The Effect of Consumers and Mutualists of Vaccinium membranaceum at Mount St. Helens: Dependence on Successional Context Suann Yang 1 * ¤a , Eelke Jongejans 2 , Sylvia Yang 3¤b , John G. Bishop 4 1 School of Biological Sciences, Washington State University, Pullman, Washington, United States of America, 2 Radboud University Nijmegen, Institute for Water and Wetland Research, Department of Experimental Plant Ecology, Nijmegen, The Netherlands, 3 Department of Biology, University of Washington, Seattle, Washington, United States of America, 4 School of Biological Sciences, Washington State University, Vancouver, Washington, United States of America Abstract In contrast to secondary succession, studies of terrestrial primary succession largely ignore the role of biotic interactions, other than plant facilitation and competition, despite the expectation that simplified interaction webs and propagule- dependent demographics may amplify the effects of consumers and mutualists. We investigated whether successional context determined the impact of consumers and mutualists by quantifying their effects on reproduction by the shrub Vaccinium membranaceum in primary and secondary successional sites at Mount St. Helens (Washington, USA), and used simulations to explore the effects of these interactions on colonization. Species interactions differed substantially between sites, and the combined effect of consumers and mutualists was much more strongly negative for primary successional plants. Because greater local control of propagule pressure is expected to increase successional rates, we evaluated the role of dispersal in the context of these interactions. Our simulations showed that even a small local seed source greatly increases population growth rates, thereby balancing strong consumer pressure. The prevalence of strong negative interactions in the primary successional site is a reminder that successional communities will not exhibit the distribution of interaction strengths characteristic of stable communities, and suggests the potential utility of modeling succession as the consequence of interaction strengths. Citation: Yang S, Jongejans E, Yang S, Bishop JG (2011) The Effect of Consumers and Mutualists of Vaccinium membranaceum at Mount St. Helens: Dependence on Successional Context. PLoS ONE 6(10): e26094. doi:10.1371/journal.pone.0026094 Editor: Darren Mark Evans, University of Hull, United Kingdom Received June 13, 2011; Accepted September 19, 2011; Published October 20, 2011 Copyright: ß 2011 Yang et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was partially funded with a Mazamas Graduate Student Research Grant, a Washington Native Plant Society Research Grant, a WSU Betty Higinbotham Travel Award and a National Science Foundation Graduate Research Fellowship to Suann Y, National Science Foundation award DEB-0089843 to JB and National Science Foundation award DEB-0614065 and Netherlands Organisation for Scientific Research veni grant 863.08.006 to EJ. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: suy14@psu.edu ¤a Current address: Presbyterian College, Clinton, South Carolina, United States of America ¤b Current address: Department of Environmental Science and Policy, University of California Davis, Davis, California, United States of America Introduction The extreme intensity of the disturbance that results in primary succession is generally considered to be responsible for the differences in community assembly between primary and secondary succession. Ecologists have identified a variety of processes whose importance is magnified during primary succession, including amelioration of the physical environment, dispersal limitation, facilitative interactions and stochastic assembly [1,2,3,4,5,6,7,8,9]. In contrast, the effect of consumers on successional plant communities is regarded as more important in secondary succession [10,11] and in marine systems [12,13,14,15]. Similarly, because of their relative scarcity in primary succcesion, mutualists are also thought to be more important in secondary succession, and the ability to grow and reproduce without their aid is considered an important attribute of primary successional plant colonists [8]. Although interactions with consumers and mutualists are considered relatively unimportant for primary succession, a variety of studies indicate that they may strongly affect colonization of plant populations. For example, in early succession, consumers may temporarily escape their enemies and cause unusually large effects on plant population growth and spatial spread [15,16,17,18]. Likewise, the temporary absence of mutualists, such as pollinators (e.g., [19], mycorrhizae [20] and nitrogen- fixing symbionts [e.g., 21,22] may disadvantage or temporarily exclude colonizing plant species that are dependent upon them. These studies suggest that the limiting effects of biotic interactions on colonizing plants can be greatly amplified during primary succession. This temporary inflation may be caused by successional properties of interaction webs. Primary successional sites, being the most intensely disturbed, generally have few species, low productivity, and support fewer trophic levels [23], whereas secondary successional sites generally possess more complex sets of interacting species. Under these circumstances, consumers may anomalously impact a primary successional plant population, because secondary consumers or competitors that might weaken the interaction are temporarily lacking. These temporarily strong effects of biotic interactions may then translate to higher negative interaction strengths (i.e., effects on population dynamics) in primary successional communities. Howev- er, it is not yet known whether the impact of biotic interactions is PLoS ONE | www.plosone.org 1 October 2011 | Volume 6 | Issue 10 | e26094