Endemic island floras are often vulnerable to anthropogenic impacts and introduction of invasive non-native plant species (Adsersen, 1989; Jäger et al., 2009; Sakai et al., 2002). Islands typically have high proportions of endemic species due to their geographic isolation, and as the most isolated archipelago on the planet, the Hawaiian Islands (HI) have a particularly high degree of endemism (~ 90% for angiosperms, and 74% for ferns and lycophytes) (Kier et al., 2009; Sakai et al., 2002; Vernon & Ranker, 2013). Similar to other isolated island archipelagoes, the Hawaiian endemic flora evolved in the absence of large mammalian herbivores, is disharmonic, and its species often have low population densities, restricted geographical ranges, and specific environmental requirements (Graham et al., 2017; Sakai et al., 2002; Zimmerman et al., 2008). On the other hand, indigenous species are components of native floras that have broader geographic distributions, and as such might have greater tolerance of shifts in environmental conditions or community composition, and less reliance on specific pollination or dispersal mutualisms (Buckley and Catford 2016; Ainsworth and Drake 2020), which theoretically could lend to greater resilience when compared with endemic native species. Unlike many endemic island plants, which have a relatively short evolutionary history, many of them having originated from recent adaptive radiations, indigenous island species generally have longer evolutionary histories, demonstrating a proven track record of longer-term survival (Sakai et al. 2002); furthermore, occasional immigration from surrounding geographic areas may facilitate indigenous species success on islands by providing opportunities for geneflow and population persistence (Shepherd et al. 2009).
Altogether, native plant species have experienced substantial declines since humans arrived in the HI, with 366 taxa officially listed as threatened or endangered, and 48 additional species under consideration for listing, accounting for 44% of the threatened and endangered species in the USA (Hawaiʻi Department of Land and Natural Resources 2013). These declines are largely attributable to anthropogenically caused increases in disturbance frequency and intensities in the HI, as well as competition from introduced plant species, many of which have growth rates and competition advantages over natives with increased resources, potentially leading to competitive displacement of native species (Woodcock 2003; Daehler 2003; Blumenthal 2006). Native island plants are also often reported to decline in response to feral ungulate grazing and disturbance, as many island ecosystems have no evolutionary history of grazing mammals (Merlin and Juvik 1992). However, while various abiotic site conditions related to anthropogenically caused disturbance and resource availability have been shown to influence the invasion process (Strayer et al. 2006; Gallien and Carboni 2017), it is relatively unknown to what degree indigenous and endemic Hawaiian plant species are differentially affected by these factors in the face of invasion across abiotic gradients in the long-term.
Where previous surveys have been conducted, resurveying becomes a powerful approach to understand long-term trends in forest communities (Condit 1995). It was therefore the objective of this study to revisit plots previously established by Wirawan (1978) and Welton (1993) to document if/how indigenous and endemic Hawaiian plant species’ respective abundances have changed, and how those changes are related to abiotic conditions in these plots. Our expectations were as follows: 1) Previous and current availability of resources (In this study, understory light availability and precipitation) will be negatively correlated with both endemic and indigenous plant cover (Daehler 2003; Blumenthal 2006); furthermore, increasing rainfall on sloped environments, which are common in Hawaii’s forests, is often associated with physical disturbances (e.g., treefalls, landslides), and many non-native invasive plants are known to increase in abundance in disturbed environments (Deb and El-Kadi 2009). 2) Elevation and slope will be positively correlated with endemic and indigenous plant cover, as areas of greater slope and elevation will be less accessible to feral ungulates (especially feral pigs) and have a reduced likelihood of anthropogenic disturbance (Chardon et al., 2019; Chynoweth et al., 2010). Native plants may also decline as invasive plants spread (Chance et al. 2019), and areas of higher elevation may have fewer invasive species present due to the fact that many non-native plant species were first introduced at low elevations, and have only gradually expanded their range upwards (i.e., invasion lag) (Pauchard et al. 2009). 3) Endemic species, which generally have shorter evolutionary histories than indigenous species, and which often have narrow environmental tolerances when derived from adaptive radiations, will tend to be more vulnerable to decline than indigenous species (Sakai et al., 2002).