Changes In Argentine Ant Trophic Position As A Function of Time

The ecological effects of species introductions can change over time, but an understanding of how and why they do remains hindered by the lack of long-term data sets that permit investigation into underlying causes. We employed stable isotope analysis to estimate how trophic position changes as a function of time for the Argentine Ant, a widespread, abundant, and ecologically disruptive introduced species. Previous research at a site in southern California (Rice Canyon, San Diego Co.) found that Argentine Ant δ15N values were higher at the leading edge of invasion than at those same sites in years subsequent to invasion. To assess if a reduction in relative trophic position over time is a typical feature of ant invasions, we expanded the temporal and spatial scale of sampling and measured δ15N values of the Argentine ant at three locations with a known or inferred history of invasion: Rice Canyon (the site of the original study), the Sacramento River Valley (Yolo and Solano Cos., CA), and San Nicolas Island (Ventura Co., CA). Resampling Rice Canyon in 2019, 16 years after the original survey, revealed a significant increase in Argentine ant δ15N values. At the two other locations, Argentine ant δ15N values were independent of time since invasion (Sacramento River Valley) or position relative to the invasion front (San Nicolas Island). These findings suggest that post-invasion reductions in trophic position may not be a general phenomenon or could reflect transitory ecological processes that require finer-scale temporal sampling than was possible to achieve in the present study. Our findings are nonetheless consistent with the results of recent studies, which found that the effects of Argentine ant invasions persist over decadal time scales.

Redescription of Gnorimosphaeroma oregonense (Dana, 1853) (Crustacea, Isopoda, Sphaeromatidae), designation of neotype, and 16S-rDNA molecular phylogeny of the north-eastern Pacific species

Gnorimosphaeroma oregonense (Dana, 1852) is revised, a male neotype is designated, photographed, and illustrated; the species occurs from Vancouver British Columbia to the central California coast. 16S-rDNA sequences (~650 bp) for all available ethanol preserved species of Gnorimosphaeroma were used to hypothesize their relationships. Our analyses revealed a sister taxon relationship between the fully marine G. oregonense and the brackish to freshwater species, G. noblei. The oyster associated and introduced G. rayi is sister to a previously not recognized or identified, but genetically distinct, Gnorimosphaeroma sp. collected at two sites in San Francisco Bay. Gnorimosphaeroma sp. is probably also a western Pacific species based on its genetic relationship to G. rayi. Photographic comparisons are offered for G. oregonense (marine), G. noblei (freshwater), G. rayi (introduced), G. sp. (presumably introduced), and G. insulare (San Nicolas Island). Records of the holdings at the Los Angeles County Museum of Natural History are summarized. Without material available north of Vancouver through Alaska, the range of G. oregonense could not be genetically verified. This review includes a diagnosis and description of the genus Gnorimosphaeroma Menzies, 1954.

Redescription of Gnorimosphaeroma oregonense (Dana, 1853) (Crustacea: Isopoda: Sphaeromatidae), designation of neotype, and 16S-rDNA molecular phylogeny of the north-eastern Pacific species

Gnorimosphaeroma oregonense (Dana, 1852) is revised, a male neotype is designated, photographed, and illustrated; the species occurs from Vancouver British Columbia to the central California coast. 16S-rDNA sequences (~650 bp) for all available ethanol preserved species of Gnorimosphaeroma were used to hypothesize their relationships. Our analyses revealed a sister taxon relationship between the fully marine G. oregonense and the brackish to freshwater species, G. noblei. The oyster associated and introduced G. rayi is sister to a previously not recognized or identified, but genetically distinct, Gnorimosphaeroma sp. collected at two sites in San Francisco Bay. Gnorimosphaeroma sp. is probably also a western Pacific species based on its genetic relationship to G. rayi. Photographic comparisons are offered for G. oregonense (marine), G. noblei (freshwater), G. rayi (introduced), G. sp. (presumably introduced), and G. insulare (San Nicolas Island). Records of the holdings at the Los Angeles County Museum of Natural History are summarized. Without material available north of Vancouver through Alaska, the range of G. oregonense could not be genetically verified. This review includes a diagnosis and description of the genus Gnorimosphaeroma Menzes, 1954.

Rhizoliths identified as prehistoric filing tools for fishhook production on San Nicolas Island, California

Chemical analysis of archeological objects can provide important clues about their purpose and function. In this study, we used scanning electron microscopy (SEM) and chemical spectroscopy (SEM-EDS and XRD) to identify a white residue present on cylindrical rhizoliths from a component at an archaeological site (CA-SNI-25) on San Nicolas Island, California, dated ca. AD 1300 to 1700. The residue was found to consist of biogenic calcite and aragonite particles, different in composition and morphology from the CaCO3 particles in the rhizoliths, but identical to marine shell material. These results, together with observations of surface micro-wear patterning on fishhooks and rhizoliths, replicative experiments, in situ spatial analysis, and other archaeological evidence, show that rhizoliths were used as files in a larger tool kit for crafting shell fishhooks. Our findings shed new light on the technological innovations devised by Native Americans to exploit the rich marine resources surrounding the Channel Islands, and provide the first analytical evidence for the use of rhizoliths as a production tool.