Diversity responses to precipitation gradients differ between older and younger islands of Hawai‘i

Studying the interaction between macroevolutionary and ecological factors is critical for understanding the principles of diversity regulation and predicting the effects of human activities. Here, we use the geological chronology of the Hawaiian archipelago as a testbed to examine the interaction between island age and climatic factors (i.e., precipitation) on contemporary patterns of tree taxonomic diversity. To this end, we estimated patterns of tree species diversity from 375 forest plots spread across steep precipitation gradients and different substrate ages on a younger island (Hawai‘i; ~ 0.5 million years old), an intermediate-aged island (Maui Nui complex; ~ 2 million years old), and an older island (O‘ahu; ~ 3 million years old). We found a clear positive relationship between precipitation and diversity on the oldest island (O‘ahu), but no such relationship on the two younger islands (islands in the Maui Nui complex and Hawai‘i). We also found high species turnover between drier and wetter environments on the oldest island, which suggests ecological specialization on these habitat types, but not on the younger islands. However, when we included plots that were highly invaded by alien species, the effect varied and precipitation had a larger effect on diversity and turnover on the younger islands. This could be because the younger islands may be more vulnerable to invasions. Our results suggest that the response of diversity to climate variation differs substantially across the Hawaiian Islands, possibly because of differences in the age of the islands; however, biological invasions are degrading this signature. Local diversity responses to a steep precipitation gradient are stronger on older Hawaiian Islands, likely due to longer timescales for macroevolutionary processes. Species turnover to distinct precipitation conditions varies across islands of the Hawaiian archipelago, with older islands exhibiting greater precipitation-driven ecological specialization. Alien species alter local diversity responses to precipitation, particularly on the youngest island of the archipelago. The presence of alien species is modifying the pattern of species turnover across distinct precipitation conditions, with dry and mesic habitats on intermediate-age islands showing higher species turnover. Biological invasions are currently reshaping plant diversity patterns in the Hawaiian archipelago.