The grasslands of California represent one of the most dramatic biological invasions in the world: 9.2 million hectares (almost 25% of the entire state) are dominated by non-native plants. Jim Reichman and I have received funding from the National Science Foundation to evaluate an untested hypothesis of biological invasion: invasive plant species, which can be weedy, highly productive forms, may increase the relative abundance of generalist native consumers, increasing consumer pressure on native species above levels experienced in their evolutionary history. This hypothesis suggests that exotic species gain an advantage indirectly through their effect on native consumers (i.e. apparent competition) rather than through direct competition with native species.

Furthermore, we are characterizing how differences in consumer behavior among patches change the spatial extent of apparent competition. This component will provide an important link between behavior, trophic interactions, and biological invasions and gives rise to another untested possibility: apparent competition and patch geometry may determine the outcome of biological invasion. By investigating the spatial dynamics of community-level controls on invasion, this work will provide ‘rules of thumb’ to aid conservation biologists in prioritizing areas for protection and to guide the design of restoration efforts. More generally, it will provide insight into how processes that change geometry (e.g. habitat fragmentation) affect the overall course of invasions.

To further characterize the role of native consumers in affecting native plant restoration, I am collaborating with Marti Witter, an ecologist at the Santa Monica Mountains National Recreation Area. This collaboration uses exclosures to monitor the impact of various consumers on seeds and seedlings of Nassella pulchra. Preliminary results already suggest that consumers play a vital role in the outcome of restoration efforts.

We have also been exploring the role of apparent competition with other emerging invasive species beyond the grasslands of California. We are collaborating with Humberto Dutra and Dr. Robert Marquis (University of Missouri, St. Louis), to examine how two different resources (food and predation risk) provided by the invasive shrub Lonicera maackii might generate apparent competition with native plants.

Orrock, J. L., R.D. Holt, and M. L. Baskett. Refuge-mediated apparent competition in plant-consumer interactions. In press.
Ecology Letters.

Orrock, J. L. and M. S. Witter. In press. Multiple drivers of apparent competition reduce re-establishment of a native plant in invaded habitats.
Oikos.

Watling, J. I. and J. L. Orrock. In press. Measuring edge contrast using biotic criteria helps define edge effects on the density of an invasive plant.
Landscape Ecology.

Motheral, S. M., and J. L. Orrock.  In press. Gastropod herbivore preference for seedlings of two native and two exotic grass species.
American Midland Naturalist.

Orrock, J. L. and J. L. Hoisington-López.  2009. Mortality of exotic and native seeds in invaded and uninvaded habitats.
Acta Oecologica 35: 758-262. [e-mail me for a reprint]

Seabloom, E. W., E. T. Borer, B. A. Martin, and J. L. Orrock.  2009. Effects of long-term consumer manipulations on invasion in oak savannah communities.
Ecology 90: 1356-1365. [Reprint]

Orrock, J. L., M. S. Witter, and O. J. Reichman.  2009.  Native consumers and seed limitation constrain the restoration of a native perennial grass in exotic habitats.
Restoration Ecology 17: 148-157. [Reprint]

Orrock, J. L., M. S. Witter, and O. J. Reichman.  2008.  Apparent competition with an exotic plant reduces native plant establishment. 
Ecology 89: 1168-1174. [Reprint]