Predation by the Introduced BrownWidow Spider (Araneae: Theridiidae) May Explain Local Extinctions of Native BlackWidows in Urban Habitats

Abstract Invertebrates that successfully colonize new habitats often share life history characteristics including high fertility, rapid development, and early maturation. Since its introduction into Florida, USA, the non-native Brown Widow, Latrodectus geometricus (Kock 1841, Araneae: Theridiidae), has rapidly expanded its range into urban areas as far north as Kansas and as far west as California. During its expansion, the Brown Widow has displaced Florida's Southern Black Widow, L. mactans (Fabricius 1775, Araneae: Tjerodoodae) and California's Western Black Widow, L. Hesperus (Chamber lin & Ivie 1935, Araneae: Theridiidae). Here, based on a field survey and controlled laboratory experiments, we report possible causes for the rapid disappearance of Florida's Southern Black Widows. Our field survey revealed that Brown Widows have twice the fertility potential as Southern Black Widows. In experiments comparing development, we show that sub-adult Brown Widows grew faster and matured earlier relative to Southern Black Widows. In our experiments on cohabitation with neighbors, bold Brown Widows were six times more likely to kill and consume shy Southern Black Widows than bold cobweb spiders and three times more likely to cohabitate with bold cobweb spiders than with shy Southern Black Widows. Our model of maternal risk-management revealed that competition for scarce prey was not a significant cause of offspring mortality for Latrodectus species. Hence, Brown Widows are not predating Black Widows or other cobweb spiders because prey is scarce. To our knowledge, this study is the first to suggest that aggressive predation by Brown Widows is a significant factor contributing to the local extinction of the shy Southern Black Widow in urban structures.