Fijian Hylaeus bee study estimates sharp genetic population decline

Fiji’s Hylaeus navai bees have undergone an estimated 99% decline in effective population size since European colonisation, according to a genomic study published in Ecology and Evolution on 5 October 2026. Reported by Phys.org on 12 October, the finding raises concern for a canopy-dwelling bee whose modeled suitable habitat covers only a small fraction of Fiji’s land area. The estimate measures genetic population size, rather than the number of bees alive today.

How the Hylaeus navai decline was estimated

Tegan A. Edwards and colleagues combined lowland and canopy sampling with mitochondrial and genome-wide data and species-distribution modeling to investigate endemic Fijian Hylaeus bees. Field collection took place between 14 April and 11 May 2025. The team generated genome-wide data from 94 specimens across four species; 85 specimens were H. navai, which therefore dominated the population-level analysis.

The genomic reconstruction yielded an approximately 99% fall in H. navai’s effective population size since European colonisation. The size of the inferred fall varied substantially with the study’s approach: estimates ranged from 81.6% under its most conservative approach to 99.9% under its least conservative one. The authors also inferred declines of 61%, 76% and 82% over three separate 10-year periods in the species’ demographic history.

Effective population size describes a population through its genetic history; it is not a direct headcount. Consequently, the percentage cannot be read as proof that 99% of individual bees have disappeared, or used on its own to calculate how many remain. The researchers also found low observed heterozygosity and elevated inbreeding, results they describe as consistent with a genetic bottleneck.

How much suitable habitat remains in Fiji

The model has a further qualification: some sites that were difficult to access lay at higher elevations, creating a possible sampling bias in the habitat estimates. The study also detected genetic structure among islands that may point to incipient speciation. Its authors say more systematic research is needed before that possibility can be resolved.

Why canopy sampling changes the picture

The difficulty of finding forest bees was already apparent in a separate 2024 study of Pacific Hylaeus. Its researchers described six new Fijian species, among eight new species reported across Fiji, French Polynesia and Micronesia. They argued that canopy sampling revealed bees conventional approaches had missed, with implications for how forest diversity is interpreted and managed.

That earlier work gives context to the new study’s use of both lowland and canopy collection. It also helps explain why an apparently narrow known range needs careful interpretation: survey methods and access to forest habitat affect what researchers can detect. The 2026 estimate adds genetic evidence of a steep decline in H. navai, while its habitat model remains subject to the access limitation identified by its authors.

What caused the decline remains unproven

The researchers discuss several pressures that could have interacted over time, including habitat modification, cyclones, competition from native and introduced pollinators, and the invasive African tulip tree as a possible ecological trap. Their genomic reconstruction does not show which of these caused the inferred losses, or establish how much any individual pressure contributed. These are proposed explanations for future investigation, not demonstrated causes of the estimated population change.

Further work is needed, the study says, to test whether African tulip tree nectar harms Fijian Hylaeus and how warming and cyclone damage affect remaining highland habitat. Those questions matter for deciding which conservation measures would address an actual threat, rather than one that is merely plausible from current evidence.

The authors say their decline estimates support a formal assessment under the International Union for Conservation of Nature’s criteria and align with criteria for Critically Endangered. The paper does not itself establish that H. navai has received that listing. A formal assessment, better coverage of difficult high-elevation sites and tests of the proposed threats would help clarify both the species’ status and the practical response.