Ecology, 96(4), 2015, pp. 1052–1061 Ó 2015 by the Ecological Society of America Apparent competition and native consumers exacerbate the strong competitive effect of an exotic plant species JOHN L. ORROCK, 1,5 HUMBERTO P. DUTRA, 2 ROBERT J. MARQUIS, 3 AND NICHOLAS BARBER 4 1 Department of Zoology, University of Wisconsin, Madison, Wisconsin 53704 USA 2 Department of Natural Sciences, Life University, Marietta, Georgia 30060 USA 3 Department of Biology and Whitney R. Harris World Ecology Center, University of Missouri, St. Louis, Missouri 63121 USA 4 Department of Biological Sciences, Northern Illinois University, DeKalb, Illinois 60015 USA Abstract. Direct and indirect effects can play a key role in invasions, but experiments evaluating both are rare. We examined the roles of direct competition and apparent competition by exotic Amur honeysuckle (Lonicera maackii ) by manipulating (1) L. maackii vegetation, (2) presence of L. maackii fruits, and (3) access to plants by small mammals and deer. Direct competition with L. maackii reduced the abundance and richness of native and exotic species, and native consumers significantly reduced the abundance and richness of native species. Although effects of direct competition and consumption were more pervasive, richness of native plants was also reduced through apparent competition, as small-mammal consumers reduced richness only when L. maackii fruits were present. Our experiment reveals the multiple, interactive pathways that affect the success and impact of an invasive exotic plant: exotic plants may directly benefit from reduced attack by native consumers, may directly exert strong competitive effects on native plants, and may also benefit from apparent competition. Key words: Amur honeysuckle; apparent competition; Busch Wildlife Conservation Area, Missouri, USA; direct effects; indirect effects; invasive plants; Lonicera maackii; plant recruitment. INTRODUCTION Invasive species can have transformative effects on the habitats they invade, altering the diversity of native plants and animals (Mack et al. 2000, Vila et al. 2011), affecting the dynamics of disease (Allan et al. 2010, Mack and Smith 2011), and potentially altering ecosys- tem function (Vila et al. 2011). Mounting evidence suggests that many invasive plants can have strong effects on native taxa via both direct effects (i.e., interactions between two species) and indirect effects (White et al. 2006), i.e., effects of exotic species on native species that are mediated through a third species. For example, invasive plants may have direct effects on native plant communities via competition for resources, such as light (Levine et al. 2003). They may also affect native plants indirectly by intensifying consumer pres- sure (i.e., apparent competition; Holt 1977) by increas- ing the abundance of consumers (Noonburg and Byers 2005) or by altering consumer foraging behavior (Orrock et al. 2010a). Despite evidence that both direct effects of competitors and consumers can affect the success of exotic organisms (Levine et al. 2004, Parker et al. 2006) and evidence that indirect effects like apparent competition can be important in the context of invasions (White et al. 2006, Orrock et al. 2008, Enge et al. 2013), the relative importance of both competition and apparent competition is seldom studied because it would require simultaneous manipulation of both direct competitive interactions and consumer access to com- petitors (Chaneton and Bonsall 2000, Orrock et al. 2010b). Understanding the relative importance of direct competition and the indirect effect of apparent compe- tition is critical because the likelihood of an invasion and the effects of an invasion on native taxa depend upon the ecological forces that promote exotic organism success. For example, if invasion is largely caused by apparent competition, then manipulation of consumer pressure may be sufficient to alter the course of the invasion (Orrock et al. 2010b). If exotic plants are superior competitors but also create greater consumer pressure on native plants (i.e., both direct competition and apparent competition), then theory suggests that invasions are expected to be more likely and may be more difficult to reverse (Orrock et al. 2010b). One difficulty in examining the interplay of competi- tion and apparent competition is that both can arise via multiple mechanisms; as such, determining whether either (or both) is important requires evaluating the multiple mechanisms that may be involved. For example, plant establishment and abundance may be limited by direct competition for light, competition for Manuscript received 16 April 2014; revised 12 August 2014; accepted 15 August 2014; final version received 9 September 2014. Corresponding Editor: T. J. Valone. 5 E-mail: jorrock@wisc.edu 1052