The effect of a pipeline corridor constructed through an ecological reserve in Southern California was investigated by assessing plant species composition and soil chemistry. A homogeneous plant community comprised primarily of exotic annuals was found along the entire length of the corridor. This community has low similarity to the adjacent native plant communities. Soil organic matter was significantly less on the disturbed corridor than in contiguous undisturbed areas. Both available nitrogen and extractable phosphorus values were greater in the disturbed corridor. By contrast, total nitrogen was significantly higher outside the pipeline. The more labile litter of the exotic annuals allows increased mineralization along the corridor than does the more recalcitrant litter of the native perennial shrubs in the undisturbed areas. Once established, the weedy exotic annual litter may completely turn over organic matter and nitrogen, favoring the persistence of the weedy annuals. These exotic annuals appear to be moving into three of the native communities - grassland, coastal sage, and oak woodland - that have less organic matter and a more open plant canopy. Poor restoration efforts can lead to the establishment of such exotics, subsequent invasion into the surrounding undisturbed habitat, and degradation of the reserve.