The complex plant-bee networks that sustain the local environment could be radically disrupted by climate change, according to new research at UC–Santa Cruz.
by Neil Khosla
Nov. 10, 2015—Fueled by an insatiable thirst for pollen, a honeybee dives toward a stout white wildflower. I crouch down to watch the bee shovel through heaps of golden powder just as Angie Ashbacher points out a sonorous buzzing a few feet up the trail. It plays in concert with the honeybee’s soprano hum. “Do you hear that?” she asks. “That’s Bombus vosneskenskii, the yellow-faced bumblebee.” All I hear is an incomprehensible drone, but Ashbacher, an ecology graduate student at UC–Santa Cruz, hears the bee calling out its name.
We scan the flower-speckled field until our eyes rest on a plump, yellow-striped bee gliding through a patch of false dandelions. “You can hear differences in the tone of their buzzing,” Ashbacher explains, quickly swooping a net over the bumblebee. “The bumblebees have a much lower tone than the honeybees.”
With an effortless flick of her wrist, mastered through countless hours of practice, she swiftly tangles the bee in the net. She takes a vial out of her backpack and gently slips the bee inside, just long enough to identify it. Sure enough, it’s the yellow-faced bumblebee. Ashbacher jots down the flower species it visited and sets the bee free. It scurries out of the vial and zips off to one of the thousands of other flowers at Marshall Field.
UCSC doctoral student Angie Ashbacher has found that the relationship between plants and bees cools as the mercury rises. Photo courtesy Jessie Schneider.
These 74 pristine acres of protected wildlife lie at the northern reaches of the UCSC campus. Hikers and nature enthusiasts explore vibrantly flowered trails like these around Santa Cruz, but most do not notice the gradual changes happening at their feet. Subtle temperature rises are pushing plants and bees out of their homes, triggering ripple effects that spread across the Central Coast.
Ashbacher is developing a method to predict these effects as part of her Ph.D. research. Here in Santa Cruz County, no one has mapped the intricate networks among pollinating bees and the plants they visit. Ashbacher’s model networks will help ecologists predict how local plants and bees will react to climbing temperatures—and how farms might make better use of wild species to adjust.
Bee Networks and Flower Hubs
Ashbacher came to UCSC after earning her master’s degree at UC San Diego, where she studied how climate change affects plant communities. She found that native plants are better equipped to survive droughts than exotic ones. That work inspired her current project. “This study is similar, but it’s more complex,” she says. “Now I’m looking at an interaction instead of just a response.”
That interaction is the famous pollination dance of plants and bees. Many researchers have studied domesticated honeybees, which have been disappearing at an alarming rate over the last several decades. In May, the White House announced its first long-term plan to protect pollinating bees and other insects. The tiny workers contribute to 35 percent of the food produced worldwide; economists estimate their annual value at $24 billion.
However, farm-raised honeybees are not the only important pollinators. Recent studies show that wild bees also pollinate crops, including many of California’s important cash crops, such as almonds and avocados. Wild bees also need wildflowers to survive, making it essential to protect natural plant-bee relationships near farms—and in our neighborhoods.
Some species are a lot more embedded in natural communities than others. These “hubs” have tremendous influence within plant-bee networks. A bee hub is a gregarious bee species that visits various types of plants, and a plant hub is a flowering plant species that hosts a multitude of buzzing visitors.
Think of hubs as your most popular friends on Facebook. These friends share posts with hundreds of people on the Internet, akin to how flowering hub species feed multiple types of bees in their network. When a popular friend deletes her Facebook account, the people who depended on her for those stimulating Buzzfeed links are left to scour their news feed for posts from other friends. Likewise, when a plant hub species vanishes from a network, the bees that visited it must seek other sources of food. Sometimes this means they will leave the network, too.
Of her 10 research sites, Ashbacher says Marshall Field has the most diverse assortment of plants and bees. Since there is no existing dataset of plant-bee networks, she watches bees flit among flowers to paint her own picture of the living Santa Cruz landscape. Once organized, her seemingly random array of data blooms into an ornate web of interconnected species.
