Plant-herbivore interactions in native and invasive Phragmites australis subspecies
Invasive species are a pervasive disturbance afflicting a variety of ecological communities. One prevalent invader of North American wetlands is the common reed, Phragmites australis australis. Originally native to Eurasia, over the past 200 years, this species has successfully invaded many areas of North America, outcompeting a number of native plants, including a native subspecies of Phragmites (P. australis americanus). Relatively little is known about how the native and invasive subspecies differ, especially in regard to traits that may influence herbivory. Previous studies have found that snail grazers are deterred from feeding on invasive Phragmites by a chemical deterrent, but it is unknown if the native subspecies also possesses this defense. Studies comparing native and invasive Phragmites have demonstrated native Phragmites to be more susceptible to some herbivorous insects. Whether other herbivores prefer native over invasive Phragmites is unknown, as are the traits that may influence palatability differences in these two subspecies. In this study, we compared native and invasive Phragmites in protein content and defensive chemicals. These traits were then compared with laboratory-based feeding preferences by a slug and snail herbivore, as well as grazing scars on plants growing in the field.
We identified and collected native and invasive Phragmites plants growing side-by-side along the Choptank River. We analyzed leaves from plants collected from the field and found that native Phragmites had significantly more soluble protein. In addition, native leaves had significantly more grazing damage by insect herbivores than the invasive subspecies. Feeding preferences of the periwinkle snail (Littoraria irrorata) differed. When offered freeze-dried, ground native and invasive Phragmites, the snail ate small amounts of both Phragmites subspecies equally. Offering snails a palatable food containing the chemical extract of native or invasive Phragmites showed similar results, suggesting that native and invasive Phragmites are equally chemically deterrent to this snail species. In contrast, the dusky slug (Arion subfuscus) significantly preferred invasive to native Phragmites, and defensive chemicals in the invasive subspecies did not deter this herbivore. Interestingly, this slug invasive in North America and is native to Europe, where it coincides with the invasive subspecies of Phragmites.