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.

Facilitation in Caribbean coral reefs: high densities of staghorn coral foster greater coral condition and reef fish composition.

Recovery of the threatened staghorn coral (Acropora cervicornis) is posited to play a key role in Caribbean reef resilience. At four Caribbean locations (including one restored and three extant populations), we quantified characteristics of contemporary staghorn coral across increasing conspecific densities, and investigated a hypothesis of facilitation between staghorn coral and reef fishes. High staghorn densities in the Dry Tortugas exhibited significantly less partial mortality, higher branch growth, and supported greater fish abundances compared to lower densities within the same population. In contrast, partial mortality, branch growth, and fish community composition did not vary with staghorn density at the three other study locations where staghorn densities were lower overall. This suggests that density-dependent effects between the coral and fish community may only manifest at high staghorn densities. We then evaluated one facilitative mechanism for such density-dependence, whereby abundant fishes sheltering in dense staghorn aggregations deliver nutrients back to the coral, fueling faster coral growth, thereby creating more fish habitat. Indeed, dense staghorn aggregations within the Dry Tortugas exhibited significantly higher growth rates, tissue nitrogen, and zooxanthellae densities than sparse aggregations. Similarly, higher tissue nitrogen was induced in a macroalgae bioassay outplanted into the same dense and sparse aggregations, confirming greater bioavailability of nutrients at high staghorn densities. Our findings inform staghorn restoration efforts, suggesting that the most effective targets may be higher coral densities than previously thought. These coral-dense aggregations may reap the benefits of positive facilitation between the staghorn and fish community, favoring the growth and survivorship of this threatened species.