Native Consumers and Seed Limitation Constrain the Restoration of a Native Perennial Grass in Exotic Habitats

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 experimental 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 restoration success: averaged across propagule density, consumers 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 establishment, 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 persistent, 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, especially when combined with restoration strategies that reduce competition with exotic plants.

Evaluating Restoration Methods across a Range of Plant Communities Dominated by Invasive Annual Grasses to Native Perennial Grasses

“…No linear increasing trend in predation was found over the study period, suggesting that the granivores did not learn the feed locations and, therefore, did not cue in on the feeding stations. Seed consumption has been found to reduce recruitment by 30 to 50% (Orrock et al 2008). Ants and rodents have been found to be responsible for most of the predation (Predavec 1997).…”

Multiple drivers of apparent competition reduce re-establishment of a native plant in invaded habitats

“…pulchra seeds added to restoration plots leads to increases in recruitment and N . pulchra adults (Orrock et al 2009).…”

“…Large consumers at our study site were mule deer Odocoileus hemionus . Live‐trapping of small‐mammal consumers in and around exclosures confirmed the presence of these consumers and demonstrated that exclosures were highly effective in manipulating their access; although a single deer mouse was captured in a single no‐consumer access exclosure, an average of 5 times more consumers were captured outside of no‐consumer exclosures (Orrock et al 2009). Our exclosures did not manipulate access by arthropod or avian consumers.…”

“…Although many factors may have contributed to the invasion of this system, including grazing, alteration of fire regimes, use of land for agriculture, and direct seeding (Heady 1977, Jackson 1985, Stromberg and Griffin 1996), several characteristics of this system may set the stage for apparent competition. Consumers are often ubiquitous (Lidicker 1989, Schiffman 2007) and can have large effects on plant populations and communities (Batzli and Pitelka 1970, Borchert and Jain 1978, Rice 1987, Lidicker 1989, Reichman and Seabloom 2002, Schiffman 2007, Orrock et al 2008, 2009), although most studies focus on the effect of one or a few consumer species within the larger suite of consumers, and studies often focus on exotic grazers (reviewed in Lidicker 1989, Schiffman 2007). Perennial native grasses are frequently seed‐limited (Hamilton et al 1999, Seabloom et al 2003, Orrock et al 2009), and evidence suggests that consumers may limit plant recruitment (Orrock et al 2009) and long‐term abundance in the community (Seabloom et al 2009).…”

“…We experimentally manipulated consumer access to areas seeded with the native perennial grass, Nassella pulchra , to directly examine whether apparent competition affected re‐establishment and whether the strength of apparent competition differed depending upon the suites of consumers with access. Nassella pulchra is thought to have once been a dominant component of California grasslands (Heady 1977) and is commonly the focus of restoration efforts (Seabloom et al 2003, Moyes et al 2005, Orrock et al 2009). Our experiment was designed to focus only on the indirect effect of apparent competition (i.e.…”