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The Stock Island tree snail, Orthalicus reses reses, went extinct in its native range in the Florida Keys in 1992. Fortunately, O. r. reses has been introduced elsewhere and further reintroductions are currently planned. Before these reintroductions are implemented, it is important to try and determine which factors were most likely to have caused the decline and extinction. While habitat destruction was probably the ultimate reason why there were so few tree snails, it is likely that an interaction of habitat fragmentation and the invasion of an exotic predator caused the final decline that lead to the extinction in 1992. We examined the last 93 O. r. reses shells to infer cause of death. In addition, using surrogate Florida tree snails, Liguus fasciatus, we conducted experiments on two previously unstudied causes of mortality: predation by red imported fire ants, Solenopsis invicta, and mortality from falls caused by wind.
We found that the majority of the last O. r. reses shells were intact, indicating that mammalian and bird predation were not the greatest causes of mortality. Mortality caused by wind knocking tree snails onto the rocky hammock substrate appeared to be a potential source of mortality, but few of the tree snails exhibited signs of breakage. Mortality from fire ants appears to be one of the most likely causes of the recent decline and extinction of MO. r. reses. Experiments indicated that in a semi-natural enclosure, fire ants were capable of killing all ages of L.fasciatus, even during aestivation. Fire ants are currently found throughout the last known habitat of O. r. reses and were first discovered in this area at the time of the decline.