Combined effects of plant competition and insect herbivory hinder invasiveness of an introduced thistle

Abstract

The biotic resistance hypothesis is a dominant paradigm for why some introduced species fail to become invasive in novel environments. However, predictions of this hypothesis require further empirical field tests. Here, we focus on evaluating two biotic factors known to severely limit plants, interspecific competition and insect herbivory, as mechanisms of biotic resistance. We experimentally evaluated the independent and combined effects of three levels of competition by tallgrass prairie vegetation and two levels of herbivory by native insects on seedling regeneration, size, and subsequent flowering of the Eurasian Cirsium vulgare, a known invasive species elsewhere, and compared its responses to those of the ecologically similar and co-occurring native congener C. altissimum. Seedling emergence of C. vulgare was greater than that of C. altissimum, and that emergence was reduced by the highest level of interspecific competition. Insect leaf herbivory was also greater on C. vulgare than on C. altissimum at all levels of competition. Herbivory on seedlings dramatically decreased the proportion of C. vulgare producing flower heads at all competition levels, but especially at the high competition level. Competition and herbivory interacted to significantly decrease plant survival and biomass, especially for C. vulgare. Thus, both competition and herbivory limited regeneration of both thistles, but their effects on seedling emergence, survival, size and subsequent reproduction were greater for C. vulgare than for C. altissimum. These results help explain the unexpectedly low abundance recorded for C. vulgare in western tallgrass prairie, and also provide strong support for the biotic resistance hypothesis.

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

“…Herbivory and propagule pressure effects on C. vulgare seedlings are likely to differ in disturbed (current study) vs. undisturbed grasslands. Interspecific competitors were shown to reduce C. vulgare seedling emergence, increase seedling mortality, and dramatically increase seedling mortality caused by herbivory (Suwa and Louda 2012). These findings are consistent with a recent study of the co-occurring native thistle, C. altissimum , and they suggest an important role of insect herbivory, rather than intraspecific density dependence, in constraining the effects of propagule pressure on thistle seedling density.…”

“…Therefore, the possibility that C. altissimum density contributes to patterns in herbivory effects on C. vulgare seedlings remains open for subsequent research. Further, Suwa and Louda (2012) observed greater herbivory and higher mortality due to herbivory for C. vulgare as compared to the native congener, C. altissimum. This is consistent with the observation that native herbivores, and their impacts on plant performance, are often more severe for nonnative plants than for the native congeners (Parker et al 2006).…”

“…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).…”

“…Because C. vulgare is a disturbance specialist (Suwa and Louda 2012), we provided a mechanical disturbance prior to planting the seeds. Our study tests two main questions focused on the initial seedling establishment stage of a nonnative plant invasion.…”

Biological control and precipitation effects on spotted knapweed (Centaurea stoebe): empirical and modeling results

“…Low levels of mortality can result in greater resource availability, resulting in more or larger individuals. However, negative impacts from herbivore addition can overcome initial advantages such as strong seedling establishment (Suwa and Louda 2012). Biological control can reduce the threshold of the level of control needed for populations to be reduced or eradicated.…”