Genetic Scorecard Indicator - Great yellow bumblebee

Genetic studies show low diversity, with reduced heterozygosity and allelic richness. Despite no signs of recent bottlenecks or inbreeding, effective population sizes are critically low (~25). Populations are genetically sub-structured, reflecting isolation and limited gene flow. The species now occupies less than 10% of Scotland’s 10 km grid squares. Formerly widespread in the UK, the species has declined by ~80% since the early 20th century due to habitat loss and agricultural intensification (Charman et al., 2010; Phelan et al., 2021; Bibby, 2025).

Population definitions:

Populations defined by geographic boundaries and genetic clusters. The species is present in five primary population centres. These groupings are based on known occurrences from recent surveys (2023–2024) and reflect major geographic separations reinforced by limited dispersal capability and habitat discontinuity. While earlier studies often grouped these into three broader regional populations, the current understanding recognises five biologically meaningful and geographically isolated units (Bumblebee Conservation Trust, 2024; Buglife, 2024). 

Ne500: The proportion of populations that have an effective population size of more than 500. 

PM: Proportion of populations that existed in 2000 that still exist in 2025.

Functional variation

Limited flight range (~1 km), dependence on deep-flowered forage plants, and widespread floral scarcity restrict both dispersal and gene flow. Habitat fragmentation further reduces genetic connectivity, leading to increased risks of inbreeding, disease susceptibility, and reduced adaptive potential. This is exacerbated by small colony sizes and isolated nesting, particularly in low-density coastal populations and could cause the loss of functional diveristy  (Phelan et al., 2021; Malone, 2025; Buglife, 2024).

Divergent lineages

There is currently no evidence of divergent evolutionary lineages within the UK populations. While strong population structure is observed between islands and regions, this likely reflects recent isolation rather than long-term divergence (Charman et al., 2010). 

Hybridisation/ Introgression

There are no known hybridisation risks or evidence of introgression from other Bombus species. The species’ ecological specialisation and geographic isolation limit opportunities for hybridisation with other closely related taxa. 

Low turnover - constraints on adaptive opportunities

The univoltine life cycle, low reproductive output, and strong dependence on specific habitats reduce opportunities for adaptation. Combined with small Ne, this constrains the potential for adaptive responses to environmental change. Populations are also highly vulnerable to stochastic events such as poor weather years, local habitat loss, or disease outbreaks. 

Bombus distinguendus faces serious genetic risks due to very small effective population sizes (Ne ~25), a highly fragmented distribution, and restricted gene flow. Of the five known population centres, only the Outer Hebrides appears relatively stable, while the others remain small and vulnerable, with no evidence of genetic recovery. 

In situ genetic threat level

In situ Risk for Scotland: Serious
In situ Risk for UK: Serious

Low diversity, steep population declines, specific habitat requirements and limited reproductive output.

Confidence in in situ threat level

Confidence score for Scotland: High
Confidence score for UK: High

This is a well-studied species in Scotland.

There are currently no known ex situ conservation programmes for Bombus distinguendus. While the species has been the focus of in situ efforts, no captive breeding, genetic banking, or insurance populations exist (Bumblebee Conservation Trust, 2024).

Population

Demographic

Genetic