Native Recovery: Breaking the Cycle of Invasion

Although concepts such as thresholds, and sustainable trajectories are critical to most restoration projects, there has been little research done in this realm. Exotic populations may have to be decreased to a sufficiently low level for a sufficient duration for the community to reorganize on a different trajectory and be able to resist further invasion. Although there have been few tests of whether sustained control efforts can redirect the community on a sustainable trajectory, they are gaining widespread use in restoration as heuristic devices (Suding et al. 2004). In addition to a desired scenario, a threshold effect leading to an alternative trajectory may be undesirable: the exotic addressed by the control efforts could be replaced by other problematic exotics rather than the desired native species. This cycle of degradation where exotics replace other exotics may be potentially widespread although there have been few published accounts for plants.

Artichoke Thistle (Cynara cardunculus; CYCA) is a deeply-rooted perennial thistle that is a problematic invader in disturbed grasslands, especially in coastal California regions. It has invaded large areas (over 4,000 acres) of the Nature Reserve of Orange County (NROC), adjacent to UCI. The NROC, working with the Nature Conservancy (TNC), initiated a control program for CYCA involving direct application of herbicide to individual plants. Thousands of acres have been treated annually (and often continually) since 1994. While there is no question as to dramatic decline of CYCA due to this program, it is unclear what is replacing CYCA in the control areas, and whether the program will be sustainable following a major reduction in the active control program planned for 2015. This presents a great opportunity to test important assumptions and questions necessary to better understand how exotic control programs influence plant community dynamics.