Competition, Seed Limitation, Disturbance, and Reestablishment of California Native Annual Forbs

Abstract

Abstract. Invasion by exotic species is a major threat to global diversity. The invasion of native perennial grasslands in California by annual species from the southern Mediterranean region is one of the most dramatic invasions worldwide. As a result of this invasion, native species are often restricted to low-fertility, marginal habitat. An understanding of the mechanisms that prevent the recolonization of the more fertile sites by native species is critical to determining the prospects for conservation and restoration of the native flora. We present the results of a five-year experiment in which we used seeding, burning, and mowing treatments to investigate the mechanisms that constrain native annuals to the marginal habitat of a Californian serpentine grassland. The abundance and richness of native species declined with increasing soil fertility, and there was no effect of burning or mowing on native abundance or richness in the absence of seeding. We found that native annual forbs were strongly seed limited; a single seeding increased abundance of native forbs even in the presence of high densities of exotic species, and this effect was generally discernable after four years. These results suggest that current levels of dominance by exotic species are not simply the result of direct competitive interactions, and that seeding of native species is necessary and may be sufficient to create viable populations of native annual species in areas that are currently dominated by exotic species.

Plant invasions and extinction debts

“…We assume that as invasions increasingly relegate native species to isolated patches (7,9,19), they can generate extinction debts via two main impacts (Fig. 1A): effects on patch size and effects on the dispersal permeability of the habitat matrix between patches.…”

“…The habitat is derived from serpentine parent material but is topographically heterogeneous, with rocky hummocks interspersed by more finely textured clay soils. Native annual forbs and native perennial grasses dominated this area before European annual grass invasion (9,23,24), but the forbs now occur on small rocky refugia, surrounded by a matrix of exotic grasses (8,9,19) (Fig. 1B).…”

“…However, recent syntheses show that competition from introduced species, and plant invaders in particular, has only rarely resulted in extinction (3)(4)(5)(6). This trend has emerged because, in the short term at least, invasive plants do not completely extirpate native plant species but rather reduce their distribution and abundance, often restricting them to isolated habitat refugia (7)(8)(9). Despite well-established cases of native plants occupying distinct refugia and outperforming invasive plant species in those habitats (8,10,11), the long-term dynamics of native species in these refugia are poorly understood.…”

“…We then parameterize and apply this model to understand the potential for invasive plant impacts on native annual plant persistence in a spatially heterogeneous serpentine soil landscape in California. Invasions are widespread in these landscapes, and they have likely reduced the connectivity and extent of local native patches within the broader metapopulation (9,19). Serpentine landscapes support a disproportionate number of rare and threatened plant species, and therefore have a high conservation value (20).…”

Efficacy of Exotic Control Strategies for Restoring Coastal Prairie Grasses

“…Recovery of Mediterranean grasslands is limited both by competition from numerous exotic grasses and forbs and a lack of native propagules Gaertner et al 2009;Seabloom et al 2003). Therefore, restoration strategies typically include efforts to both reduce exotic plant cover and reintroduce native species Stromberg et al 2007).…”