Here are results from the first two years of the California Bumble Bee Atlas!
Where we found the bees
In 2023 the project grew in size, with 355 volunteers submitting bumble bee observations--twice as many as in 2022. Together, volunteers and project staff completed 678 point surveys, making observations of 6,544 bumble bees! In its first two years, the project has netted 10,009 total bee observations made during 1,102 point surveys, as well as during incidental sighting surveys. We resampled many of the same localities as in 2022, but also expanded the geographic scope of the project. Here's a map of all observations you submitted to Bumble Bee Watch in 2022-2023:
In 2023 the project grew in size, with 355 volunteers submitting bumble bee observations--twice as many as in 2022. Together, volunteers and project staff completed 678 point surveys, making observations of 6,544 bumble bees! In its first two years, the project has netted 10,009 total bee observations made during 1,102 point surveys, as well as during incidental sighting surveys. We resampled many of the same localities as in 2022, but also expanded the geographic scope of the project. Here's a map of all observations you submitted to Bumble Bee Watch in 2022-2023:
Bumble bee species frequencies in our surveys
One way of understanding the California Bumble Bee Atlas results is to compare species by frequency (or relative abundance) in the data. The graph below shows the percent of identifiable records in the CABBA dataset that are accounted for by each species in 2022 (blue triangles) and 2023 (green squares). As you can see, B. vosnesenskii is by far the most common bumble bee species in California, while others, such as B. occidentalis, are much less common. One thing that's interesting to note is that some species were more or less common in 2023 than the previous year. A great example is B. crotchii, a candidate for state endangered species listing, which was roughly twice as common in 2023 than 2022. Bumble bee populations are known to fluctuate from year to year, and our survey may help to explain what drives that variation.
One way of understanding the California Bumble Bee Atlas results is to compare species by frequency (or relative abundance) in the data. The graph below shows the percent of identifiable records in the CABBA dataset that are accounted for by each species in 2022 (blue triangles) and 2023 (green squares). As you can see, B. vosnesenskii is by far the most common bumble bee species in California, while others, such as B. occidentalis, are much less common. One thing that's interesting to note is that some species were more or less common in 2023 than the previous year. A great example is B. crotchii, a candidate for state endangered species listing, which was roughly twice as common in 2023 than 2022. Bumble bee populations are known to fluctuate from year to year, and our survey may help to explain what drives that variation.
Relationships with plants
Over the first two years of the survey, we have now observed bumble bees foraging for nectar and pollen on plants belonging to 342 plant genera! We don't know the plant species name for all observations, but the number of plants used by bumble bees in California must far exceed the number of genera we have observed being used. One lesson from the Bumble Bee Atlas thus far is that bumble bee ecology varies across the state's complex geography, and this applies to host plant use among other factors. In the table below, we highlight the top 5 (in order 1-5) most commonly visited plants, separated by ecoregion. The number of ecoregions in which each is a "top 5" plant is summarized in the final column, and the table is sorted according to this value. So, the plants near the top of the table are the most widely important to bees across multiple types of habitats, while those near the bottom are "top 5" for at least one ecoregion, but may be only regionally important within the state. Please note, plants not given a number on this list were not necessarily ignored by bumble bees in that ecoregion (they just didn't make it onto the top 5 list). Keep in mind also that individual bee species vary in their host plant preferences (see below!) and that there are many other important plants for bumble bees in California that didn't make this top 5 list. One further caveat to bear in mind is that the list reflects the places we surveyed, which are not necessarily reflective of the diversity of plant habitats across the state.
We can use this list as an index of general California bumble bee plant diet preference, which might be useful in conservation and restoration efforts.
