A Century of Vegetation Change on an Offshore Islet1

Offshore islets are features of archipelagos and coastal continental regions. Because of their small size, they can be considered microcosms for studies of island biogeography and community assembly. The Hawaiian Islands include at least 54 islets, which provide safe habitat for nesting seabirds and monk seals due to the absence of mammalian predators and laws restricting human activity. Because the Hawaiian islets are uninhabited by people, they offer unique opportunities to examine community dynamics in relatively undisturbed habitats. Mānana Islet, also known as Rabbit Island, is a 25.5-hectare islet located off Kaupō Beach in Waimānalo, O‘ahu Island, that has been designated a Seabird Sanctuary by the state of Hawai‘i. European rabbits (Oryctolagus cuniculus) were introduced to Mānana in the 1880s for food and game but were eradicated by 1985 through a state—initiated hunting and poisoning program. Across a nearly 80-yr period (1927–2008), spanning rabbit habitation and eradication, the vegetation of Mānana has been surveyed or reported 21 times, providing a longitudinal record of plant diversity on the islet. To extend this record, a new vegetation survey was conducted in 2023, replicating previous methods. Rabbits had devastating effects on the vegetation of Mānana, which then increased in plant species richness after rabbit eradication. The most recent surveys have detected an increase in nonnative species, highlighting biological invasion as an ongoing threat to these islet habitats. In the most recent survey, a total of 43 plant species were detected, including seven new records, six of which are non-native. How these patterns in species richness correspond to measures of diversity based on abundance and evenness remains unclear. Additional restoration of Mānana Islet may increase native biodiversity, supporting nesting seabirds and ensuring these diverse coastal islets persist in the future.