Impacts and success of the invasive spider Cheiracanthium mildei (Miturgidae) in vineyards and surrounding natural ecosystems

Invasive species pose a widespread threat to natural and managed ecosystems. Reduced species diversity has often been associated with the increased success of invasive species, although the mechanisms that drive this pattern remain ambiguous. Invasive species may be more successful in species-poor settings, or they may contribute to the reduced diversity of invaded habitats by negatively impacting native species. The nocturnal wandering spider Cheiracanthium mildei has invaded a range of habitats in North America, and frequently dominates the arthropod predator fauna in vineyard ecosystems of California. In the following dissertation, I use a combination of surveys and manipulative experiments to examine the relationship between the impacts and success of this invasive spider and species diversity. Major findings include: (1) across four regions in California, vineyards dominated by C. mildei were characterized by lower numbers of spiders and reduced spider species diversity; (2) C. mildei dominated low-diversity vineyards surrounded by vineyard monoculture in Napa County, California, but was less successful in vineyards adjacent to natural vegetation, where spider diversity and numbers of native wandering spiders were higher; (3) dominance of C. mildei along transects declined dramatically between vineyard and surrounding oak woodland, while native wandering spiders showed an opposing pattern, providing evidence that C. mildei has a critical adaptive advantage in the disturbed setting of the vineyard; (4) success of dominant spiders in vineyards, including C. mildei , is related more to habitat preferences or species interactions than to aerial dispersal rates from natural habitat; (5) in manipulative field experiments, C. mildei appeared to drive food web interactions in vineyards, reducing numbers of native spiders and herbivores.