Abstract
We aimed to understand the regional population dynamics of the endemic fern lineage Diellia (D. erecta, D. erecta f. alexandri, D. falcata, D. mannii, D. pallida, and D. unisora) in mesic forests on the Hawaiian Islands. In particular, we were interested in whether studying life-stage structure would contribute to setting conservation management priorities and understanding regional dynamics. A repeated field survey of historically recorded population locations and most of the known populations were performed between 1999 and 2005. Distribution data available since 1838 show that these ferns have become extinct from 86% of the formerly recorded locations and the number of recorded extinction events exceeds the number of recorded appearance events. A significantly stronger cumulative impact of alien animals and plants distinguishes the habitats of extinct populations from other sites. The status of 19 local populations was assessed on the basis of stage structure as 'dynamic' (2 populations, sporelings predominate), 'normal' (8 populations), or 'regressive' (9 populations, mature plants predominate). Population size was negatively dependent on soil disturbance by alien animals. The existence of small regressive Diellia populations possibly indicates a delayed extinction process due to habitat deterioration. The study of life-stage structure, together with information about population size, is a useful tool to evaluate conservation management priorities and understand regional dynamics in conditions where demographic and precise distribution data are lacking. We conclude that the whole lineage is critically endangered and the impact of alien ungulates is obviously the main threat.