Unnatural Selection: Urban life may be forcing the hand of evolution — in real time


Cassidy Johnson

Unnatural Selection

By Sports & Health Editor

If you’d asked Walter Marcisz 10 years ago where the sweet spots for monk parakeet sightings in Chicago were, he would have rattled off a long list. As former president of the Chicago Ornithological Society and a lifelong resident, he would know.

He would have said to set up shop in Hyde Park or Jackson Park, or to camp out in a number of places on the South Side where the monk parakeet population began to explode at the turn of the millennium before fading in recent years. 

Marcisz has seen the monk parakeet population fluctuate in number since the late 1970s. The 100th Street exit on the Chicago Skyway, a toll road stretching from Indiana into Chicago, is one of the last holdouts of the monk parakeet. He estimates there are no more than a couple dozen nests left.

“Nowadays, their main stronghold is [there],” Marcisz said. “They build their nests at about 100th Street. It’s basically an area between Ave. L and Ave. M.”

Whether it is the wild coyotes that are beginning to claim territory in downtown Chicago or the evolution of the indoor biome, city life is spilling over into the natural world and molding it in the process.

A decade ago, areas with larger concentrations of monk nests—such as the village of Burnham—could have claimed nearly 40 nests on their own. If you wanted to see the big stick nests the parakeets built, you would have to look no further than the tops of utility poles, which, for the birds, were reminiscent of the trees in which they would normally nest but provided added protection because of their height—at least before the utility companies began to catch on, Marcisz said. 

“The minute a parakeet builds a nest on a utility structure or cell tower, they pretty typically take them down right away,” Marcisz said. “They’re kind of at war with the utility companies.”

A byproduct of the pet trade, monk parakeets are technically an invasive species in parts of the U.S. The bright, emerald- and lime-green birds are about the size of blue jays and native to South America, but they can withstand the temperature extremes of the Midwest. This clash between monk parakeets and the infrastructure of Chicago is just one example of how the adaptive events taking place at the junction of urban life and the natural world actually force the hand of evolution right before our very eyes. 

In her February 2015 paper published in the journal Trends in Ecology & Evolution, Marina Alberti, a professor in urban design and planning and director of the Urban Ecology Research lab at the University of Washington, said one of the great challenges for managing ecology in the future will be understanding the role we play in driving changes in evolutionary dynamics.

“We know that evolution is happening, and we have increasing evidence that it’s happening very rapidly,” Alberti said. “The idea that evolution affects ecosystems is not new, but until very recently scientists have assumed that the changes in those feedback loops were not occurring on a contemporary timescale. Now we have initial evidence that this is happening.” 

As people build a growing web of cities across the globe, the processes that take place in the surrounding ecosystems change, as well as the structure of those habitats. These shifts occur because, in cities, people selectively determine which species can and cannot coexist. That is what’s happening to the monk parakeet. The result: Evolutionarily adaptive changes in the behavior and appearance of plants and animals that could go beyond genetic makeup. For example, research has shown that certain rodents have shrunk over time in accordance with human population density.

“We have specifically reduced native vegetation, we fragment nature and vegetation, we change biotic interactions,” Alberti said. “We change the temperature, we produce a number of novel disturbances such as pollutants, noise and light. All of those are changes in the habitat and whichever species are more adaptable to them, those are the ones that we selectively prefer.” 

In a November 2011 paper published online in the journal Urban Environments, researchers looked at changes in the northern Illinois monk parakeet population between 1970 and 2010. Their numbers curiously became distorted over the course of the 40 years.

“At first, people were reporting a nest or two, but the nests did not persist from year to year,” said Stephen Pruett-Jones, an associate professor in the Department of Ecology and Evolution at the University of Chicago and lead author of the 2011 paper. “In the early 1970s, a small colony settled along Lake Shore Drive, around 53rd Street. That was a population that persisted. In other words, the population kind of blinked in and out until this colony got started, and that was the colony that really established a solid breeding population in Hyde Park.”  

According to Pruett-Jones, the population began to expand throughout the 1980s and 1990s, growing exponentially in the Chicago region before the original group, holed up in its Hyde Park fortifications, took a nose dive. Where hundreds of the feral birds once flitted from tree to tree, scavenging for sticks to shore up their hardy nests, there are now thought to be less than 20.

“At the same time the birds in Hyde Park were declining in numbers, the number of locations they were nesting in and the numbers in the greater Chicago region appeared to still be increasing,” Pruett-Jones said. “This is going to sound confusing, but we ourselves are a bit confused.”

The researchers found a skew in population on a national level as well. At the same time that the number of monk parakeets tallied in surveys fell, the number of places the birds were seen may have actually been rising, meaning fewer parakeets were clustered in places where they were likely to be seen and counted. But birders and scientists alike are at a loss as to why the parakeets, far from their natural habitat, would have clustered in the city only to mysteriously disperse, if that is in fact what has happened. The birds are either dying off or spreading out.

“If the decline is real, then nobody has any idea as to why they’re going down in numbers,” Pruett-Jones said. “It could be a disease or it could be something else. We just have no idea.”

One factor that influences the size, distribution and success of animal populations is the variety and types of plants that make up their habitats. 

“For animals, plants are the habitat, that’s what they live in, what they eat, that’s where they build their nests or their homes,” said Emily Minor, associate professor of biological sciences at the University of Illinois at Chicago. “In [Chicago’s] residential neighborhoods, people are dictating most of the plants … people are deciding what to put in their yard. In general, the animals are responding to the plants that people are putting out.”

Minor, who co-authored the 2011 Urban Environments paper and has done additional work on the monk parakeet with Pruett-Jones, said her lab has found a positive relationship between varieties of flowers and bees. For example, the more varied the flora, the more bees abound. The same goes for birds.

