Case assistant professor of biology, Dr. Sarah Diamond, was thrust into the limelight last year when her work on a tiny harbinger of climate change, the acorn ant, was covered in the New York Times article, What Makes a City Ant? Maybe Just 100 Years of Evolution. The George B. Mayer Chair in Urban and Environmental Studies summarized her work at a recent Inside Science presentation at the Cleveland Museum of Natural History.
Dr. Sarah Diamond found a correlation between epigenetic, or heritable changes in gene function that do not involve changes in the DNA sequence, and a common situation happening in cities, something called urban heat island effect, which, as the name suggests, are pools of elevated temperatures found throughout heavily paved and built over areas.
As The Times explained, “It can often take millenniums for organisms to evolve. But for crumb-size acorn ants in Cleveland, a single human life span may have been enough for them to become adapted to city living.”
From a practical standpoint, scientists looking to replicate the effects of rapid, contemporary warming on living species will find what they need in cities.
“Cities are a globally replicable experiment,” she says. “Urban heat islands are an ideal proxy for the rapid temperature rise that will occur everywhere in the future.”
Higher urban temperatures have a pronounced effect on acorn ant colonies — and other invertebrates in the insect kingdom. Cleveland’s typical northern temperatures and a growing amount of vacant and sometimes unintentionally “natural” land make it an ideal site to observe changes. The changes she observed are within a window of 20 ant generations.
That relatively quick shift in tolerance to higher temperatures means that Diamond, when rearing ants in the lab at a higher temperature than their forested cousins, produced rapid and clear results. Diamond found a correlation between temperature and a physical condition (tolerance to heat). To confirm her laboratory results, she places the ants in city lots with some natural cover.
“Cleveland has a big shift in temperatures along the rural to urban gradient,” Diamond observes. “There’s a big shift in thermal tolerance (in the ants) from rural to urban: a full degree. That might not sound like a lot, but it is. It’s evidence of an evolutionary change.”
The temperature tolerance suggests an adaptability to climate change, she adds.
To further test how cities will be impacted by climate change, Diamond conducted a round of acorn ant experiments in Cleveland, Cincinnati and Knoxville (high, mid and low latitude ranges). Since cities do heat and cool at different rates, the ants’ tolerance was more noticeable in Cleveland and Knoxville. Diamond does not have a ready explanation for Cincinnati, except to say that the work is “settling the debate that cities are leading to increased plasticity in heat tolerance.”
That may sound like good news, but it is actually cause for concern. The urban populations of acorn ants lose their ability to shift their metabolic rate to keep up with warming, Diamond discovered. “With thermal adaptation, there is less variation in heat and cold tolerance in urban areas. High latitude (Cleveland) had a more broad thermal performance. Low latitude (Knoxville) had a narrow range.”
“As we warm the climate,” she concludes, “we’ll see a loss of colony stability.”