Effects of plant competition and insect herbivory on invasive Cirsium vulgare (Bull thistle)

Although an increasing number of studies have documented invasions by introduced species, the mechanisms underlying the success or failure of such biological invasions remain poorly understood. Plant competition and feeding by insects (i.e. foliage herbivory) are two potentially important factors that are known to limit plant invasion success in a few cases. However, the synergistic effects of these factors on invasive species have rarely been examined. Here, we present results from a field experiment conducted to study the effects of competition exerted by neighboring plants, foliage herbivory by insects, and the interaction of these factors on the success of established plants of an introduced invasive plant species, bull thistle (Cirsium vulgare). Preliminary data from the experiment in the summer of 2006 suggest that both insect herbivory and competition from neighboring plants severely reduced C. vulgare plant biomass and number of flower heads produced. However, presence or absence of the neighboring vegetation (i.e. plant competition) did not affect either the proportion of damaged leaves or the number of flower heads. We also did not find any evidence of combined effects of competition and herbivory on plant performance. However, if native plant and insect communities act synergistically to resist biological invasions by introduced plants, then this might occur in an earlier life stage. Therefore, we are now examining the effects of plant competition and insect herbivory on the seed-to-seedling stage transition of the introduced C. vulgare. To date, we conclude that insect herbivory and plant competition have independent, additive effects on the invasive C. vulgare once plants are established.