"The life of toheroa in the inter-tidal zone appears to be a high-risk high-reward one, their numbers blooming rapidly and enormously in response to ample food," he said.
"The downside for them is mass mortality events from being caught by waning tides and high temperatures on the beach, or retreating to deeper water for a slower, less productive life as food diminishes."
The various alternative theories had been undermined by the massive eruption of Mount Pinatubo in 1991, which resulted in a sharp drop in temperatures back to levels of the early 1900s. (The cooling effects of the eruption were also demonstrated by the heaviest frosts he could recall, inflicting significant plant damage).
Few of the factors behind alternative theories had changed, but westerly winds returned, as did the plankton and the toheroa, which remained on the beach until easterly winds resumed and temperatures returned to current levels.
One of the most persistent toheroa beds had been situated immediately south of Waipapakauri Ramp, where the majority of vehicles accessed the beach.
"Having observed how rapidly toheroa can move off the beach, using the returning action of sweeps, it is hard to imagine that they would have persisted so long there if they were stressed," Mr Austen added.
"Many were dug up by tourists, only to be replaced in the sand when the tourists were warned about hefty fines or were confronted by locals.
"A simple 'eggsperiment,' burying two eggs 20mm in damp sand then running over them with a 4x4 left them intact. That probably falls short of scientific rigour, but seems a reasonable test of vehicle impact, at least on adult toheroa.
"Nature being the dynamic force that it is, we may see sporadic recovery of toheroa on 90 Mile, but without a return to the conditions that allowed them to flourish historically any substantial return is unlikely."
Warning?Mr Austen also pondered whether toheroa were an isolated case or the "deceased canary in the coal mine." The northern scallop fishery had been decimated, and while few doubted that overfishing has been a contributing factor, the fishery had been closed for some time, and there should have been some recovery.
Reports from divers indicate there was little evidence of that.
"The influence of strong prevailing winds affecting the ability of scallop spat to settle and grow on was well evidenced in early May 2012, when hundreds of tonnes of scallops were cast up on to 90 Mile Beach following heavy swells and large tides after a perigee moon," he said.
"Though it is common local knowledge that there have always been scallop beds off 90 Mile, a beach casting of this magnitude was totally unheard of. Notably the scallops were of very consistent size, indicating that they were possibly from a single or possibly two spatfalls.
"A prolonged spell of calmer weather saw scallop numbers explode, indicating that these calmer conditions were more conducive to the successful settlement and on-growth of spat.
"Over the same period the remnants of the east coast scallop population were rolling up on east coast beaches with regularity, in response to the increase in easterly swell events and predomination of easterly winds, which have also caused considerable erosion on east coast beaches."
It was "worrying indeed," he added, that such an apparently minor temperature change could have such a dramatic effect on marine species. There are obvious positives associated with warming, such as for the avocado industry, but it was native species that had adapted over millennia that were most at risk as a result of change.
