Weeds keep up the attack on riverbed
An invasion of exotic weed species along the Ashley/ Rakahuri River continues to put riverbed breeding bird species at risk, clogging potential nesting sites.
Ashley/Rakahuri Rivercare Group (ARRG) member Grant Davey said that without any decent flooding since May 2014, weeds had ‘‘taken over’’.
Using aerial photographs and satellite images, ARRG had been compiling information on the correlation between weeds, bare gravel levels and bird numbers.
‘‘If you plot a graph showing the flow at the gorge and the amount of bare gravel, it’s pretty clear,’’ Davey said.
ARRG would apply again this year for funding to clear gravel islands for potential nesting sites and estimated around 73 hectares of cleared space would give the birds a ‘‘good chance to nest successfully’’, Davey said.
Last year, islands were cleared in August to allow nesting for ngutupare (wrybill), tuturiwhatu (banded dotterel), poaka (pied stilt), torea (oystercatcher), tarapiroe (black fronted tern) and tarapuka (black-billed gull). Davey said that between 2006 and 2014 there were consistent big floods along the Ashley/Rakahuri River.
‘‘We don’t have records back terribly long, I can only go back to 2004, but it’s the worst we’ve had.
‘‘My estimate now ... is around about 30 hectares [of bare gravel], about a sixth of what was available in 2006.’’
Davey said a flow of more than 100 cumecs (cubic metres per second) would flush out loose weeds, such as lupin, ‘‘pretty effectively’’.
But there was also a problem developing along the river with increasing numbers of gorse, broom and willows growing further into the riverbed.
‘‘Not only is more weed developing, we’re getting more nasty stuff which would be difficult to flush out.’’
Davey said weeds could rapidly regrow along the riverbed, meaning that ‘‘until someone comes up with a way of getting rid of nasty exotic weeds’’, ARRG would continue clearing the weeds and studying their effects.
‘‘There’s spots where it was clear in 2015 and now you have metre high lupin,’’ he said.
‘‘We could quite easily get back to only a couple of nesting wrybills. You get a couple of seasons of that and they might not come back.’’