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

The researchers built what is known as a total-evidence phylogeny of feliforms, the branch of the mammalian order Carnivora that includes cats, hyenas, civets, linsangs, mongooses and their many extinct relatives. Total-evidence approaches combine morphological data from both living and fossil species with molecular data from living ones, allowing extinct taxa to be placed on the tree with their dates of occurrence rather than being dangled loosely from unresolved branches. This tip-dated framework is crucial for studying macroevolution, because it preserves the temporal dimension of evolutionary change. Fossils are not simply decoration on a molecular tree; they are data points whose anatomy and age jointly constrain inferences about how traits evolved through deep time.

With this phylogeny in hand, Barrett and Hopkins modeled the evolution of two key traits across feliform history: body mass and what they call the dental toolkit, measured as the relative blade length of the lower carnassial, the modified shearing tooth that is the signature innovation of the carnivoran jaw. By fitting alternative evolutionary models to these traits across the tree, the authors could detect where evolution followed different rules, testing the classical idea of adaptive zones first articulated by George Gaylord Simpson in the 1940s and 1950s. Adaptive zones are, in essence, distinct regions of ecological and functional opportunity, each with its own adaptive landscape that shapes the direction and tempo of trait evolution for the lineages occupying it.

The analysis identified three adaptive zones within feliforms. The first, which the authors term the ancestral cataract of carnivory, encompasses small-bodied, ecologically flexible taxa in which dental evolution appears largely stochastic, drifting without strong directional pressure. The second, the broad ecology cursor, is associated with lineages in which forelimb dexterity is restricted and prey processing relies more heavily on the head, favoring running adaptations and cranial specializations. The third, the soft-flesh specialist, is defined by intense selection for slicing-dominated dentitions in hypercarnivores, animals whose diets consist almost entirely of meat and whose teeth have been progressively simplified into blades at the expense of crushing and grinding surfaces.

Crucially, the framework does not stop at the feliform branch of the tree. Extending their synthesis across terrestrial mammalian carnivores more broadly, the authors propose a fourth zone, the versatile omnivore, representing a distinct adaptive regime of dietary and morphological flexibility at large body size. Bears are perhaps the most familiar modern occupants of this zone, and their fossil relatives, along with giant amphicyonids and other big-bodied generalists, suggest it has been repeatedly occupied throughout carnivoran history. Large-bodied omnivores retain generalized dentition despite their size, reflecting an adaptive landscape in which dietary breadth, rather than specialization, is the winning strategy.

The most consequential finding concerns where selection on teeth actually operates. Across all of these regimes, dental morphology experiences strong, consistent selection only in hypercarnivores. In the soft-flesh specialist zone, the demands of slicing meat impose a tight functional constraint, and the length of the carnassial blade becomes a reliable predictor of diet. Everywhere else, dental evolution is closer to a random walk. In taxa with broader diets, tooth shape drifts in ways that are weakly tied to what the animal actually eats, because generalized dentitions can process many food types and individual morphological changes carry little functional cost. This single insight elegantly explains why recent attempts to infer diet from dental measurements have produced frequent misclassifications: those methods work superbly for extreme specialists and poorly, sometimes badly, for everyone else.

The finding carries immediate practical implications for paleontology. Estimates of ancient diets underpin reconstructions of past food webs, predator-prey dynamics and ecosystem structure, and they inform debates about extinction drivers, competition among sympatric predators and responses to climate change. If diet predictions from teeth are reliable only for hypercarnivores, then paleobiologists can place greater confidence in dietary reconstructions for saber-toothed nimravids, dirk-toothed barbourofelids and the most committed flesh-slicers of the fossil record, while treating generalized taxa with appropriate caution. The new framework essentially supplies a filter: it tells researchers when a morphological proxy can be trusted and when it is likely to mislead.

The study also resonates with a rich body of prior work on carnivoran ecomorphology. Decades of research by Blire Van Valkenburgh and colleagues documented the iterative evolution of hypercarnivory, particularly in canids, where repeated incursions into extreme meat eating were followed by elevated extinction risk, a pattern that underscores how specialization can be both an ecological triumph and an evolutionary dead end. Work on elbow-joint morphology by Ki Andersson and Lars Werdelin illuminated the evolution of cursorial locomotion, and analyses of skull shape and biting biomechanics by Figueirido, Tseng, Slater and others mapped the functional landscapes that different feeding strategies impose. Barrett and Hopkins knit these threads together into a single macroevolutionary model in which different trait complexes, teeth, limbs and skulls, may respond to different adaptive landscapes operating simultaneously on the same animal, a form of mosaic evolution that the authors documented previously for feliform morphological disparity.

