Hibiscadelphus giffardianus is one of many imperiled species that is endemic to Hawaii Volcanoes National Park. Conservation efforts for this and other species have been complicated by a slew of ecological threats in recent history, including the introductions of numerous non-native plants and animals. In 2010, ecologists and managers identified invasive Rattus rattus as one of the primary threats to H. giffardianus and other rare plants of Hawai’i Volcano National Park’s upland forests. The aim of the current study is to evaluate the degree to which R. rattus treatment may improve this imperiled species’ ability to complete its life cycle in nature. Through a Before-After-Control-Impact (BACI) experimental design, we controlled the rat population (using poison bait stations) in one of two stands where H. giffardianus grows, while the other stand served as a control. We monitored rat activity, the production of buds, flowers, and fruit, as well as the recruitment of seedlings, on and around each tree (n = 173) in both stands beginning four months prior to treatment through four months after treatment. We used ANOVA and Tukey’s Honest Significant Difference test to analyze differences between the two stands before and after treatment.
Evidence of rat activity in the treated stand decreased to an all-time low through the 4-month post-treatment observation period, but remained high in the control stand. Flowers, live fruit, and open fruit were more abundant after treatment compared to pre-treatment and control levels. While fruit set was highest following rat control, recruitment remained low. Two possible explanations for this exist. First, a number of non-native competitors and predators may be affecting other stages of the H. giffardianus life cycle, including mice, Kalij pheasants (Lophura leucomelanos) and grasses. While reduction in the rat population may yield increased fruit set, the seeds reaching the forest floor may fail to establish as seedlings in the presence of these threats. Another possible explanation is that a longer period of rat population control may be necessary for seedling establishment. H. giffardianus germination typically takes 1.5-3 months, but the additional time required for fruit to open and release seed may extend the timeline beyond the 4-month observation period. Managers of ecosystems containing imperiled plants benefit from a holistic perspective of species’ entire life cycles to determine the point at which management actions become cost-effective.