Community-level consequences of a plant invasion: Effects on three habitats in coastal California

Biological invasion by normative species is a global phenomenon that has the capacity to dramatically alter native communities. However, surprisingly few studies have quantified the effects of exotic plant species on the communities they invade, or have considered how these effects vary among habitat types or seasons. Here, we used both comparative and experimental field studies to investigate the influence of Cape ivy (Delairea odorata; Asteraceae), an invasive evergreen vine native to South Africa, on three habitat types in coastal regions of northern California (coastal scrub, willow riparian, and alder riparian). In the comparative study, plots invaded by Cape ivy contained 36% fewer native plant species and 37% fewer normative taxa, and this pattern persisted across habitat types and seasons. The richness of grass and forb species was lower in invaded plots, whereas fern and shrub richness did not vary among zones. Native species richness was significantly lower with increasing cover of Cape ivy, but this was not the case for normative species. In addition, invasion by Cape ivy was associated with a 31% decrease in species diversity as well as an 88% decrease in the abundance of native seedlings and a 92% decrease in normative seedlings compared to uninvaded areas. After 2 yr, a Cape-ivy reduction experiment yielded similar results, with a 10% increase in the richness of native species compared to control plots, and a 43% increase in the richness of normative taxa. Forb species richness increased significantly when Cape-ivy cover was reduced, whereas shrub richness decreased slightly and no effects were detected for ferns and grasses. We also found that Cape-ivy reduction led to a 32% increase in plant species diversity, an 86% increase in the abundance of native seedlings, and an 85% increase for normative seedlings. In all cases, the effects of Cape-ivy reduction were consistent across habitat types. Collectively, our results indicate that this invader has significantly changed the composition of three different habitat types, and its control should be a major priority. However, our data also indicate that Cape ivy had negative effects on the richness of both native and normative plant species. Such findings suggest that a consequence of removing Cape ivy from invaded areas may be to facilitate the proliferation of other normative species.