Ideas in Conservation: The Case of Declining Bumblebee Populations

News has been all abuzz about declining global bee populations for several years now. From talk of cell phone radiation disorienting bees to overexposure to pesticides and, particularly, insecticides threatening populations, bees have made headlines for their growing threatened status. But while bee populations, in general, are on the decline, a new study published this past week in Proceedings of the Royal Society, B delved a bit deeper into the story using bumblebee species and found nearly one-third of species examined were declining--and all of them closely related.

 Results from this new study show that individuals from the older Thoracobombus (top) subgenus are significantly more susceptible to extinction than those from Pyrobombus (bottom), which currently shows almost no population decline.
Photo sources: https://www.flickr.com/photos/giaa/5073361366/ and https://www.flickr.com/photos/29697818@N03/7304148034/

Bumblebees are fuzzy, flying juggernauts of insects, the largest of which lives in Chile and reaches a body length of 1.6 inches - "a monstrous, fluffy, ginger beast," as British scientist David Goulson described it in his book A Sting in the Tale. With 260 species, bumblebees populate almost every continent around the globe (Antarctica and Australia being the exceptions), preferring higher altitudes and latitudes where temperatures are cooler, which raises concern. As the global climate continues to rise, this penchant for cooler climate may have dire consequences for many bumblebee species and possibly explain their declining populations. But that might be only one factor. Habitat loss and fragmentation of land repurposed for agriculture, increased use of pesticides and insecticides, and transmission of pathogens between species all may play a part, but nobody is certain.

"[T]he global picture of bumblebee decline is still fragmentary," wrote Marina Arbetman and colleagues, the authors of the study. "[N]o previous study has evaluated worldwide patterns of bumblebee decline as well as their potential predictors within a phylogenetic framework." 

Traditional methods of determining conservation status stem from collecting information pertaining to an organism's biology: where it lives; how its populations are distributed across a town, a country, a continent; what habitat it prefers; what the major threats are to that habitat; what its ecological role is in that habitat. From this information, a conservation status is determined: least concern, near threatened, endangered, critically endangered. 

That's exactly what the International Union for the Conservation of Nature (IUCN) did for bees in Europe back in 2014. They found about eight percent of nearly 2000 bee species in Europe had declining populations, which seems small, but that's because 79 percent of species had insufficient population data to draw any conclusion -- they were only seeing the tip of the iceberg. Even so, bumblebees, making up less than four percent of bee species in Europe, were almost entirely accounted for and over a quarter were threatened or near-threatened.

Traditional conservation status methods, however, exclude what some argue is an important puzzle piece: the phylogenetic diversity of a group. Phylogenies--the evolutionary relations of organisms--act as an excellent proxy for the functional diversity of organisms, which in turn helps scientists grasp what role those species play in an ecosystem and what services they perform for both the ecosystem and for humans. 

"[T]he conservation of higher phylogenetic diversity is likely to safeguard the evolutionary legacy of bumblebees," wrote Nicolas Vereecken, an assistant professor at the Université Libre de Bruxelles in Belgium, earlier this year and who was not an author on this paper. He believes that the conservation of phylogenetic diversity of bumblebees may protect their potential to face environmental change and ultimately play a pivotal role in the maintenance of ecosystem processes and services. "The time is now ripe for the incorporation of phylogenetic diversity as an alternative biodiversity metric into conservation planning to avoid worst-case losses of long branches from the bumblebee tree of life."

Using a New Tool

Arbetman and colleagues looked into the global phylogenetic diversity of bumblebees, asking if the species that were declining fell into specific lineages that play critical ecological roles as well as what factors may contribute to their declining populations, such as range size, presence of certain parasites, and even tongue length, a characteristic that can indicate the bee's specialization to feeding on specific flowers.

The study found that roughly one-third of the bumblebee species evaluated were declining in populations, and the highest proportion of these fell within three subgenera of bumblebees rather than being scattered randomly across the bumblebee tree. One of those subgenera, the Thoracobombus, is the second largest bumblebee subgenus and also one of the oldest.

"[Twenty-two] of the 52 recognized species in [Thoracobombus]...have a well-established extinction risk status," wrote the authors. "From them, almost two-thirds of these 22 species...are declining." The authors also concluded that the loss of this as well as the subgenus Cullumanobombus, another of the subgenera severely threatened, would result in higher losses of phylogenetic diversity than would be predicted from random extinctions of bumblebees. 

"Thus, from a phylogenetic perspective, species of these subgenera deserve the highest conservation priority," concluded the authors.

In striking contrast, bumblebees from the largest subgenus Pyrobombus showed almost no indication of population decline or threatened status. "Out of the 50 species belonging to this subgenus, 32 species (64%) had well-established extinction risk status...with only two species...being threatened," wrote the authors. 

The difference may stem from the other correlates that they found: bumblebees with small ranges, long tongues, and almost no pathogens--all indications of specialization to some degree--showed higher susceptibility to extinction. This becomes even more worrying considering that global commercial exchange and transportation of bee colonies has increased over the past decade, giving prime opportunity for species to invade and potentially out-compete these vulnerable bumblebee lines; the process that has already begun in Chile, where Bombus dahlbomii -- the largest bumblebee species in the world and a member of the declining Thoracobombus subgenus -- is threatened by commercial bees with pathogens previously unknown to the area.

Broader Questions

Overall, this study provides a precise method of determining organisms to prioritize in conservation efforts as well as a key to determining the species crucial to an ecosystem or the preservation of a lineage, an otherwise daunting challenge considering that many inhabitants are declining simultaneously. 

But why should this matter to us?

"Bumblebees are an essential component in our agroecosystem," wrote Vereecken. "[T]heir decline represents a major threat for the sexual reproduction -- and hence survival -- of wild flowers and several pollinator-dependent crops alike." 

Bumblebees play not only a crucial role in natural ecosystems but also our own crop productions through what conservationists call "ecosystem services." We ultimately depend on pollinators like bumblebees, bats, hummingbirds, and various other insects to help produce our food from year-to-year, which in turn save companies, farms, and consumers millions of dollars. This dramatic impact has led some conservation organizations to shift their focus from traditional "preservation of biodiversity" conservation campaigns to ecosystem service campaigns when discussing the need for conservation. People seem to understand money better. 

But Arbetman and colleagues employ a method and present results that hinge on the principle of preserving diversity. From a market's view, though, only a species that optimally performs the desired service is necessary--the others are theoretically expendable. This raises concerns for some conservationists: Focus on a species' ecosystem service neglects the complete picture of natural interactions. After all, nothing in nature is actually independent of the other parts. 

So, is there a way we discuss both economic benefits to a market and people and still emphasize biodiversity preservation? A group of environmental scientists from the United States and France believes they have found a way.

I'll talk about that in the next post.