Dietary behavior of the mangrove monitor lizard (Varanus indicus) on Cocos Island, Guam, and strategies for varanus indicus eradication

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2016-05
Authors
Ehrhard, Seamus Patrick
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Mautz, William
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Tropical Conservation Biology & Environmental Science
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The mangrove monitor lizard (Varanus indicus), a large invasive predator, can be found on all areas of the 38.6 ha Cocos Island at an estimated density, in October 2011, of 6 V. Indicus per hectare on the island. Plans for the release of the endangered Guam rail (Gallirallus owstoni) on Cocos Island required the culling of V. Indicus, because the lizards are known to consume birds and bird eggs. Cocos Island has 7 different habitats; resort/horticulture, Casuarina forest, mixed strand forest, Pemphis scrub, Scaevola scrub, sand/open area, and wetlands. I removed as many V. Indicus as possible from the three principal habitats; Casuarina forest, mixed scrub forest, and a garbage dump (resort/horticulture) using six different trapping methods. Cage traps and garbage barrels were highly effective in capturing medium to large adults, while snake traps were the only trapping method that effectively captured neonate monitor lizards. An air rifle with pellet shot removed the most individual V. Indicus and was effective in capturing all sizes of the lizards. Polyvinyl chloride pipe retreats and monofilament live snares were much less effective. After 11 months of trapping and shooting on Cocos Island, V. Indicus density was reduced to an estimated one V. Indicus per ha. Live captured V. Indicus were euthanized then weighed, measured, and dissected to analyze diet. Combined with earlier dietary data, I compared the diet of V. Indicus before and after an earlier rodent eradication using a prior diet analysis when rodents were present. In contrast to data published on V. Indicus from mainland Guam, rodents did not constitute a large percentage of the V. Indicus diet on Cocos Island prior to rodent eradication. However, 20 months post-rodent eradication, there were increased numbers of reptile eggs, earthworms, and insect larvae in the stomach contents of V. Indicus. Comparison of the percent occurrence of ingested items from the three different habitats showed that garbage dump greatly differed from the Casuarina forest and the mixed scrub forest. Comparison of diet between the mixed scrub forest and Casuarina forest revealed that while both populations of V. Indicus were consuming high percentages of crabs, the species of crab consumed differed between the two areas. Dietary differences were quantified using the Importance Index, which analyzes prey importance in relation to predator body size. Combined prey Importance Index with the prey frequency Index, showed that birds are the most important prey item followed by crabs. Although birds are found in only 4% of V. indicus with identifiable stomach contents, birds are the most important component in terms of dietary energy acquisition.
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Biology, Cocos Island, Diet, Guam, Herpetology, Lizard, Varanus
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48 pages
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