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.