Less birds is a good thing?
According to the Dominion Post Environment Minister Nick Smith said the clean-up effort and the reduced number of dying birds was encouraging, with regards to New Zealand’s worst environmental disaster with the grounding of Rena on the Astrolabe Reef.
Although at first glance a reduced number of dying birds may appear to be a good thing, the underlying process driving this trend is not clear. It could be that most birds at risk to the oil spill have already died, and the reduced number of dying birds is in fact because there are no more birds alive left to die.
Statistically, what are confounded here are the population size and the probabilities of detection and impact (from the oil spill). When the total number of birds being found dead has declined, we don’t know if the population size has remained relatively stable, but the number being detected and impacted has declined, or if the population size has declined, but the number being detected and impacted has remained constant. In both cases the calculation of the absolute number of birds being recovered comes out the same, but under the first scenario, the impact of the oil spill has declined, whereas in the second, the oil spill is having the same impact.
I hope for the sake of the birds it is the first scenario, as Nick Smith hopes, because as Minister of Conservation Kate Wilkinson says, “it’s not their fault.”
James Russell is a quantitative ecologist jointly appointed in the School of Biological Sciences and the Department of Statistics. He was the 2012 Prime Ministers Emerging Scientist prize recipient. See all posts by James Russell »
James.. interesting point. At last count a little over 1000 birds have been found dead. Some of these will have died for reasons other than the oil spill of course as dead birds are always found on our beaches. One can assume many more have died that have not been found. Nonetheless this is an awful situation and the public outcry is justified and concern for the wildlife of the region is heartening.
What if all the native birds killed by the “mammalian Mafia” (the suite of introduced pests) were dumped on the beach at Tauranga? What would the outcry be? Here’s an interesting statistic you may be familiar with, from John Innes at Landcare.
Statistics quoted in a New Zealand Journal of Ecology and published on-line this month, were used to produce the multimillion death toll, which Mr Innes said was an under-estimate and did not include the adult, usually a female, which had been incubating the eggs or caring for the chicks. It also did not include introduced species such as blackbirds and thrush.
Mr Innes said forest covered 23 per cent of New Zealand, an area totalling 5.98 million hectares. He said if there were five native bird nests to each hectare in any nesting season, that was 29.9 million nests. Of those, 73 per cent failed – a total of 21.827 million nests and, at an average of two eggs per nest, a total of 42.654 million chicks which failed to fly from the nest.
Predators are blamed for 61 per cent – 26,628,940 – of chick and egg losses
http://www.stuff.co.nz/waikato-times/news/3129546/Mammal-mafia-waging-war-on-native-birdlife
26.6 Million per year. Where is the outcry? Out of sight, out of mind, but it’s real enough for out native birds 24-7.
13 years ago