Species Background

Carpobrotus edulis (highway iceplant, hottentot fig) is a succulent, mat-forming perennial plant native to South Africa that grows almost like a vine, spreading from an initially central stem and able to overgrow adjacent plants growing low to the ground (Figure 7.1). Stems root at the nodes as they grow and individual branches can live independently if severed from the initial main stem. Individual plants can live for decades. Stems layer over one another forming mats that can reach up to 30 cm

FIGURE 7.1 Transect through a highly invaded backdune community at Vandenberg Air Force Base, California. The mat forming C. edulis can be seen along the entirety of the transect. (See color plate.)

or more in depth and more than 8 m in diameter (D'Antonio, personal observation). The growth form can be thought of as a dense blanket that carpets the soil surface. Stems grow up onto short stature shrubs and subshrubs and eventually completely cover them. Occasionally stems are found blanketing shrubs up to 1 m tall, although this is not typical.

Carpobrotus edulis is considered an invasive weed in California, Australia, France, Spain, and the Balearic Islands of the Mediterranean. It was apparently introduced into California in the early 1900s from South Africa because of its potential to stabilize dune soils. By the 1980s it was recognized as an invasive species of native coastal habitats (e.g., Hoover 1970, Zedler and Scheid 1988). D'Antonio (1990) demonstrated that its indehiscent fruits are eaten by a variety of native vertebrates that inadvertently disperse the seed to new locations.

Controls over the invasion of C. edulis into "native" or unmanaged plant communities have been well studied in California. Seedlings of the species are readily consumed by native generalist herbivores, slowing rates of invasion into most habitats (D'Antonio 1993). Establishment probabilities were lowest in coastal scrub sites due to extremely high rates of herbivory by rabbits. Seedlings in dune sites suffer from physiological stress and some herbivory resulting in intermediate establishment probabilities. Seed establishment probabilities are highest in grassland sites where soil disturbance by animals is highest and native vegetation is not competitive once C. edulis is established.

We chose to use C. edulis as a case study for several reasons. It is an ideal species to assess the impacts of organisms with unique traits, because it has succulent leaves and a mat-forming prostrate growth form that is largely unique in comparison to most other native Califor-nian plant species in the habitats where it is invading. It grows low to the ground, which also contrasts with the generally taller growth forms that dominate several of the habitats where it invades. This is in contrast to many autogenic engineers that appear to introduce taller, more complex structures. Because the species invades a range of coastal habitats and invades relatively slowly, we could measure impact across different habitats at different stages of invasion within any one habitat type. While C. edulis has been demonstrated to have negative effects on the growth of neighbors (e.g., D'Antonio and Mahall 1991), mechanisms of impact are not fully understood. From the perspective of engineering, its dense blanket-like form alters the soil surface, the rooting environment, and the chemistry of the soil. At a larger scale, its provision of fruits to animals at a time of year when little else is available (D'Antonio 1990a) could have significant trophic-level impacts. Figure 7.2 presents a schematic of the array of impacts caused by this species. In following text we briefly

Homogenizes structural complexity of vegetation

Homogenizes structural complexity of vegetation

Alters water availability

FIGURE 7.2 Conceptual diagram of multiple pathways of impact caused by C. edulis.

Alters water availability

FIGURE 7.2 Conceptual diagram of multiple pathways of impact caused by C. edulis.

discuss these, using preliminary and unpublished data to make several points about impacts and the extent to which C. edulis functions as an ecosystem engineer.

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