Mating to extinction
There might be a two-step extinction underway in New Zealand. In the first step, native grey ducks are breeding with exotic mallard ducks and creating a hybrid bird. Some believe the New Zealand grey duck is now pretty much extinct as a pure species.
In the second step, a Southern Hemisphere lice that’s known to inhabit grey ducks in Australia and was assumed to inhabit the birds here too wasn’t found by researchers from Massey University and Te Papa in a recent study.
While that’s not evidence that the lice has died out here, it does remind that extinctions can have knock-on effects for other species. But it’s complex.
Mallards were introduced to New Zealand early in the 20th century and were established by 1910, according to a 2011 study. It showed a minimum of 30,000 mallards were released by acclimation societies up until the 1970s. It also found that private releases were impossible to count but considerable.
Mallards and grey ducks can mate. Their hybrid offspring can mate as well – and on through generations.
The process is known as ‘‘extinction through hybridisation’’. It also affects pied stilt and kakı¯ birds, which interbreed, and probably played a role in the extinction of the kurı¯ dog, but data are lacking.
These species are essentially mating their way to extinction. It is an aspect of evolution.
Bird watchers, hunters and scientists have known about these hybrid grey-mallard birds for decades.
The result is that ‘‘few pure grey ducks may now exist’’, according to NZ Birds Online, an authoritive collaboration by Te Papa, DOC and the Ornithological Society.
The conservation status of greys is ‘‘nationally critical’’, the worst state before extinction.
The trouble is that grey ducks and hybrids tend to look like female mallards. It’s hard to know what you’re looking at.
It also means hunters don’t know if they are shooting pure grey ducks, hybrids or female mallards. Hunting of all three is legal.
However, Dr Murray Williams, a biologist at Victoria University, said habitat loss probably accounted for more grey duck loss than hunting. And now to step two, the lice. Dr Mary Morgan-Richards, a professor in wildlife evolution at Massey and colleagues, got hold of 40 hybrid ducks and looked at the lice they carried.
Each duck was host to about 60 feather lice from three different species.
The research group then looked at mitochondrial DNA sequences from the ducks and from their lice.
‘‘Although we found both grey duck and mallard duck [genetics] we found no evidence of local New Zealand lice,’’ she said.
That was unexpected. ‘‘All the lice sequences were the same as mallard lice’’ from the Northern Hemisphere.
In theory, there ought to have been Southern Hemisphere grey duck lice although its existence has not been established.
The results suggested that ‘‘we may already have lost endemic parasite lineages due to hybridisation’’, MorganRichards wrote.
‘‘Based on the evidence from Australian waterfowl lice, there’s a lineage of grey duck lice that we didn’t find in New Zealand and we thought we would,’’ she said in an interview.
‘‘You’d expect to see that lineage of lice – or one that’s unique to New Zealand or at least different,’’ she said.
But when something is missing, it’s hard to say it ever existed.