A cave in the Kahurangi National Park's Ōparara Basin will close for a year to protect the rare spiders which live in it.
Nelson cave spider egg sac in Crazy Paving Cave in Kahurangi National Park's Ōparara Basin. Photo: Supplied / DOC / Richard Rossiter
With a leg span of 13cm and a 3cm long body, the Nelson cave spider is New Zealand's largest arachnid.
Department of Conservation senior biodiversity ranger Scott Freeman said surveys had shown spider egg sac numbers were decreasing in the Crazy Paving Cave, which could be due to the number of human visitors.
Only one egg sac had been seen since 2018.
"The number of spiders seen have actually increased from about 2019 onwards, possibly due to a decrease in visitor numbers associated with Covid. However, only one egg sac has been seen since 2018.
"Breeding is the real long-term driver of the population so we want to close the cave to see if this will allow breeding to improve. Closing the cave means we can monitor the spider population's response to the removal of human visitors."
The cave gets its name from its floor of ancient, fragile, fine deep sediment which has dried out slowly, cracking and curling into what looks like large, distorted paving stones. It is a dry cave where the sediment has stayed in place unchanged for hundreds of years.
Nelson cave spider in Crazy Paving Cave in Kahurangi National Park's Ōparara Basin. Photo: Supplied / DOC / Richard Rossiter
Nelson cave spiders are also found in Golden Bay. They are the only spider protected by the Wildlife Act 1953.
They are thought to be directly descended from the earliest true spiders and may be the missing link between primitive spiders - from the time of Gondwana 350 million years ago - and modern spiders.
Scientists have estimated that baby Nelson cave spiders take two to three years to mature. Most other spiders complete their entire life cycle in a year.
They live near cave entrances and dine mostly on cave wētā, finding their prey by vibration.
The young are born and raised in egg sacs which hang from the cave ceiling of the cave almost like small golf balls. Each sac can contain up to 50 small spiders.
The Crazy Paving Cave will close for visitors from 1 June for 12 months.