Brett Smith for redOrbit.com – Your Universe Online
The relationships among species change over time as shifts in an ecosystem begin to affect the organisms living in them.
Climate change has placed a new emphasis on studying these shifting relationships and a team of biologists from the Midwest sifted through historical scientific logs to find that the plant-pollinator relationships in their area have been significantly altered over the past 120 years.
Working out of Washington University in St. Louis (WUSTL), biologists Tiffany Knight and Laura Burkle discovered that 50 percent of the local bee species in central Illinois had vanished, some pollinators were active before their associated plants had bloomed, some plants weren’t visited as frequently, and the bees that did visit weren’t carrying as much usable pollen.
According to their report in Science Express, the team based their research on the historical data sets of 19th century naturalist Charles Robertson, which were collected near the small town of Carlinville.
Between 1887 and 1916, Robertson meticulously recorded the visits of over 1400 pollinators, such as butterflies and bees, to 460 plant species. He even described several hundred insect species previously unknown to science; many of which were named after him.
Burkle said she enjoyed the detective work that came with sifting through Robertson´s records.
“It was like solving a mystery, she said, trying to deduce what he had done from old ledgers, specimen i.d. tags, and his privately published book Flowers and Insects (sic).”
To compare the relationships of Robertson´s time to those of today, the WUSTL researchers collected flowers from Robertson´s network of plants.
“To keep our project manageable,” Knight said, “we re-collected a subset of the network Robertson collected, focusing on one plant community: forest spring ephemeral plants. We looked at 26 plant species in this community, which were associated with 109 bees in Robertson’s time.”
“If any community is going to be affected by climate change,” she noted, “it would be this one, because the plants flower soon after the winter snow melts.”
The team initially found that many of the bees associated with their collection were no longer living in the area. Of the 532 pairings found in the Robertson subset — 406 had been lost; however 120 new pairings had been gained. Of the broken connections, Forty five percent were due to a missing bee species.
The re-collection also showed that the timing between the bees and the plants had been offset. On average, the plants were flowering 9.5 days earlier than they had in Robertson´s time. The biologists found that the bees were active about 11 days earlier as well.
The team also found that activity cycles for the plants and pollinators had sped up. The flowers were seen blooming around eight fewer days and the insects flew for 22.5 fewer days. This compressed timeline resulted in less overlap and less time for successful pollination.
The team discovered another problem with Robertson´s data set — the naturalist did not keep records on the frequency of pollinator visits.
“Robertson didn’t keep track of how much time he spent in the field watching each flower, so we couldn’t get visitation rates from his data,” Knight said. “But of course we searched the literature to see whether anybody had published on the Carlinville network since Robertson, and one person had.”
John Marlin, a research affiliate at the University of Illinois’s Prairie Research Institute, had recollected part of Robertson´s network in 1970 as a part of the Illinois Natural History Survey.
“Marlin’s dataset gave us visitation rate, a quantitative measure of pollination we otherwise wouldn’t have had,” Knight said. “Comparing the visitation rates we measured to Marlin’s, we discovered that the bees were making fewer trips to the flowers than they had in the 1970s.”
The team focused their study on one plant, Claytonia virginica, also known as ‘spring beauty,´ because it is visited by a wide variety of bees.
“Marlin counted 0.59 bee arrivals per minute and we counted 0.14 arrivals,” Knight said. “So even those some interactions are still present, they’re weaker.”
Both Robertson and Marlin had stored their bees in the Illinois Natural History Survey, many still containing the pollen they were captured with.
Burkle and Knight picked six bee species from the collection that were caught while visiting Claytonia and washed Robertson’s, Marlin’s, and their own specimens for pollen.
“We gave the bee a gentle bath and washed its pollen off onto a microscope slide and then we fluffed it back up with a hair dryer,” Knight says.
Although they were all caught off the same flower, the team found that many of these bees had been more loyal to Claytonia in the past than they were now.
“I was surprised by how tenuous a lot of these plant-bee interactions are,” Burkle said. “We’ve pushed on these communities a lot, and they are pretty robust, but at the same time, they are compromised, and more compromised than I was expecting them to be.”