Island biogeography explores how biodiversity in island ecosystems arises and is maintained. The topographical complexity of islands can drive speciation by providing a diversity of niches that promote adaptive radiation and speciation. However, recent studies have argued that phylogenetic niche conservatism, combined with topographical complexity and climate change, could also promote speciation if populations are episodically fragmented into climate refugia that enable allopatric speciation. Adaptive radiation and phylogenetic niche conservatism therefore both predict that topographical complexity should encourage speciation, but they differ strongly in their inferred mechanisms. Using genetic (mitochondrial DNA (mtDNA) and single-nucleotide polymorphism (SNP)) and morphological data, we show high species diversity (22 species) in an endemic clade of Fijian Homalictus bees, with most species restricted to highlands and frequently exhibiting narrow geographical ranges. Our results indicate that elevational niches have been conserved across most speciation events, contradicting expectations from an adaptive radiation model but concordant with phylogenetic niche conservatism. Climate cycles, topographical complexity, and niche conservatism could interact to shape island biodiversity. We argue that phylogenetic niche conservatism is an important driver of tropical island bee biodiversity but that this phylogenetic inertia also leads to major extinction risks for tropical ectotherms under future warming climates.

Anthropogenic climate change and invasive species are two of the greatest threats to biodiversity, affecting the survival, fitness and distribution of many species around the globe. Invasive species are often expected to have broad thermal tolerances, be highly plastic, or have high adaptive potential when faced with novel environments. Tropical island ectotherms are expected to be vulnerable to climate change as they often have narrow thermal tolerances and limited plasticity. In Fiji, only one species of endemic bee, Homalictus fijiensis, is commonly found in the lowland regions, but two invasive bee species, Braunsapis puangensis and Ceratina dentipes, have recently been introduced to Fiji. These introduced species pollinate invasive plants and might compete with H. fijiensis and other native pollinators for resources. To test whether certain performance traits promote invasiveness of some species, and to determine which species are the most vulnerable to climate change, we compared the thermal tolerance, desiccation resistance, metabolic rate, and seasonal performance adjustments of endemic and invasive bees in Fiji. The two invasive species tended to be more resistant to thermal and desiccation stress than H. fijiensis, while H. fijiensis had greater capacity to adjust their CTMAX with season, and H. fijiensis females tended to have higher metabolic rates, than B. puangensis females. These findings provide mixed support for current hypotheses for the functional basis of the success of invasive species, however, we expect the invasive bees in Fiji to be more resilient to climate change due to their increased thermal tolerance and desiccation resistance.

The genus Homalictus Cockerell has not been taxonomically reviewed in the Fijian archipelago for 40 years. Here we redescribe the four known species and describe nine new ones, bringing the number of endemic Homalictus in Fiji to 13 species. We provide identifications keys to all species. Most of the species diversity (11 species) have their distributions over 800 m asl (meters above sea level; highlands), and with only two species under 800 m asl (lowlands). We highlight the vulnerability of the highland-restricted species to a warming climate, and document the potential extinction of one highland species. The new species described here are H. atritergus sp. nov., H. concavus sp. nov., H. groomi sp. nov., H. kaicolo sp. nov., H. nadarivatu sp. nov., H. ostridorsum sp. nov., H. taveuni sp. nov. H. terminalis sp. nov. and H. tuiwawae sp. nov.. [Zoobank URL: urn:lsid:zoobank.org:act:71318BEC-40CD-470F-A1E7-0E1FD18A6459]