Abundance of and Floral Herbivory on Exotic Bull Thistle versus Native Tall Thistle in Western Tallgrass Prairie

An important, yet poorly quantified mechanism to explain the failure of some exotic species to increase and spread is that indigenous natural enemies provide ecosystem resistance to invasiveness. To evaluate this idea, we hypothesized that spillover of native thistle-feeding floral insect herbivores onto Eurasian bull thistle (Cirsium vulgare) from the prairie native tall thistle (C. altissimum) helps limit bull thistle population growth and spread throughout the western tallgrass prairie region in Nebraska. To test this hypothesis, we quantified both the occurrence of bull thistle, a known invasive species worldwide, and the floral herbivory on it by native insect herbivores in relation to occurrence and floral herbivory on tall thistle across the region. We quantified plant occurrence by recording presence/absence of each thistle in 1600 m x 30 m (0.1 mile x ~100 ft) quadrats along four radiating 16 km (10 mile) transects at nine sites stratified across three longitudes and three latitudes that span the range of tallgrass prairie in eastern Nebraska. For flowering bull thistle plants, we recorded distance to the nearest tall thistle, along with size, reproductive effort, and floral damage for both thistles. Bull thistle occurred in \u3c1% of all quadrats, while tall thistle occurred in \u3e20%. Although plant size, reproductive effort, and insect floral herbivory varied significantly among the nine sites, no strong patterns occurred with longitude or latitude for either thistle. These results suggest that thistle occurrence and growth in this region are not controlled by geographic variation in large-scale physical or climatic variables. Floral herbivory on both species was high, with a tendency to be lower in the north. The proportion of flower heads damaged was as high, and sometimes higher, on bull thistle (68 – 83%) as on native tall thistle (64 – 78%). Herbivory on flower heads of bull thistle increased with plant proximity to a flowering adult of native tall thistle. The results of this study provide strong support for both the ecosystem resistance hypothesis and the spillover hypothesis as important in limiting abundance of this potentially invasive plant in the western tallgrass prairie region