Patterns of endangerment in the Hawaiian flora.  The Hawaiian Islands have one of the highest proportions of rare and endangered species in the United States because of high rates of endemism, habitat loss and degradation of the environment through the impacts of feral animals and invasive alien plant species.  In collaboration with Warren L. Wagner (Smithsonian Institution), Dr. Sakai is working on ecological correlates and phylogenetic patterns of endangerment in the native Hawaiian flora.

Restoration ecology. Dr. Weller, in collaboration with Dr. Robert Cabin ( Brevard College ) and Dr. David Lorence ( National Tropical Botanical Garden ) has worked for a number of years monitoring community changes with exclusion of large herbivores such as feral deer, goats, and pigs in mesic forests of Kauai . Fenced and unfenced control plots in Mahanaloa Valley show significant differences in the understory after only a few years of fencing.

Invasive species-Invasibility of fountain grass. A significant threat to dry sites in the Hawaiian Islands is fountain grass (Pennisetum setaceum), an aggressive invasive alien in Hawaii that now carpets large areas of the Big Island of Hawaii, posing a significant fire threat to this area. In Arizona , fountain grass is beginning to spread rapidly, and in southern California , fountain grass is present but localized.  Dr. Jessica Poulin, who finished her doctoral degree in 2005 in the Sakai/Weller lab, investigated the relative importance of genetic and environmental factors in the different patterns of invasiveness of fountain grass in these three areas.  Using molecular markers, she found no genetic variation among populations from these three areas, but significant differences in growth between common garden plots with different amounts of precipitation.  These results have significant implications for the invasiveness of fountain grass in different regions with potential changes in precipitation patterns with global change.