On-site and in-kind: Compensatory mitigation of California Coastal Zone habitat impacts between 2010 and 2018.

Planning development while minimizing negative impacts to sensitive habitats poses a challenge for global natural resource management. After impacts from development are avoided and minimized, remaining adverse impacts may be offset using compensatory mitigation. Along the California coast, the California Coastal Commission (CCC) regulates development and subsequent mitigation for allowable impacts. We reviewed publicly available CCC staff reports for approved projects that impacted coastal habitats and required compensatory mitigation from 2010 to 2018. The median project size was approximately 728 square meters and almost all permanent impacts were mitigated at a >1:1 ratio, with regional and habitat-specific planning regulations driving some variation across the state. We found that wetlands were the most frequently impacted and had higher mitigation ratios. Temporary impacts were almost always mitigated at a 1:1 ratio. While most mitigation was on-site and in-kind, mitigation that was required off-site had a median distance of 4.7 km from the site of impact. Restoration was the most frequent mitigation action, over creation, enhancement, or preservation, but proportions of each action varied across habitat types. While our findings suggest no net loss of habitat area within the California Coastal Zone, the net change in ecosystem function is dependent on the performance of the mitigation projects. This review is only the first step in evaluating the success of compensatory mitigation along California's coast.