Now, when we tell folks we are studying bees, the most common question is: "So what's happening to all the bees?" Despite the unforgettable moment when they were stung, most people don't appear to hold a grudge. They are genuinely concerned about the health and well-being of bees.
Clearly no blog about bees would be adequate without some mention of bee declines, though to be honest, I am loathe to write about it. Why? Because it is such a hard question to answer. Understanding bee declines is complicated. As Winnie the Pooh was fond of saying: "You never can tell with bees..."
First, it depends on the bee species. Some broad brush-stroke reporting on the part of the media has convinced most of the country that all bees are suffering from colony collapse disorder, a condition that affects only one species: the domesticated honey bee (Apis mellifera).
Setting honey bees aside for this post (we will come back to them later), in the U.S. alone there are around 4,000 species. Each has a unique life history--they forage at different times of the year, they nest in different kinds of materials, they experience varying levels of sociality, and they specialize on different plants. To lump all bees together and label them as "experiencing declines" makes as much sense as saying "All women are bad drivers" or "Men never read the instructions". Obviously, it depends on the person; equally so, it depends on the bee.
Second, by definition, a population decline means that there used to be X number of individuals, and now, some years later, there are X - Y number of individuals. The problem with studying bee declines is that we don't know 1) X or 2) how many years "some years later" should be. Consider this: the number of scientists studying bees in the early 1900s could be counted on both hands and feet. Today the number is orders of magnitude greater. As a measure of this change, the number of papers published in 1975 that had to do with bees was around 100. The number of papers published in 2014 related to bees was over 2000!
Incredible, isn't it? For our purposes, the point is that there were so few bee studies prior to 1970 that is hard to know what bee populations used to look like. Even starting in 1970, the number of those studies that focused on counting how many bee species and individuals were in an area are very few.
Thus, establishing that there is a decline is harder than you might think. For the majority of the species in the United States, we don't just plain don't know. However, based on our knowledge of what a bee needs to survive, we can surmise that many modifications that have been made to bee habitat over the last 50 years probably aren't beneficial to native bees. Huge swaths of the midwest that used to be wild prairie are now giant monocultures of corn, wheat, and soy--none of which are particularly useful to bees. Acres and acres of ground-nesting habitat have been paved over, potentially eliminating homes for many North American species. Urban sprawl isolates once continuous areas of vegetation, often fragmenting bee populations and diminishing their health. Insecticides sprayed on crops can have the unfortunate side-effect of killing all insects, regardless of what kind they are. Several pathogens (some known to be, and others hypothesized to be, introduced) also infect bees.
And as if that's not enough, bee populations appear to naturally fluctuate hugely. As an example, take a snapshot from a study of orchid bees (Euglossini) collected in the same location over 20 years in Panama: roughly 400 individuals were collected one year, nearly 1000 the next, and just over 300 the year after. Depending on which two years you look at, it can seem that the bees are increasing (Year One to Year Two), decreasing (Year Two to Year Three), or staying the same (Year One to Year Three). Which reflects the reality for Euglossini?
This is why scientists often look for 'trends', usually established over the biggest possible span of years. The trend among those orchid bees, by the way, was that a few species had declined, but the majority had remained unchanged. Unfortunately, data sets as long-term as the one in Panama are relatively rare.
One finally complication: bees are just as variable in space as they are through time. Many bee species can't travel very far (as in, less than a football field). If the floral and nesting resources they need are in close proximity, they may have no need to move for generations. A study of bees on creosote bushes in Arizona found that within just 3 miles, the bee species visiting for floral resources might change as much as 67%!
Keeping those caveats in mind, there are two questions we need to ask: Is there evidence of a decline for native wild bees? And, if so, to what can these declines be attributed?
In later posts I'll dress the second question. For now, I'll break down what we know about overall trends and what we know from old datasets... i.e. Is there actual evidence of a decline?
