A demographic approach to evaluating the impact of stressors on bumble bee colonies

Abstract

1. Natural and anthropogenic stressors threaten the sustainability of bumble bees and evaluating their impact is essential to the stewardship of these valuable pollinators. Demographic modelling provides a framework for testing hypotheses about the impacts of stressors, but it has not previously been applied to bumble bees. 2. I therefore formulated a demographic model for a bumble bee colony and then quantified the impact of two stressors, pesticide exposure and spider predation, by perturbing it with their known effects. 3. By simulating a laboratory exposure of Bombus terrestris L. to dietary imidacloprid (a neonicotinoid insecticide), I tested whether the observed colony decline was explained solely by a toxic effect on the fecundity of the foundress queen. By simulating field observations of B. terricola Kirby, I tested whether predation by crab spiders reduced colony fitness sufficiently to provide an adaptive explanation for avoidance behaviours seen when bumble bees encounter spiders. 4. In B. terrestris, a dose‐appropriate decrease in fecundity predicted the observed colony decline, which implicates this as a principal mechanism of toxicity. In B. terricola, doubling the rate of spider predation reduced a colony's production of new queens by 11%, which implies that spider avoidance is highly adaptive. 5. These analyses illustrate the utility of demographic modelling for quantifying the impacts of stressors on bumble bees. In the future, models of this type could be used to investigate a wider range of stressors and to produce thereby knowledge and tools useful for safeguarding bumble bees and the pollination services that they provide.

Integrating vital rates explains optimal worker size for resource return by bumblebee workers

“…Therefore, evaluating individual performance may not provide the ecological insight into size variation than evaluating the collective performance of individuals at the colony level (Beshers & Fewell, ; Bourke, ; Duarte, Weissing, Pen, & Keller, ; Gardner & Grafen, ; Hölldobler & Wilson, ; Wilson & Wilson, ). Despite these challenges, recent studies have begun developing demographic models for bumblebee colonies to understand landscape effects and stressors on colony dynamics (Bryden, Gill, Mitton, Raine, & Jansen, ; Cresswell, ; Crone & Williams, ). Our study makes an important contribution into providing ecological insight into the size‐based trade‐offs among workers as well as a methodological approach that allows us to integrate multiple empirical measurements of fitness to better understand size‐based trade‐off.…”

“…Despite these challenges, recent studies have begun developing demographic models for bumblebee colonies to understand landscape effects and stressors on colony dynamics (Bryden, Gill, Mitton, Raine, & Jansen, 2013;Cresswell, 2017;Crone & Williams, 2016). Our study makes an important contribution into providing ecological insight into the size-based trade-offs among workers as well as a methodological approach that allows us to integrate multiple empirical measurements of fitness to better understand size-based trade-off.…”

Bumble‐BEEHAVE: A systems model for exploring multifactorial causes of bumblebee decline at individual, colony, population and community level

“…Six contrasting bumblebee models have recently been published (Banks et al., ; Bryden, Gill, Mitton, Raine, & Jansen, ; Cresswell, ; Crone & Williams, ; Häussler, Sahlin, Baey, Smith, & Clough, ; Olsson, Bolin, Smith, & Lonsdorf, ). However, while useful in exploring the impact of individual stressors, such as food availability (Crone & Williams, ) or pesticides (Bryden et al., ; Cresswell, ), none as yet have the structural realism to incorporate multiple stressors or competition, operating at different organisational levels (individual or colony or population). They have limited flexibility to incorporate feedback mechanisms that may buffer the colony against environmental stress, or indeed lead to spiralling collapse.…”

“…Six contrasting bumblebee models have recently been published (Banks et al, 2017;Bryden, Gill, Mitton, Raine, & Jansen, 2013;Cresswell, 2017;Crone & Williams, 2016;Häussler, Sahlin, Baey, Smith, & Clough, 2017;Olsson, Bolin, Smith, & Lonsdorf, 2015).…”

“…However, while useful in exploring the impact of individual stressors, such as food availability (Crone & Williams, 2016) or pesticides (Bryden et al, 2013;Cresswell, 2017), none as yet have the structural realism to incorporate multiple stressors or competition, operating at different organisational levels (individual or colony or population).…”