While the concept of invasibility has been a subject of much study for half a century, there is little consensus as to how a community resists invasion or what components most confer resistance. In communities living in stressful environments, however, abiotic conditions can be determining factors of invasibility. The utility of edaphic manipulation of abiotic conditions as a counter to invasive species establishment and dominance remains unknown. Serpentine grasslands are widely recognized as prime ecological refugia for native species and have two components which are thought to confer their resistance to invasion by non-native plants: low Ca:Mg ratio and low water availability. As a consequence of this, not only do some native plants persist only in serpentine, but also the herbivorous insects and other community members that depend of them become dependent upon serpentine grasslands as well. In an effort to provide additional habitat for the threatened and serpentine-restricted Bay Checkerspot butterfly, Euphydryas editha bayensis, we experimentally altered a non-serpentine site to mimic a nearby serpentine site formerly occupied by the butterfly.
First, attempts to lower the Ca:Mg ratio through addition of MgSO4 gave inconsistent results. Some of the plots with added Mg had lower Ca:Mg ratios similar to serpentine, but most did not. However, we did detect a small but significant decrease in the invasive community in the plots with additional Mg and found higher E. editha host plant individuals in the plots with the most Mg added. Second, we altered the soil depth through the addition of gravel beds to determine the effects of water stress on invasibility. We found that shallow soils had less water content and correspondingly had significantly less invasive species richness and lower invasive species cover. The results demonstrate the utility of site-specific plant invasibility reduction as a possible means of restoration and illustrate how the manipulation of edaphic conditions to mimic stressful environments can hinder invasive species establishment and improve native species persistence.