BEE PHYLOGENY AND DIVERSITY
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BEE PHYLOGENY AND DIVERSITY
Phylogenetics, Evolution, Life Histories, Species Richness
Globally, there are over 20,000 species of bees and over 4,000 in the United States alone, but many people are only familiar with the honey bee! What are these other species, and how did they evolve?
When biologists use the term diversity it can have a number of different meanings.
First, diversity can refer to the total number of species in a group. Flowering plants are diverse because there are over 350,000 described species. Beetles are a diverse group because they include over 750,000 described species (roughly ¾ of all animals on earth). A more precise term for the number of species in a group would be species richness. But diversity can also refer to something else: the phenotypic variation among the members of a group. Flowering plants are extraordinarily diverse because they come in all sizes and shapes and they inhabit almost every possible terrestrial habitat. Likewise, beetles are an extraordinarily diverse group because they are enormously variable in size, shape, coloration, life history, and ecology.
Evolutionary History of Bees
How do bees compare in these measures of diversity, including species richness and phenotypic diversity? We know, from studies of the fossil record and phylogeny, that bees arose approximately 120 million years ago in the mid-Cretaceous. They evolved from hunting wasps, a group of solitary, mostly ground-nesting, non-social wasps that prey on other insects (including flies, crickets, caterpillars and many others). One evolutionary lineage of the hunting wasps began to combine pollen with insect prey and eventually evolved into bees feeding on a diet of pure pollen. Bees are essentially vegetarian wasps!
Figure 1. The vast majority of the ~20,000 described bee species are solitary (figure adapted from Danforth, Minckley, and Neff, 2019).
The Diversity of Bees and their Life Histories
Once the transition to pollen feeding had taken place, bees diversified rapidly into many habitats and they evolved diverse ways of exploiting flowering plants. Today, bees include over 20,000 described species divided into seven families, and over 500 general. In North America alone there are over 4,000 bee species and in New York we estimate that there are approximately 420 bee species. It is worth comparing bee species richness to other groups that you may be familiar with – vertebrates such as birds, mammals and reptiles. There are only 4,000 mammal and 7,000 bird species on earth, so bees are five times more species-rich than mammals and three times more species-rich than birds.
Bees are also phenotypically diverse. Bees range widely in body size from the smallest bee (Perdita minima; just 2 mm in length) to the largest bee (Megachile pluto; up to 40 mm in length). They vary widely in coloration, from all-black bees, to yellow, blue, and shiny green bees. They also differ in hairiness — some bees are rotund, fluffy species (like bumble bees) while others are sleek, relatively hairless, wasp-like creatures (see the bee diversity poster under the resources tab). Bees also exhibit a diversity of life histories. There are solitary bees, social bees, brood parasitic bees, and social parasites. Figure 1 shows the proportion of bee species that fall into each of these four broad categories.
In solitary bees, each female builds her own nest, forages for food, lays eggs, and defends the nest against parasites and predators. Solitary bees tend to be less aggressive than social bees and are therefore less conspicuous to humans. But they comprise the vast majority of bee species on earth (77%).
In social bees, which comprise just 9% of all bee species, there are multiple females per nest — sometimes hundreds to even thousands. In the colony, only queens reproduce, and most females are sterile workers that forage for food, build the nest, and defend against attackers (including humans). Honey bees and bumble bees are two groups of social bees that you are likely already quite familiar with. But there are other social bees that are less well known, including the social sweat bees (Halictidae).
Figure 2. Subfamily phylogeny of bees showing the relative abundance of each life history category. Most subfamilies consist of exclusively solitary species (figure from Danforth, Minckley, and Neff, 2019).
In brood parasitic bees, females enter the nests of solitary bees and lay their eggs in open or closed brood cells. The eggs hatch and the parasite larva kill the host egg or larva and then consumes the pollen nectar provisions. Brood parasitic bees — also called “cuckoo bees” — comprise approximately 13% of all bee species. Brood parasitic bees don’t need to collect and carry pollen, so they lack the scopa and associated structures for pollen transport (see Bee Anatomy). They also tend to be far less hairy and more heavily armored than their pollen-collecting hosts. Cuckoo bees are beautiful, wasp-like creatures that you might see flying low over the ground searching for host nests in the spring or summer.
A final life history category is the social parasites. These comprise a very small slice of bee diversity — just 0.3% of all bee species. Social parasites, as the name implies, only attack social hosts. Their behavior and mode of attack are quite different from those of the brood parasites we introduced above. In socially parasitic bees, a parasitic queen enters the nest of the social host (usually a closely related species) and either kills or incapacitates the host queen. The social parasite then exploits the host workers to rear her offspring. In fact, social parasites never produce workers — they only co-opted the workforce of their hosts. What a devious strategy! Some bumble bee species are social parasites that attack other bumble bee species. You can recognize the socially parasitic bumble bees because females lack the scopa for carrying pollen.
All bees are social – in fact, as we have seen above, only a relatively small percentage of bee species are social. The vast majority of bees live solitary lives.
- All bees build a nest – Only the solitary and social bees are nest-making. The brood parasitic bees and the social parasites do not build nests. Instead, they attack the nests of their solitary and social hosts.
All bees make honey – only a very smaller percentage of bees make honey. The true honey bees (genus Apis; just 12 described species) store honey as a form of winter fuel in order to survive year-round. Other social bees, such as the stingless bees of the tropics (~300 species), store honey as well. The remaining bees (95% of species) do not store honey.
