Specialist bee species prefer abundant hosts

How do bees choose which flowers to visit? Some bees will visit almost any bloom, while others are more discerning. How, and whether, bees choose to specialize in a single type of flower or pollen is a question that has puzzled entomologists and ecologists for years.

Now, a team of scientists led by ecologist Colleen Smith — currently based at the University of California, Santa Barbara, but conducting the research while working at the University of Ottawa — outlines why some bee species specialize in visiting one type of plant over another. They concluded that specialized bees tend to focus on the species that are most abundant in an ecosystem – at least in the eastern United States. The findings were published last month in the journal of the Ecological Society of America Ecology.

“Most environments have a few common species that are really abundant, so most of the plants bees encounter come from a few common plants,” explains Smith. “Bees are much less likely to find rare plants. Plant abundance could be a mechanism that promotes specialization – and thus speciation.”

Of the thousands of bee species in the United States, about a quarter are pollen specialists who use only pollen from plants within a genus, species, or family. Only a minority of plants are hosts to these pollen specialists.

“Some plant taxa, such as willow and sunflower, support tons of bee species, and most do not support any particular bee species,” says Smith. “Why do some plants support specialists? What causes bees to specialize in one type of plant? That’s what we want to measure.”

Two competing hypotheses prevail in the field: One proposes that bees may specialize when the plants they prefer have poor pollen quality. Another opinion is that bees specialize in the most abundant taxa or plant species in their ecosystem.

To decide between the two, Smith’s team tested each hypothesis. First, they conducted a field study in Ottawa to see if generalist bees also collect pollen from specialist bee host plants. If specialist bees target low-quality host plants or active toxic pollen, generalists will avoid those plants. That’s not what they found, suggesting that pollen quality did not affect specialization.

To test the plant abundance hypothesis, they turned to a large public data source: citizen scientists uploading images to the iNaturalist app.

“It’s difficult to get a standard measure of plant abundance of all plant genera in the Eastern United States if you’re going to sample yourself,” says Smith. “We use data from a phone application, iNaturalist. Because people record where they see different plant species, we can opportunistically use this data to derive regional plant abundances.

“Our research shows that specialist bees are more likely to use a lot of plants than rare plants,” said Smith. “This is strong support for what many anecdotally suspect. However, this is the first time that there has been a quantitative test of various crops. One of the interesting things about this research is that it supports some of the ecological theories that explain why specialization developed in the first place.”

Future research might test the same hypothesis in the western United States – a specialist bee diversity center – to see if the pattern holds there too.

For non-researchers the implications for increasing bee species are clear: Plant your garden with plants that host both specialist and generalist bees.

“By growing these abundant host plants that specialists and species generalists visit, you support these potentially more vulnerable bee species in addition to the largest numbers of bees,” said Smith. “By paying attention to what specialists in the field of flora need, you will help more species of bees.”

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