Insect herbivory on juveniles of a threatened plant, Cirsium pitcheri, in relation to plant size, density and distribution

Pitcher's thistle (Cirsium pitcheri [Torr.] Torrey & Gray) is a characteristic, rare endemic plant in the dunes around the Great Lakes. Although insect herbivory has been reported, little is known about its frequency, magnitude, or impact on plant population dynamics. We quantified insect feeding on juvenile rosettes in relation to plant size, density, and location, since these variables could determine either present patterns of feeding or reflect past herbivory. The data were taken along the disturbance and successional gradient from lakeshore to forest at five sites, arrayed from north to south within Sleeping Bear Dunes National Lakeshore, Michigan, U.S.A., in 1993 and 1994. First, both the frequency and magnitude of insect feeding, especially on new central leaves and upper rootcrowns, increased with plant size. Average plant size increased from north to south in the park and from foredune to inland zones, helping to explain higher levels of insect herbivory documented in the southern sites and inland zones. Insect herbivory on multiple-rosetted plants, caused by compensatory growth in response to apical meristem damage, was also highest on large plants, especially in the south and inland. Second, insect herbivory decreased with density of all plants, but increased with the density of large juveniles. The density of large juveniles increased from north to south and from foredune to inland zones, while total densities did the opposite. Thus, higher herbivory in the south and inland correlated with the higher density of larger juveniles in these areas, consistent with the resource concentration hypothesis. Overall, insect herbivory was greater where recruitment success was lower, size at maturation higher, and occurrence of larger juveniles was more frequent. Thus, we hypothesize that insect herbivory increases time to maturation and that it contributes to reduced persistence of Pitcher's thistle in the more stabilized, late successional portions of the dune system. Consideration of this biotic interaction should be included in management and restoration plans for Pitcher's thistle.