The disastrous decision to introduce non-native snails to Tahiti led to the extinction of 56 species of tree snails. Yet a handful survived. Using the world’s smallest complete computer, biologists have explained the most puzzling of these anomalies in Communications Biology, perhaps improving our chances of preserving what remains.
Snails may hold little interest to the general public, but the land snails of the South Pacific make up a grossly disproportionate share of the creatures whose extinction is humanity's fault. The tree snails of the Society Islands (including Tahiti), for example, were called the; “Darwin finches of the snail world.” Their extraordinary abundance, with at least 61 species adapted to slightly different environmental niches, represented a treasure-trove for biologists seeking to understand the workings of evolution in microcosm. A century ago lighter-shelled snails such as Partula hyalina were reported as occurring more often in areas with more sunlight. Long before this, P. hyalina’s white shells made it particularly important to Polynesians, who used them for jewelry and leis.
Then came the foolish decision to introduce the giant African land snail to the south Pacific to diversify available food sources. Even worse was the subsequent decision to introduce the rosy wolfsnail, a predatory species from North America to undo the previous mistake. This disaster was settled upon in 1974, forty years after Australia’s effort to control one invasive species with another became a globally recognized disaster.
"The endemic tree snails had never encountered a predator like the alien rosy wolfsnail before it's deliberate introduction. It can climb trees and very quickly drove most of the valley populations to local extinction," Professor Diarmaid Ó Foighil of the University of Michigan said in a statement. Among the few survivors, most persisted at high altitudes or in isolated valleys. Foighil wanted to know why P. hyalina was the exception. He suspected their white shells were a clue, reflecting so much sunlight it could survive in conditions that would overheat its dark-shelled counterparts.
The problem was that to confirm this, Foighil needed detailed records of snails' sunlight exposure, and at the time no sensors were available whose weight would not impede the snails’ movements.
Enter the Michigan Micro Mote (M3). Although microscopic devices capable of performing simple computations exist, its makers claim the M3 is the smallest “complete” computer, one that can input, process, store, and output data. In its first field application, the M3 was combined with a battery and solar cell, allowing the sunlight to be measured based on how fast the battery charged.
P. hyalina is protected, and couldn’t be subjected to the indignity of sensor attachment. Instead Foighil and colleagues stuck the sensors to the wolfsnail shells, and placed them on leaves next to where the nocturnal P. hyalina slept.
Their findings revealed P. hyalina can spend the day in areas with sunlight 10 times stronger than the rosy wolfsnail tolerates, allowing it to escape the intruder’s predations. Almost certainly the now-extinct snails lacked this capacity.
Besides solving a single mystery, Foighil believes; “The M3 really opens up the window of what we can do with invertebrate behavioral ecology,” allowing scientists to track species previously considered far too small.