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.