Effects of Nutrient Addition and Competition on Biomass of Five Cirsium Species (Asteraceae), Including a Serpentine Endemic

Endemic plant species are often thought to be restricted to marginal habitats as a result of a trade‐off between fitness in high‐stress environments and competitive ability in benign environments. Here we provide a unique test of this hypothesis by manipulating competition and habitat quality across a gradient and contrasting closely related species that differ in their habitat specializations. We compared the competitive abilities of five Cirsium species that co‐occur in northern California and range from an endemic species to a noxious invader. The endemic species Cirsium fontinale var. fontinale is restricted to low‐nutrient serpentine habitats. We conducted a greenhouse experiment to compare the competitive abilities of all native species with those of conspecifics (intraspecific competition) and with those of the invasive species Cirsium vulgare (interspecific competition) under five nutrient treatments. We hypothesized that C. fontinale would have a poor competitive ability in a high‐nutrient environment compared with its congeners due to a stress‐competition trade‐off. In contrast to our expectations, however, we found that C. fontinale competed well, even in high‐nutrient environments. It was the only native species that showed no significant difference in biomass between intraspecific‐ and interspecific‐competition treatments. Its inability to expand its range beyond the serpentine habitat is likely explained by factors other than poor competitive abilities in less stressful environments. Understanding the mechanisms that explain why endemic species are restricted to harsh abiotic environments requires removing them from their specialized habitats and comparing their fitnesses and competitive abilities in more benign environments.