Protection from Natural Enemies in Managing Rare Plant Species

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Abstract
Natural enemies such as pathogens, herbivores, and seed predators can substantially limit the abundance of plants, including rare species. Vulnerability to particular enemies is likely to differ between life‐history stages. We hypothesized that short‐term protection of juvenile plants from herbivores can be used to increase population growth of rare species and thus improve the probability of long‐term persistence. Using the federally listed (threatened) Pitcher's thistle (Cirsium pitcheri ) as a model, we experimentally excluded insect herbivores from juvenile rosettes to evaluate the potential benefits of deliberate insect control as a tool for management of rare species. Herbivore effects varied spatially across the local environment. Excluding insects in portions of the habitat where herbivory was high had direct benefits, including a 53% decrease in juvenile plant mortality (60% to 7%) and a 10‐fold increase in seed production of juveniles that matured and flowered. In other areas, where herbivore‐induced juvenile mortality was relatively low, excluding insects either increased seed production of plants that flowered or had no major effect. Our data also suggest indirect benefits to the metapopulation via potential improvement in dispersal among patches. Temporal variation in growing conditions occurred between years, suggesting that multiple‐year exclusions would be most effective. Our study suggests that small–scale manipulation of often inconspicuous interactions between rare plants and their natural enemies can be an effective, relatively low‐cost tool for the management and restoration of rare plant species.