Cities as a glimpse of the future

This is a guest post by our Research Associate Elsa Youngsteadt about the work and meaning behind her new research published in Global Change Biology.

About a year ago, I found myself sitting ruefully in a patch of chiggery grass by the side of the road near the little town of Bahama, North Carolina, waiting for a tow truck. I had stuck the lab pickup firmly in a ditch. It was tilted at an embarrassing, sickening angle and had one wheel lodged against the mouth of a culvert. Helpful passers-by with chains and four-wheel drives kindly offered to pull me out, but really only made matters worse.

My memory is already fuzzy about the sequence of events, but somewhere in there—

Gloomy scales and the beetle that loves them. Each white or gray bump is a gloomy scale. The twice stabbed lady beetle is one of their predators. Photo: S.D. Frank

Gloomy scales and the beetle that loves them. Each white or gray bump is a gloomy scale. The twice stabbed lady beetle is one of their predators. Photo: S.D. Frank

between slipping into the ditch, the failed rescue attempts, and the final arrival of the giant tow truck—I did actually hike into the woods and get what I came for: eight slender red maple branches, clipped from trees growing in NC State’s Hill Forest.

I found my way to this particular spot, ditch and all, by following the trail of a plant biologist who had collected maple branches there more than 40 years ago during the height of the Nixon administration and the Vietnam War. In those days, the forest was cooler. The fevered dog days of summer now average about 1.4 degrees C (about 2.5 degrees F) hotter than they did then—and that should make a difference to the trees and the insects that live on them.

Specifically, it ought to make a difference to gloomy scale insects. These little sap-sucking insects seem to like it hot. My colleague Adam Dale has been studying gloomy scales in the city of Raleigh, and he’s found that street trees in the hottest parts of the city have far more scales—sometimes 200 times more–than those in the cooler parts of the city.

The scales drink tree juices, so more scales are bad for trees. A couple of degrees

Sad, bedraggled, gloomy scale infested red maple trees. Photo: SD Frank

Sad, bedraggled, gloomy scale infested red maple trees. Photo: SD Frank

warming can make the difference between a stately shade tree and a sad, bedraggled specimen with dead branches, sparse leaves, and grimy, scale-encrusted bark.

We thought that if warming gives scales such a powerful boost in the city, global warming could do the same thing for scale insects in rural forests. But we still had no direct evidence that what happens in the city represents what happens in rural areas over time.

This seemed like hard evidence to get. Unlike birds and butterflies, the drab, millimeter-long gloomy scale has not invited enthusiastic long-term monitoring. But perhaps we could scavenge scale-insect information from another source—and this is why I became extremely grateful to scores of plant biologists like the one who archived a foot-long maple twig from Hill Forest in 1971.

These historical plant specimens are stored in collections known as herbaria, where they are affixed to stiff pieces of paperboard, labeled, and stacked in mothball-scented cabinets. It turns out that many of these old twigs still have scale insects intact, stuck firmly but inconspicuously to the spots where they once lived.

An herbarium specimen used in the study. Photo: EK Youngsteadt

An herbarium specimen used in the study. Photo: EK Youngsteadt

It made perfect sense that they would be there, but it still felt outlandish when, only 12

branches into my first search in the UNC Herbarium, there was a gloomy scale—the same species that burdens our urban red maples. It was beautifully preserved, looking like it was collected last week instead of 30 years ago. Even on 100-year old branches, the scales looked perfect.

So I counted them. And kept counting them on more than 300 historical specimens from the southeastern US, then matched up their abundance with historical temperatures for the year and location where each specimen was collected.

There it was: During relatively cool historical time periods, only 17% of branches had scale

Gloomy scale covers preserved on an old herbarium specimen. Photo:EK Youngsteadt

Gloomy scale covers preserved on an old herbarium specimen. Photo:EK Youngsteadt

insects. But during relatively hot periods, 36% were infested. In other words, scale-infested branches were more than twice as common during hot periods than cool periods—exactly as we would expect if scale insects benefit from warming in rural forests as they do in the city. Furthermore, the most heavily infested twigs were ones that had grown at temperatures similar to those of modern urban Raleigh.

But the historical specimens weren’t the whole story. The past several years have been warmer than even the historically warm time periods, so to test our prediction, we needed to go back to places where those old branches were originally collected, and see if their scale infestations had actually gotten worse.

Thanks to the careful records of those past plant collectors, I was able to track down 20 of the forest sites across North Carolina where red maple branches were collected in the ‘70s, ‘80s, and ‘90s (and only put the truck in a ditch at one of them). At 16 of the 20 sites, gloomy scale populations were denser than they were on the original branches from the same locations. Overall, I found about five times more scales in 2013 than in the earlier decades.

Careful records and herbarium tags from the past helped Elsa relocate the collection sites. Photo: EK Youngsteadt

Careful records and herbarium tags from the past helped Elsa relocate the collection sites. Photo: EK Youngsteadt

This isn’t good news, but it’s also not time to panic about gloomy scales killing our forests. Although the rural scale insects clearly benefited from warming, just as they do in Raleigh, they still never got as abundant as the ones we see in town. The reasons for that difference are an open question (I have some guesses, but that’s a different story). So, although I’d put money on gloomy scales getting more common in rural North Carolina over the next several decades, I wouldn’t yet say how much more common.

But this really isn’t just about gloomy scale. It’s about cities as an advance guard of climate change. If we can look at scales’ response to urban warming and correctly predict their increased abundance due to global warming, can we do it for other organisms, too? Can we do it for functions, like pollination and biological control of pests?

