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Researchers worldwide are racing to understand how bees are responding to environmental change and what is causing their decline, but what do we know so far, and how do we know it?
Since 2008, when the first reports of colony collapse disorder (CCD) hit the news, there has been a rising awareness in the media and among the general public that bees are a threatened and potentially vulnerable group of organisms that merit conservation. Because bees are our most important pollinators (see Mason Bees as Pollinators) there are obvious reasons to be concerned about their health and well-being. Natural ecosystems depend on bees for pollination and a major proportion of human agricultural production, especially pollinator dependent crops like apples, blueberries, strawberries, cranberries and among many others, depend on healthy populations of bees.
What is the hard scientific evidence that bees are in decline? In this module we examine what it means for an organism to be “in decline” and the empirical evidence that bees are in decline.
There are a number of ways one might recognize bee declines. First, a bee species might be gradually becoming less common or abundant over time. A gradual or steady decline in abundance would be a clear indication of decline and might set off alarm bells for a conservation biologist. Likewise, the geographic range of the species might be shrinking. We would become very worried if a widespread species was shrinking in geographic range. A declining range would indicate that local populations were going extinct. Finally, one might ask if there is a decline in species richness (the observed number of species present in an area) over time. All three of these measures — decline in abundance, reduction in geographic range, and decrease in regional species richness, have been used to document bee decline.
Figure 1. A pinned Osmia californica labeled with the location where it was collected, date, collector, and identification (Image: © Cory Sheffield)
Concerns about bee decline were first triggered by massive losses of honey bee colonies over the winter of 2007/2008. These losses were real — many colonies died that winter across the U.S. and beekeepers had no clear explanation for these losses. But you will also recall from the Bee Phylogeny and Diversity module that honey bees (Apis mellifera) are not native to the US. In fact, despite the dramatic declines in honey bees in the US in 2007/2008, honey bee populations globally are actually increasing in abun- dance. Honey bees, like cows, are domesticated species in much of the world and they are far from a threatened species.
We focus here on declines of wild, non-managed (native) bees, which comprise the majority of bee species on earth. Let's start with a basic, but fundamental, question. How would we know if a bee species, or any organism, is in decline? This is not a dumb question! To document the decline of a species, we must know at least two things: (1) what was its distribution or abundance in the past and (2) how does that compare to its current distribution or abundance. If we had a time machine and could transport ourselves to another time, this would be easy. Unfortunately, a time machine has not yet been developed so we need to find another way.
Museum collections offer one possibility. Natural history museums like the American Museum of Natural History in New York [www.amnh.org/], the National Museum of Natural History in Washington, DC [naturalhistory.si.edu/], and the Cornell University Insect Collection, in Ithaca NY [cuic.entomology.cornell.edu/] have collections of bees that have been accumulated over decades going back to when people started collecting insects in the 18th and 19th centuries. Each specimen has associated data – where it was collected, when it was collected, what flower it was visiting, and even who collected it (Figure 1). Therefore, museums house key information that can be used to document the geographic range and abundance of bees in the past. By comparing how widespread or common a bee is in historical collections to how common and widespread a bee is today, we can uncover how that bee species has fared over time. In fact, museums are real-life time machines. We can use them to travel back in time and reconstruct the past. Most of what we know about bee decline comes from careful comparisons, using historical and current collections, of the past and present abundance and geographical distribution of bees.
Figure 2. In their 2019 study, Powney et al. overlaid the United Kingdom with a grid of 1km squares and calculated the average probability each square was occupied by individuals of 139 species of bees during a given year, referred to as occupancy in the figure. Between 1980 and 2012 occupancy declined with some years showing drastic declines, represented by red circles (Figure adapted from Powney et al. 2019).
A number of studies, both in Europe and North America, have documented declines in wild bee species over time. Bumble bees (genus Bombus) provide some of the most thoroughly documented cases of bee decline. Bumble bees are large, attractive, easily identifiable bees that are common in the Northern Hemisphere. Large numbers of bumble bee specimens exist in insect collections. The Cornell University Insect Collection, houses thousands of bumble bee specimens collected since 1865, when Cornell was founded, to the present. In North America, we know of a number of bumble bee species that are in decline or near extinction. Bombus terricola, Bombus occidentalis, and Bombus affinis, all widespread and abundant bumble bee species in the past, have undergone significant range reductions (see this module’s activity). In the western US, Bombus franklini is now thought to have gone extinct. But bumble bees are not the only bee group in decline. While less well-known and considerably less well-studied, many other solitary, social and brood parasitic bees are also threatened.
A number of studies have documented declines in a wide range of bee species, including solitary, social, and brood parasitic species. One study by Ollerton et al. (2014) analyzed the loss of bee species in Britain from 1850 to 2014 and found that 13 bee species, including solitary, social, and brood parasitic species, have gone extinct over that time period. The peak of extinction occurred from 1930 to 1960. This period coincides with rapid agricultural intensification and the conversion of flower-rich meadows and hedgerows to agricultural fields. While some crop plants can benefit native pollinators, conversion to wind-pollinated field crops, such as wheat, corn, and soybeans, can have a negative impact on bees.
