroos prior to modern management did result in alarming declines, and the few studies of likely impacts of climate change on large kangaroos suggest a negative effect on these species.
Thankfully, though, and despite recent political wrangling over the issue, it is my view we currently have a carefully managed, well-run and scientifically sound kangaroo monitoring program, which informs the setting of sustainable harvest quotas, which, in turn, should safeguard kangaroo populations well into the future. – Karl Vernes
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This article was originally published on The Conversation. (Reblogged by permission). Read the original article.