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Invasive Brown Tree Snake On Guam Leads To Explosion In Spider Populations

Demise of 10 of 12 native birds leaves spider population largely unchecked.

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Invasive brown tree snakes (Boiga irregularis) on Guam have caused yet another imbalance in the island's ecosystem: The proliferation of spiders. The brown tree snake has devastated Guam's bird population so severely over the last 60 years that only two of 12 birds native to Guam now exist, and the absence of certain birds that eat spiders and other insects has caused an explosion in spider populations, according to a study in the journal PLoS One.

Researchers have found that the spider population on Guam compared to other nearby islands in their study was more dramatic than what was encountered in previous small scale studies. The scientists counted spider webs in Guam's jungles and on the islands of Saipan, Tinia, and Rota which do not have brown tree snakes and found spider densities on Guam 40 times greater than these islands with birds during the wet season and 2.3 times greater during the dry season. They also found that the near absence of birds on Guam resulted in one species of spider spinning larger webs than the same species on the other islands that have normal bird populations. 


Prior to the introduction of the brown tree snake on Guam, there were ten native birds that were known to eat insects: Bridled White-eye (Zosterops conspicillatus), Rufous Fantail (Rhipidura rufifrons), Micronesian Starling (Aplonis opaca), Mariana Swiftlet (Aerodramus bartschi), Cardinal Honeyeater (Myzomela rubratra), Nightingale Reed-Warbler (Acrocephalus luscinia), Mariana Crow (Corvus kubaryi), Micronesian Kingfisher (Todiramphus cinnamominus), Guam Flycatcher (Myiagra freycineti), and the Guam Rail (Gallirallus owstoni).