Should I stay or should I go? The influence of resource regeneration in flowers on foraging behaviour of bumblebees

Abstract

It is well established that bumblebees play a vital role in the ecosystem, not only as pollinators but as providers of other ecosystem services. Thus, it is important to have a good understanding of their habitat requirements and ecology. Bumblebees can learn to associate floral cues with rewarding and unrewarding flowers. The availability of resources in flowers is very dynamic as it varies during the day, the course of a flower’s lifetime and with environmental conditions. Consequently, it is important for bumblebees to track changes in resource levels and adjust their foraging behaviour accordingly in order to maximise their energy gain and therefore colony growth. Much within the dynamic of behavioural responses to changes in floral resource levels is poorly understood. This study aims fill some of these knowledge gaps.

To do so, resource content in flowers was manipulated, by excluding flowers from foraging. The time bumblebees spent per flower was then compared between excluded patches and adjacent controls in two flower species, Trifolium repens and Lotus corniculatus. The same was aimed for in the spring flower Anemone nemorosa, but due to low bumblebee visitation, focus was put on pollen development instead.

I found that bumblebees adjusted their foraging behaviour to differences in resource levels and adapted their behaviour in direct response to declining resource levels during the observation period. The different visiting bumblebee species responded to the initial variation in resource levels in the same way, but generally spent different times on flowers, probably due to physiological differences between the bumblebee species. Water availability further influenced resource levels in flowers. In A. nemorosa, pollen presented by the flowers throughout their lifetime followed a bell curve – increasing at the beginning and then decreasing toward the end of life.

Popular Abstract

How to fill the shopping basket - bumblebees’ choices

Bumblebees are not only charismatic fluffy animals, they also play an important role in ecosystems. During their search for food, they pollinate not only wildflowers, but also the crops we eat. As a result, we humans are directly benefitting from the work they do. Given that many bumblebee species are facing a decline, it is vitally important to understand how bumblebees make foraging choices in order to find appropriate measures to ensure their continued survival.

Bumblebees are so-called “central place foragers”. They leave their nest to search for pollen and nectar aiming to return with a full shopping basket. A bumblebee’s world is full of floral grocery stores, but these stores do not keep their shelves stocked at all times. Pollinators taking away nectar and pollen from flowers cause resource levels in flowers to vary. Resources like nectar are regenerated in flowers. But the resource regeneration varies depending on environmental factors, the age of the flower, and it often follows a daily pattern. Thus, bumblebees are constantly faced with a decision of where to go to maximize their net energy gain and help their colony to grow.

I aimed to fill some of the knowledge gaps in the complex relationship between changing resource levels in flowers and the foraging behaviour of bumblebees. To do so, I manipulated resource levels to be higher than normally in white clover (Trifolium repens) and bird’s-foot trefoil (Lotus corniculatus) by making them inaccessible to foragers for some time. I then looked the time bumblebees spent on resource rich flowers compared to on adjacent flowers with lower resource levels. My goal was to do the same with the wood anemone (Anemone nemorosa), but low bumblebee abundance made it impossible to do so. Instead, I looked at the pollen development throughout the flower’s lifetime.

The bumblebees fine-tuned their behaviour over the course of an observation period, spending less time per flower with sinking resource levels. Furthermore, even though the different visiting bumblebee species reacted in the same way, they varied in the time they spent on the flowers. That was probably due to differences in their tongue lengths. The study was conducted during an ongoing drought. The bumblebee behaviour was impacted by the water availability. Bumblebees spent less time on flowers with lower water provision.

The wood anemone presented pollen throughout its life span. The amount of pollen presented varied over time, increasing in the beginning and then declining towards the end of its lifetime.

Bumblebees visit patches depending on resource availability and distribute themselves in the landscape accordingly. The knowledge acquired in this study adds to our understanding of bumblebee behaviour and the way they disperse in their environment. Foraging models have been made to predict how bumblebees distribute in a given landscape. The knowledge gained here can add detail to those models with which conservation actions for bumblebees can be better planned.

Master’s Degree Project in Conservation Biology 30 credits 2018/2019
Department of Biology, Lund University

Supervisors: Ola Olsson & Johanna Yourstone
Department of Biology, Biodiversity unit

• author: Samnée, Annika
• supervisor: Johanna Yourstone; Ola Olsson
• organization: Degree Projects in Biology
• course: BIOM02 20191
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8994934
• date added to LUP: 2019-09-13 16:09:58
• date last changed: 2019-09-13 16:09:58