What people plant in their yards is a more significant determinant for the size of nearby bird populations than whatever forest preserves or parks may be close by. Even the prevalence of house cats can wreak havoc on the success of winged creatures in Chicago’s neighborhoods.

When analyzing exhaustive lists of parakeet sightings compiled by citizen scientists, Minor and her postdoctoral researcher Amélie Davis found that, in the southern half of the U.S., climate and vegetation seemed to be the environmental factors the monks were responding to as their distribution changed. Conversely, northern populations, like those found in Illinois, are drawn to urban spaces.

“It didn’t much matter what the environmental conditions were as long as there were lots of people there,” Minor said. “We speculated that that’s because they were really reliant on people in these locations, they were relying on the food or perhaps the warmth the cities were creating. They couldn’t survive in these northern locations without people nearby.”

As mysterious as the case of the monk parakeet might be, there are many others beyond these mossy-colored creatures that are changing in response to our actions, according to Joel Brown, an evolutionary ecologist and professor in the Department of Biological Sciences at the University of Illinois at Chicago.

“In any kind of urban setting, if a species is able to survive or thrive, it will always first acclimate,” Brown said. “The species that live around us in our human-dominated landscapes are going to evolve, but right now the million-dollar question is how fast and how much? The two are probably related.”

In Chicago, the European dandelion was not always capable of springing up from the soil and flowering in less than a week, Brown says. But regular lawn mowing coaxed the plant into keeping its leaves much lower to the ground to avoid being clipped, speeding up the rate at which it produces spores. Even the pests that buzz around our heads while we mow the lawn are changing—pesticide resistance in summertime stinging insects like yellow jackets is an evolutionary adaptation that is commonly seen nowadays.

In the early 1990s, Brown and his colleagues set up “Project Squirrel” in order to monitor changes in the gray and fox squirrel population in the Chicagoland area, zip code by zip code, neighborhood by neighborhood. Running the numbers of the respective urbanized squirrel populations against those made available by preserves such as the Morton Arboretum, Brown and his team have drawn conclusions about how similar species of animals function, both in ideal and adaptive habitats.

“The Morton Arboretum provides a wonderful study area for fox and gray squirrels because they’re operating pretty much as they would in the wild,” Brown said. “People don’t feed them, and there is a full complement of predators—coyotes, red-tailed hawks and foxes.”

Though the two species have different survival strategies, they basically maintain their numbers in much the same way they always have without humans influencing their daily experience. The ratio of fox squirrels to gray ones has essentially remained static inside the walls of the arboretum. The numbers of their city-dwelling counterparts have not. Since 1992, the Oak Park population of squirrels has flipped, from about 80 percent fox and 20 percent gray then, to 20 percent fox and 80 percent gray now, Brown said.

Fox squirrels tend to live on the margins of their habitats and thrive in the face of danger. They are risk-takers and are better adapted to evading predators. Grays, on the other hand, do better deep in the woods, where there are fewer predators that could easily detect them based on fur color. Brown said this large percentage swap between the two populations likely flipped because of dog leash laws being enacted, which effectively eliminated a large percentage of the squirrels’ urban predators. 

“Unwittingly, Oak Park has become a much safer place for squirrels and, as a consequence, the grays are outcompeting the foxes,” Brown said. “Now the best indicator of where you will find squirrels in Oak Park is where there are the lowest number of registered cats and dogs.”

Alberti’s paper makes note of shrinking fish sizes as a byproduct of human influence on animal evolution on a short timescale. About 20 years ago, Brown had firsthand experience of this working with a cod fishery off the coast of Massachusetts and Canada. The fishermen began to notice the cod they were catching were much smaller than those originally being netted and recorded by the 13 colonies.

“There were two schools of thought—one is that if you heavily harvest a species, they will simply be smaller because they won’t have as much of a chance to grow old,” Brown said. “The other is that a heavily harvested species will just tend to breed at a smaller size.”

Alberti stressed that although fostering and maintaining an awareness of the ecological influence of human behavior in urban environments is important to the preservation of plants and animals, humans are the ones that will end up suffering or benefiting from implementing the foresight. 

“We think on a timescale that is not planetary,” Alberti said. “We think on a much shorter timescale as we organize society and make decisions.”

Alberti said people often think of eco-evolutionary feedback as being low-risk for long-term human well-being because we think of evolution in the traditional sense—taking place in incremental changes over immense spans of time. Though this has been proved scientifically not to be the case, she remains hopeful that when necessity breeds invention, humans will rally and be able to engage in including the long-term future of the planet in our shorter-term plans. The pyramids and other architectural feats would not have been built without the capacity to imagine a scope of progress thousands of years long. The same goes for writing and legislating policies that take processes such as deforestation into consideration—being cognizant not only of the impact they will have on future generations but of immediate, less obvious ramifications. 

Brown calls this school of thought “evolutionary enlightened management”—making an effort to stay mindful not only of the ecological consequences of trying to preserve a species or ecosystem, but also the evolutionary consequences that every living thing will incur. 

“After 20 years of fussing about it, it’s a done deal,” Brown said. “We know there are behavioral changes, like with the squirrels, and there have been dramatic and demonstrated genetic changes, too. We have these robins that don’t migrate anymore, we have these Canada geese that don’t migrate anymore, these various species that live in our backyards. The challenge for urban areas is that without having a concerted research program to look at all of it, we’re going to end up like the fish. It’s kind of like watching your kids grow up too fast. ‘Oh my gosh, when did that happen?’ In this case we’re going to be in for many, many situations that are sometimes humorous and sometimes a bummer where we’ll wake up and say, ‘We were responsible for the rapid evolution of that species.’”