Shining a light on species coexistence: visual traits drive bumblebee communities

Abstract

Local coexistence of bees has been explained by flower resource partitioning, but coexisting bumblebee species often have strongly overlapping diets. We investigated if light microhabitat niche separation, underpinned by visual traits, could serve as an alternative mechanism underlying local coexistence of bumblebee species. To this end, we focused on a homogeneous flower resource—bilberry—in a heterogeneous light environment—hemi-boreal forests. We found that bumblebee communities segregated along a gradient of light intensity. The community-weighted mean of the eye parameter—a metric measuring the compromise between light sensitivity and visual resolution—decreased with light intensity, showing a higher investment in light sensitivity of communities observed in darker conditions. This pattern was consistent at the species level. In general, species with higher eye parameter (larger investment in light sensitivity) foraged in dimmer light than those with a lower eye parameter (higher investment in visual resolution). Moreover, species realized niche optimum was linearly related to their eye parameter. These results suggest microhabitat niche partitioning to be a potential mechanism underpinning bumblebee species coexistence. This study highlights the importance of considering sensory traits when studying pollinator habitat use and their ability to cope with changing environments.

Looking across the gap: Understanding the evolution of eyes and vision among insects

“…[72,73] This plasticity in eye development within bumblebees has been suggested to produce workers with different visual abilities that allows optimal foraging in alternative niches with varying light conditions, [72] therefore facilitating microhabitat niche partitioning within species, as suggested above for partitioning between fly and butterfly species. [73] A new study of the hummingbird hawkmoth Macroglossum stellatarum has shown that the superposition eyes of this insect scale hypoallometrically with body size and that smaller hawkmoths have fewer but wider ommatidia. [74] This allows these insects to maintain the balance between acuity and contrast sensitivity despite differences in body size.…”

“…Recent papers have described striking natural variation in eye size within and between bumblebee species that are predicted to cause differences in their vision based on morphological measurements. [ 72,73 ] This plasticity in eye development within bumblebees has been suggested to produce workers with different visual abilities that allows optimal foraging in alternative niches with varying light conditions, [ 72 ] therefore facilitating microhabitat niche partitioning within species, as suggested above for partitioning between fly and butterfly species. [ 73 ]…”

“…[ 72,73 ] This plasticity in eye development within bumblebees has been suggested to produce workers with different visual abilities that allows optimal foraging in alternative niches with varying light conditions, [ 72 ] therefore facilitating microhabitat niche partitioning within species, as suggested above for partitioning between fly and butterfly species. [ 73 ]…”

Multiple axes of visual system diversity in Ithomiini, an ecologically diverse tribe of mimetic butterflies

“…In apposition compound eyes, each facet typically projects light along a single rhabdom within an ommatidium, separated from adjacent ommatidia by light-absorbing pigments ( Land, 1989 ). However, anatomical and physiological parameters such as total eye size, shape, lens diameter, acuity, sensitivity, number of ommatidia and pupillary response vary, even between closely related species ( Bartholomée et al, 2023 ; Greiner, 2006 ; Greiner et al, 2004a ; Land, 1989 ; Narendra et al, 2013 ; Scales and Butler, 2016 ; Somanathan et al, 2009 ; Warrant, 2001 ). For example, shifts to a nocturnal lifestyle have led to larger ommatidial facets and rhabdom diameters in the apposition eye morphology of the sweat bee, Megalopta genalis , compared with its close relatives ( Greiner et al, 2004a ; Warrant, 2017 ).…”

“…For example, shifts to a nocturnal lifestyle have led to larger ommatidial facets and rhabdom diameters in the apposition eye morphology of the sweat bee, Megalopta genalis , compared with its close relatives ( Greiner et al, 2004a ; Warrant, 2017 ). Microhabitat partitioning within communities of Hawaiian damselflies and hemiboreal bumblebees has also resulted in rapid shifts in eye sensitivity at a much finer ecological scale ( Bartholomée et al, 2023 ; Scales and Butler, 2016 ). Spatial variation in light abundance and spectral composition within habitats has also promoted compound eye regionalisation whereby different eye regions are fine-tuned to optimally receive ecologically relevant stimuli (e.g.…”