Older Hawaiian Islands Show Stronger Tree Diversity Responses to Rainfall Than Younger Ones

The Hawaiian archipelago has long served as a natural laboratory for studying biodiversity, and a new study published in Frontiers of Biogeography adds a striking twist to that tradition. Researchers found that native tree diversity responds to rainfall gradients very differently depending on the age of the island, with the oldest island in their analysis showing a clear positive relationship between precipitation and diversity while the two younger islands showed little to no such relationship. The findings suggest that millions of years of macroevolutionary time leave a measurable imprint on how local communities respond to climate, but that biological invasions are rapidly erasing this geological signature.

Lead author M. Paola Barajas Barbosa of the German Centre for Integrative Biodiversity Research, together with Tiffany Knight, Renske Onstein and Jonathan Chase, took advantage of the archipelago’s linear chronology, where each major island rises abruptly from the sea as a volcanic mountain. Every island experiences dramatic climate variation, from roughly 300 millimeters of rain per year on the dry leeward side to more than 10,000 millimeters per year on the wet windward side. Because the islands successively increase in age along the hotspot chain, the team could compare how tree communities respond to nearly identical precipitation gradients on substrates of very different geological ages.

The researchers analyzed tree abundance and composition from 375 forest plots compiled in the Open Nahele database, spanning the youngest island of Hawai’i at approximately 0.5 million years old, the intermediate-aged Maui Nui complex at roughly 2 million years old, and the older island of O’ahu at about 3 million years old. Plots ranged from 100 to 10,000 square meters in area and contained a minimum of 15 trees, defined as individuals with a trunk diameter of at least 5 centimeters measured at 1.3 meters height. The team excluded Kaua’i, the oldest main island at roughly 5 million years, because too few plots were available for analysis.

To control for sampling effort across plots of different sizes, the researchers applied coverage-based rarefaction, standardizing diversity estimates to a sample completeness of 80 percent. They computed taxonomic diversity using Hill numbers, a framework that expresses diversity as the effective number of equally abundant species. Hill number q = 0 corresponds to species richness, where all species count equally, while q = 1 weights species by abundance and q = 2 emphasizes dominant species. Comparing these orders allowed the team to detect whether diversity patterns were driven by rare or common species, and the results held regardless of the weighting used.

When the analysis included only relatively uninvaded plots, defined as those with no more than 40 percent alien individuals, and only native tree species, a total of 282 plots and 79 species, the pattern was unmistakable. O’ahu, the oldest island in the study, showed a strong positive relationship between mean annual precipitation and local tree diversity. On Maui Nui and Hawai’i, the two younger islands, no such relationship was evident. The interaction between island and precipitation was statistically significant in all models, and the results remained qualitatively similar when precipitation was truncated to a common range across the three islands.

The team also measured species turnover, or beta diversity, using a coverage-based metric called betaC, comparing pairs of plots and relating turnover to the absolute difference in mean annual precipitation between them. On O’ahu and Maui Nui, native species turnover increased with precipitation difference, indicating ecological specialization, with distinct species assemblages occupying dry and wet habitats. On the youngest island, Hawai’i, turnover was actually negatively related to precipitation difference. The researchers attribute this to versatile native generalists such as Metrosideros polymorpha, Cheirodendron trigynum and Ilex anomala, which tolerate enormous ranges of rainfall, from recent lava flows to substrates millions of years old, and to lower competition on a species-poor young island.

Soil development offers a complementary explanation for the low diversity on Hawai’i Island. The youngest island has nutrient-rich but poorly developed soils, and several of its wettest plots, receiving more than 5,000 millimeters of rain per year, sit on substrates only 0 to 3,000 years old from recent eruptions. Undeveloped soils limit opportunities for niche partitioning and species coexistence. In contrast, intermediate-aged substrates on O’ahu have well-developed, diverse soils that positively affect diversity. The models also showed that substrate age had a negative effect on diversity overall, while mean annual temperature, covering a limited range of 8 to 23 degrees Celsius, showed almost no effect.

When the researchers expanded the dataset to all 375 plots and included both native and alien species, 142 species in total with 63 alien, the patterns changed in unexpected ways. Rather than simply weakening the island-age signature, invasions created new patterns. On Hawai’i, diversity and turnover relationships with precipitation became positive once highly invaded plots were included, partly because invaded wet plots still contained high native diversity, often threatened by the invasive strawberry guava Psidium cattleianum, and partly because different invaders prefer different rainfall regimes. Leucaena leucocephala, pines and Schinus terebinthifolia favor drier plots, while Psidium cattleianum and Myrica faya favor wetter ones, generating turnover between habitats. On Maui Nui, however, turnover declined with precipitation difference when invaded plots were included, suggesting that alien species are negatively affecting native diversity in the wetter parts of that island complex.

The authors conclude that geological time not only promotes overall diversity but also shapes the ability of species to specialize in distinct environmental conditions, with older islands providing more opportunity for colonization, speciation and adaptation to specific rainfall regimes. With 89 percent of the archipelago’s flowering plants endemic, the Hawaiian flora carries a deep evolutionary signature, yet the ongoing wave of invasions is fundamentally reshaping local tree diversity patterns and the ecosystem services they provide. As alien species continue to spread across the steepest precipitation gradients on Earth’s most famous island chain, the geological fingerprint of millions of years of evolution is being rewritten within decades.

Subject of Research: How island age and precipitation gradients shape tree taxonomic diversity across the Hawaiian archipelago

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

Article References: Barajas Barbosa, M. P., Knight, T. M., Onstein, R. E., & Chase, J. M. (2025). Diversity responses to precipitation gradients differ between older and younger islands of Hawai‘i. Frontiers of Biogeography, 18, Article e131129. https://doi.org/10.21425/fob.18.131129

Image Credits: AI Generated

DOI: 10.21425/fob.18.131129

Keywords: Hawaiian Islands, tree diversity, precipitation gradients, island age, macroevolution, species turnover, biological invasions, alpha diversity, beta diversity, volcanic islands, native species, soil development