Demographic and Evolutionary Impacts of Native and Invasive Insect Herbivores on Cirsium Canescens

Abstract

Abstract. Invasive species have the potential to alter trade-offs leading to selection in the populations they invade. Here we quantify the demographic and selective effects of herbivory by native insects and the introduced floral feeder Rhinocyllus conicus on Platte thistle (Cirsium canescens), a sparse monocarpic thistle endemic to North America. Rhinocyllus first invaded the Platte thistle population in 1993. Since then, its numbers have increased exponentially, while the Platte thistle population size has decreased. Data from 11 years were analyzed to determine how demographic rates varied with plant size and damage by native insects and Rhinocyllus. Individual growth, survival, flowering probability, and seed set were all size dependent. Damage to vegetative structures did not influence demographic rates; damage to flower heads did because Platte thistle is seed limited. These analyses were used to parameterize a series of integral projection models (IPMs) that investigated the effects of floral herbivory on the population growth rate , equilibrium population size, and the evolutionary stable (ES) flowering strategy. The IPMs showed that native insects have significant impact on the equilibrium population size and , but not the ES flowering strategy, because they use the flowers of different-sized plants indiscriminately. In contrast, Rhinocyllus has the potential to drive Platte thistle extinct. Rhinocyllus preferentially fed and oviposited on the flowers of larger plants and therefore selected for a reduction in flowering size. However, as the thistle population went into decline, this pattern reversed. Thus, selection imposed by an invader may be complex and will reflect behavioral interactions between herbivore and host, as well as demographic changes in the host population.

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

“…This outcome is consistent with mounting evidence documenting significant effects of insect herbivory on parameters of fitness of both native and introduced thistle species (Louda et al 1990;Louda and Potvin 1995;Jackson 1998;Maron et al 2002;Rose et al 2005;Eckberg 2008;Suwa 2008).…”

Indirect interaction between two native thistles mediated by an invasive exotic floral herbivore

“…Sampling procedures for wavyleaf thistle paralleled those used previously in studies of Platte thistle (Louda 2000;Russell and Louda 2004;Rose et al 2005). To quantify flowering and floral herbivory, we sampled adult plants at two sites at each preserve [1991][1992][1993][1994][1995][1996][1997][1998][1999][2000][2001].…”

“…These measurements are sufficient to derive the shape of the plant flowering phenology distribution and to quantify R. conicus oviposition levels (Louda 1998(Louda , 2000Rose et al 2005). On each date, we recorded diameter and developmental stage of all of the flower heads on each marked plant.…”

“…The causes of such variation among plant populations and years, however, are poorly understood for coevolved plant-insect interactions, and even less so for interactions involving invasive exotic insects. A better understanding of the mechanisms underlying variation in herbivory is critical to predicting the conditions under which herbivores, native or exotic, will affect either plant population densities (Louda 1982b(Louda , 1983Louda and Potvin 1995;Kelly and Dyer 2002;Maron et al 2002, Rose et al 2005 or distributions (Louda 1982a(Louda , 1983Louda et al 1987;Smith 1987, Louda andRodman 1996).…”

“…In these prairies, R. conicus oviposits not only on wavyleaf thistle, but it also has heavily impacted a second, native thistle, Platte thistle (C. canescens Nutt.) Louda and Arnett 2000;Louda et al 2003;Rose et al 2005).…”