Urbanization disrupts latitude-size rule in 17-year cicadas.

Holly L. Menninger,Nana S. Boateng,DeAnna E. Beasley,Robert R. Dunn,Clint A. Penick

doi:10.1002/ece3.3879

Abstract

Many ectotherms show a decrease in body size with increasing latitude due to changes in climate, a pattern termed converse Bergmann's rule. Urban conditions—particularly warmer temperatures and fragmented landscapes—may impose stresses on development that could disrupt these body size patterns. To test the impact of urbanization on development and latitudinal trends in body size, we launched a citizen science project to collect periodical cicadas (Magicicada septendecim) from across their latitudinal range during the 2013 emergence of Brood II. Periodical cicadas are long‐lived insects whose distribution spans a broad latitudinal range covering both urban and rural habitats. We used a geometric morphometric approach to assess body size and developmental stress based on fluctuating asymmetry in wing shape. Body size of rural cicadas followed converse Bergmann's rule, but this pattern was disrupted in urban habitats. In the north, urban cicadas were larger than their rural counterparts, while southern populations showed little variation in body size between habitats. We detected no evidence of differences in developmental stress due to urbanization. To our knowledge, this is the first evidence that urbanization disrupts biogeographical trends in body size, and this pattern highlights how the effects of urbanization may differ over a species’ range.

Highlights

Urban habitats are characterized by increased temperatures and higher prevalence of environmental stressors that impact the biological processes of organisms relative to those living in rural environments (Grimm et al, 2008; Oke, 1973)
If cicadas at the southern end of their range are already living close to their physiological maximum, we expected additional heat imposed by cities and other urban stressors to result in a reduction in body size and/or an increase in fluctuating asymmetry
We used geometric morphometric techniques to test for measurement error, assess presence of other developmental asymmetries, and extract fluctuating asymmetry (FA) and wing size values using MorphoJ software (Klingenberg, 2011; Klingenberg & Monteiro, 2005)

Summary

| INTRODUCTION

Urban habitats are characterized by increased temperatures and higher prevalence of environmental stressors that impact the biological processes of organisms relative to those living in rural environments (Grimm et al, 2008; Oke, 1973). These two patterns are known, respectively, as Bergmann’s rule and converse Bergmann’s rule (Bergmann, 1848; Mousseau, 1997) and have been found to hold often, if not always (Blanckenhorn & Demont, 2004) In ectotherms, these patterns are likely due to direct effects of temperature and seasonal length on the fitness and physiology of individual organisms. If cicadas at the southern end of their range are already living close to their physiological maximum, we expected additional heat imposed by cities and other urban stressors to result in a reduction in body size and/or an increase in fluctuating asymmetry. By comparing rural and urban cicadas across their range, we evaluate how urbanization can have different impacts on development for cicadas and potentially other broadly distributed ectotherms

| MATERIALS AND METHODS

| DISCUSSION

DATA ACCESSIBILITY