Invertebrate responses to plant invasions can help delineate the drivers of biodiversity and community patterns, thus guiding the conservation and restoration of diverse native ecosystems. Although it is well known that invasive species have pervasive effects on native communities and ecosystems, our knowledge of the relative contributions of, and interactions among, key factors determining invader impacts is limited. Among potentially key factors is invader abundance, but few studies have examined its relationship to community structure. Those studies have shown varying relationships between invertebrate assemblages and invader cover, illustrating the need for additional studies on the forms and mechanisms of relationships between invertebrate biodiversity and invasive plant abundance.
For my dissertation work, I asked what effect non-native plant invasions are having on entire arthropod assemblages, and how that effect is related to the degree of invasion. I further asked how the responses by component feeding guilds contribute to these relationships. I answered these questions by conducting a meta-analysis and review of existing studies, then investigating the effects of one invader, Carpobrotus edulis, on coastal dune arthropods at four different coastal California sites. At Vandenberg Air Force Base, Montaña de Oro State Park, and San Buenaventura State Beach, I utilized unmanipulated Carpobrotus populations to compare paired invaded and uninvaded patches and to investigate a gradient of Carpobrotus infestation. At Coal Oil Point Reserve, I conducted an experimental restoration trial, removing varying amounts of Carpobrotus and re-vegetating with native dune plants.
The meta-analysis showed negative effects of non-native plant invasions on arthropod richness, which are in part determined by the abundance of the invader, and are greatest at high levels of invader abundance (70% cover or greater). My field research showed that these effects vary by site, season, and sampling technique/stratum, and are apparently modulated by the characteristics of both the invader and the invasion itself, including growth patterns, litter production, invader abundance, and native plant diversity. Even a relatively small amount of Carpobrotus edulis has negative effects on arthropod abundance, richness, and composition, with an exponential decrease in abundance and richness and a shift in composition well before 50% Carpobrotus cover. Changes in feeding guilds and composition helped to suggest the drivers of these patterns, and revealed a particularly strong effect on fossorial arthropods, likely related to the dense rooting structure of Carpobrotus. Thus, while the negative effects of non-native plant invasions typically increase with increasing invader cover, the form of this relationship is determined by the attributes of both the invader and the invaded system.