DISCUSSION

Across most measures of diversity, we found that there was interaction between the response of local diversity to precipitation and island age. Specifically, there was a positive precipitation-diversity relationship on the oldest island (O’ahu; 3 - 2.6 million years), but no such correlation on the younger islands of Maui Nui (2.2 – 1.2 million years) and Hawai’i (0.5 - 200,000 years). In addition, the turnover of native species across contrasting habitats (e.g., dry vs. wet), is higher on the older island. However, the presence of non-native species eroded these distinctions in patterns of diversity among the islands, and even reversed them in the case of Maui Nui, where there was a slight negative relationship between diversity and precipitation and local communities in different conditions became more similar to one another.
While our data and analyses cannot specifically test the mechanisms that might underlie the pattern of increasing local native diversity with increasing precipitation in older but not younger islands, our results provide hints to the potential mechanisms. For example, if the mechanism of a longer time for macroevolutionary processes to promote specialists on older islands were responsible for the observed patterns (e.g., via new adaptations or colonization of pre-adapted specialists (Pontarp & Wiens, 2017)), then we might expect to see endemic or limited range species contributing to the higher species richness in sites with higher precipitation on the older islands. We find some support for this. For example, we found older island endemics such as Psychotria hexandra and Sapindus oahuensis in our plots on O’ahu, that do not occur on Maui Nui or Hawai’i (Wagner and Khan, 2023). Further, several species, including Bobea sandwicensis , Clermontia kakeana , Cyanea angustifolia , Hibiscus arnottianus andSantalum freycinetianum are found in our plots on O’ahu and Maui Nui, but do not occur on the youngest island of Hawai’i (Wagner & Khan, 2023).
Our observed patterns of low diversity in plots across the entire precipitation range of Hawai’i as well as the low species turnover across plots could have an alternative explanation other than island age per se. Instead, the youngest island (Hawai’i) has lower local heterogeneity in soil types compared to the older islands which have higher soil diversity (Deenik, J. & McClellan, 2007). Low soil diversity can create fewer opportunities for species to coexist (via niche partitioning) or frequency-dependent feedbacks through the soil (Eppinga et al., 2018). Furthermore, the commonly low topographic complexity of young volcanic islands (such as Hawai’i) also limits the diversity of island habitats via reduction environmental heterogeneity (Barajas-Barbosa et al., 2020). Within island variation of substrate age may also play a role on diversity. On the youngest island, Hawai’i some plots located in the wettest areas of the island (> 5000 mm/year) are also on substrates with the youngest age (0 -3000 years; Figure S9a) (Sherrod et al., 2007). Such young substrates may limit diversity (Figure S9b-c) as a result of more recent disruptive volcanic activity. We find that three native species tended to be present in many plots across Hawai’i: Metrosideros polymorpha (Myrtaceae),Cheirodendron trigynum (Araliaceae) and Ilex anomala(Aquifoliaceae). The species name ‘polymorpha’ was given toMetrosideros polymorpha to describe the versatility of this species, which can occur on very recent lava flows as well as soils that are millions of years old. While all three of these common species prefer moist habitats, they can all tolerate a range of annual precipitation levels (Barton et al., 2021). This, together with the potential for lower competition on younger islands with lower species diversity (Borregaard et al., 2016; Craven et al., 2019), may allow such generalist species to be successful across the young Hawai’i island.
Our result of a positive relationship between native species turnover and the precipitation difference among plots on O’ahu and Maui Nui, but not Hawai’i, reinforces the idea that there has been more time for macroevolutionary processes to generate species that specialize in different environments and habitat types, as well as for new specialized colonists to establish in those habitats. This may relate to a high niche differentiation occurring on older islands; whereby diversity is promoted in wet environments, potentially due to the more favorable abiotic conditions than in dry environments (Spasojevic et al., 2014). This result is also consistent with the idea that macroevolutionary time provides more opportunities for species specialization into different habitat types; yet, the high heterogeneity in soil types and topography of older islands may also allow more species to coexist in the wetter environments (Laliberté et al., 2013; Mason et al., 2012).