I hope we can start watching urban ecosystems for problem insects and using that information to stand forewarned about future ecological changes in natural areas. The experiments we have made by paving our cities and making them heat up may have much more to tell us about how organisms will handle future warming.

This post is based on a new study:

Youngsteadt, E., Dale, A.G., Terando, A.J., Dunn, R.R. and Frank, S.D. 2014. Do cities simulate climate change? A comparison of herbivore response to urban and global warming. Global Change Biologydoi: 10.1111/gcb.12692.  PDF

Advertisements

Hot in the City: urban heat and the future of trees

In this guest post our PhD student, Emily Meineke, discusses her new paper in PLoS One and the the city life of plants, pests, and people…..

Most people live in cities. We walk between tall buildings and eat food grown thousands of

Emily hard at work counting scales in climate chambers. Photo by Becky Kirkland/NC State University Communications

Emily hard at work counting scales in climate chambers. Photo by Becky Kirkland/NC State University Communications

miles away. Urbanization brings big changes, and these changes have consequences—good and bad—for the creatures live with us, from microbes to mammals. Two big questions are: How do we change the species around us, and how do they change us? Now that most people live in cities, urban species are at the center of these questions.

More so than in any other ecosystems, humans engineer the conditions in which other species live in cities. This engineering includes intentional consequences (streets and their traffic) but also unintended ones, such as the urban heat island effect.

Imagine what cities used to look like before industrialization: a cart, a horse, and a dirt road. Imagine that cart becomes a car. Then the dirt under that car becomes a sidewalk, and the wooden shop beside the car becomes a modern building. Every part of the built environment that was wood or some other soft material becomes plastic, cement, or metal. Most of the materials in cities now are impermeable, meaning water can’t get in, and heat gets trapped. Cities are seas of man-made hardscape that hold in heat during the day and slowly release it at night, so that when surrounding rural areas are cooling off for the evening, cities are still heating themselves up from everywhere. This excess heat, the urban heat island effect, is changing the urban fauna, including us.

Thermal map of the Raleigh urban heat island.

Thermal map of the Raleigh urban heat island.

Hotter cities disadvantage certain groups of our own species that are sensitive to high temperatures. They can also kill some plants, whether via drought or the direct effects of heat. However, the vast majority of animals that live around us need heat from outside their bodies to survive. Some urban warming could be a good thing for some of these species, including some insects, spiders, and roly polies, just to name a few.

In this big story, I study one small but (I think) consequential piece: how urban warming affects scale insects (photo), tree pests closely related to aphids that insert their straw-like mouthparts into trees and suck out their phloem. Sometimes they kill street trees, but more often they weaken them.

My new research, published in PLoS One, shows that urban warming causes scale insects

Oak lecanium scale on willow oak twigs. Photo by Becky Kirkland/NC State University Communications

Oak lecanium scale on willow oak twigs. Photo by Becky Kirkland/NC State University Communications

to become more abundant in cities. Warm a street tree and the scale insects on it become much, more dense. This happens even in the lab, though the scale insects from warm trees seem better able to take advantage of the heat. It may be they have evolved so as to like it hot, or at least hotter. Warming-driven scale insect outbreaks are likely to be bad news for urban plants. And because urban plants remove pollutants from the air and cool cities, they might be bad news for us too.

Knowing what urban heat does to pests can help us predict changes as global temperatures climb. Overall, between killing or weakening trees via drought and pest insects and causing human health issues, urban warming seems like bad news.

This is not to say I begrudge the scale insects. They do what they have long done; they suck (plant juices). They are fascinating and poorly understood, and I plan on spending the next years of my life figuring out their mysteries. The good news is, we know a lot about how to mitigate the effects of urban warming. Plant a tree. Paint your roof white. Or, better yet, plant a tree on your roof. But when you do, keep an eye on the scale insects and tell me what you see. They will be there, I guarantee you. They always are – the real question is how abundant they will be, whether or not they have prospered in the luxuriance of urban warmth.

There will be more to this story (I’m just starting my thesis), so stay tuned. Or if you aren’t the sit and wait type, go outside and start looking at what else is going on in the ecology of the city. Scales are just the tip of the urban life-berg. There are beneficial microbes living inside the scales. Sometimes the scale may be the unlucky victim of a parasitoid wasp. Go deeper and you might find that the offspring of that parasitoid wasp growing inside the scale could also be unlucky, themselves the host of a hyper-parasitoid wasp. No one said urban life was simple, it is just relevant and more so every day.

This work was supported by a grant from the USGS Southeast Regional Climate Science Center to RRD and SDF. RRD was also supported by NASA Biodiversity Grant (ROSES-NNX09AK22G) and an NSF Career grant (0953390). SDF was also supported by grants from USDA Southern Region IPM (2010-02678), North Carolina Nursery and Landscape Association, the Horticultural Research Institute, and the USDA IR-4 Project. EKM was also funded by the NCSU Department of Entomology, and an EPA STAR Fellowship.

The research described in this paper has been funded wholly or in part by the United State Environmental Protection Agency (EPA) under the Science to Achieve Results (STAR) Graduate Fellowship Program. EPA has not officially endorsed this publication and the views expressed herein may not reflect the views of the EPA.

The project described in this publication was supported by Grant/Cooperative Agreement Number G10AC00624 from the United States Geological Survey.  Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the USGS