No interaction between competition and herbivory in limiting introduced Cirsium vulgare rosette growth and reproduction

Abstract

Both competition and herbivory have been shown to reduce plant survival, growth, and reproduction. Much less is known about whether competition and herbivory interact in determining plant performance, especially for introduced, weedy plant species in the invaded habitat. We simultaneously evaluated both the main and interactive effects of plant neighbors and insect herbivory on rosette growth and seed reproduction in the year of flowering for Cirsium vulgare (bull thistle, spear thistle), an introduced Eurasian species, in tallgrass prairie in 2 years. Effects of insect herbivory were strong and consistent in both years, causing reduced plant growth and seed production, whereas the effects of competition with established vegetation were weak. The amount of herbivore damage inflicted on rosettes did not depend on the presence of neighbor plants. We also found no interaction between competition and herbivory on key parameters of plant growth and fitness. The results of this study contradict the hypothesis that competitive context interacts with insect herbivory in limiting the invasiveness of this introduced thistle. Further, the results provide additional, experimental evidence that high levels of herbivory on established rosettes by native insects exert significant biotic resistance to the invasiveness of C. vulgare in western tallgrass prairie.

Native insect herbivory limits population growth rate of a non-native thistle

“…These results are based on a simple, two-stage matrix model; population growth rate is not influenced by the number of stages included in a matrix model (Tenhumberg et al 2009;Salguero-Gómez and Plotkin 2010). These findings are consistent with experimental studies showing that insect herbivory often severely reduce the fecundity of thistles (Louda and Potvin 1995;Rose et al 2005Rose et al , 2011Suwa et al 2010). Similar to the native C. altissimum, the late flowering time of C. vulgare combined with the occurrence of late-season herbivores, such as Homoeosoma eremophasma (Pyralidae) (Takahashi 2006), likely prevent successful compensatory seed production (Rose et al 2011).…”

“…Despite the clipping, however, the insecticide treatment increased heterospecific plant cover (ESM 4). Heterospecific competitors can reduce growth and seed production of C. vulgare (Suwa et al 2010;. Therefore, the increase in heterospecific cover associated with the insecticide treatment makes our estimate of the effect of herbivory on C. vulgare population growth a conservative one.…”

“…These observations lead to the question What limits this species in establishing populations in uninvaded areas? We know that native insects that specialize on the co-occurring native tall thistle [Cirsium altissimum (L.) Spreng] in tallgrass prairie spillover and cause a 50-88% reduction in the seed production of C. vulgare (Louda and Rand 2002;Takahashi 2006;Suwa et al 2010), leading to our further investigation of plant population level effects of insect herbivory. Previous work to model the population growth rate of C. vulgare relied heavily on data from its native range or the native congener C. altissimum (Tenhumberg et al 2008).…”

Insect herbivory and propagule pressure influence Cirsium vulgare invasiveness across the landscape

“…Photo credit: Masaru Takahashi. Suwa et al 2010). Their median estimate of predispersal seed predation (76.5%) combined with our 58% median reduction in seedling density indicates that herbivory can reduce lifetime fitness of C. vulgare by .90%.…”

“…Further, in a parallel study, we found that any subsequent compensatory density-dependent processes did not alter predictions for population size and dynamics suggested by the herbivory effects on the seedling stage (J. O. Eckberg, B. Tenhumberg, and S. M. Louda, unpublished data). Native insect herbivory also impacts later life stages of C. vulgare, reducing rosette survival, probability of flowering, and seed production (Louda and Rand 2002, Suwa et al 2010, Suwa and Louda 2012, all of which can reduce population growth rate. For instance, floral herbivory can reduce the population growth rate of C. vulgare by 70% (Tenhumberg et al 2008).…”