INTRODUCTION

One of life’s most unique features is its varied types and forms–collectively known as biodiversity (Díaz & Malhi, 2022; Magurran, 2021; Wilson, 1988). For centuries, ecologists and evolutionary biologists have strived to understand the patterns of biodiversity, and the mechanisms that cause it, for example, across latitudinal gradients (Hillebrand, 2004) or across islands of different sizes, ages and levels of isolation (Gillespie & Baldwin, 2010; Warren et al., 2015). Most patterns of biodiversity variation can be attributed to a combination of historical biogeography (including macroevolution) and contemporary environmental conditions (e.g., precipitation, temperature) (e.g., Brown et al., 2013; Keil & Chase, 2019; Sandel et al., 2020; Thomas et al., 2020). Today, however, the human footprint can have as great, or greater, influence on biodiversity and can obscure natural patterns of biodiversity variation (Nogué et al., 2021; Russell & Kueffer, 2019).
Volcanic islands have long served as a laboratory to study patterns of biodiversity (Whittaker et al., 2017). The Hawaiian archipelago, for example, has been formed as part of a hotspot volcanic system that has led to a sequence of islands that range in age (Clague, 1996). The four largest islands of the Hawaiian archipelago vary in age from ca 4.7 million years on Kaua’i to 3 - 2.6 million years on Oʻahu, to 2.2 - 1.2 million years on the Maui Nui complex (including formerly connected islands of Lānaʻi, Molokaʻi and Kahoʻolawe: hereafter ‘Maui Nui’) to ca 0.5 - 200,000 years on the island of Hawaiʻi (Price & Clague, 2002). This hotspot age gradient provides evolutionary biologists and ecologists with a model system in which to study macroevolutionary processes, such as species diversification (Baldwin, 1997; Givnish et al., 2009; Oberlander et al., 2004), as well as the effect of time on contemporary ecological processes that shape biodiversity patterns (Barton et al., 2021; Craven et al., 2019).
Recently, Craven et al., 2019 used a compiled database of forest plots sampled across the Hawaiian archipelago (Craven et al., 2018) to ask whether the island age gradient known to influence tree diversity across the islands, where older islands harbor higher species diversity (Borregaard et al., 2017; Rosemary G. Gillespie, 2016), was also reflected at local ecological scales. They found that plots on the oldest island of Kaua’i had higher native species diversity locally (and more rare species) for a given sampling effort, compared to younger islands, such as Hawai’i. These patterns were obscured, however, when both native and alien species were included in the analyses, suggesting that the introduction of alien species may erode the signature of time on species diversity. While the analyses of Craven et al. (2019) suggests a macroevolutionary signature on local ecological patterns, they were not able to dissect the interaction of ecological and macroevolutionary processes simultaneously.
To disentangle the ecological from macroevolutionary effects, we examine the interaction between island age and local environmental conditions. To do so, we take advantage of another unique feature of the Hawaiian archipelago, where each island experiences a high climatic variation, as each of them abruptly rises from the sea as a mountainous volcano. Most notably, each island has a very wet side on the windward (east) side of the island where the air rises and loses water heading over the volcano in the form of precipitation, and a very dry side on the leeward (west) side of the island (Figure 1). As a result, each island has an abrupt precipitation gradient that can range from over 10000 mm/year on the wet side to about 300 mm /year on the dry side (Figure 1). Since precipitation is known to play a crucial role in driving patterns of diversity of many species, especially trees (Keil & Chase, 2019; Sandel et al., 2020), this provides an important environmental gradient to consider when examining patterns of species diversity within islands.
Based on previous work, we expect there to be more species on the older islands (Craven et al., 2019; Price, 2004), more species in wetter than drier regions (Adler & Levine, 2007; Esquivel-Muelbert et al., 2017) and higher species turnover between habitats with different amounts of precipitation compared to among those with similar precipitation (Givnish, 1999; Idárraga‐Piedrahíta et al., 2022). What is unknown, however, is how island age and precipitation interact. We hypothesize (hypothesis 1) that there has been more time for macroevolutionary processes to facilitate the evolution or colonization of precipitation specialists on older islands (Segovia et al., 2020), resulting in stronger responses of multiple facets of local diversity and species turnover (beta diversity) to precipitation on older compared to younger islands. We further examine whether patterns of Hawaiian tree diversity in response to precipitation across the island age gradient are altered when we include alien species into the analysis. We hypothesize (hypothesis 2) that the spatial biodiversity patterns observed for native forest plots across the island age and precipitation gradients might be weakened when we include alien species with no macroevolutionary history on these islands.
To test these hypotheses, we investigate several facets of species diversity, including diversity based on species identities (taxonomic diversity), evolutionary relationships (phylogenetic diversity) and trait diversity (functional diversity). These three diversity facets aid understanding why diversity may vary among or within islands (Chao et al., 2014, 2021). For instance, if patterns of functional and/or phylogenetic diversity relationships with precipitation differ from taxonomic diversity, niche conservatism and environmental filtering may play a role by constraining phylogenetically-related species that share similar traits. Here, we used sample-controlled analyses of tree diversity data from an extensive database of forest plots distributed across the main islands of the Hawaiian archipelago (Craven et al., 2018). We specifically had enough data from plots on the youngest island of Hawai’i, the intermediate-aged Maui Nui and the older island of O’ahu (Figure 1), to test how the response of multiple facets of tree species diversity to precipitation gradients varied across islands.