The Hunger Gaps: when flower supply fails to meet bee demand

Timberlake, T.P., Vaughan, I.P. and Memmott, J. 2019. Phenology of farmland floral resources reveals seasonal gaps in nectar availability for bumblebees. Journal of Applied Ecology 56: 1585-1596. DOI: 10.1111/1365-2664.13403

The difference between bees and bees

The past decade has seen a strong increase in public concern for the well-being of bees. Warnings about declining bee numbers have led to appeals for broad action to “Save the Bees”, which have figured in everything from climate marches to corporate marketing campaigns. In light of this, many would argue that public awareness about the importance of bees for human food production and biodiversity has never been higher.

Image 1. Protester at the People’s Climate March in New York City, 2014. (Image credit: canopic, Flickr).

But which bees are we talking about? In my native Sweden, a number of big corporations this year have sought goodwill from the public by announcing  that they’re placing honeybee hives on their premises to counteract declining bee numbers. My bee researcher colleagues point out in a recent opinion piece (in Swedish) that honeybees are not the bees we should be worrying the most about. They’re a domesticized species, highly unlikely to face extinction, and their most important role as pollinators lies in agriculture and horticulture, where they can be introduced to boost the yields of flowering crops. Wild bees, on the other hand, are the indispensable pollen-transporters necessary for maintaining diverse plant communities in nature. While it’s of course a simplification to separate honeybees and wild bees into purely “agricultural pollinators” and “natural pollinators” – wild insects are for example significant contributors to crop yields worldwide – it’s a helpful distinction to make when considering the singular importance of wild bees: they are vital for the upkeep of diverse natural ecosystems.

insects composite
Image 2. Examples of flower-visiting bees. On the left, a honeybee (Apis mellifera) on a rapeseed crop (Photo credit: Sandra Lindström). On the right, two types of wild bee: the middle photo shows an early-nesting bumblebee (Bombus pratorum) (Photo credit: Ivar Leidus), and the photo on the far right shows a tawny mining bee (Andrena fulva) (Photo credit: Jerzy Strzelecki) .

Yet, the confusion in the discourse about bee declines persists. An example from the past week shows the BBC detailing reports of declines of managed honeybees in Russia and illustrating the article with pictures of wild bumblebees. While this miscommunication can be highly frustrating for pollination ecologists, and a spoke in the wheel of any efforts to inform about the consequences of wild bee declines, there are several points where honeybees and wild bees are facing similar challenges. One of these points is simple enough to understand: there just doesn’t seem to be enough food to go around throughout the year.

Every bee’s gotta eat

While the total amount of floral resources is important for determining how many wild pollinators an area can sustain, an equally important – and relatively poorly explored – factor is the timing of these resources. Plants only function as food sources for bees while they are flowering, and most plant species only flower during a restricted part of the growing season. This means that any given area can only be considered a truly good bee habitat if there’s a continuous supply of open flowers throughout the activity period of the bees. In other words, the habitat needs to provide not only diversity in the total number of plant species, but also diversity in the timing of the flowering of these plant species.

Supply and demand in the farmland

A recent paper from pollinator researchers at the University of Bristol in the UK elegantly illustrates this conundrum. The researchers investigated floral phenology – the seasonal timing of different events in an organism’s life cycle – at a number of farmland locations in Southwestern UK, with the purpose of seeing if there was a continuous, stable food supply for the local wild bees. They also quantified the nectar demands of common farmland bumblebees so that they could compare the supply and demand of nectar throughout the season – in other words, checking to see if sufficient amounts of nectar were available for the whole time the bumblebees were buzzing around out there, demanding it.

The researchers estimated how much sugar each individual bumblebee could be expected to need during a day in order to go about their daily lives. They then compared this with the amount of nectar theoretically available to each individual bee over different periods throughout the season. In this way, they could identify periods where there was a mismatch between the supply and the demand of nectar.

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Fig. 1. The graph shows how the amount of nectar available per unit area drastically changed over the season. (Source: Timberlake et al. 2019).
Hunger gaps and nectar rollercoasters

In the summary of their data collection, they provide some staggering numbers: over the two years of the study they conducted 137 farm visits, walked 2664 transects, and counted close to half a million flowers. Anyone who has ever, like me, been tasked with walking transects to count bees and flowerheads – a sometimes tedious task – will find these numbers dauntingly impressive.

What this elegantly designed, meticulous study revealed was that there were periods during the growing season where the nectar supply at each farm was insufficient to fill the collective demands of local bumblebees (Fig. 1-2). Bumblebees require a continuous, steady floral supply throughout the season to keep the colony running. With continuity being the issue, the fact that there are periods of nectar surplus (for example during the spring bloom; see Fig. 1) is not enough to counteract these “hunger gaps”, where demand greatly exceeds supply.

fig 2
Fig. 2. The red line indicates the nectar requirement of an individual bumblebee, and the black line indicates the estimated nectar availability for each individual bee at any given time. Red areas therefore indicate periods of nectar deficit where demand exceeds supply at each of the four study sites (i.e. one graph details one site). (Source: Timberlake et al. 2019).
Let’s save all the bees!

I mentioned at the start that equating “bees” with “honeybees” is always problematic, as honeybees are just one species and a domesticized one at that. This is still true, and important to keep in mind, but it doesn’t mean that initiatives designed to help honeybees can’t also be beneficial to wild bees, and perhaps especially so when it comes to dealing with these kinds of “hunger gaps”. Initiatives that aim to close the flowering gaps in a given landscape – like encouraging people to manage their garden so that it provides blooms throughout the season – will benefit all bees, irrespective of the original target group. In many areas, beekeepers and farmers work together to close flowering gaps during the growing season. While such initiatives are designed as win-win scenarios for boosting both honeybee hives and crop pollination, they could be expanded and systematized to include conservation goals for wild bees, and help these “nature’s maintenance workers” to ensure the upkeep of natural ecosystems.

The Bristol researchers finish by stressing that any bee-saving initiative will need to keep in mind that timing is everything: phenological mismatches in nectar supply and bee demand can upend even the best-intentioned bee conservation plans if the risk of such mismatches is not considered in the planning stage. So, when you next find yourself in a meadow, or a garden, or a flowering road verge, or any seemingly bee-friendly habitat, take a moment to look around and consider: will there still be something for them to eat if they buzz in again a week later?

Reviewed by: Brittany Maule.

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Pernilla Borgstrom

Pernilla Borgstrom

I have a PhD in ecology from the Swedish University of Agricultural Sciences (SLU) and have spent a lot of time in the past half decade thinking about what role interactions between plants and insects play in determining how an ecosystem responds to environmental change. Outside of my research, I'm interested in figuring out how to broadly communicate science in ways that will both inform and inspire people on issues regarding climate change, biodiversity loss and ecosystem degradation.

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