How nectar resources affect pollinators.
Pollinators are vital for our ecosystem and economy, playing a crucial role that we can't overlook! Unfortunately, we're witnessing alarming declines in both pollinator species abundance and diversity, which also impacts our precious wild plants. Factors like habitat loss, pests, pesticides, and climate change all contribute to this concerning trend.
This research delves into the diverse floral nectar resources available to pollinators in the UK, uncovering nectar traits that influence foraging choices. Sugar content in nectar, especially glucose, plays a key role in which flowers pollinators choose, while fructose and sucrose also make a difference!
With nectar quality and quantity being essential for pollinator preferences, these findings pave the way for initiatives aimed at boosting pollinator diversity and numbers and also shines a light on the nectar feeding habits of a variety of pollinators, including many rare and underappreciated species, helping us better understand and protect these incredible creatures!
Why study nectar?
Flowers are often taken for granted, yet they play a crucial role in supporting pollinators and our ecosystem. Without pollination, our food supply would be drastically diminished, and the world would lose much of its beauty. It's surprising how little research has focused on the floral resources that flowers offer to pollinators. While we have a better understanding of pollen and its nutritional value (likely due to the ease of collecting and analysing it) nectar remains a significantly underexplored area in pollinator nutrition.
​
This was something which I was keen to change.
​
Using the 'Historic nectar assessment reveals the fall and rise of floral resources in Britain' by Baude, et al. (2016) as a starting point I began by selecting flowers to nectar sample.
Nectar sampling.
Having selected sixty flower species which bloomed throughout the year I then needed to sample their nectar.
​
The first step was to prevent pollinators from foraging on the flowers so there would be some nectar to sample. To do this plants were netted the evening before sampling with a fine nylon mesh. Nectar from 10 flowers was collected with micro-capillary tubes and the average volume produced was recorded. High performance anion exchange chromatography analysis (HPAEC) revealed the percentage and volume of sucrose, glucose and fructose present in the nectar (see Table 1).
Pollinator surveys.
Identifying the pollinators that were visiting these flowers was also a crucial aspect of this research.
To achieve this, I conducted timed surveys of pollinators for each flower species from which I had collected nectar. A group of twenty flowers was observed for ten minutes at six different times during the day and the species of any insect feeding on the nectar was documented.
​
One concerning result was the lack of Lepidoptera (butterflies and moths) seen. After 60 hours of observations, spread over six months only 134 Lepidoptera were recorded feeding on nectar. Habitat loss and a lack of suitable breeding and feeding sites is a key factor in the decline of these species.
Results.
Figure 1. The strength of association shown by pollinators at an order and genus level to the percentage and volume of glucose, fructose and sucrose.
Glucose.
A significant positive correlation was observed between the highest number of pollinators and the percentage and volume of glucose found in the sampled nectars. All pollinator orders (Hymenoptera, Diptera, and Lepidoptera), as well as Hymenoptera from the genera Apis (honey bees), Andrena (mining bees), and Lasioglossum (base-banded furrow bees), exhibited this positive association.
Fructose.
Only Hymenoptera at an order level showed a positive correlation with the percentage and volume of fructose, with Apis and Bombus (bumble bees) being the sole genera of pollinators that demonstrated a positive association with fructose volume.
Sucrose.
At the order level, no pollinators exhibited a statistically significant relationship with the percentage or volume of sucrose. While Bombus showed a positive correlation with both the percentage and volume of sucrose, Andrena displayed a negative association with sucrose percentage (see Figure 1).
Nectar volume.
​Bombus were the only genus of pollinator that demonstrated a positive relationship to the volume of nectar produced by flowers, and although the overall response of all pollinators to the volume of nectar offered by the sampled flowers was not statistically significant the results did demonstrate an interesting plant pollinator dynamic.
​
The five plant species which offered the highest mean nectar volumes had disproportionally fewer pollinators foraging on them. Nectar from these flowers can only be accessed by specialist pollinators which have physical adaptations to access the nectar.
​
Conversely, disproportionally high pollinator numbers were recorded foraging on four plant species which offered lower mean nectar volumes. These flower species are typically grown as crops and while an individual flower offers only a small amount of nectar a crop produces a significant nectar resource (see Figure 2).
Figure 2. The association between the total number of pollinators foraging and the mean nectar volumes offered by the flowers. Dark blue = 50% confidence interval; light blue = 95% confidence interval.
What does this all mean?
Sucrose is a di-saccharide and has a higher calorific value than glucose and fructose which are mono-saccharides, which makes them easier to digest. Pollinators showed a preference for glucose, and to a lesser extent fructose in sampled nectars and the molecular make up of these sugars may be a primary driver of flower selection during foraging of many important pollinator groups.
​
​Honey bees and bumble bees are social insects that care for and continually feed their larvae until they pupate. During egg laying solitary bees leave a provision of food for larvae to feed on but provide no further care for their young, and the larval stages of diptera and lepidoptera feed themselves and receive no care from adults. The results from this study found that social insects, and in particular bumble bees responded strongly to the volume of the individual sugars available in nectar and bumble bees also showed a preference for sucrose. This could be linked to the social pollinators need to source a greater volume of quality nectar for brood feeding and bumble bees may find the higher calorific sucrose easier to digest due to their larger size.
​
The volume of nectar available is a driver of pollinator floral selection. However, factors such as the morphology of the flower and the spatial distribution of flowers also influence pollinator feeding selection.
Find out more.
You can read more about this research in the Journal of Apiculture Research - 'Potential use of floral nectar sugar characteristics in plant selection for pollinator habitats' by following this link.
