Narrow pollen diets are associated with declining Midwestern bumble bee species

Area of Science:

• Ecology
• Conservation Biology
• Entomology

Background:

• Many bumble bee (Bombus) species face range and abundance declines globally.
• A leading hypothesis suggests declining species have narrow pollen-foraging niches, limiting their adaptability.
• This hypothesis remains under-explored in North America due to data gaps on bumble bee diets.

Purpose of the Study:

• To investigate the relationship between pollen-foraging niche breadth and population status in North American bumble bees.
• To quantify the diets of 12 bumble bee species in Michigan using museum specimens.
• To assess contemporary distribution and status of these species compared to historical data.

Main Methods:

• Pollen diets of 12 Michigan bumble bee species were analyzed from museum specimens.
• Species' contemporary status and distribution were surveyed across Michigan.
• Dietary niche breadth was compared between species with stable and declining populations.

Main Results:

• Six of the 12 studied bumble bee species showed range contractions exceeding 50% compared to pre-2000 baselines.
• Species with declining ranges collected significantly fewer pollen types (narrower dietary niches) than stable species.
• No significant difference in pollen types collected was found based on bumble bee tongue length.

Conclusions:

• Results support the hypothesis that narrower dietary niches increase bumble bee species' risk of decline.
• The causal relationship between niche width and decline requires further investigation, potentially involving climatic factors.
• Understanding bumble bee dietary niche is crucial for predicting population trajectories and informing conservation strategies.