Smell of A Hotter Planet
With one glance at Ashbacher’s network models, it's clear why hub species are important. Only a few links represent the connections of specialized species, but hub species have several strands radiating outward, creating a dense nexus. “If you focus in on the hubs, then you can focus on the species that have the greatest impact on the entire community,” says Ashbacher. “You can lose big groups of species quickly if you lose just one hub species.”
When the heat rises, plant-bee networks can start to lose their buzz. Higher temperatures affect a bee’s ability to detect a flower’s pollen reward, Ashbacher has observed. “The bee doesn’t know what the reward looks like until it’s on the flower,” she says. “The flower is giving all these cues: 'Look at me, I’m big, I’m yellow, I’m awesome, come visit me!'” But when parched by warm climates, flowers lose some of this advertising power.
A yellow-faced bumblebee, a hub species, gathers pollen from a manzanita. Photo by JKehoe_Photos on Flickr.
Ashbacher is not sure what causes this failure to communicate, but she has some ideas. She conducted experiments with plants grown under different temperature conditions in growth chambers at UCSC’s greenhouse. There, she sniffed out an interesting interaction.
“On the plants grown in better [cooler] conditions, the chamber would fill with a scent, and the bees would immediately visit the plant,” says Ashbacher. “On the [warmer] end of the stress spectrum, all of a sudden the bees couldn’t care less about what was in that chamber. I could put them on the plant and say, ‘This is a plant, eat!’ and they would just fly away.”
The pleasant aromas of a blossoming flower do not just help florists sell bouquets. They also act as a perfume, attracting bees to help the plant reproduce. Like any good perfume, the flower’s alluring fragrances are expensive to make. If the weather gets too hot to handle, plants spend less energy on producing scents. Instead, they just try to survive. When flowers stop emanating these enticing smells, some bees have a tough time finding food. They may abandon certain areas.
By analyzing historical data, Ashbacher discovered that warming climates already have affected California’s bumblebee populations during the past 30 years. “When temperatures are higher, bumblebee diversity is lower,” she says. While factors such as pollution and urban expansion also harm bee and plant populations, she believes rising temperatures are the main reason for the flight of bumblebees from Central Coast fields.
Santa Cruz Bees and Native Plants
Ashbacher’s research is only just budding. After collecting enough data, she plans to remove hub species from her model networks to predict how losing certain species would transform the bee-plant connections around Santa Cruz. Such forecasts, she says, could help growers and residents protect important bees and plants from global warming.
Protecting wild bees starts simply by fostering better building and gardening habits, Ashbacher says. “As we build our houses or parks, it’s important to consider the native bee community,” she says. “First, ask ‘What are we putting here? Let’s put some native plants here, and provide some habitats for the bees.’ Otherwise, you’re just wiping out the habitat that’s really important for them.” Ashbacher’s models could also inform farmers about wildflowers to plant near their fields that would help wild bees navigate to their crops.
The yellow-faced bumblebee, the one Ashbacher netted in Marshall Field, is an up-and-coming hub species. Its Santa Cruz population has soared over the past 50 years, and its network dwarfs even your most popular friend’s Facebook pages. That’s good for its own livelihood, but it means many other bumblebee species are disappearing. For instance, Ashbacher tells me that she only recently saw a California bumblebee for the first time.
“Its name is Bombus californicus, so you think it would be everywhere in California,” says Ashbacher. “I’ve seen one in three years of working at these field sites.”
As Ashbacher is discovering, we have a huge influence on how plant-bee networks evolve. So the next time you venture onto a flowering trail and hear a low buzzing noise, look around for a hovering yellow face. Watch the bumblebee bury herself deep into the juicy center of a wildflower. Look for golden crumbs of pollen clinging to her body, and think about where she might travel next. She’s the center of attention of a blooming web of species, and she’s recently added a lot of friends to her network. If she had a status update, it might read: “Firmly planted in Santa Cruz networks #hublife.”
Neil Khosla majored in ecology and evolutionary biology at UC Santa Cruz. He wrote this story for SCIC 160: Introduction to Science Writing.