Competition between Lythrum salicaria and a rare species: combining evidence from experiments and long-term monitoring

The rare endemic plant Sidalcea hendersonii (Henderson's checker-mallow) occurs in tidal marshes of the Pacific Northwest and may be threatened by Lythrum salicaria (purple loosestrife), a European invader plant. We compared the abundances of Lythrum and Sidalcea in a wetland in British Columbia (Canada) in 1999 to those measured in 1979 to track changes in both species. Although the frequency of Sidalcea decreased by more than 50%, and that of Lythrum increased by almost 20%, there was no significant relationship between the changes of the two species. We assessed the potential effects of competition by Lythrum on Sidalcea in field and patio experiments. In the field, we measured the response of Sidalcea to the removal of Lythrum over a two-year period and compared this to the response of Sidalcea to the removal of native species and in unmanipulated control plots. Removal of Lythrum significantly improved the vegetative performance of Sidalcea compared to the removal of randomly selected native plants and the control treatment in the first year. In the second year, the performance of Sidealcea did not differ significantly with treatment. Removals did not influence the reproductive performance of Sidalcea in either year. A one-year additive experiment, carried out in pots, compared the competitive effect of Lythrum on Sidalcea with that of two native species. Lythrum's impact on Sidalcea was not consistently stronger than that of the native species. Collectively, these results do not indicate a strong impact of Lythrum on the reproduction or abundance of Sidalce

Competition between Lythrum salicaria and a rare species: combining evidence from experiments and long-term monitoring

The rare endemic plant Sidalcea hendersonii (Henderson’s checker-mallow) occurs in tidal marshes of the Pacific Northwest and may be threatened by Lythrum salicaria (purple loosestrife), a European invader plant. We compared the abundances of Lythrum and Sidalcea in a wetland in British Columbia (Canada) in 1999 to those measured in 1979 to track changes in both species. Although the frequency of Sidalcea decreased by more than 50%, and that of Lythrum increased by almost 20%, there was no significant relationship between the changes of the two species. We assessed the potential effects of competition by Lythrum on Sidalcea in field and patio experiments. In the field, we measured the response of Sidalcea to the removal of Lythrum over a two-year period and compared this to the response of Sidalcea to the removal of native species and in unmanipulated control plots. Removal of Lythrum significantly improved the vegetative performance of Sidalcea compared to the removal of randomly selected native plants and the control treatment in the first year. In the second year, the performance of Sidealcea did not differ significantly with treatment. Removals did not influence the reproductive performance of Sidalcea in either year. A one-year additive experiment, carried out in pots, compared the competitive effect of Lythrum on Sidalcea with that of two native species. Lythrum’s impact on Sidalcea was not consistently stronger than that of the native species. Collectively, these results do not indicate a strong impact of Lythrum on the reproduction or abundance of Sidalcea

Variable success of biological control of Lythrum salicaria in British Columbia

Purple loosestrife, Lythrum salicaria, has invaded North American wetlands over the last 200 years. A biological control project was started in British Columbia, Canada, in 1993 with the introduction of Galerucella calmariensis, a leaf- feeding beetle of European origin. To evaluate the success of the biological control project in the Lower Mainland of British Columbia, we monitored the defoliation levels of Lythrum at 10 release sites of the control agent, identified factors associated with among-site variability in defoliation levels, and determined the effect of defoliation on the performance and abundance of Lythrum. In tidal areas, the beetles could not establish due to tidal disturbances. The control agent established at all non- tidal sites, but defoliation levels varied considerably. Defoliation levels were negatively associated with predation on Galerucella eggs, suggesting that general predators may be able to prevent or delay successful biological control of Lythrum. Eggs on Lythrum stems surrounded by water experienced lower predation rates than eggs on stems on land, indicating that standing water acts as a partial refuge against predators for Galerucella eggs. Flower-bud density, length of inflorescence, and plant biomass per m² were all negatively related with the intensity of beetle feeding, and stem height was marginally reduced with increased beetle grazing. Dry biomass per m² of Lythrum appeared to decline drastically only at sites where beetle grazing was intense

Impact and biological control of Lythrum salicaria in southwestern British Columbia

Purple loosestrife, Lythrum salicaria, is a Eurasian plant that has invaded North American wetlands over the last 200 years. As a precautionary measure, but without detailed knowledge of the ecological impact of Lythrum on native species, a biological control project was started in the 1990s with the introduction of Galerucella calmariensis, a leaf-feeding beetle of European origin. To measure the ecological impact of Lythrum, I assessed the effect of Lythrum on a rare, native plant species, Sidalcea hendersonii. To evaluate the biological success of the biological control project, I investigated the effectiveness of the biological control agent and identified factors associated with among-site variability in defoliation levels. Lythrum's impact on Sidalcea was not stronger than the impact of native species, possibly owing to the early emergence in the spring and the tall growth of Sidalcea. The abundance of Lythrum in a marsh where it co-occurs with Sidalcea did not increase significantly from 1979 to 1999, which suggests that the invasion process was largely completed by 1979. The effectiveness of the control agent was dependent on site condition. In tidal areas, the beetles could not establish due to the tidal disturbances. At sites where the control agent established, the reproductive performance and dry biomass of Lythrum were strongly reduced by beetle feeding damage in the same year, but stem density only declined at one site, Chilliwack, after severe herbivory over a four-year period. At most other non-tidal sites, plant stem density increased, and stem density changes were inversely related to the average beetle feeding damage during the previous four years. Beetle feeding damage was associated with predation on G. calmariensis eggs, but not predation on any other life-history stage, or plant quality. The densities of crawling, invertebrate predators were not generally related to predation levels, suggesting that predation did not vary in response to predator abundance, but to predator attack levels. I discuss the level of success of this biological control project and the need for control methods in tidal areas.Science, Faculty ofZoology, Department ofGraduat

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Département de biologi