- At least five species of bumble bee have experienced severe declines over the last 100 years, with the most notable declines in the last 30 to 40 years. The Western Bumble Bee (Bombus occidentalis) used to be the most common bumble bee species west of the Mississippi. It is now hardly ever seen in most of its range In the east, the American Bumble Bee (Bombus pennsylvanicus) is much reduced from all but the western-most areas of its range, and some areas in the south. Bombus affinis and B. terricola are similarly hard to find. Most dramatically, Bombus franklini seems to be almost entirely (if not entirely) extinct. In contrast, many other bumble bee species do not appear to have changed in their distributions (or in abundance) at all. Why are some kinds of bumble bees in decline? Scientists aren't certain, but several hypotheses are being investigated: Could it be that the species in peril are ones more susceptible to the fungus Nosema bombi? Could it be that the lower genetic diversity seen in declining species is the cause of their decline? Or could it be that bumble bees are a hold-over from a different, cooler, era, whose extinction is inevitable as the world warms? The verdict is out.
- A study in 2013 looked at all bee records for the northeast over the last 140-plus years. The authors picked out 187 species that had been relatively commonly collected and looked for trends in population size based on how often the bee has been collected over that time span. Three species have declined significantly and dramatically--two of those three were bumble bees. One was Macropis patellata, a specialist on loosestrife (Lysimachia). A little under 29% have declined a little bit. And a little over 27% have increased in abundance. The authors tried to find commonalities among the species that have declined; while the evidence was weak, it did suggest that bigger bees might be more likely to be in decline, as might specialists.
- In the midwest, near Carlinville Illinois, Charles Robertson (one of this U.S.'s earliest pollination ecologists) studied 441 flowering plants. For each plant, he meticulously recorded every flower-visitor for two years. The result is a fantastic record of nearly all bee species in that part of the country and their floral preferences. For two years in the 70s, two scientists resampled 24 of the plant species that Robertson had sampled. They found 82% of the bees Robertson had found on those same plants species, as well as 22 species Robertson didn't collect.
- To add to this dataset, the area was again resampled in 2009 and 2010, this time focusing on 26 flowering plant species. Of the 109 bee species Robertson had recorded on those 26 plants, the scientists found only 54. However, many of the species that Robertson had collected might be considered 'incidentals' on those plants. For example, one missing in 2009 that Robertson had seen 100 years before was Andrena erythrogaster, a likely specialist on willows (Salix). As they did not collect on willows, it is possible the bee is not extinct, but is just not commonly found on other plants; i.e. Robertson's record may have represented a fairly rare bee/flower connection. Also, roughly a dozen of the bees missing in 2009 are parasitic species; bees that do not visit flowers for pollen, but only for nectar. Their absence, again, may not represent a loss of those species in the area, but just that they weren't on these particular flowers. All of which is to say there has been some change in bee visitors to these particular plants in the last 100 years, but the significance of this change and what it tells us about bee declines in the midwest is hard to say.
- Some species have dramatically expanded their ranges. Take the example of the squash bee, who has expanded its range from the southwest all the way to Maine in the last 5,000 to 7,000 years ago, both as North America has warmed, and as humans have planted squashes outside its 'natural' range. Or the more than two dozen introduced species in the U.S. and Canada; Anthidium manicatum has made it from the east coast to the west in less than 50 years.
So what can we conclude from these few studies? Some bumble bee species are in serious trouble; though the reason why is unclear, the species most in decline are also the ones most susceptible to Nosema. The status of other species in the U.S. is largely unknown, though there is a little evidence that specialist bees and large-bodied bees have disproportionately shifted (decreased?) their ranges. Many species appear not to have changed their distributions, and some species are more common.
Perhaps the conclusion we will eventually reach is that, just as "Global Warming" as a catch-phrase has proved inaccurate, so too will "Pollinator Declines". Perhaps "Pollinator Shifts" more accurately reflects the trends we are seeing, as some species retreat to cooler refugia, and others embrace the warmth of the once-inaccessible northern reaches (the majority of the world's species thrive in the heat). Overall diversity may not be changing in North America, but the bees making up the neighborhood, and the 'hotspots' of biodiversity, may be shifting.