D3339: Differences in life-history strategies may explain the rapid displacement of native black widows by invasive brown widows in urban southern California

 
Invasive species pose a global threat to biodiversity, often by displacing native species. Predicting the outcomes of interactions between invasives and natives is critical to developing management plans, and knowledge about life-history strategies may provide insight into the consequences of these interactions. Brown Widows (Latrodectus geometricus) were first established in Southern California in the early 2000s, and rapidly displaced native Western Black Widows (L. hesperus) in many urban areas. To date, the mechanism(s) underlying this rapid displacement have not been identified. We hypothesized that the life-history attributes of invasive Brown Widows confer advantages that have facilitated their rapid displacement of native Western Black Widows. We compared the life-history attributes of field-mated, captive females (n = 31 L. geometricus; n = 16 L. hesperus) from Northridge, California by quantifying (1) body mass, (2) egg sacs produced over 3 mo, (3), estimated egg mass, (4) clutch size (eggs per egg sac) and mass, and (5) relative clutch size and mass. Relative to L. geometricus, L. hesperus had twice the body mass, produced 61% more eggs per clutch, and eggs that were twice as large, which translated into 71% larger clutch mass. However, the relative clutch size of Western Black Widows was 87% lower than that of Brown Widows, and they also produced more egg sacs over 3 mo. Our data suggest a competitive reproductive advantage for the invasive species that has likely played a role in the rapid displacement of Western Black Widows by Brown Widows in parts of Southern California.