Figure 3. A) Non-bombus species, B) Bombus species, and C) exotic (introduced) species show different trends in species richness overtime (figure from Bartomeus et al. 2013).
In another study, Powney et al. (2019) examined the change in the geographic ranges of 214 bee species between 1980 and 2012 using museum data. Twenty-five per- cent of the bee species showed declines in geographic range. These declines were most pronounced in solitary bees (32% declined). In addition, many habitat and host-plant specialist bees (bees that were already rare) declined (Figure 2).
But Britain is not the only region where bee declines have been documented. Bee declines have been documented in the eastern US as well. Bartomeus et al. (2013) analyzed data from 30,000 specimens collected in eastern North America from 1872-2011 (140 years). There was sufficient data to analyze patterns of abundance in 187 species and 53 of these were found to be in decline. Declining species included solitary, social, and brood parasitic bees. Three bumble bee species (Bombus affinis, B. pensylvanicus and B. ashtoni) were in rapid decline suggesting they were headed to extinction. Host plant specialists seemed to be the most vulnerable, and habitat loss seems to be the main cause of species decline. Non-native species tended to be increasing in abundance (Figure 3). This study provides a worrisome picture of bee conservation status in our area.
A recent study by Zattara and Aizen (2021) took a similar approach to analyze evidence of bee decline from around the world. They used a massive dataset of over 9 million specimen records. Records included both pinned insects in entomological collections as well as visual observations (photos) available via iNaturalist and other citizen science platforms. They reported a drop of 8% in the number of bee species collected or observed in the 2000s, and in the 2010s there was a reduction of approximately 25%. Declines occurred in all bee families and in all continents. Declines appear to be most pronounced among rare species as well as among habitat and host-plant specialists, as shown in the regional studies from Britain and North America. Museum data are not perfect, but they do point to a worrisome drop in bee species richness over the period from 1990 to the present.
In summary, these studies paint a concerning picture about bees. Analysis of historical collections provide evidence that some bee species are going extinct before our eyes, that many bee species are shrinking in geographic range, and that globally we are seeing a decline in species richness. In the next module, we will explore some of the causes of bee decline and what you can do to help preserve native bee populations.
Abundance - The number of individuals of a particular species
Geographic range - The area in which a species can be found
Species richness - The number of species in a region, landscape, or community
While many species of bees are in decline, some of the best-studied are bumble bees. In this activity, we will use open data to assess how the geographic range of a federally listed endangered species, the rusty patched bumble bee (Bombus affinis), has changed and watch a short documentary on the subject.
Species facing extinction are generally characterized by their declining ranges and abundances. To untangle a species' risk of decline we need to understand how population dynamics and geographic range change overtime. Museum collections provide us with that information. Insect collections in the United States maintain specimens dating back well over a hundred years and many collections are now starting to be digitized and shared in freely accessible online databases.
GBIF.org is one of the largest databases for biological specimens, including bees. Researchers and conservationists use the data provided through these databases in their research. In fact, many studies which have looked at bee declines used data available from GBIF, but you don't have to be a university researcher to access this data; it is freely available to view and download.
The maps above show observations of rusty patched bumble bees from between 1980 and 2019, available from GBIF (GBIF.org (17 March 2021) GBIF Occurrence Download https://doi.org/10.15468/dl.du92cc). Each map shows observations reported on GBIF in each decade. Using the maps above, we will explore how rusty patched bumble bees have declined over the last 40 years.
1. Use the guide below to draw the Extent of Occurrence for each decade on the maps above.
Extent of Occurrence (EOO) is one method for estimating a species conservation risk and its likelihood of extinction from human causes and natural threats. Species with a smaller EOO are more likely to face extinction from a given threat (e.g., habitat loss from development, drought, fire, invasive species, etc.) than species with a larger EOO. EOO is not a direct estimation of range but instead used to better understand how a species may respond to threats based on the range it occupies. However, we will consider the EOO to be a rough estimate of range for simplicity in this example.
To visualize EOO, draw the smallest possible convex polygon around all rusty patched bumblebee observations in each decade. See below for an example of a convex vs. concave polygon.
1. Compare each decade:
Follow photographer Clay Bolt as he searches for the rusty patched bumble bee and learns how scientists and conservationists are working to save the species.
(www.youtube.com/watch?v=P7TF8PvAdnE)
1. How many wild bee species are native to North America?
2. When was the rusty patched bumble bee last seen in Great Smoky Mountains National Park?
3. True/False – The rusty patched bumble bee disappeared from most of the eastern United States in just over a decade.
4. Sam Droege compares his photographs of bees to what other type of photographs?
5. Where are rusty patched bumble bees still found?
6. True/False - Male bumble bees sting.
7. What is a gyne?
8. What three things do pollinators need?
Bee Decline - What Do We Know? Module PDF
Bee Decline - What Do We Know? Activity PDF
Bee Decline - What Do We Know? Activity Answers PDF
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