Original article Elevated atmospheric CO 2 alters the arthropod community in a forest understory Jason Hamilton a , Arthur R. Zangerl b,1 , May R. Berenbaum b, c , Jed P. Sparks d , Lauren Elich a , Alissa Eisenstein b , Evan H. DeLucia c, e, * a Department of Biology and Environmental Studies Program, Ithaca College, Ithaca, NY 14850, USA b Department of Entomology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA c Institute of Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA d Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853, USA e Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA article info Article history: Received 19 September 2011 Accepted 4 May 2012 Available online 12 June 2012 Keywords: Spiders Insects Global change Carbon isotope Nitrogen isotope abstract The objective of this study was to determine the extent to which overall population sizes and community composition of arthropods in a naturally occurring forest understory are altered by elevated CO 2 . The Free Air Concentration Enrichment (FACE) method was used to fumigate large, replicated plots in the Piedmont region of North Carolina, USA to achieve the CO 2 concentration predicted for 2050 (w580 mll 1 ). In addition, the extent to which unrestricted herbivorous arthropods were spatially delimited in their resource acquisition was determined. Stable isotope data for spiders (d 13 C and d 15 N) were collected in ambient and elevated CO 2 plots and analyzed to determine whether their prey species moved among plots. Elevated CO 2 had no effect on total arthropod numbers but had a large effect on the composition of the arthropod community. Insects collected in our samples were identified to a level that allowed for an assignment of trophic classification (generally to family). For the groups of insects sensitive to atmospheric gas composition, there was an increase in the numbers of individuals collected in primarily predaceous orders (Araneae and Hymenoptera; from 60% to more than 150%) under elevated CO 2 and a decrease in the numbers in primarily herbivorous orders (Lepidoptera and Coleoptera; from 30 to 45%). Isotopic data gave no indication that the treatment plots represented a “boundary” to the movement of insects or that there were distinct and independent insect populations inside and outside the treatment plots. A simple two-ended mixing model estimates 55% of the carbon and nitrogen in spider biomass originated external to the elevated CO 2 plots. In addition to changes in insect perfor- mance, decreases in herbivorous arthropods and increases in predaceous arthropods may also be factors involved in reduced herbivory under elevated CO 2 in this forest. Ó 2012 Elsevier Masson SAS. All rights reserved. 1. Introduction Increases in atmospheric CO 2 from anthropogenic inputs have the potential to stimulate plant productivity (Long et al., 2006); however, predicted increases in productivity from enhanced photosynthesis and water use efficiency may be reduced by increased losses to insect herbivory (Aldea et al., 2005; Hamilton et al., 2005). Growth under elevated CO 2 changes myriad chem- ical and structural properties that affect the suitability of plant material to herbivorous insects (Robinson et al., 2012). Increased herbivory may result from elevated consumption rates on the part of herbivores compensating for reduced availability of growth- limiting foliar nitrogen (Bezemer and Jones, 1998) or impaired defense signaling and reduced defense investments in foliage grown under elevated CO 2 (Zavala et al., 2008). In contrast with simplified agro-ecosystems, the response of complex plant and animal communities to global change is highly variable (Tylianakis et al., 2008). Recent studies have documented that loss of foliage to arthropod herbivores decreases under elevated CO 2 in woody communities (Hamilton et al., 2004; Knepp et al., 2005; Stiling and Cornelissen, 2007), and that the fitness and in some cases the population size of herbivorous insects may decline in communities exposed to elevated CO 2 (Hillstrom and Lindroth, 2008; Hillstrom et al., 2010). The factors mediating this response are not well understood, although Knepp et al. (2007) demonstrated that elevated CO 2 reduced the nutritional quality of foliage of two species of Quercus for Antheraea polyphemus, and * Corresponding author. Tel: þ1 217 333 6177, þ1 217 778 533; fax: þ1 217 244 2057. E-mail addresses: jhamilton@ithaca.edu (J. Hamilton), maybe@illinois.edu (M.R. Berenbaum), jps66@cornell.edu (J.P. Sparks), laurengeryan@gmail.com (L. Elich), alissaeisenstein@gmail.com (A. Eisenstein), delucia@illinois.edu (E.H. DeLucia). 1 Deceased. Contents lists available at SciVerse ScienceDirect Acta Oecologica journal homepage: www.elsevier.com/locate/actoec 1146-609X/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.actao.2012.05.004 Acta Oecologica 43 (2012) 80e85