The conditions of climate change—elevated temperatures, increased carbon dioxide levels and the accumulation of nitrogen in the biosphere—are difficult to replicate in a laboratory setting. This has complicated scientists' efforts to understand the potential ecological impacts of global warming. But, according to a new paper published in Global Change Biology, cities exhibit many of these factors, making them a petri dish in which researchers can glimpse how the earth's biological systems will be altered in the coming decades.
"Cities have all these things that match up with the future," says Elsa Youngsteadt, an entomologist at North Carolina State University and an author on the new study.
The density, development and emissions in urban environments all conspire to elevate the temperature inside cities, creating what are called urban heat islands. Youngsteadt's study focused on the urban heat island of Raleigh, North Carolina, a city with approximately 430,000 people, according to the 2010 census. "Raleigh can be between three and five degrees celsius warmer than nearby areas," says Steven Frank, the study's principal investigator and an entomologist at NCSU. That matches climate change predictions for 50 to 100 years in the future.
"Cities represent climate that we'll probably see in our lifetime. Or our children's lifetime," Frank adds.
To test whether cities model future climate conditions, the researchers looked at how the population of the bug Melanaspis tenebricosa has been altered by temperature changes in both urban and rural environments. The pests, casually called gloomy scales, feed on red maple trees—a common tree in cities across the east coast of the U.S. The bugs drink fluids inside the maples which, on a large enough scale, causes branch dieback and even tree death.
The team collected gloomy scales from 55 trees on 32 streets in Raleigh and compared them to samples taken in the surrounding rural environment. They found that inside cities, where temperatures were higher, the bugs were more abundant—jeopardizing the health of maples.
To confirm that it was in fact temperature changes that influenced the insects' population increase, the researchers examined historical collections of gloomy scales taken in rural environments between 1895 and 2013. The found that, in hotter time periods and locations, they were also more abundant.
"That's what leads us to believe temperature is one of the most important factors," says Frank, who calls the team's study "the first real evidence" that cities can be used to predict impacts of global warming.
As the researchers point out, however, it is important to note that the relationship between urban environments and future climate conditions isn't perfect. Several factors inherent to cities may also be influencing the growth of gloomy scale populations. Because of buildings and impenetrable surfaces such as sidewalks, for example, the bugs' predators may have a more difficult time hunting them. Further, it may be the case that the trees are simply more stressed in cities, making them more susceptible to pests.
Still, at least in the case of gloomy scales, cities appear to provide an important model for understanding how the bugs may spread in the coming century as rural areas begin to heat up.
If using urban environments as a model for future climate conditions proves more broadly applicable, Youngsteadt says, it may help researchers predict which insect species are set to become pests. "Which pests are becoming more common in cities? Those are the ones I'd want to keep an eye on in rural environments."
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