Brood cell – A chamber in bee nests which houses bees as they develop from egg to adult and the pollen provision (food resource) stored within (see Nesting and Mating Behavior in Mason Bees)
Brood parasitic or “cleptoparasitic” – In brood parasitic bees, females do not build nests or collect pollen and nectar for larval nutrition. In fact, these bees lack the structures for gathering, manipulating and carrying pollen. Instead, they enter the nests of free-living, pollen-collecting bees and lay their eggs in either open or closed brood cells. The adult female or her first instar larva kills the host egg or larva, and the brood parasite then consumes the pollen provisions of the host bee. Female brood parasites are often heavily armored to defend themselves against the attack of the host female. Brood parasites are often, but not always, closely related to their hosts.
Social – Social bees exhibit three key features that distinguish them from solitary bees. In social bees there is:
- Reproductive division of labor – meaning some females reproduce (the queen) and others remain as sterile workers.
- Cooperative brood care – meaning females care for offspring that are not their own.
- Overlap of generations – meaning multiple generations (mother and daughter) remain together over time.
When these conditions are permanent (meaning a female, who is a worker remains a worker for her entire life) we refer to these societies as “eusocial”. When these conditions are temporary (meaning a female may serve as a worker for some part of her life but then later assume the role of queen) we refer to these societies as “cooperatively breeding”.
Socially parasitic – Social parasites enter the nests of social bees and kill or replace the host female as the primary egg layer. These bees only attack social hosts and are often closely related to their hosts (e.g., the subgenus Psithyrus in bumble bees).
Solitary – a single, adult female builds and occupies each nest. She constructs her own brood cells, provisions them with pollen and nectar, guards her own nest, and lays her own eggs. Approximately three-quarters of all bees are solitary (Figure 1).
Global Bee Diversity — Data Analysis
Using a series of figures we will explore global bee diversity, bee abundance, and the relationship between species richness and annual precipitation.
Part 1: Global Diversity
To highlight the impressive diversity of bee species, we will explore the variation in the number of species worldwide. The following figures, created using species richness data available from Discover Life, a free web tool that maintains a global encyclopedia of life (www.discoverlife.org/). In this activity, we will look at bee richness in five regions of interest: Australia, Brazil, North America (Canada, Mexico, and the United States), Southern Africa (South Africa, Botswana, Namibia, and Mozambique), and Spain.
To help you orient yourself, Figure 1 from Danforth et al. shows the family-level phylogeny of bees. For this activity, we will focus on the global distribution of these families and how species richness varies in each region.
Interpreting the Figures
Figure 2 shows the species richness (number of species) per area in millions of square Kilometers of reach region. Australia, Brazil, and North America have similar species richness per unit area even though they occupy very different bioregions. Southern Africa shows moderately more species richness per unit area, possibly the result of the arid environment. Similarly, Spain's dry Mediterranean climate makes it a bee biodiversity hotspot. The relatively small size and lack of environmental diversity likely contribute to the high species richness seen in this area.
Figures 4 and 5 examine variation in the species richness within each family across the five regions of interest. The proportion of total species richness in each family varies between regions. Colletidae makes up over 50% of the total species richness in Australia, and Apidae makes up over 50% of Brazil's total richness. Figure 5 highlights the number of species in each family in each region.
Figure 2. Species richness by region.
Figure 5. Bar plots showing the number of species in each bee family by region.
Describe how species richness per unit area varies across the five regions in Figure 1. Why might we see this relationship?
How are bee families distributed across the globe? Which families are found in the most regions? Which families are found in the fewest regions?
Which bee family is most common in each region and what proportion of total species richness does it account for in the region?
Part 2: Bee Abundance and Climate
Now that we have a better understanding of the immensity of global bee diversity, lets home in on one of the many drivers of variation in bee abundance— local climate, specifically annual precipitation. In this part, we shift our focus from species richness (number of species) to bee abundance (total number of bees).
Interpreting the Figures
Figure 6a, adapted from Koh et al. 2016, shows the relative mean bee abundance across the United States, dark blue represents regions of high abundance and low abundance are light yellow. The deserts of the west host the greatest bee abundance which corresponds to the lowest annual precipitation (Figure 6b). However, you may notice that this does not explain all of the variation in abundance across the country.
Where is the bee abundance the highest? Where is bee abundance the lowest?
What is the relationship between annual precipitation and bee abundance?
What areas of the country do not follow the pattern you described in question 2? What other factors might impact bee abundance in these areas?
Figure 6. (A) Relative mean bee abundance (Koh et al. 2016). (B) Average annual precipitation between 1981 and 2010 (PRISM Climate Group, OSU).
Danforth, B. N., Cardinal, S., Praz, C., Almeida, E. A. B., & Michez, D. (2013). The impact of molecular data on our understanding of bee phylogeny and evolution. Annual Review of Entomology, 58, 57–78. https://doi.org/10.1146/annurev-ento-120811-153633
Koh, I., Lonsdorf, E. V., Williams, N. M., Brittain, C., Isaacs, R., Gibbs, J., & Ricketts, T. H. (2016). Modeling the status, trends, and impacts of wild bee abundance in the United States. Proceedings of the National Academy of Sciences of the United States of America, 113(1), 140–145. https://doi.org/10.1073/pnas.1517685113