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Berkeley BIOLOGY 1B - Species Interactions Reverse Grassland Responses to Changing Climate

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DOI: 10.1126/science.1136401 , 640 (2007); 315Science et al.K. B. Suttle,to Changing ClimateSpecies Interactions Reverse Grassland Responses www.sciencemag.org (this information is current as of February 10, 2007 ):The following resources related to this article are available online at http://www.sciencemag.org/cgi/content/full/315/5812/640version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/cgi/content/full/315/5812/640/DC1 can be found at: Supporting Online Materialfound at: can berelated to this articleA list of selected additional articles on the Science Web sites http://www.sciencemag.org/cgi/content/full/315/5812/640#related-content http://www.sciencemag.org/cgi/content/full/315/5812/640#otherarticles, 5 of which can be accessed for free: cites 11 articlesThis article http://www.sciencemag.org/cgi/collection/ecologyEcology : subject collectionsThis article appears in the following http://www.sciencemag.org/help/about/permissions.dtl in whole or in part can be found at: this articlepermission to reproduce of this article or about obtaining reprintsInformation about obtaining registered trademark of AAAS. c 2007 by the American Association for the Advancement of Science; all rights reserved. The title SCIENCE is a CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the on February 10, 2007 www.sciencemag.orgDownloaded fromSpecies Interactions ReverseGrassland Responses toChanging ClimateK. B. Suttle,1*† Meredith A. Thomsen,2Mary E. Power1Predictions of ecological response to climate change are based largely on direct climatic effects onspecies. We show that, in a California grassland, species interactions strongly influence responsesto changing climate, overturning direct climatic effects within 5 years. We manipulated theseasonality and intensity of rainfall over large, replicate plots in accordance with projections ofleading climate models and examined responses across several trophic levels. Changes in seasonalwater availability had pronounced effects on individual species, but as precipitation regimes weresustained across years, feedbacks and species interactions overrode autecological responses towater and reversed community trajectories. Conditions that sharply increased production anddiversity through 2 years caused simplification of the food web and deep reductions in consumerabundance after 5 years. Changes in these natural grassland communities suggest a prominent rolefor species interactions in ecosystem response to climate change.Impacts of recent climate change on plantsand animals are already evident, as geograph-ic distributions shift poleward (1, 2) andtoward higher elevations (3, 4), phenologicalevents advance in time (5–7), and some speciesdisappear altogether (8). With further climatechange still expected, prediction of future im-pacts has become critical to conservation plan-ning and management. To forecast ecologicalchange under continued climate warming, how-ever, we need a better understanding of therelative importance of direct responses by indi-vidual species to climate versus responses medi-ated by changing interactions with resources,competitors, pathogens, or consumers (9–14). Weimposed projected future precipitation regimesover grassland in northern California to evaluatethe importance to ecosystem response of directeffects on grassland species versus indirect ef-fects arising from species interactions.Much of the California coastal region ex-periences a Mediterranean climate, characterizedby wet winters and long summer droughts. Eco-logical responses to climate change in regionswith Mediterranean climate regimes may bestrongly driven by the redistribution of water intime and space (15). Changes in seasonal wateravailability that affect plant phenology, for ex-ample, could lead to temporal mismatch betweenresource availability and consumer demand (16),which can have important effects on resourceflow and ecosystem function (17). General cir-culation models developed at the Hadley Centrefor Climate Prediction and Research (HadCM2)and the Canadian Centre for Climate Modelingand Analysis (CCM1) (18) predict substantialincreases in precipitation over most of Californiabut differ in the projected seasonality of theseincreases. The Hadley model calls for all addi-tional rain to fall during the current winter rainyseason, whereas the Canadian model projectsincreased rainfall extending into the current sum-mer drought. The discrepancy between the twoscenarios may be critical to the fate of grass-land ecosystems in California, where summerdrought severely constrains plant growth andthe timing of rainfall is more important to an-nual production and species composition thanthe amount (19–22).In 2001, we began a large-scale rainfall ma-nipulation in a northern California grassland toexamine the consequences of these two projectedregimes for production and diversity of grasslandplants and invertebrates. In a grassland at theAngelo Coast Range Reserve in Mendocino County,California (39° 44' 17.7″ N, 123° 37' 48.4″ W),18 circular 70-m2plots were subjected to one ofthree watering treatments: a winter addition ofwater (January through March), a spring addi-tion of water (April through June), and anunmanipulated ambient control (Fig. 1). Eachwatered plot received about 44 cm of sup-plementary water over ambient rainfall per year,roughly a 20% increase over mean annual pre-cipitation but within natural variability in bothamount and timing at the study site (fig. S1). We1Department of Integrative Biology, University of Califor-nia, Berkeley, CA 94720, USA.2Department of Biology,University of Wisconsin, La Crosse, WI 54601, USA.*Present address: Earth and Planetary Science, Universityof California, Berkeley, CA 94720, USA.†To whom correspondence should be addressed. E-mail:[email protected]. 1. (A ) Bird’s-eye view of experimental communities in July 2002. Anearby road is visible as a gray strip, top right. Research de scribed hereis from 18 open-grassland plots (18 additional plots were used in sep-arate resear ch). (B) Schematic representation o f an experimental p lot,shown as partitioned for measurement of plant biomass (30 900-cm2subplots, small squares), p lant


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Berkeley BIOLOGY 1B - Species Interactions Reverse Grassland Responses to Changing Climate

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