How timing shapes scientific stories

This blog post is provided by Rachel Dickson and tells the #StoryBehindThePaper for the paper “Behavioral responses by a bumble bee to competition with a niche-constructing congener”, which was recently published in the Journal of Animal Ecology.
Rachel Dickson has spent the past ten years working as a plant-pollinator ecologist. She has studied bumble bees in Argentina, honey bees in Kenya and the phenologies and interactions of plants and pollinators at the Rocky Mountain Biological Lab. She is currently learning techniques in agriculture to combine her love of science with growing food.  

We live in a time when working as an ecologist is both exciting and downright terrifying. This research was done on the precipice of a species extinction; it is likely that I and fellow co-authors were the last people to observe an abundant population of one of the world’s largest bumble bees, Bombus dahlbomii, also known as the Patagonian bumble bee or “flying mouse”. Only five years after our study, it is no longer possible to ask follow up research questions as dahlbomii has nearly gone extinct in Patagonia. During the past two decades, dahlbomii has suffered dramatic population declines due largely to the invasion of Bombus terrestris, the buff-tailed bumble bee. This invader spreads parasites to dahlbomii and has overwhelmed ecosystems as an aggressive competitor. Our study examined the relationship between dahlbomii and terrestris in a national park in Argentine Patagonia.

Fuchsia magellanica is shrub native to the temperate regions of Chile and Argentina and has two primary pollinators; native bumble bee dahlbomii and native hummingbird Sephanoides sephaniodes. Fuchsia shrubs have ornamental flowers that hang down and have basal nectar tubes, meaning bumble bees who visit through the front of the flower grasp onto the red sepals and reach their proboscises up through the long purple corolla tubes in order to access nectar. Because of the floral morphology, only long-tongued bumble bees like native dahlbomii are able to effectively reach the nectar. This means that short-tongued invader, terrestris can access only a fraction of the nectar through visiting the flower legitimately.

One of our study plants blooming next to a stream in the Valdivian rainforest. (Photo credit: Lawrence Harder)

At the beginning of our study, both bumble bee species visited flowers legitimately 100% of the time. On Feb 2, the story of the ecosystem began to rapidly change. Luckily, our methodology had us conducting pollinator surveys every day of the week, so we caught the sudden shift to robbing. On Feb 2, the invasive bumble bees began chewing holes in the corolla tubes of flowers, a “cheating” behaviour known as primary nectar robbing that is used to gain nectar rewards without providing the flower with the service of pollination. The holes in the flowers remain until the flower dies, incentivizing robbers to continue robbing throughout the flower’s lifespan. Visitors that continue to use these established holes are called “secondary robbers”.

Native bumble bee (left) and invasive bumble bee (right) secondary robbing Fuchsia flowers simultaneously. (Photo credit: Nick Rosenberger)

Out of the total 2,542 robbing visits observed during the 2.5 months of observations, we recorded only 28 primary robbing visits by 20 invasive bumble bee individuals. This means that only 1.3% of our recorded robbing visits were primary, compared to the 98.7% that were secondary. Only three days after the first observed primary robbing event, the invasive bumble bees were secondary robbing flowers almost exclusively. In other words, we could have missed observing primary robbing visits almost entirely if we had not been conducting observations every day.

During the first ten days of February, mean abundance of invasive bumble bees increased almost 20-fold, which was 72 times faster than the simultaneous 27% increase in Fuchsia flowers. This growth was about 13 times faster than the maximal growth of terrestris colonies, so they must have recruited individuals that were previously feeding on other plants to come join the robbing party. As one would expect, the number of robbed flowers corresponded directly to the density of robbers. After this phenomenon, terrestris abundance generally declined until late March, after which terrestris almost entirely stopped visiting Fuchsia.

Meanwhile, during the period of aggressive robbing by terrestris, the abundance of native dahlbomii visitors per plant declined. Then, after terrestris stopped visiting Fuchsia in late March, the abundance of dahlbomii increased sixfold within ten days. When dahlbomii abundance was likely increasing due to its phenology, its abundance on Fuchsia remained consistently low until 10 days after terrestris almost entirely stopped visiting Fuchsia. Their abundance then increased at a rate much higher than would have been possible from only colony growth, meaning once terrestris was out of town, dahlbomii individuals who had switched over to foraging on other plant species returned to forage on Fuchsia.

Typical nectar robbing holes in a Fuchsia flower. (Photo credit: Nick Rosenberger)

We categorized the actions of terrestris as ecological niche construction (ENC) because terrestris altered its environment in order to create future benefits (increased access to nectar) affecting dahlbomii by altering an abundant food source. Ecological niche construction happens within a species’ lifetime and can change the structure of communities. In our study system, terrestris acts as the constructor species (robber) and increases its foraging returns, while dahlbomii acts as the bystander, and experiences increased competition for one of its primary food sources. The constructor activity of terrestris (robbing) created an environmental modification (holes in many Fuchsia flowers and increased terrestris  visitation) which resulted in three behavioural outcomes for the bystander species, dahlbomii.

Dahlbomii assorted themselves among all three different responses to environmental modification: tolerance (front tube visits) adoption (secondary robbing) and avoidance (visits to other flower species) that rendered all options equally rewarding. This flexibility in response is possible because like most bumble bees, dahlbomii is a behaviourally flexible ecological generalist. For other species that are specialists and/or not behaviourally flexible, adoption is not a possible outcome.

Native bumble bee visiting a Fuchsia flower legitimately. Photo credit: Rachel Dickson)

We first observed dahlbomii nectar robbing just five days after B. terrestris began. It was remarkable to witness the established evolutionary mutualism between dahlbomii and Fuchsia change so rapidly in response to the competitive pressure and behaviour of an invader. This research produced novel and exciting results that would have benefitted from further questioning and examination. Unfortunately, due to the rapid decline of B. dahlbomii, it is no longer possible to ask questions of these same species interacting in this ecosystem. It is difficult to not feel a sense of despair when the organisms and ecosystems we grow to love change and disappear before our eyes. However, it is extremely important to look at losses like these as more motivation to continue to tell the stories of all the precious life surrounding us that cannot speak.

Supporting information for main article:

Arbetman, M. P., Meeus, I., Morales, C. L., Aizen, M. A., & Smagghe, G. (2013). Alien parasite hitchhikes to Patagonia on invasive bumblebee. Biological Invasions15, 489– 494.

Montalva, J., Dudley, L. S., Sepúlveda, J. E., & Smith-Ramírez, C. (2020). The Giant Bumble Bee (Bombus dahlbomii) in Mapuche Cosmovision. Ethnoentomology4, 1-11.

Read the paper

Rosenberger, N. M., Aizen, M. A., Dickson, R. G., & Harder, L. D. (2022). Behavioural responses by a bumble bee to competition with a niche-constructing congener. Journal of Animal Ecology, 91, 580– 592.

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