Differential change in indigenous versus endemic plant cover over time in an invaded Hawaiian landscape

Abstract

Abstract Endemic island plant species under threat due to anthropogenic activities may be more susceptible to decline than their indigenous counterparts. Our study aimed to determine whether endemic and indigenous plant cover in Hawaiian forests have been differentially affected by resource availability, disturbance, and invasion over time. We resurveyed 25 plots (400 m2 each) across gradients of elevation, precipitation, slope, and understory light availability in forested conservation areas on the island of Oʻahu, Hawaiʻi, USA. We assessed how these abiotic factors relate to absolute cover (previous, current, and change over time) of endemic and indigenous Hawaiian plants, as well as that of non-native species. Endemic species had a net loss of cover (-45%) since previous surveys 27 to 45 years earlier, while indigenous species had a net gain (+ 29%), as did non-natives (~ 1%). Endemic species cover was associated with higher elevations in both prior surveys and in 2018, and with steeper plot slopes in prior surveys. However, change in endemic cover over time was negatively correlated with slope, likely due to landslide disturbance or the proliferation of shade-tolerant invaders in low-light conditions where no disturbance was evident. Change in endemic species cover over time was not clearly related to resource availability (light and precipitation). Given the differences in responses of endemic and indigenous species to abiotic conditions, and that the endemic species in this study were more susceptible to decline than indigenous species, the results of this study indicate that generalizations about natives as a single group can be misleading.