Methodologically, the study demonstrates the power of modern Bayesian phylogenetic toolkits. The total-evidence tree was inferred using tip-dated approaches in BEAST 2, integrating fossil occurrences as sampling events along branches, and the resulting maximum clade credibility tree served as the scaffold for model comparison. Trait evolution was modeled using Ornstein-Uhlenbeck and related processes implemented in packages such as Geiger and mvMORPH, allowing the authors to compare regimes of stabilizing selection, random drift and adaptive peaks across the tree. The data, including metric measurements from museum specimens spanning living feliforms and newly examined hyaenid and nimravid fossils, along with all analysis code, have been released openly through Zenodo and figshare, making the framework transparent and extensible. Follow-up work can now ask whether the same zones can be detected in caniforms, the other great carnivoran radiation, and whether marine carnivores obey parallel rules.

Ultimately, what makes this study compelling is that it reconciles two observations that have long seemed contradictory: the remarkable convergence of carnivore form and the unreliability of form as an indicator of function. Convergence is real, but it is concentrated in the zones where selection is strongest, above all the soft-flesh specialist regime where slicing teeth are non-negotiable. Outside those peaks, morphological similarity can be coincidental, and dissimilarity can be meaningless. By mapping where in the adaptive landscape teeth are locked to diet and where they roam freely, Barrett and Hopkins have given evolutionary biologists and paleontologists alike a predictive model for one of the most iconic radiations in the history of terrestrial vertebrates, and a sharper set of tools for reading the ecological lives of predators long extinct.

The concept of adaptive zones has a long intellectual pedigree stretching back to Simpson’s foundational work on tempo and mode in evolution, and it has since been applied to systems as varied as cichlid fishes with their pharyngeal jaws, phytophagous insects, and mammals that acquired the hypocone, a cusp widely regarded as a key innovation opening herbivorous niches. What distinguishes the new feliform analysis is that it treats the adaptive zone not merely as a descriptive category but as a testable statistical regime, asking whether trait evolution within each zone obeys stabilizing selection around an optimum or drifts idly. This quantitative framing connects the study to a mature comparative-methods literature on Ornstein-Uhlenbeck models, in which the strength of selection and the location of adaptive peaks can be estimated directly from trait data distributed across a phylogeny.

The energetic dimension of carnivore ecology also deserves emphasis. Body mass and diet are tightly coupled in terrestrial carnivores because meat is a patchy, energetically expensive resource, and small predators can subsist on invertebrates and mixed foods that would never sustain a large-bodied hunter. This scaling relationship helps explain why the ancestral cataract of carnivory is populated by small, flexible taxa, while the versatile omnivore zone is defined by large size combined with dietary breadth, a combination that requires generalized teeth capable of processing both flesh and plant material. The carnassial blade, meanwhile, is a structure whose functional performance degrades gracefully across many diets, which is precisely why its length carries little information except at the hypercarnivorous extreme.

There is also a conservation angle worth noting. Living feliforms occupy all three identified zones, and understanding which lineages sit under strong functional constraint may inform expectations about their vulnerability to prey depletion and habitat change, since specialists dependent on intact vertebrate communities face narrower margins than flexible generalists.

Subject of Research: Adaptive zones and evolutionary regimes in the dental and ecological evolution of feliform carnivores

Article Title: Adaptive zones of feliforms and evolutionary regimes within terrestrial mammalian carnivores

Article References: Barrett, P. Z., & Hopkins, S. S. B. (2026). Adaptive zones of feliforms and evolutionary regimes within terrestrial mammalian carnivores. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-026-03176-1

Image Credits: AI Generated

DOI: 10.1038/s41559-026-03176-1

Keywords: feliforms, adaptive zones, hypercarnivory, carnassial, total-evidence phylogeny, macroevolution, ecomorphology, diet prediction, Carnivora, paleontology, mosaic evolution, Ornstein-Uhlenbeck models