Measuring the effects of invasive species on the demography of a rare endemic plant

The impacts of invasive species are among the greatest threats to the persistence of native species and communities. Yet most work on rare plants has focused on issues such as habitat fragmentation and genetic diversity, while few studies have quantified the impacts of invasive plants on native ones or investigated the underlying mechanisms of those impacts. I used removal experiments to assess the effects of invasive grasses on the seedling and adult demography of an endangered California endemic, Oenothera deltoidesssp. howellii. Invasive plant removal significantly increased O. deltoides seedling recruitment, but had no effect on adult plants. Differences in seedling recruitment were primarily driven by greatly increased seedling emergence rates in removal plots, although there was also some evidence of higher seedling survivorship with invasives removal. Differences in habitat type strongly influenced both the effects of removal treatments and O. deltoides demography, with areas that support natural recruitment showing weaker treatment effects and higher overall recruitment, but lower adult survivorship, compared to those under restoration through planting. These results indicate that inhibition of germination due to reduced soil disturbance, rather than resource competition, appears to be the strongest impact of invasive plants on this rare endemic. Although previous work has documented the importance of changed disturbance regimes in generating invasion impacts, invader effects on rare plants are generally presumed to result from resource competition. Studies like this one highlight the need for a greater emphasis on understanding the mechanisms by which invasive plants impact native ones, and the importance of such information in designing conservation and management strategies.