Native pollinators feel the sting of habitat loss
U-T San Diego
by Deborah Sullivan Brennan
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Credit: John Baker |
In James Hung’s collection at UC San Diego is a kaleidoscope of native bees, many of which bear little resemblance to the honeybees and bumblebees we know.
The biggest are grape-sized and glossy black, while the tiniest could sit on the head of a pin or slip unnoticed into a flower. Some are iridescent, jeweled shades of blue and green, and others rusty red.
Squadrons of the insects buzz through coastal sage scrub and desert of San Diego, helping pollinate native plants and making the county a hot spot for bee diversity. Hung, a PhD candidate at the university, has collected more than 350 different native bee species, and estimates that there are as many as 600 species in the region.
“There are more bees than the number of birds, and San Diego is famous for having a phenomenal number of birds,” said Hung, a graduate student at the university, who began studying the insects as an undergraduate at Dartmouth College in New Hampshire.
Over time, however, drought and development are eroding that diversity. So Hung aims to catalogue the county’s bees and describe the nuanced ways they interact with plants and other pollinators to sustain native flora and agricultural crops.
“There is intrinsic interest in preserving biodiversity for its own sake,” he said. “People do care if an endangered species goes extinct, or a common species becomes endangered. But I think it will resonate with people a lot more if they know how important bees are for functions of our entire ecosystem.”
Hung, 26, was born in Taiwan and moved to Vancouver, Canada at age 10. As a child, he was fascinated by insects and cultivated his own collections.
“As a kid I didn’t know that much about bees,” he said. “I would catch things I knew I could rear, so I’d have captive insect zoos in my home.”
Far from being wary of his unusual pets, his parents were “super supportive” of his hobby, he said.
“My mom would drive me to get grasshoppers for my pet praying mantis, or drive me to streams to get small fish for my water scorpions,” he said.
It wasn’t until he was an undergraduate that he began learning about bees in earnest.
One of his professors at Dartmouth, ecologist Doug Bolger, had collected jars of miscellaneous insects from pitfall traps in San Diego. Bolger had not had time to sort and catalogue the specimens, so Hung took on the task for his senior honors thesis.
It was “exacting and somewhat tedious” work, Bolger acknowledged. But the variety of bees captivated Hung.
“He was very interested in just the different kinds of bees that were there, so he took great pleasure in identifying those,” Bolger said. “He travelled to different museums to get help identifying the species, because it’s not easy.”
Hung decided to continue his study of bees through a doctoral program at UC San Diego, where he could study the region’s bees in their native habitat.
Near the university’s campus in La Jolla is the Scripps Coastal Reserve, a stretch of coastal sage scrub that’s one of 39 sites in the UC Natural Reserve System, and among the local areas where Hung studies native bee populations.
Invasive mustard dots its slopes, but many small barrel cactus, buckwheat and other native plants bloom there as well.
On a flowering cactus, a bomber of a bee showed up, a hulking, tan-colored Diadasia. The native is unmistakably bigger than the honey bee, and feeds exclusively on cactus, Hung said.
While honey bees maintain complex social structures within hives, or colonies, native bees are solitary or maintain loose social structures, and most nest underground, Hung said. Diadasia, constructs earthen tubes from its burrows, which can extend half a meter underground.
Nearby, Hung pointed out the tell-tale structure: “This soil turret is the thing that earns the bee its common name, chimney bee.”
Inspecting a buckwheat plant bursting with creamy flowers, Hung swiped a delicate, yellow bee, Perdita rhois, also known as a small mining bee.
“Isn’t it cute?” he asked. “So gorgeous.”
Throughout the reserve are non-native mustard plants – mementos from Spanish settlement of California. Buzzing around them were European honey bees, whose role in agriculture – and recent decline due to colony collapse – have been well-publicized.
But a closer look revealed a small, dark insect that could easily be mistaken for a winged ant or fly. It’s a bee in the genus Halictus, a common San Diego native.
Unlike other natives that forage only on specialized plants, this one isn’t picky – really not picky. It earned its common name of sweat bee by licking perspiration from human skin.
“It’s highly generalist,” Hung said. “It will go to almost any plant that blooms and gives out pollen.”
Although the ubiquitous mustard scattered around the reserve provides food for honeybees and some species such as the sweat bee, they’re crowding out native plants that other bees need to survive.
That’s the kind of ecological process that Hung is trying to unravel. So he’s working with undergraduate students to eradicate mustard to see how neighboring plants and insects respond.
“Hopefully we’ll show that if we can suppress mustard enough, the native plants will come back, and as the native plants come back, hopefully the native bees will return,” he said.
The sheer variety of San Diego bees, and their role in the coastal environment, is a new area of exploration, Hung said.
Although Southwest bees had been studied in some depth in Riverside County and in Arizona, they had not received the same attention in San Diego, Hung said. But bee populations here face different problems than those in rural areas.
An aerial view of the coast, he said, shows “little finger of canyons buried in a sea of urbanization. This is not something we can easily reverse.”
Those fingers, or fragments, of habitat are the focus of his research. Since he began graduate work in 2010, Hung has surveyed bees in large expanses of open space, such as Mission Trails, the Elliott Chaparral Reserve and the Otay-Sweetwater unit of San Diego National Wildlife Refuge.
He compared those samples to smaller, but well preserved swathes of habitat including the Scripps reserve, Chollas Creek Park, Pasatiempo Open Space and Paseo Del Rey Park. These are the signature canyons and fields that give San Diego its spacious, natural feel. But they’re deceptively disturbed by surrounding civilization.
He found less variety of bees in the smaller habitat parcels, even when those sites have the same plant types and density as the big areas. In some fragments, certain bees that normally use that habitat type are completely missing.
“Even if a small habitat retains a lot of the vegetation features of coastal sage scrub, bee diversity is reduced,” he said. “A nice fragment will have about 65 percent of the diversity of the larger habitat.”
The species that tend to thrive in fragmented or disrupted habitat were the “generalist” bees – those that can feed on many different flowering plants. But the “specialists” – species that need a specific type of vegetation – fared poorly. In some cases Hung as quantified the disappearance of species that were barely known before.
“James has meticulously documented the loss of bee diversity,” said UCSD ecologist David Holway, Hung’s doctoral advisor.
As local bees decline, native plants could suffer without the help of pollinators. Cultivated crops could also be affected, because farm systems are more productive with multiple bee species serving as pollinators.
“I really do think bee diversity matters, and I believe my research will show that and raise awareness of how important it is to conserve them,” Hung said.
To read more about native bees, visit Hung’s bee blog.
