Native Consumers and Seed Limitation Constrain the Restoration of a Native Perennial Grass in Exotic Habitats John L. Orrock, 1,2,3 Martha S. Witter, 4 and O. J. Reichman 5 Abstract Native consumers and seed limitation may be particularly important in the restoration of native plants where they have been displaced by exotic plants. We used experimen- tal exclosures and seed additions to examine the role of native mammalian consumers and seeding density (500 or 1,000 seeds/m 2 ) in affecting the establishment of a native perennial grass, Purple needlegrass (Nassella pulchra), in the grasslands of California. To focus solely on consumers and propagule density, experimental areas were tilled and weeded. Consumers were important determinants of res- toration success: averaged across propagule density, con- sumers reduced N. pulchra seedling recruitment by nearly 30%, reduced seedling height by 44%, reduced plant establishment by 52%, and reduced reproductive tiller length by 43%. Small rodents affected seedling establish- ment, especially where seeding density was high but did not affect seedling height. Plots accessible by squirrels and rabbits exhibited significantly decreased seedling height and plant establishment, whereas there was no additional impact of allowing large consumers (i.e., deer) access. Despite strong, spatially variable effects of consumers, doubling seed density led to nearly doubled N. pulchra establishment on average. Consumer effects were persis- tent, shaping N. pulchra abundance in the subsequent growing season and remaining evident over 18 months after the experiment was initiated. Our work suggests that, despite strong consumer effects, seed addition may be a viable strategy for restoration of N. pulchra in invaded areas where it has been displaced by exotic plants, espe- cially when combined with restoration strategies that reduce competition with exotic plants. Key words: exotic plants, herbivory, granivory, grassland. Introduction Consumers are capable of affecting plant populations (e.g., Crawley 2000; Goheen et al. 2004), changing the structure and composition of plant communities (e.g., Bartholomew 1970; Hulme 1996; Howe & Lane 2004; Howe et al. 2006), and influencing the success of invasive plants (Mitchell et al. 2006). The effect of consumers may vary depending upon consumer foraging mode (Reichman 1979; Goheen et al. 2004), their preference for particular plants (Batzli & Pitelka 1970; Borchert & Jain 1978; Howe & Lane 2004; Howe et al. 2006), and particular plant life stages (Howe & Brown 1999). Given these varied con- sumer impacts, understanding how different consumers affect native plants is of paramount importance because consumers play a key role in the success of conservation (e.g., McGraw & Furedi 2005) and restoration efforts (Hatch et al. 1999; Howe & Brown 1999; Dyer 2003; Martin & Wilsey 2006). The effect of consumers on plant populations may also depend upon the density of propagules available (Crawley 2000; Turnbull et al. 2000; Clark et al. 2007). When seed density is high, it may overwhelm the ability of consumers to limit plant populations (Crawley 2000). However, high seed density may also attract mobile con- sumers (Holt & Kotler 1987) or lead to increases in the size of consumer populations (Holt 1977), thus increas- ing consumer pressure on plants. Interactions among consumers and seed density are also possible, e.g., avian granivores may be attracted to high seed densities, whereas rodent consumers may be more effective at altering community structure at low seed densities (Howe & Brown 1999). Interactions among consumers and seed density are of particular interest in the context of restoration efforts in areas with invasive plants be- cause native plants in invaded habitats may be seed limi- ted (Hamilton et al. 1999; Seabloom et al. 2003) and seed limitation may be driven by consumers (e.g., Orrock et al. 2006). Moreover, native consumers may target native plants (Mitchell et al. 2006). As such, when levels of natural seed production are reduced due to competi- tion with exotic plants (e.g., Dyer & Rice 1999), consum- ers may constrain restoration of native plants. 1 National Center for Ecological Analysis and Synthesis, University of California, Santa Barbara, CA 93101, U.S.A. 2 Department of Biology, Washington University, Saint Louis, MO 63130, U.S.A. 3 Address correspondence to J. Orrock, email orrock@wustl.edu 4 Santa Monica Mountains National Recreation Area, National Park Service, Thousand Oaks, CA 91360, U.S.A. 5 Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93101, U.S.A. Ó 2008 Society for Ecological Restoration International doi: 10.1111/j.1526-100X.2008.00384.x 148 Restoration Ecology Vol. 17, No. 1, pp. 148–157 JANUARY 2009