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Abstract The big-headed ant,Pheidole megacephala, is an ecologically disruptive invader of tropical and subtropical environments worldwide. In April 2014 an established infestation ofP. megacephalawas discovered in a residential neighborhood in Costa Mesa, Orange County, California, and in 2019 a second infestation was found in a residential neighborhood (Talmadge / City Heights) in San Diego, San Diego County, California. Although big-headed ants are regularly detected in commerce in California, the records from Costa Mesa and Talmadge / City Heights represent the first established infestations documented from the state. In 2024 and 2025, four additional infestations were discovered or confirmed in other residential neighborhoods in San Diego. To assess whether or notP. megacephalawill expand its range in this region, we delineated infestations in Costa Mesa and Talmadge / City Heights in 2023 and 2024 and compared this species to another widespread invader, the Argentine ant (Linepithema humile), with respect to desiccation tolerance and δ15N. The delineatedP. megacephalainfestations extend over multiple hectares of suburban and urban development, with the Talmadge / City Heights infestation exceeding 100 ha and the Costa Mesa infestation exceeding 10 ha. Between 2023 and 2024 the size of the Talmadge / City Heights infestation increased by 12 ha. Comparisons of the two focal species revealed overlapping δ15N values and estimates of desiccation tolerance. Our findings indicate that established populations ofP. megacephalawill continue to spread in urban environments in coastal southern California and potentially cause impacts comparable to those resulting from invasion by the Argentine ant. 

Abstract Predicting where introduced species will establish and spread remains a central goal of invasion biology. While climate data are often used to forecast potential occurrence at regional scales, environmental filtering can limit susceptibility to invasion at finer scales. Factors underlying such filtering are important to identify, especially when they influence spread into protected areas set aside for conservation. Useful information about factors limiting invasion can be obtained from historical resurveys. Between 2022 and 2024, we used pitfall traps and visual surveys to resurvey 111 sampling points in two protected areas in coastal San Diego County, California (Torrey Pines State Reserve and the Point Loma Ecological Conservation Area) that were originally surveyed for the non-native Argentine ant between 1995 and 1997. The multi-decade time span between the surveys coupled with the observed distributional limits, which have either contracted (Torrey Pines) or appear static (Point Loma), indicate that the Argentine ant has reached the limits of its ability to invade these sites. At Torrey Pines the soil types with the lowest water retention values were the least invaded in the original survey and were overrepresented among retractions observed in the resurvey. These findings are consistent with experimental work demonstrating the central role of soil moisture in limiting Argentine ant spread in seasonally dry areas. Variation in precipitation combined with changes in human water use will likely continue to influence the distribution of the Argentine ant in semiarid regions. These results generally illustrate the value of historical resurveys in clarifying limits to invasion. 

Abstract Intertidal environments receive energy from marine ecosystems in the form of marine wrack, which makes up the base of a food web that includes both intertidal and terrestrial consumers. Consumption of wrack by terrestrial consumers can elevate their abundance and alter how they interact with organisms in adjacent terrestrial environments. Although rarely documented, terrestrial invaders may exploit marine wrack subsides and potentially disrupt intertidal and terrestrial food webs. Here, we examine consumption of marine wrack resources by the introduced Argentine ant (Linepithema humile), which occurs commonly on beaches in southern California. In controlled trials the Argentine ant readily scavenged arthropod detritivores (amphipods and flies) abundant in wrack. In spite of obvious risks (e.g., exposure to tides, desiccation, thermal stress) associated with intertidal foraging, Argentine ant activity on beaches was comparable to that in spatially-paired, scrub environments. Foraging on beaches allowed ants to access higher densities of arthropod prey and carrion compared to those found in scrub environments. Stable isotope analyses provide evidence for extensive assimilation of marine-derived resources. Values of δ¹⁵N and δ¹³C for the Argentine ant were higher at beach sites than at scrub sites, and Argentine ant δ¹⁵N values broadly overlapped those of intertidal consumers at beach sites. Although ants are known to forage in intertidal environments, this study provides a novel example of an introduced ant species exploiting a cross-boundary subsidy.