Impact of an invasive predatory lizard on the endangered Hawaiian tree snail Achatinella mustelina: a threat assessment

All 41 species of the Hawaiian tree snail genus Achatinella were listed as endangered in the early 1980s, primarily due to predation by invasive species. Today, only 9 species are estimated to remain. The recent discovery of A. mustelina in gut contents of Jackson's chameleons Trioceros jacksonii xantholophus, a predatory arboreal African lizard introduced to Hawaii (USA) in 1972, was the first documentation of environmental impact by any reptile on native Hawaiian taxa. In this study, we assessed the predatory threat posed by chameleons to A. mustelina by (1) determining shell digestion and passage rates in daily fed and fasting individuals and (2) using these data to estimate time of ingestion of snails found in guts of field-collected chameleons. Our results indicate that daily food intake increases passage rate. Well fed chameleons passed largely intact A. mustelina shells in similar to 4 to 5 d, whereas starved individuals retained shells in their stomachs until complete digestion in similar to 8 d. Shell mass was digested at similar to 12.5% d-1. In total, 8 A. mustelina were found in 45 field-collected lizards, and estimated time of ingestion, based on shell condition, was similar to 3 to 4 d prior to capture in all cases. Given a predator density of 45 per 2000 m2 (0.5 acre) and assuming a feeding frequency of 8 snails every similar to 3 to 4 d, we conservatively estimate that 45 chameleons could consume 730 to 974 snails yr(-1), per 2000 m(2). Given the low growth and reproductive rates of Hawaiian tree snails, this level of impact is probably unsustainable even over the short term, and immediate control action is warranted.