Following the removal of an invasive species, when can natives recover without additional intervention? How can we assess when additional intervention is necessary? Is there some threshold reduction for sustainable restoration to occur? When can the application of ideas relating to novel ecosystems optimize biodiversity and ecosystem services?
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 an extended 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). Aside from the desired scenario, a threshold effect may lead to an alternative and undesirable trajectory: 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). 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 the dramatic decline of CYCA due to this program, it is unclear what is replacing CYCA in the treated areas, and whether the program will be sustainable following a major reduction in control efforts 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.
Restoration across large and complex systems requires a full understanding of the lands marked for restoration and careful prioritization of restoration sites based on current and historical site conditions, condition of the surrounding lands and logistical limitations to management (i.e., access and resources). The expanse of NROC contains many restoration sites that provide a full gradient of restoration efforts. These efforts range from sites receiving no management to those receiving high inputs of resources such as: plant material, soil amendments and frequent maintenance. Gradients such as these, when also occurring along environmental and land use history gradients, provide further opportunity to examine thresholds and begin to understand drivers of restoration success. Identifying key drivers of restoration success within the environment, land use histories and previous management is critical to increasing the likelihood of successful restoration and land management programs.