The Impact of Two Introduced Herbivores on the Population Ecology of Lythrum Salicaria: Implications for Plant Performance, Reproduction and Community Diversity

Abstract

The release of biological control agents into the environment is inherently risky: assessment of those risks through on-going, post-release monitoring is very important. Herbivores have the potential to inflict multiple impacts on a host plant’s performance and reproduction. Previous research demonstrates that the effects of herbivory on plants include changes to plant architecture, biomass allocation, flowering time and reproductive success, to list a few. Moreover, when herbivory significantly impacts the population ecology of a dominant community member, other species can be indirectly affected, ultimately influencing plant community ecology. Here I describe an investigation into the impacts of two introduced herbivorous biological control agents: the leaf beetle Galerucella calmariensis (Coleoptera: Chrysomelidae), and the flower-feeding weevil, Nanophyes marmoratus (Coleoptera: Brentidae) on several characteristics of the host plant species, invasive purple loosestrife (Lythrum salicaria), and its surrounding community. I collected data on 18 invaded communities from around eastern Ontario, including information on feeding damage and the density of each species of biological control, along with data on purple loosestrife’s height and biomass, inflorescence length, inflorescence number and fruit production. The history of each site’s colonization by Galerucella was also considered. I discovered that the density of both Galerucella and Nanophyes at a site was negatively associated with Lythrum fruit production. However, herbivore density was not significantly associated with Lythrum biomass, height or the species richness of the surrounding plant community. This study, conducted 20 years after the initial Ontario release of Galerucella, demonstrates that although vegetative traits of Lythrum do not appear to be significantly impacted by the presence of Galerucella or Nanophyes, reproductive traits are. Twenty years is likely too short a time period to adequately assess the impacts of the release on community species richness, although my data indicate that communities with smaller Lythrum plants tend to have higher species richness. This study covered a small geographical area and data collection was conducted for a single season only; adding additional years and/ or sites is recommended.