The concomitant change observed for taxonomic, phylogenetic and functional diversity with increasing precipitation suggests that native lineages, functional groups and species numbers change differently with precipitation among islands. In O’ahu, all three types of diversity increased with increasing precipitation. This suggests that the higher taxonomic diversity present in wetter plots is composed of species that are functionally different and that come from more distinct evolutionary lineages compared to those in dry habitats. For example, one 500 m2 plot on the older island of O’ahu located in a wet area contained six different tree species (i.e., Acacia koa, Clermontia kakeana, Cordyline fruticose, Hibiscus arnottianus, Metrosideros polymorpha, and Metrosideros tremuloides ). These species are also functionally diverse, including a nitrogen-fixing tree (Acacia koa ) that is tall and has high leaf nitrogen content,Metrosideros polymorpha , which has low leaf nitrogen content and specific leaf area but high wood density, Clermontia kakeana,which is shorter with low wood density, but high specific leaf area and leaf nitrogen content. These species are also quite phylogenetically distinct (e.g., including both monocots and dicot). This result is consistent with the idea that resource rich (i.e., wet) environments can promote (via e.g., high coexistence) functional diversity, whereas dry environments can limit it (Spasojevic et al., 2014; Xu et al., 2017). Furthermore, as lineages generally have a limited ability to adapt to dry environments and species tend to retain their ecological preferences through evolutionary time through niche conservatism (Ringelberg et al., 2023; Segovia et al., 2020), adaptation and colonization of lineages in dry areas of the older island is also limited. This likely influenced the lower phylogenetic diversity we observed in O’ahu’s arid environments. Lastly, the result that phylogenetic and functional diversity increase with precipitation on the older, but not younger islands supports the idea that macroevolutionary time plays a key role allowing more pre-adapted lineages to colonize and establish in wet environments. At the same time, macroevolutionary time increases functional diversity in high precipitation environments where distinct plant ecological strategies can co-exist.
While we hypothesized that the spatial biodiversity patterns might weaken when we included heavily invaded plots (e.g., through homogenization), we actually found distinctly new patterns rather than a weakening of existing patterns (Figure 2g-l). For example, we found no trends in diversity or species turnover with precipitation on the youngest island of Hawaii when only plots dominated by native species were examined, but these relationships became positive when we included the highly invaded plots. This may be for two reasons. First, several of the invaded plots that were in wet areas contain relatively high native tree diversity (in terms of species number), and thus contributed to the trend for increasing tree diversity with increasing precipitation. These plots in wet areas were often invaded by Psidium cattleianum, a highly abundant invasive tree that may eventually displace these native trees, but has not yet done so (Barton et al., 2021). Second, many of the invasive species have preferences for different precipitation conditions. For example, the invaders Leucaena leucocephala ,Pinus spps and Schinus terebinthifolia tend to occur more in drier plots, while Psidium cattleianum and Myrica fayaoccur more in wet plots, creating turnover between wetter and drier plots. The increase in taxonomic diversity with precipitation in Hawai’i is accompanied by an increase in functional diversity, suggesting that alien species contribute to new trait variation in the wet environments of Hawai’i (Figure 2g-l and Figure S4).
In contrast, on Maui Nui, there was a decrease in forest diversity with increasing precipitation when we included heavily invaded plots (Figure 3b). This was particularly so for taxonomic and functional diversity. That is, local diversity of both native and alien species on Maui Nui was higher in more arid than in wet environments when we included heavily invaded plots and alien species. This could be because Maui Nui were uniquely invaded by arid-favoring species, or because alien species have a greater establishment and impact on native species diversity in wetter environments. While we cannot definitively test among these hypotheses, the former would not explain why species turnover across plots declines as precipitation becomes more different on Maui Nui, as the opposite pattern would be expected.