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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 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. 

Non-native plant species can disrupt plant–pollinator interactions by altering pollinator foraging behavior, which can in turn affect levels of interspecific pollen transfer between native and nonnative plant species. These processes may be amplified in cases where introduced plant species act as magnet taxa that enhance pollinator visitation to other plant species. We investigated these interactions on Santa Cruz Island (Santa Barbara Co., California) between non-native fennel (Foeniculum vulgare), a widespread and abundant invader, and the endemic Santa Cruz Island buckwheat (Eriogonum arborescens), which broadly overlaps fennel in its local distribution and blooming phenology. A fennel flower removal experiment revealed that this invader acts as a magnet species by increasing insect visitation to adjacent buckwheat flowers. Analysis of the amount of pollen carried on the bodies of insect pollinators (i.e., pollen transport) revealed that 96% of visitors to buckwheat flowers carried fennel pollen and 72% of visitors to fennel flowers carried buckwheat pollen. Pollen transport analyses and visitation rate data further suggest that members of three bee genera (primarily Augochlorella) may be responsible for the majority of fennel pollen deposited on the stigmas of buckwheat flowers (i.e., pollen transfer) and vice versa. Lastly, fennel pollen transport appeared to occur at a larger spatial scale than the magnet effect that fennel plants exert on floral visitors to neighboring buckwheat plants. The ability of fennel to act as a magnet species, coupled with the fact that it is widespread invader with known allelopathic capacities, suggests that future studies could evaluate if the transfer of fennel pollen adversely affects native plant reproduction in areas where fennel is introduced.