Although the spread of invasive, exotic species is a global phenomenon recognized as a critical source of environmental change, few studies have quantified the impact of invasive plants on the communities they invade or considered the degree to which plant groups with different life histories, life forms and geographic origins vary in response to invasion. In addition, those studies that have examined such impacts rarely consider whether these effects are reversed with invader removal, or whether invaders have residual effects that persist even after their removal. We evaluated the response of a coastal dune community in northern California to invasion by a South African succulent, common iceplant (Carpobrotus edulis), as well as the community response to iceplant removal. To further assess the impacts of iceplant, we evaluated its above and below-ground influences on the germination and growth of an exotic annual grass, Bromus diandrus.
Results/Conclusions
Our results show that iceplant has substantial effects on the composition of an invaded plant community and that these impacts were not always reversed with invader removal. Abundance of native annual forbs and abundance and species richness of native perennial forbs was lower in iceplant-invaded areas than in uninvaded ones. In contrast, exotic annual grasses exhibited greater abundance in invaded areas, while no such patterns emerged for exotic annual forbs. Iceplant removal increased the abundance and species richness of both native annual and perennial forbs, as well as the abundance and richness of exotic annual forbs. In contrast to the positive relationship we found between iceplant invasion and exotic annual grass abundance, iceplant removal had no impact on exotic annual grasses. Also in contrast, we found that B. diandrus grown in iceplant patches or in soil where iceplant had been removed had poorer germination and growth than B. diandrus grown in soil not influenced by iceplant. This suggests that iceplant affects soil properties and that these effects remain even after removal. These findings illustrate that plants varying in life form, life history and geographic origin respond differently to invasion, and that removal will not necessarily reverse invader impacts.