Over the first two years of the survey, we have now observed bumble bees foraging for nectar and pollen on plants belonging to 342 plant genera! We don't know the plant species name for all observations, but the number of plants used by bumble bees in California must far exceed the number of genera we have observed being used. One lesson from the Bumble Bee Atlas thus far is that bumble bee ecology varies across the state's complex geography, and this applies to host plant use among other factors. In the table below, we highlight the top 5 (in order 1-5) most commonly visited plants, separated by ecoregion. The number of ecoregions in which each is a "top 5" plant is summarized in the final column, and the table is sorted according to this value. So, the plants near the top of the table are the most widely important to bees across multiple types of habitats, while those near the bottom are "top 5" for at least one ecoregion, but may be only regionally important within the state. Please note, plants not given a number on this list were not necessarily ignored by bumble bees in that ecoregion (they just didn't make it onto the top 5 list). Keep in mind also that individual bee species vary in their host plant preferences (see below!) and that there are many other important plants for bumble bees in California that didn't make this top 5 list. One further caveat to bear in mind is that the list reflects the places we surveyed, which are not necessarily reflective of the diversity of plant habitats across the state.
We can use this list as an index of general California bumble bee plant diet preference, which might be useful in conservation and restoration efforts.
Did different species of bees favor different host plants? Yes, they did! As one example, the graph below compares host plant use by B. crotchii and B. melanopygus, within the lower elevation range of the former (i.e., we filtered the data to only include B. melanopygus records from within the range of B. crotchii). These are the top 15 forage plants for each species, sorted in terms of their frequency as hosts for B. crotchii, and a missing dot means the bee did not use that plant at all. We can immediately see big differences between the two: the most important plants for B. crotchii were milkweeds (Asclepias species, accounting for more than 20% of the species' diet), while B. melanopygus didn't use milkweed flowers at all, and most often fed at flowers of Salvia and Ceanothus species.
Interestingly, we have discovered some differences in host plant preference between males and females of the same species. Using B. crotchii as an example, the graph below shows the observation frequency of some of the most important host plants for males and females. In this plot, we considered only female bee observations made within the flight season of the males, so both could be expected to have the same choice of plants. We can see that some plants--Salvia, for example--are important to both sexes, while others are more important to males (e.g., milkweeds; Asclepias) or females (e.g., lupine; Lupinus). For some plants, this difference reflects the fact that only females forage for pollen, which is not always found at the same plants where both sexes gather nectar. For others such as milkweeds, where bees do not gather pollen, the differences observed between male and female bees in preference could be related to other factors, such as nectar chemistry.
We also documented that plant community composition and structure affected bumble bees seen in our surveys. We asked volunteers to estimate the percent cover of flowering plants (below left) and number of plant species in flower (bottom right) during point surveys. Both of these variables were significantly positively correlated with both the abundance and species richness (not shown here) of bumble bees. This shouldn't surprise us, and in fact, this pattern is well-documented for bees in other areas. (Please note, we asked you to assign flower species number to categories (0, 1-2, 3-5, 6-10, and 10+ species) rather than telling us the exact number you counted. Here we have made that variable ordinal, assigning integers to each category in sequence.)
Taken together, these results regarding host plant use reveal much about the bumble bees we are surveying for!
Taken together, these results regarding host plant use reveal much about the bumble bees we are surveying for!
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Other factors influencing our data
Host plant relationships are critical to understanding the results of this project, but many other factors also influence the abundance and diversity of California bumble bees. One of these is precipitation--itself a key driver of plant growth and flowering. We found that bumble bee diversity in our point surveys is strongly, positively correlated with the amount of winter precipitation received at the survey site. (Here, winter is defined as December-February. Bee abundance is also positively correlated with precipitation.)
We also found that diversity was positively correlated with net primary productivity (NPP), an estimate of how fast plants accumulate biomass as they grow. NPP is correlated with some of the other variables that affect bumble bee distributions, including precipitation.
Host plant relationships are critical to understanding the results of this project, but many other factors also influence the abundance and diversity of California bumble bees. One of these is precipitation--itself a key driver of plant growth and flowering. We found that bumble bee diversity in our point surveys is strongly, positively correlated with the amount of winter precipitation received at the survey site. (Here, winter is defined as December-February. Bee abundance is also positively correlated with precipitation.)
We also found that diversity was positively correlated with net primary productivity (NPP), an estimate of how fast plants accumulate biomass as they grow. NPP is correlated with some of the other variables that affect bumble bee distributions, including precipitation.
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Credits: bumble bee species thumbnail graphics on this page from Williams et al. 2014, Bumble Bees of North America: an Identification Guide.
